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Down Syndrome

Down Syndrome

Definition

Down syndrome (DS) is the most common cause of mental retardation and malformation in a newborn. It occurs because of the presence of an extra chromosome. It was first described in 1866 by Dr. John L. H. Down (18281896), an English physician.

Down syndrome occurs about once in every 800 births. It is estimated that about 6,000 children are born with DS each year in the United States.

Description

Chromosomes are the units of genetic information that exist within every cell of the body. Twenty-three distinctive pairs, or 46 total chromosomes, are located within the nucleus (central structure) of each cell. When a baby is conceived by the combining of one sperm cell with one egg cell, the baby receives 23 chromosomes from each parent, for a total of 46 chromosomes. Sometimes, an accident in the production of a sperm or egg cell causes that cell to contain 24 chromosomes. This event is referred to as nondisjunction. When this defective cell is involved in the conception of a baby, that baby will have a total of 47 chromosomes. The extra chromosome in Down syndrome is labeled number 21. For this reason, the existence of three such chromosomes is sometimes referred to as trisomy 21.

In a very rare number of Down syndrome cases (about 1-2%), the original egg and sperm cells are completely normal. The problem occurs sometime shortly after fertilization; during the phase where cells are dividing rapidly. One cell divides abnormally, creating a line of cells with an extra chromosome 21. This form of genetic disorder is called a mosaic. The individual with this type of Down syndrome has two types of cells: those with 46 chromosomes (the normal number), and those with 47 chromosomes (as occurs in Down syndrome). Some researchers have suggested that individuals with this type of mosaic form of Down syndrome have less severe signs and symptoms of the disorder.

Another relatively rare genetic accident which can cause Down syndrome is called translocation. During cell division, the number 21 chromosome somehow breaks. A piece of the 21 chromosome then becomes attached to another chromosome. Each cell still has 46 chromosomes, but the extra piece of chromosome 21 results in the signs and symptoms of Down syndrome. Translocations occur in about 3-4% of cases of Down syndrome.

Down syndrome occurs in about one in every 800-1,000 births. It affects an equal number of boys and girls. Less than 25% of Down syndrome cases occur due to an extra chromosome in the sperm cell. The majority of cases of Down syndrome occur due to an extra chromosome 21 within the egg cell supplied by the mother (nondisjunction). As a woman's age (maternal age) increases, the risk of having a Down syndrome baby increases significantly. For example, at younger ages, the risk is about one in 4,000. By the time the woman is age 35, the risk increases to one in 400; by age 40 the risk increases to one in 110; and by age 45 the risk becomes one in 35. There is no increased risk of either mosaicism or translocation with increased maternal age.

Down syndrome sometimes occurs together with such other developmental disorders as Rett syndrome. Although such double diagnoses are very rare, it is important for parents to recognize that the presence of one chromosomal abnormality does not exclude the possibility that their child may have a second anomaly.

Causes and symptoms

While Down syndrome is a chromosomal disorder, a baby is usually identified at birth through observation of a set of common physical characteristics. Babies with Down syndrome tend to be overly quiet, less responsive, with weak, floppy muscles. Furthermore, a number of physical signs may be present. These include:

  • flat appearing face
  • small head
  • flat bridge of the nose
  • smaller than normal, low-set nose
  • small mouth, which causes the tongue to stick out and to appear overly large
  • upward slanting eyes
  • extra folds of skin located at the inside corner of each eye, near the nose (called epicanthal folds)
  • rounded cheeks
  • small, misshapen ears
  • small, wide hands
  • an unusual, deep crease across the center of the palm (called a simian crease)
  • a malformed fifth finger
  • a wide space between the big and the second toes
  • unusual creases on the soles of the feet
  • overly-flexible joints (sometimes referred to as being double-jointed)
  • shorter than normal height

Other types of defects often accompany Down syndrome. About 30-50% of all children with Down syndrome are found to have heart defects. A number of different heart defects are common in Down syndrome, including abnormal openings (holes) in the walls that separate the heart's chambers (atrial septal defect, ventricular septal defect). These result in abnormal patterns of blood flow within the heart. The abnormal blood flow often means that less oxygen is sent into circulation throughout the body. Another heart defect that occurs in Down syndrome is called Tetralogy of Fallot. Tetralogy of Fallot consists of a hole in the heart, along with three other major heart defects.

Malformations of the gastrointestinal tract are present in about 5-7% of children with Down syndrome. The most common malformation is a narrowed, obstructed duodenum (the part of the intestine into which the stomach empties). This disorder, called duodenal atresia, interferes with the baby's milk or formula leaving the stomach and entering the intestine for digestion. The baby often vomits forcibly after feeding, and cannot gain weight appropriately until the defect is repaired.

Other medical conditions that occur in patients with Down syndrome include an increased chance of developing infections, especially ear infections and pneumonia ; certain kidney disorders; thyroid disease (especially low or hypothyroid); hearing loss ; vision impairment requiring glasses (corrective lenses); and a 20-times greater chance of developing leukemia (a blood disorder).

Development in a baby and child with Down syndrome occurs at a much slower than normal rate. Because of weak, floppy muscles (hypotonia), babies learn to sit up, crawl, and walk much later than their normal peers. Talking is also quite delayed. The level of mental retardation is considered to be mild-to-moderate in Down syndrome. The actual IQ range of Down syndrome children is quite varied, but the majority of such children are in what is sometimes known as the trainable range. This means that most people with Down syndrome can be trained to do regular self-care tasks, function in a socially appropriate manner in a normal home environment, and even hold simple jobs.

As people with Down syndrome age, they face an increased chance of developing the brain disease called Alzheimer's (sometimes referred to as dementia or senility). Most people have a six in 100 risk of developing Alzheimer's, but people with Down syndrome have a 25 in 100 chance of the disease. In addition to an increased risk of developing Alzheimer's, patients with DS show the first signs of the disease much earlier than most people, often in their early 40s. Alzheimer's disease causes the brain to shrink and to break down. The number of brain cells decreases, and abnormal deposits and structural rearrangements occur. This process results in a loss of brain functioning. People with Alzheimer's have strikingly faulty memories. Over time, people with Alzheimer's disease will lapse into an increasingly unresponsive state. Some researchers have shown that even Down syndrome patients who do not appear to have Alzheimer's disease have the same changes occurring to the structures and cells of their brains. A new questionnaire was published in 2004 to help doctors evaluate adults with Down syndrome for symptoms of Alzheimer's-related dementia.

As people with Down syndrome age, they also have an increased chance of developing a number of other illnesses, including cataracts, thyroid problems, diabetes, and seizure disorders.

Diagnosis

Diagnosis is usually suspected at birth, when the characteristic physical signs of Down syndrome are noted. Once this suspicion has been raised, genetic testing (chromosome analysis) can be undertaken in order to verify the presence of the disorder. This testing is usually done on a blood sample, although chromosome analysis can also be done on other types of tissue, including skin. The cells to be studied are prepared in a laboratory. Chemical stain is added to make the characteristics of the cells and the chromosomes stand out. Chemicals are added to prompt the cells to go through normal development, up to the point where the chromosomes are most visible, prior to cell division. At this point, they are examined under a microscope and photographed. The photograph is used to sort the different sizes and shapes of chromosomes into pairs. In most cases of Down syndrome, one extra chromosome 21 will be revealed. The final result of such testing, with the photographed chromosomes paired and organized by shape and size, is called the individual's karyotype.

Treatment

No treatment is available to cure Down syndrome. Treatment is directed at addressing the individual concerns of a particular patient. For example, heart defects will many times require surgical repair, as will duodenal atresia. Many Down syndrome patients will need to wear glasses to correct vision. Patients with hearing impairment benefit from hearing aids.

A drug known as piracetam received some attention in the treatment of Down syndrome patients in the mid-1990s. Piracetam is a so-called "smart drug" that is marketed in Europe and Japan to normal adults hoping to increase their cognitive abilities. It is also sold to skiers and mountain climbers as a remedy for loss of concentration at high altitudes. Although some European researchers have studied piracetam as a possible treatment for dementia in Alzheimer's disease, none of these trials have shown as of 2004 that the drug is of any benefit to Alzheimer's patients. Piracetam is not approved for use in the United States; several large shipments of it were seized by the Food and Drug Administration in 2004. Piracetam can be obtained via the Internet but is not recommended by mainstream medical practitioners.

In 1998 the European company licensed to produce piracetam, UCB Pharma in Belgium, issued a statement discouraging its use in children with Down syndrome. The company obtained orphan drug status for piracetam from the FDA in the early 2000s and has conducted a controlled trial of the drug as a possible treatment for muscle spasms (myoclonus) in children.

While some decades ago, all Down syndrome children were quickly placed into institutions for lifelong care. Research shows very clearly that the best outlook for children with Down syndrome is a normal family life in their own home. This approach, however, requires careful support and education of the parents and the siblings. It is a life-changing event to learn that a new baby has a permanent condition that will effect essentially all aspects of his or her development. Some community groups exist to help families deal with the emotional effects of this new information, and to help plan for the baby's future. Schools are required to provide services for children with Down syndrome, sometimes in separate special education classrooms, and sometimes in regular classrooms. This educational practice is called mainstreaming or inclusion.

Prognosis

The prognosis in Down syndrome is quite variable, depending on the types of complications (heart defects, susceptibility to infections, development of leukemia) of each individual baby. The severity of the retardation can also vary significantly. Without the presence of heart defects, about 90% of children with Down syndrome live into their teens. People with Down syndrome appear to go through the normal physical changes of aging more rapidly, however. The average age of death for an individual with Down syndrome is about 50-55 years. The most common cause of death is heart disease.

Still, the prognosis for a baby born with Down syndrome in the early 2000s is better than ever before. Because of modern medical treatments, including antibiotics to treat infections and surgery to treat heart defects and duodenal atresia, life expectancy has greatly increased. Community and family support allows people with Down syndrome to have rich, meaningful relationships. Because of educational programs, some people with Down syndrome are able to hold jobs.

Men with Down syndrome appear to be uniformly sterile (meaning that they are unable to have offspring). Women with Down syndrome, however, are fully capable of having babies. About 50% of these babies, however, will also be born with Down syndrome.

Prevention

Efforts at prevention of Down syndrome are aimed at genetic counseling of couples who are preparing to have babies. A counselor needs to inform a woman that her risk of having a baby with Down syndrome increases with her increasing age. Two types of testing is available during a pregnancy to determine if the baby being carried has Down syndrome.

Screening tests are used to estimate the chance that an individual woman will have a baby with Down syndrome. At 14-17 weeks of pregnancy, measurements of a substance called AFP (alpha-fetoprotein) can be performed. AFP is normally found circulating in the blood of a pregnant woman, but may be unusually high or low with certain disorders. Carrying a baby with Down syndrome often causes AFP to be lower than normal. This information alone, or along with measurements of two other hormones, is considered along with the mother's age to calculate the risk of the baby being born with Down syndrome. These results are only predictions, and are only correct about 60% of the time.

The only way to definitively establish (with about 98-99% accuracy) the presence or absence of Down syndrome in a developing baby, is to test tissue from the pregnancy itself. This is usually done either by amniocentesis or chorionic villus sampling (CVS). In amniocentesis, a small amount of the fluid in which the baby is floating is withdrawn with a long, thin needle. In chorionic villus sampling, a tiny tube is inserted into the opening of the uterus to retrieve a small sample of the placenta (the organ that attaches the growing baby to the mother via the umbilical cord, and provides oxygen and nutrition). Both amniocentesis and CVS allow the baby's own karyotype to be determined. A couple must then decide whether to use this information in order to begin to prepare for the arrival of a baby with Down syndrome, or to terminate the pregnancy.

Once a couple has had one baby with Down syndrome, they are often concerned about the likelihood of future offspring also being born with the disorder. Most research indicates that this chance remains the same as for any woman at a similar age. However, when the baby with Down syndrome has the type that results from a translocation, it is possible that one of the two parents is a carrier of that defect. A carrier conveys the genetic defect to the next generation but does not actually have the disorder. When one parent is a carrier of a translocation, the chance of future offspring having Down syndrome is greatly increased. The specific risk requires evaluation by a genetic counselor.

KEY TERMS

Chromosome The structures that carry genetic information. Chromosomes are located within every cell, and are responsible for directing the development and functioning of all the cells in the body. The normal number is 46 (23 pairs).

Karyotype The specific chromosomal makeup of a particular cell.

Mental retardation A condition where an individual has a lower-than-normal IQ, and thus is developmentally delayed.

Mosaic A term referring to a genetic situation, in which an individual's cells do not have the exact same composition of chromosomes. In Down syndrome, this may mean that some of the individual's cells have a normal 46 chromosomes, while other cells have an abnormal 47 chromosomes.

Nondisjunction A genetic term referring to an event which takes place during cell division, in which a genetic accident causes an egg or sperm cell to have 24 chromosomes, rather than the normal 23.

Orphan drug A term for a drug that treats a rare disease, defined by the Food and Drug Administration (FDA) as one that affects fewer than 200,000 Americans. The FDA has an Office of Orphan Products Development (OOPD), which offers grants to researchers to develop these products.

Translocation A genetic term referring to a situation during cell division in which a piece of one chromosome breaks off and sticks to another chromosome.

Trisomy The condition of having three identical chromosomes instead of the normal two.

Resources

BOOKS

Beers, Mark H., MD, and Robert Berkow, MD, editors. "Congenital Anomalies." Section 19, Chapter 261 In The Merck Manual of Diagnosis and Therapy. Whitehouse Station, NJ: Merck Research Laboratories, 2004.

Tierney, Lawrence, et al. Current Medical Diagnosis and Treatment. Los Altos, CA: Lange Medical Publications, 2001.

PERIODICALS

Chen, Harold, MD. "Down Syndrome." eMedicine December 6, 2004. http://www.emedicine.com/med/topic567.htm.

Egan, J. F., P. A. Benn, C. M. Zelop, et al. "Down Syndrome Births in the United States from 1989 to 2001." American Journal of Obstetrics and Gynecology 191 (September 2004): 1044-1048.

Evans, J. G., G. Wilcock, and J. Birks. "Evidence-Based Pharmacotherapy of Alzheimer's Disease." International Journal of Neuropsychopharmacology 7 (September 2004): 351-369.

Leonard, H., L. Weaving, P. Eastaugh, et al. "Trisomy 21 and Rett Syndrome: A Double Burden." Journal of Paediatrics and Child Health 40 (July 2004): 406-409.

Prasher, V., A. Farooq, and R. Holder. "The Adaptive Behaviour Dementia Questionnaire (ABDQ): Screening Questionnaire for Dementia in Alzheimer's Disease in Adults with Down Syndrome." Research in Developmental Disabilities 25 (July-August 2004): 385-397.

Tanner, Lindsey. "Study: Drug May Hurt Syndrome Kids." Chicago: Associated Press April 12, 2001.

Tyler, C., and J. C. Edman. "Down Syndrome, Turner Syndrome, and Klinefelter Syndrome: Primary Care throughout the Life Span." Primary Care 31 (September 2004): 627-648.

ORGANIZATIONS

National Down Syndrome Congress. 1605 Chantilly Drive, Suite 250, Atlanta, GA 30324-3269. (800) 232-6372.

National Down Syndrome Society. 666 Broadway, 8th Floor, New York, NY 10012-2317. (800) 221-4602. http://www.ndss.org.

National Organization for Rare Disorders (NORD). 55 Kenosia Avenue, P. O. Box 1968, Danbury, CT 06813-1968. (203) 744-0100. Fax: (203) 798-2291. http://www.rarediseases.org.

United States Food and Drug Administration (FDA). 5600 Fishers Lane, Rockville, MD 20857-0001. (888) INFO-FDA. http://www.fda.gov.

OTHER

Food and Drug Administration (FDA). "Grants Awarded by the OOPD Program." http://www.fda.gov/orphan/grants/previous.htm.

Food and Drug Administration (FDA). "Refusal Actions by FDA as Recorded in OASIS for China (Mainland). August 2004." http://www.fda.gov/ora/oasis/1/ora_oasis_c_cn.html.

Leshin, Lem, MD. "Piracetam and Down Syndrome." http://www.ds-health.com/piracet.html.

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Down Syndrome

Down syndrome

Definition

Down syndrome is the most common cause of mental retardation and malformation in a newborn. A genetic disorder, it occurs because of the presence of an extra chromosome.

Description

Chromosomes are units of genetic information that exist within every cell of the body. Twenty-three distinctive pairs, or 46 total chromosomes, are located within the nucleus (central structure) of each cell. When a baby is conceived by combining one sperm cell with one egg cell, the baby receives 23 chromosomes from each parent, for a total of 46 chromosomes. Sometimes, an accident in the production of a sperm or egg cell causes that cell to contain 24 chromosomes. This event is referred to as nondisjunction. When this defective cell is involved in the conception of a baby, that baby will have a total of 47 chromosomes. The extra chromosome in Down syndrome is labeled number 21. For this reason, the existence of three such chromosomes is sometimes referred to as trisomy 21.

In a very rare number of Down syndrome cases (about 12%), the original egg and sperm cells are completely normal. The problem occurs sometime shortly after fertilization; during the phase when cells are dividing rapidly. One cell divides abnormally, creating a line of cells with an extra chromosome 21. This form of genetic disorder is called a mosaic. The individual with this type of Down syndrome has two types of cells: those with 46 chromosomes (the normal number) and those with 47 chromosomes (as occurs in Down syndrome). Some researchers have suggested that individuals with this type of mosaic form of Down syndrome have less severe signs and symptoms of the disorder.

Another relatively rare genetic accident which can cause Down syndrome is called translocation. During cell division, the number 21 chromosome somehow breaks. A piece of the number 21 chromosome then becomes attached to another chromosome. Each cell still has 46 chromosomes, but the extra piece of chromosome 21 results in the signs and symptoms of Down syndrome. Translocations occur in about 34 percent of cases of Down syndrome.

Demographics

Down syndrome occurs in about one in every 800 to 1,000 births. It affects an equal number of boys and girls. Less than 25 percent of Down syndrome cases occur due to an extra chromosome in the sperm cell. The majority of cases of Down syndrome occur due to an extra chromosome 21 within the egg cell supplied by the mother (nondisjunction). As a woman's age (maternal age) increases, the risk of having a Down syndrome baby increases significantly. For example, at younger ages, the risk is about one in 4,000. By the time the woman is age 35, the risk increases to one in 400; by age 40 the risk increases to one in 110; and by age 45 the risk becomes one in 35. There is no increased risk of either mosaicism or translocation with increased maternal age.

Causes and symptoms

While Down syndrome is a chromosomal disorder, a baby is usually identified at birth through observation of a set of common physical characteristics. Babies with Down syndrome tend to be overly quiet; less responsive; with weak, floppy muscles. Furthermore, a number of physical signs may be present. These include:

  • flat appearing face
  • small head
  • flat bridge of the nose
  • smaller than normal, low-set nose
  • small mouth, which causes the tongue to stick out and to appear overly large
  • upward slanting eyes
  • extra folds of skin located at the inside corner of each eye, near the nose (called epicanthal folds)
  • rounded cheeks
  • small, misshapen ears
  • small, wide hands
  • an unusual, deep crease across the center of the palm (called a simian crease)
  • a malformed fifth finger
  • a wide space between the big and the second toes
  • unusual creases on the soles of the feet
  • overly flexible joints (sometimes referred to as being double-jointed)
  • shorter than normal height

Other types of defects often accompany Down syndrome. About 30 to 50 percent of all children with Down syndrome are found to have heart defects. A number of different heart defects are common in Down syndrome, including abnormal openings (holes) in the walls that separate the heart's chambers (atrial septal defect , ventricular septal defect). These result in abnormal patterns of blood flow within the heart. The abnormal blood flow often means that less oxygen is sent into circulation throughout the body. Another heart defect that occurs in Down syndrome is called tetralogy of Fallot . Tetralogy of Fallot consists of a hole in the heart, along with three other major heart defects.

Malformations of the gastrointestinal tract are present in about 57 percent of children with Down syndrome. The most common malformation is a narrowed, obstructed duodenum (the part of the intestine into which the stomach empties). This disorder, called duodenal atresia, interferes with the baby's milk or formula leaving the stomach and entering the intestine for digestion. The baby often vomits forcibly after feeding and cannot gain weight appropriately until the defect is repaired.

Other medical conditions that occur in patients with Down syndrome include an increased chance of developing infections, especially ear infections and pneumonia ; certain kidney disorders; thyroid disease (especially low or hypothyroid); hearing loss; vision impairment that requires corrective lenses; and a 20-times greater chance of developing leukemia (a blood disorder).

Development in a baby and child with Down syndrome occurs at a much slower than normal rate. Because of weak, floppy muscles (hypotonia ), babies learn to sit up, crawl, and walk much later than their normal peers. Talking is also quite delayed. The level of mental retardation is considered to be mild-to-moderate in Down syndrome. The actual IQ range of Down syndrome children is quite varied, but the majority of such children are in what is sometimes known as the trainable range. This means that most people with Down syndrome can be trained to do regular self-care tasks, function in a socially appropriate manner in a normal home environment, and even hold simple jobs.

As people with Down syndrome age, they face an increased chance of developing the brain disease called Alzheimer's (sometimes referred to dementia or senility). Most people have a six in 100 risk of developing Alzheimer's, but people with Down syndrome have a one-in-four chance of the disease. Alzheimer's disease causes the brain to shrink and to break down. The number of brain cells decreases, and abnormal deposits and structural arrangements occur. This process results in loss of brain function. People with Alzheimer's have strikingly faulty memories. Over time, people with Alzheimer's disease lapse into an increasingly unresponsive state. Some researchers have shown that even Down syndrome patients who do not appear to have Alzheimer's disease have the same changes occurring to the structures and cells of their brains.

As people with Down syndrome age, they also have an increased chance of developing a number of other medical difficulties, including cataracts, thyroid problems, diabetes, and seizure disorders.

Diagnosis

Diagnosis is usually suspected at birth, when the characteristic physical signs of Down syndrome are noted. Once this suspicion has been raised, genetic testing (chromosome analysis) can be undertaken in order to verify the presence of the disorder. This testing is usually done on a blood sample, although chromosome analysis can also be done on other types of tissue, including skin. The cells to be studied are prepared in a laboratory. Chemical stain is added to make the characteristics of the cells and the chromosomes stand out. Chemicals are added to prompt the cells to go through normal development, up to the point where the chromosomes are most visible, prior to cell division. At this point, they are examined under a microscope and photographed. The photograph is used to sort the different sizes and shapes of chromosomes into pairs. In most cases of Down syndrome, one extra chromosome 21 will be revealed. The final result of such testing, with the photographed chromosomes paired and organized by shape and size, is called the individual's karyotype.

Treatment

As of 2004 no treatment is available to cure Down syndrome. Treatment is directed at addressing the individual concerns of a particular patient. For example, heart defects often times require surgical repair, as will duodenal atresia. Many Down syndrome patients need to wear glasses to correct vision. Patients with hearing impairment benefit from hearing aids.

In the mid 1900s, all Down syndrome children were quickly placed into institutions for lifelong care. Research shows, however, that the best outlook for children with Down syndrome is family life in their own home. This arrangement requires careful support and education of the parents and the siblings. Parents and other siblings face a life-changing event in receiving a new baby who has a permanent condition that will affect essentially all aspects of his or her development. Some community groups are committed to helping families deal with the emotional effects of this new situation. Schools are required to provide services for children with Down syndrome, sometimes in separate special education classrooms and sometimes in regular classrooms, a practiced called mainstreaming or inclusion.

Prognosis

The prognosis in Down syndrome is quite variable, depending on the types of complications (heart defects, susceptibility to infections, development of leukemia) of each individual baby. The severity of the retardation can also vary significantly. Without the presence of heart defects, about 90 percent of children with Down syndrome live into their teens. People with Down syndrome appear to go through the normal physical changes of aging more rapidly, however. The average age at death for an individual with Down syndrome is about 50 to 55 years.

Still, in the early 2000s, the prognosis for a baby born with Down syndrome is better than ever before. Because of modern medical treatments, including antibiotics to treat infections and surgery to treat heart defects and duodenal atresia, life expectancy has greatly increased. Community and family support allows people with Down syndrome to have rich, meaningful relationships and in some cases to hold jobs.

Men with Down syndrome appear to be uniformly sterile (meaning that they are unable to have offspring). Women with Down syndrome, however, are fully capable of having babies. About 50 percent of these babies, however, will also be born with Down syndrome.

Prevention

Efforts at prevention of Down syndrome are aimed at genetic counseling of couples who are preparing to have babies. A counselor needs to inform a woman that her risk of having a baby with Down syndrome increases with her increasing age. Two types of testing is available during a pregnancy to determine if the baby being carried has Down syndrome.

Screening tests are used to estimate the chance that an individual woman will have a baby with Down syndrome. At 1417 weeks of pregnancy, measurements of a substance called AFP (alpha-fetoprotein) can be performed. AFP is normally found circulating in the blood of a pregnant woman but may be unusually high or low with certain disorders. Carrying a baby with Down syndrome often causes AFP to be lower than normal. This information alone, or along with measurements of two other hormones, is considered along with the mother's age to calculate the risk of the baby being born with Down syndrome. These results are only predictions and are only correct about 60 percent of the time. Other screening tests measure and compare the levels of other markers present in the mother's blood. A specialized ultrasound exam measures the thickness of the back of the fetus's neck (called nuchal lucency). Thicker measurements correlate with the possibility of Down syndrome or other chromosomal abnormalities.

KEY TERMS

Chromosome A microscopic thread-like structure found within each cell of the human body and consisting of a complex of proteins and DNA. Humans have 46 chromosomes arranged into 23 pairs. Chromosomes contain the genetic information necessary to direct the development and functioning of all cells and systems in the body. They pass on hereditary traits from parents to child (like eye color) and determine whether the child will be male or female.

Karyotype A standard arrangement of photographic or computer-generated images of chromosome pairs from a cell in ascending numerical order, from largest to smallest.

Mental retardation A condition where an individual has a lower-than-normal IQ, and thus is developmentally delayed.

Mosaic A term referring to a genetic situation in which an individual's cells do not have the exact same composition of chromosomes. In Down syndrome, this may mean that some of the individual's cells have a normal 46 chromosomes, while other cells have an abnormal 47 chromosomes.

Nondisjunction An event that takes place during cell division in which a chromosome pair does not separate as it should. The result is an abnormal number of chromosmes in the daughter cells produced by that cell division.

Translocation The transfer of one part of a chromosome to another chromosome during cell division. A balanced translocation occurs when pieces from two different chromosomes exchange places without loss or gain of any chromosome material. An unbalanced translocation involves the unequal loss or gain of genetic information between two chromosomes.

Trisomy An abnormal condition where three copies of one chromosome are present in the cells of an individual's body instead of two, the normal number.

All of these screening tests are used to decide which mothers will be offered other, more definitive testing to ascertain whether the baby has Down syndrome. These more definitive tests each carry a risk of miscarriage, which is why screening tests are an important first step. The only way to definitively establish (with about 9899% accuracy) the presence or absence of Down syndrome in a developing baby is to test tissue from the pregnancy itself. This is usually done either by amniocentesis or chorionic villus sampling (CVS). In amniocentesis, a small amount of the fluid in which the baby is floating is withdrawn with a long, thin needle. In chorionic villus sampling, a tiny tube is inserted into the opening of the uterus to retrieve a small sample of the placenta (the organ that attaches the growing baby to the mother via the umbilical cord, and provides oxygen and nutrition ). Both amniocentesis and CVS allow the baby's own karyotype to be determined. A couple must then decide whether to use this information in order to begin to prepare for the arrival of a baby with Down syndrome or to terminate the pregnancy.

Once a couple has had one baby with Down syndrome, they are often concerned about the likelihood of future offspring also being born with the disorder. Most research indicates that this chance remains the same as for any other woman at a similar age. However, when the baby with Down syndrome has the type that results from a translocation, it is possible that one of the two parents is a carrier of that defect. (A carrier carries the genetic defect but does not actually have the disorder.) When one parent is a carrier of a translocation, the chance of future offspring having Down syndrome is greatly increased. The specific risk will have to be calculated by a genetic counselor.

Parental concerns

Parenting a child with Down syndrome can be both challenging and rewarding. Children with Down syndrome have a wide range of potential outcomes. Early intervention programs have been proven to be of great help in assisting these children in achieving the highest level of functioning possible. There are many support groups available for parents and siblings of Down syndrome children.

Resources

BOOKS

Hall, Judith G. "Chromosomal Clinical Abnormalities." In Nelson Textbook of Pediatrics. Edited by Richard E. Behrman et al. Philadelphia: Saunders, 2004.

Simpson, Joe Leigh. "Genetic Counseling and Prenatal Diagnosis." In Obstetrics: Normal and Problem Pregnancies. Edited by Steven G. Gabbe. London: Churchill Livingstone, 2002.

Tierney, Lawrence, et al. Current Medical Diagnosis and Treatment. Los Altos, CA: Lange Medical Publications, 2001.Periodicals

Canick, J. A. "Prenatal screening for Down syndrome: current and future methods." Clinical Laboratory Medicine 86 (June 2003): 395411.

Roizen, N. J. "Medical care and monitoring for the adolescent with Down syndrome." Adolescent Medicine 13 (June 2002): 34558.

Tyler, C. "Down syndrome, Turner syndrome, and Klinefelter syndrome: primary care throughout the life span." Primary Care 31 (September 2004): 627.

ORGANIZATION

National Down Syndrome Congress. 1370 Center Drive, Suite 102 Atlanta, GA 30338 (800) 232-6372. Web site: <www.ndsccenter.org>

National Down Syndrome Society. 666 Broadway, 8th Floor, New York, NY 10012-2317. Web site: <www.ndss.org>.

Kim A. Sharp, M.Ln. Rosalyn Carson-DeWitt, MD

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Down Syndrome

Down Syndrome

Down syndrome, also called trisomy 21, is the single most common genetic cause of moderate mental retardation. It occurs in about one of every eight hundred live births. It is caused by the inheritance of an extra copy of chromosome 21. The condition was named after an English physician, J. Langdon Down, who in 1866 published the first report describing patients with similar facial features and mental retardation. The chromosomal basis of Down syndrome was not determined until nearly a century later.

Clinical Features

Down syndrome is associated with a characteristic physical appearance, mental retardation, and specific birth defects or health conditions. The facial features, in addition to low muscle tone (called hypotonia), are often the first signs that alert a physician to a potential diagnosis of Down syndrome. These features include an up-slant of the outer corners of the eyes, small skin folds over the inner corners of the eyes, a small nose with a flat nasal bridge, a flat profile, and a large, grooved tongue that often protrudes from the mouth. Other physical characteristics can include a short neck, excess skin on the back of the neck, short hands with a single palmar crease, a wide gap between the first and second toes, and short stature. There are many individuals without Down syndrome who may have one or more of these features. It is only when the features occur together and the appropriate genetic test (chromosome studies) confirms clinical suspicion that a diagnosis of Down syndrome is made.

All individuals with Down syndrome have mental retardation, usually mild to moderate. The degree of learning disability may not be immediately apparent, since social ability generally exceeds scholastic ability. Early intervention programs are important for giving people with Down syndrome the best chance to maximize their learning potential.

Certain birth defects and health conditions are more common in individuals with Down syndrome. The most common birth defect is a congenital heart defect, affecting 40 percent to 50 percent of newborns with the condition. Although many can be repaired with surgery, congenital heart defects remain the major cause of early death among affected persons. Individuals with Down syndrome have an increased chance of experiencing hearing loss, vision problems, and thyroid disease, as well as an increased susceptibility to infections. Because of such concerns, specific guidelines for the health care of individuals with Down syndrome have been developed.

Chromosomal Basis of Down Syndrome

In 1959 French geneticist Jerome Lejeune recognized that individuals with Down syndrome have forty-seven chromosomes instead of the usual forty-six. Later, it was determined that it is an extra copy of chromosome 21 that causes the condition. It is not yet clear how the extra chromosome causes the clinical features, although it is believed that an "extra dose" of one or more of the genes on the chromosome is responsible.

There are three types of Down syndrome: trisomy 21, mosaic Down syndrome, and translocation Down syndrome. In 94 percent of cases, the extra copy of chromosome 21 stands alone (is not attached to any other chromosomes) and is present in every cell of the body. This is called trisomy 21, trisomy meaning three.

Trisomy 21 occurs due to a chromosome packaging error. Usually when the body makes its sex cells (egg or sperm cells) during meiosis , it packages up one chromosome from each pair. However, sometimes an error (nondisjunction) occurs, causing both chromosomes from a pair to get packaged together. If the sex cell with the extra chromosome is fertilized by a sex cell with the usual chromosome number, the resulting embryo will have a trisomy. If the extra chromosome is chromosome 21, the embryo will have Down syndrome. About 75 percent of embryos with trisomy 21 abort spontaneously before birth. Nondisjunction occurs by chance in the making of both egg and sperm cells, but it happens more often in egg cells as women get older. Thus, the chance of having a baby with Down syndrome increases with increasing maternal age.

Translocation Down syndrome, which accounts for 3 percent to 4 percent of cases, occurs when the extra copy of chromosome 21 is attached to another chromosome. In about one-fourth of the cases where a person has translocation Down syndrome, he or she inherited the translocation from a parent. Therefore it is important to test the parents' chromosomes in these cases, for purposes of future family planning.

The third type of Down syndrome is the mosaic type, which occurs in 2 percent to 3 percent of cases. In mosaic Down syndrome, a person has some cells with an extra copy of chromosome 21 and some cells with the usual two copies. People with mosaic Down syndrome may or may not have milder symptoms than people with "full" trisomy 21.

Testing for Down Syndrome

Cytogenetic analysis looks at the number and structure of a person's chromosomes. This test, which can be performed on a blood sample, is the test used to definitively determine if an individual has Down syndrome.

Maternal Age Risk of Down Syndrome Total Risk for all Chromosomal Abnormalities
20 1/1667 1/526
21 1/1667 1/526
22 1/1429 1/500
23 1/1429 1/500
24 1/1250 1/476
25 1/1250 1/476
26 1/1176 1/476
27 1/1111 1/455
28 1/1053 1/435
29 1/1000 1/417
30 1/952 1/385
31 1/909 1/385
32 1/769 1/322
33 1/602 1/286
34 1/485 1/238
35 1/378 1/192
36 1/289 1/156
37 1/224 1/127
38 1/173 1/102
39 1/136 1/83
40 1/106 1/66
41 1/82 1/53
42 1/63 1/42
43 1/49 1/33
44 1/38 1/26
45 1/38 1/21
46 1/23 1/16
47 1/18 1/13
48 1/14 1/10
49 1/11 1/8

Prenatal diagnosis for Down syndrome (testing for the condition during pregnancy) is possible. Chromosome studies can be performed on fetal cells collected via chorionic villus sampling (CVS) at ten to twelve weeks of pregnancy or by amniocentesis at fifteen to twenty weeks of pregnancy. Because of the link between the mother's age and the chance of Down syndrome, prenatal diagnosis for Down syndrome and other chromosome conditions is routinely offered to women thirty-five and older. Whether to pursue prenatal diagnosis is a personal decision that can only be made by the parents.

see also Birth Defects; Chromosomal Aberrations; Meiosis; Mosaicism; Nondisjunction; Prenatal Diagnosis.

Angela Trepanier

and Gerald L. Feldman

Bibliography

Evans, Mark I., et al. Fetal Diagnosis and Therapy: Science, Ethics, and the Law. Philadelphia, PA: JB Lippincott Co., 1989.

Gardner, R., J. McKinlay, and Grant R. Sutherland. Chromosome Abnormalities and Genetic Counseling, 2nd ed. New York: Oxford University Press, 1996.

Nussbaum, Robert L., Roderick R. McInnes, and Huntington F. Willard, eds. Thompson & Thompson Genetics in Medicine, 6th ed. Philadelphia, PA: W. B. Saunders, 2001.

Pueschel, Siegfried M., ed. A Parent's Guide to Down Syndrome: Toward a Brighter Future, 2nd ed. Baltimore, MD: Paul H. Brooks Publishing, 2001.

Internet Resource

Cohen, William I., ed. "Health Care Guidelines for Individuals with Down Syndrome: 1999 Revision." Down Syndrome Quarterly 4, no. 3 (1999): 1-15. <http://www.denison.edu/dsq/health99.shtml>.

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Down Syndrome

Down syndrome

A hereditary mental disorder present at birth resulting from an abnormality in the number of chromosomes; also known Trisomy 21.

Down syndrome was named after John Langdon Haydon Down, a British physician and advocate of education for the mentally retarded, who first described it in 1866. In 1959, the French pediatrician Jerome Lejeune discovered that the disorder is caused by a chromosomal abnormality. Ninety-five percent of individuals with Down syndrome have Trisomy 21, an extra chromosome in the 21st pair (altogether, they have 47 chromosomes instead of the normal 46); four percent have translocation, a chromosomal abnormality; and one percent have mosaicism. Down syndrome is characterized primarily by varying degrees of mental and motor retardation.

Most people with the disorder are retarded. Individuals with Down syndrome have I.Q.s ranging from 20 to more than 90, with the mean being 49. They are also prone to possible heart defects, poor vision and hearing , cataracts, and a low resistance to respiratory infections. Until the discovery of antibiotics, most Down syndrome children died of pneumonia before reaching adulthood. People with Down syndrome are 20 times more likely than the general population to develop leukemia and a neurological condition similar to Alzheimer's disease .

Individuals with Down syndrome have a distinctive physical appearance characterized by almond-shaped eyes (on which the condition's former alternate name mongolismwas based); a short, stocky build; a flat nose and large, protruding tongue (which makes normal speech difficult); a small skull flattened in the back; a short neck with extra skin; and small hands with short fingers. Other features include a fold of skin on the inner side of the eye; speckling at the edge of the iris; and a small amount of facial and body hair. Muscle tone is often poor, and newborns are prone to hypotonia, or "floppiness." People with Down syndrome are widely recognized to have docile temperaments; and are generally cheerful, cooperative, affectionate, and relaxed, although there are no scientific studies to confirm this. Their motor, speech, and sexual development is delayed, and their cognitive development may not peak until the age of 30 or 40. In infancy , speech development is delayed by about seven months.

Recent research has led to the conclusion that Down children are capable of expressing complex feelings, of developing richer personalities, and of mastering higher degrees of learning using adaptive strategies (such as computer-aided learning to teach reading and writing). One developmental program that began with Down children as young as 30 months old and stressed positive parent-child communication eventually enabled the children to read at a second-grade level. The theory is that early stimulation helps to develop connections in the brain that might otherwise not have developed.

Although most people with Down syndrome were institutionalized until the 1970s, those with only moderate retardation are capable of achieving some degree of self-sufficiency. Today, with changed social attitudes and expanded educational opportunities, many lead productive, fulfilling lives. In less than a century, the expected lifespan of a person with Down syndrome has increased from nine years of age in 1910 to 19 or 20 after the discovery of antibiotics to age 55 today, due to recent advancements in clinical treatment.

More than 350,000 people in the United States have Down syndrome; one baby is born with Down syndrome for every 800 to 1,000 births in the U.S. Down syndrome babies are found in every ethnic group and socioeconomic class. About 5,000 babies are born yearly in the United States with Down syndrome. Twenty to 25 percent of children conceived with Down syndrome survive beyond birth . Women over age 35 have a one in 400 chance of conceiving a child with Down syndrome. For women age 40, the incidence becomes one in 110. For mothers age 45, the incidence increases to one child in every 35. Under age 30, women give birth to Down syndrome babies at a rate of one for every 1,500 babies born. However, women under age 35 actually bear 80 percent of Down infants, and recent studies suggest that the father's age may play a role as well. Prenatal detection of Down syndrome is possible through amniocentesis and chorionic villus sampling and is recommended for pregnant women over the age of 35.

See also Mental retardation

Further Reading

Cicchetti, D. & Beeghly, M. (Eds.) Children with Down Syndrome: A Developmental Perspective. New York: Cambridge University Press, 1990.

Cunningham, C. Understanding Down Syndrome: An Introduction for Parents. Cambridge, MA: Brookline Books, 1996.

Further Information

National Down Syndrome Congress. 1800 Dempster Street, Park Ridge, Illinois 60068-1146, (708) 8237550, (800) 232NDSC.

National Down Syndrome Society. 666 Broadway, New York, New York 10012, (212) 4609330, (800) 2214602.

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Down syndrome

Down syndrome, congenital disorder characterized by mild to severe mental retardation, slow physical development, and characteristic physical features. Down syndrome affects about 1 in every 730 live births and occurs in all populations equally. It was first described in 1866 by an English physician, J. Langdon Down. In 1959 a French physician, Jerome Lejeune, discovered that the syndrome was caused by an extra chromosome. It was later discovered that this extra chromosome appears as a third chromosome attached to the 21st of the 23 pairs of chromosomes normally present in the human genome. This third chromosome gives rise to the alternate name trisomy 21.

The extra genetic material is responsible for the physical characteristics of the syndrome: low muscle tone, flattish facial features, an upward slant to the eyes and epicanthal folds (which were the basis for the former name, mongolism), a single crease across the palm, hyperflexibility of the joints, and a displastic middle phalanx on the fifth finger. People with Down syndrome have an increased incidence of infection, childhood leukemia, congenital heart defects, and respiratory problems, but modern medical treatment has improved the life expectancy from 9 (in 1910) to 55 (in 1995).

Mental retardation varies widely, from minimal to severe. The great majority of those who have the disorder attend public schools and as adults can live independently or in group homes. After age 35 individuals with the syndrome develop the neurological changes of Alzheimer's disease, and many develop the dementia that accompanies them.

Eighty percent of children with Down syndrome are born to women under 35 years of age, but the incidence of Down syndrome births does increase with age. Approximately 5% of cases are transmitted by the sperm. Amniocentesis or chorionic villus sampling can be used to detect the disorder in the fetus. Children born to women with Down syndrome have a 50% chance of having the disorder.

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Down Syndrome

Down Syndrome

Jasons Sister Anna: A Look at Someone with Down Syndrome

What Is Down Syndrome?

What Causes Down Syndrome?

Can a Pregnant Woman Find Out if Her Baby Has Down Syndrome?

What Does Life Hold for Anna and Her Family?

Resources

Down Syndrome is a genetic condition that occurs when a person has three copies of chromosome 21 rather than the usual two. People with Down Syndrome usually have a characteristic physical appearance, significantly lowered intellectual abilities, and sometimes a number of physical problems, such as heart defects.

KEYWORDS

for searching the Internet and other reference sources

Chromosomes

Mosaicism

Trisomy 21

Jasons Sister Anna: A Look at Someone with Down Syndrome

First and foremost, Jasons sister Anna is a happy, active, five-year-old girl. But she also is a girl with Down Syndrome, and Jason feels more protective of his sister than do most big brothers. When they are out at the mall, everyone says how adorable Anna is, but they also stare at her. Anna has many of the characteristic features of people with Down Syndrome. These features may include:

  • a flattened face
  • up-slanted eyes
  • low-set ears
  • a protruding tongue
  • a short neck
  • a single, straight crease across the palm
  • characteristic patterns of the ridges of the skin on her fingers, palms, and soles
  • short arms and legs
  • poor muscle tone
  • mental retardation
  • certain other health problems

All of these characteristics vary among affected people, and some people with Down Syndrome have fewer of these features than do others. Anna is somewhere in the middle of the range. She is moderately mentally retarded.

Jason researched Down Syndrome for a school report and now worries about Annas health. Many people with Down Syndrome have health problems such as heart defects, increased susceptibility* to infection, respiratory problems, and digestive problems. Childhood leukemia occurs slightly more frequently in children with Down Syndrome than in other children, and adults with the syndrome are at increased risk for Alzheimers disease.

* susceptibility
(su-sep-ti-BIL-i-tee) means having less resistance to and higher risk for infection or disease.

What Is Down Syndrome?

People have Down Syndrome when they have three copies of chromosome 21 (or parts of chromosome 21). Chromosomes provide all of the genetic information needed for the cells of the body to work properly. Normally, most of a persons cells contain 23 pairs of chromosomes, for a total of 46. The exceptions are eggs and sperm cells, which have only one set of 23 chromosomes. Most people with Down Syndrome have 47 chromosomes, instead of the usual 46, and the extra genetic material causes developmental problems. Down Syndrome can occur in three ways.

Nondisjunction

Ninety-five percent of people with Down Syndrome have Trisomy 21, meaning they have three copies of chromosome 21. This

A Brief History of Down Syndrome Research

1866: An English physician, John Langdon Haydon Down (18281896), published the first description in the medical literature of a person with Down Syndrome.

1959: Jerome Lejeune, a French physician, found the extra chromosome 21.

1990: Chromosome 21 was the first chromosome to be fully mapped*.

* mapping
locates the positions of all the genes on a chromosome.

occurs because of an error in cell division called nondisjunction. Normally, an egg or a sperm cell has only 23 chromosomes. During the cell divisions that form these reproductive cells, the 23 chromosomes first replicate* and then separate, with one set of 23 going to each new cell. If the two copies of chromosome 21 do not separate, however, the result is an egg or a sperm cell with two copies of the chromosome instead of the usual one. At least 95 percent of the time, trisomy 21 occurs when a normal sperm fertilizes an egg with two copies of chromosome 21. When cell division begins to form an embryo, the extra chromosome is then replicated in every cell of the body.

* replicate
(REP-li-kate) means to create an identical copy.

Translocation

When the extra chromosome 21 breaks off during cell division and attaches to another chromosome, it is called translocation. In this case, the total number of chromosomes is 46, but the genetic material from the extra chromosome 21 that is attached to another chromosome causes the features of Down Syndrome. Translocation accounts for 3 to 4 percent of Down Syndrome cases.

Mosaicism

The third type of Down Syndrome is called mosaicism (mo-ZAY-i-siz-im). This occurs when nondisjunction of chromosome 21 takes place in one of the initial cell divisions after the egg is fertilized, causing some cells to have 46 chromosomes and others to have 47. Only 1 to 2 percent of people with Down Syndrome have mosaicism.

What Causes Down Syndrome?

No one knows what causes the chromosomal abnormality that results in Down Syndrome, a condition that affects approximately 1 in 1,000 babies. Any woman can have a baby with Down Syndrome. It is not associated with a persons culture, race, where they live, or how rich or poor they are.

A mothers age, however, does seem to be correlated* with her risk of having a child with Down Syndrome. While 80 percent of children with Down Syndrome are born to women younger than 35, this means that 20 percent are born to women older than 35. But women over 35 only have 5 to 8 percent of all babies. In other words, older women have a greater chance of giving birth to a baby with Down Syndrome, and the risks increase as women grow older. Researchers estimate the chance of having a baby with Down Syndrome to be:

* correlated
means linked in a way that can be measured and predicted.
  • approximately 1 in 1,250 for a 25-year-old woman
  • approximately 1 in 378 for a 35-year-old woman
  • approximately 1 in 30 for a 45-year-old woman.

Can a Pregnant Woman Find Out if Her Baby Has Down Syndrome?

Down Syndrome is the most common chromosomal abnormality in humans, and there are several ways to test for it.

Screening

The triple screen test and the alpha-fetoprotein (AFP) test are commonly used to predict whether a woman is carrying a baby with Down Syndrome. They are called screening tests because they do not give a definite answer. Instead, they measure the amounts of certain substances in the mother s blood that can indicate a problem. If one of these tests is positive, it does not necessarily mean that the fetus (the developing baby) has Down Syndrome, but it does indicate that more tests should be done. Sometimes the test results are false-negatives, meaning that the test did not indicate Down Syndrome even though the fetus has it. Low levels of AFP in the mothers blood are correlated with Down Syndrome in the fetus, but the test detects only about 35 percent of cases. The triple test, which measures levels of three substances, is correct about 60 percent of the time.

Diagnostic

Pregnant women over 35, and women with positive results of screening tests, can be tested using several different diagnostic tests, such as amniocentesis (am-nee-o-sen-TEE-sis), in which the chromosomes from the fetuss cells are examined. Diagnostic tests give a definite answer, which means they are correct 98 to 99 percent of the time. For these tests, samples are extracted from the tissue or fluid surrounding the fetus or from the umbilical cord. On rare occasions, these procedures cause the mother to have a miscarriage (lose the baby before birth). Women who plan to have diagnostic tests performed should receive information and have emotional support available to help them understand the procedures and cope with test results indicating Down Syndrome and with the possibility of miscarriage.

What Does Life Hold for Anna and Her Family?

In 1910, most children with Down Syndrome did not live past the age of nine. When antibiotics were developed in the 1940s, the average child with Down Syndrome survived to age 19 or 20. By the start of the twenty-first

century, because of advances in clinical medicine, about 80 percent of people with Down Syndrome are expected to live to age 55 or longer.

Annas family learned all they could about Down Syndrome as soon as she was born. They knew Anna would learn to sit, walk, and talk somewhat later than her peers. Annas family is providing her with a stimulating home environment, good medical care, and good educational programs. They are teaching her to be a happy productive member of the community. Whether she will ever be able to go to school or to work or live independently depends on the level of her mental development. However, Annas family knows that the Americans with Disabilities Act of 1991 (ADA) will help protect Annas right to live her life free of unnecessary limitations or discrimination due to her disabilities.

See also

Genetic Diseases

Mental Retardation

Resources

Books

Cunningham, Cliff. Down Syndrome: An Introduction for Parents. Boston: Brookline Books, 1995.

Hassold, Terry J., and David Patterson. Down Syndrome: A Promising Future, Together. New York: John Wiley, 1998.

Selikowitz, Mark. Down Syndrome: The Facts. New York: Oxford University Press, 1997.

Organizations

National Down Syndrome Society, 666 Broadway, 8th Floor, New York, NY 10012-2317.

Telephone 800-221 -4602

National Down Syndrome Congress, 1800 Dempster Street, Park Ridge, IL 60068-1146.

Telephone 800-232-NDSC

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Down syndrome

Down syn·drome / ˈdoun ˈsindrōm/ (also Down's syn·drome) • n. Med. a congenital disorder arising from a chromosome defect, causing intellectual impairment and physical abnormalities including short stature and a broad facial profile. It arises from a defect involving chromosome 21, usually an extra copy (trisomy-21).

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Down Syndrome

Down Syndrome

Definition

Down syndrome is the most common chromosome disorder and genetic cause of mental retardation. It occurs because of the presence of an extra copy of chromosome 21. For this reason, it is also called trisomy 21.

Description

Chromosomes are the units of genetic information that exist within every cell of the body. Twenty-three distinctive pairs, or 46 total chromosomes, are located within the nucleus (central structure) of each cell. When the sperm cells from the father and egg cell from the mother are formed, they both undergo a reduction of their total number of chromosomes from 46 to 23. This process is called meiosis. When a baby is conceived by the combination of one sperm cell with one egg cell, the baby receives 23 chromosomes from each parent, for a total of 46 chromosomes. Occasionally, an error occurs in the reduction process. Instead of passing on 23 chromosomes to the baby, a parent will pass on 24 chromosomes. This event is called non-disjunction and occurs in 95% of Down syndrome cases. The baby, therefore, receives an extra chromosome at conception. In Down syndrome, this is an extra chromosome 21. The total number of chromosomes in individuals affected with Down syndrome is 47 instead of 46.

In approximately 1-2% of Down syndrome cases, the original egg and sperm cells cells contain the correct number of chromosomes, 23 each. The problem occurs sometime shortly after fertilization; during the phase where cells are dividing rapidly. One cell divides abnormally, creating a line of cells with an extra copy of chromosome 21. This form of genetic disorder is called mosaicism. An individual with this type of Down syndrome has two types of cells: those with 46 chromosomes (the normal number), and those with 47 chromosomes (as occurs in Down syndrome). Individuals affected with this mosaic form of Down syndrome generally have less severe signs and symptoms of the disorder.

Another relatively rare genetic accident that causes Down syndrome is called translocation. During cell division, chromosome 21 somehow breaks. The broken off piece of this chromosome then becomes attached to another chromosome. Each cell still has 46 chromosomes, but the extra piece of chromosome 21 results in the signs and symptoms of Down syndrome. Translocations occur in about 3-4% of cases of Down syndrome.

Genetic profile

Down syndrome is usually the result of an extra copy of chromosome 21 (trisomy 21). As described earlier, Down syndrome may occur because of nondisjunction within the sperm, or more commonly, the egg cell; genetic mosaicism, which occurs after conception; or translocation, which also occurs after conception.

Once a couple has had one baby with Down syndrome, they are often concerned about the likelihood of future offspring also being born with the disorder. Women who are 34 and younger have a recurrence rate of 1% for having another Down syndrome child. Women who are 35 and older are at increased risk. The specific risk varies and increases as their age increases. When a baby with Down syndrome has the type that results from a translocation, it is possible that one of the two parents is a carrier of a balanced translocation. When one parent is a carrier of a balanced translocation, the chance of future offspring having Down syndrome is greatly increased. Specific risks must be assessed by a genetic counselor.

Demographics

The incidence of Down syndrome is about one in 800 live births. It affects an equal number of male and female babies. The majority of cases of Down syndrome occur due to an extra chromosome 21 within the egg cell supplied by the mother (nondisjunction). As a woman's age (maternal age) increases, the risk of having a Down syndrome baby significantly increases. By the time the woman is age 35, the risk increases to one in 400; by age 40 the risk increases to one in 110; and, by age 45 the risk becomes one in 35. There is no increased risk of either mosaicism or translocation with increased maternal age.

Causes and symptoms

While Down syndrome is a chromosomal disorder, a baby is usually identified at birth through observation of a set of common physical characteristics. It is important to remember that not all children with Down syndrome will exhibit all of the features discussed. There is great variability in both the number and severity of characteristics. Babies with Down syndrome tend to be overly quiet, less responsive to stimuli, and have weak, floppy muscles. A number of physical signs may also be present. These include: a flat appearing face; brighter sparkles in the iris of the eye (Brushfield spots); a small head; a flat bridge of the nose; a smaller than normal, low-set nose; small mouth. This causes the tongue to stick out and to appear overly large. They have upward slanting eyes; extra folds of skin located at the inside corner of each eye, near the nose (epicanthal folds); rounded cheeks; and small, misshapen ears. They tend to have small, wide hands; an unusual deep crease across the center of the palm (simian crease); a curved little finger; a wide space between the great and the second toes; unusual creases on the soles of the feet; overly flexible joints (sometimes referred to as being double-jointed); and, a shorter than normal length.

Other types of problems often accompany Down syndrome. Approximately 30-50% of all children with Down syndrome are found to have heart defects.

Malformations of the gastrointestinal tract are present in about 5-7% of children with Down syndrome. The most common malformation is a narrowed, obstructed duodenum (the part of the intestine into which the stomach empties). This disorder, called duodenal atresia, interferes with the baby's milk or formula leaving the stomach and entering the intestine for digestion. The baby often vomits forcibly after feeding, and cannot gain weight appropriately until the defect is repaired. Another relatively common problem is a tracheo-esophageal fistula. This is an abnormal connection between the trachea (windpipe) and esophagus that interferes with both eating and breathing.

Other medical conditions occurring in persons with Down syndrome include an increased chance of developing infections, especially ear infections and pneumonia; certain kidney disorders; thyroid disease (especially low or hypothyroidism); hearing loss; vision impairment requiring glasses (corrective lenses); and a 20 times greater chance than the population as a whole of developing leukemia.

Development in a baby and child affected with Down syndrome occurs at a much slower than normal rate. Because of weak, floppy muscles (hypotonia), babies learn to sit up, crawl, and walk much later than their unaffected peers. Talking is also quite delayed. The level of mental retardation is considered to be mild to moderate in people with Down syndrome. The degree of mental retardation varies greatly among children with Down syndrome. While it is impossible to predict the severity of Down syndrome at birth, with proper education, children who have Down syndrome are capable of learning. Most children with Down syndrome can read and write and are placed in special education classes at school. The majority of individuals with Down syndrome become semi-independent, meaning that they can take care of their own needs with some assistance. Many hold noncomplex jobs.

As people with Down syndrome age, they face an increased chance of developing the brain disease called Alzheimer's (sometimes referred to as dementia or senility). People without Down syndrome have a lifetime risk of 12% for developing Alzheimer's disease. In contrast, by age 40-50, almost all persons with Down syndrome will also develop Alzheimer's disease.

As people with Down syndrome age, they also have an increased chance of developing a number of other illnesses, including cataracts, thyroid problems, diabetes, and seizure disorders.

Diagnosis

Diagnosis is usually suspected at birth, when the characteristic physical signs of Down syndrome are noted. Once this suspicion has been raised, genetic testing (chromosome analysis) can be undertaken in order to verify the presence of the disorder. This testing is usually done on a blood sample, although chromosome analysis can also be performed on other types of tissue, including the skin. The cells to be studied are prepared in a laboratory. Chemical stain is added to make the characteristics of the cells and the chromosomes stand out. Chemicals are added to prompt the cells to go through normal development, up to the point where the chromosomes are most visible, prior to cell division. At this point, they are examined under a microscope and photographed. The photograph is used to sort the different sizes and shapes of chromosomes into pairs. In most cases of Down syndrome, one extra chromosome 21 will be revealed. The final result of such testing, with the photographed chromosomes paired and organized by shape and size, is called the individual's karyotype. A female with Down syndrome will have a 47, XX + 21 karyotype; a male with Down syndrome will have a 47 XY + 21 karyotype.

Women who are over the age of 35 are offered prenatal tests to determine if their developing baby is affected with Down syndrome. A genetic counselor meets with these families to inform them of the risks and to discuss the tests that are available to make a diagnosis prior to delivery. Because there is a slight risk of miscarriage following some prenatal tests, all testing is optional. Couples must decide whether or not they desire to take this risk to learn the status of their unborn baby. Couples must also decide if they wish to know whether or not the baby the mother carries has Down syndrome. Some couples choose not to have the test because they are certain they would not choose an abortion if they find theirs is a Down syndrome baby.

Ultrasound is now available for prenatal screening. Some abnormalities associated with Down syndrome, including intrauterine growth retardation, may be detected using ultrasound. The use of ultrasound as a screening test for Down syndrome is limited by the difficulty of producing reliable sonographic images of critical fetal structures.

A counselor needs to inform a woman that her risk of having a baby with Down syndrome increases with her increasing age. Two types of testing are available during a pregnancy to determine if the baby being carried has Down syndrome.

Screening tests are used to estimate the chance that an individual woman will have a baby with Down syndrome. All pregnant women under the age of 35 are offered a maternal serum alpha-fetoprotein (MSAFP) screen. This test is normally performed at 15-22 weeks of pregnancy. The MSAFP screen measures a protein and two hormones normally found in maternal blood during pregnancy. A specific pattern of these hormones and protein can give a pregnant woman an increased risk for having a baby born with Down syndrome. Carrying a baby with Down syndrome often causes MSAFP to be lower than normal. Test results reveal only an increased risk and cannot diagnose a baby born with Down syndrome. The MSAFP test can detect up to 60% of all babies who will be born with Down syndrome. Women with an increased risk are offered amniocentesis.

The only way to definitively (with about 99% accuracy) establish the presence or absence of Down syndrome in a developing baby is to test tissue from the pregnancy itself. This is usually done either by amniocentesis or chorionic villus sampling (CVS). In chorionic villus sampling, usually performed at 10-12 weeks of pregnancy, a tiny tube is inserted into the opening of the uterus to retrieve a small sample of the placenta (the organ which attaches the growing baby to the mother via the umbilical cord, and provides oxygen and nutrition ). In amniocentesis, a small amount of the fluid in which the baby is floating is withdrawn with a long, thin needle. Both amniocentesis and CVS allow the baby's own karyotype to be determined. Both tests carry a small risk of causing miscarriage. The risk from CVS is 1% and the risk from amniocentesis is 0.5%. This small risk must be considered when deciding to perform these tests. If test information is positive for Down syndrome, a couple must then decide how to use this information to begin to prepare for the arrival of a baby with Down syndrome, to consider adoption for the baby or to terminate the pregnancy. It must be noted that while the results of prenatal tests can diagnose Down syndrome, they are unable to predict the severity of symptoms.

Treatment

No treatment is available to cure Down syndrome. Treatment is directed at addressing the individual concerns of a particular person. For example, heart defects will many times require surgical repair, as will duodenal atresia or a tracheo-esophageal fistula. Many persons with Down syndrome will need to wear glasses to correct vision. Persons with hearing impairment benefit from hearing aids.

While some decades ago, all Down syndrome children were quickly placed into institutions for lifelong care, research shows very clearly that the best outlook for children with Down syndrome is a normal family life in their own homes. This requires careful support and education of both parents and siblings. It is a life-changing event to learn that a new baby has a permanent condition that will affect essentially all aspects of development. Some community groups exist to help families deal with the emotional effects of this new information, and to help plan for the baby's future. Schools are required to provide services for children with Down syndrome, sometimes in separate special education classrooms, and sometimes in regular classrooms (this is called mainstreaming or inclusion).

As of 2001, the genetic sequence for chromosome 21 was fully determined, which may open the door to new approaches to the treatment of Down syndrome through the development of gene-specific therapies.

Prognosis

The prognosis in Down syndrome is quite variable, depending on the types of complications (heart defects, susceptibility to infections, development of leukemia) of each individual baby. The severity of the retardation can also significantly vary. Without the presence of heart defects, about 90% of children with Down syndrome live into their teens. People with Down syndrome appear to go through the normal physical changes of aging more rapidly, however. The average age of death for an individual with Down syndrome is about 50 to 55 years.

The prognosis for a baby born with Down syndrome continues to improve. Due to modern medical treatments, including antibiotics to treat infections and surgery to correct heart defects, duodenal atresia, and tracheo-esophageal fistula, life expectancy has greatly increased. Community and family support allows people with Down syndrome to have rich, meaningful relationships. Because of educational programs, some people with Down syndrome are able to hold jobs.

Most men with Down syndrome appear to be sterile (meaning that they are unable to have offspring). There has been at least one report of a male with Down syndrome who fathered a child. Some women with Down syndrome, however, are capable of having babies. Approximately 40% of women with Down syndrome are unable to become pregnant. The risk of a woman with trisomy 21 to have a child with Down syndrome is approximately 50%.

Health care team roles

Obstetricians, nurse practitioners, or family doctors often make an initial recommendation that a woman be screened for Down syndrome. Depending on the test, a physician, phlebotomist, or ultrasonographer may obtain sample materials. A laboratory technician will process sample materials. A genetic counselor, physician or other person with specialized training usually provides test results to a couple or woman. Counselors must be available to assist a woman or couple to make appropriate decisions about their baby. Pediatricians, family doctors, internists, and geriatric physicians provide care throughout the life of an individual with Down syndrome. Other health professionals, including surgeons, eye specialists, and hearing experts, provide services as needed.

Prevention

As of 2001, there is no known way to prevent Down syndrome.

KEY TERMS

Cell— A fundamental unit of living tissue. The specialized nature of organs and tissues in a human body reflects the specialized structure and function of its constituent cells.

Chromosome— The structures that carry genetic information. Chromosomes are located within every cell and are responsible for directing the development and functioning of all the cells in the body. The normal number is 46 (23 pairs).

Genetic counseling A communication process by which personal genetic risk information is translated into practical information for families. Genetic counselors are health care professionals with specialized training and experience in the areas of medical genetics and counseling.

Karyotype— The specific chromosomal makeup of a particular cell.

Mental retardation— A condition where an individual has a significantly lower-than-normal IQ, and thus is developmentally delayed.

Mosaic— A term referring to a genetic situation in which all of an individual's cells do not have the same composition of chromosomes. In Down syndrome, this may mean that some of an individual's cells have a normal 46 chromosomes, while other cells have 47 chromosomes.

Nondisjunction— A genetic term referring to an event that takes place during cell division in which a genetic accident causes an egg or sperm cell to have 24 chromosomes, rather than the normal 23.

Translocation— A genetic term referring to a situation during cell division in which a piece of one chromosome breaks off and sticks to another chromosome.

Trisomy— The condition of having three identical chromosomes, instead of the normal two.

Resources

BOOKS

Girod, Christina M. Down Syndrome. Farmington Hills, MI: Lucent Books, 2001.

Jones, Kenneth L. "Down syndrome." In Smith's Recognizable Patterns of Human Malformation, 5th ed. Edited by Richard E. Behrman et al., Philadelphia: Saunders, 1997, 8-14.

Miller, Jon F, Mark Leddy and Lewis A. Leavitt. Improving the Communication of People With Down Syndrome. Baltimore, MD: Paul H Brookes Publishers, 1999.

Pueschel, Sigfried M. A Parent's Guide to Down Syndrome: Toward a Brighter Future, 2nd ed. Baltimore, MD: Paul H. Brookes Publishers, 2000.

Van Allen, Margot and Judith C. Hill. "Congenital anomalies." In Cecil Textbook of Medicine, 21st ed. Edited by Lee Goldman and J. Claude Bennett. Philadelphia: W.B. Saunders, 2000, 150-153.

Weeks, Daniel J. Perceptual-Motor Behavior in Down Syndrome. Champaign, IL: Human Kinetics Publishing, 2000.

PERIODICALS

Kaiser, A. P., P. P. Hester, and A. S. McDuffie. "Supporting communication in young children with developmental disabilities." Mental Retardation and Developmental Disability Research Review 7, no. 2 (2001): 143-150.

Maymon, R., E. Dreazen, I. Buckovsky, Z. Weinraub, and A. Herman. "Does a 'notched' nuchal translucency indicate Down syndrome fetuses or other adverse pregnancy outcome?" Prenatal Diagnosis 21, no. 5 (2001): 403-408.

Palisano R. J., S. D. Walter, D. J. Russell, P. L. Rosenbaum, M. Gemus, B. E. Galuppi, and L. Cunningham. "Gross motor function of children with down syndrome: creation of motor growth curves." Archives of Physical Medicine and Rehabilitation 82, no. 4 (2001): 494-500.

Savulescu, J. "Resources, Down's syndrome, and cardiac surgery." British Medical Journal 322, no. 7291 (2001): 875-876.

Van Riper, M., and W. I. Cohen. "Caring for children with Down syndrome and their families." Journal of Pediatric Health Care 15, no. 3 (2001): 123-131.

Varela, A. M., L. B. Sardinha, and K. H. Pitetti. "Effects of an aerobic rowing training regimen in young adults with Down syndrome." American Journal of Mental Retardation 106, no. 2 (2001): 135-144.

ORGANIZATIONS

American Academy of Neurology, 1080 Montreal Avenue, St. Paul, Minnesota 55116. (651) 695-1940. Fax: (651) 695-2791. 〈http://www.aan.com/〉. [email protected]

American Academy of Pediatrics, 141 Northwest Point Boulevard, Elk Grove Village, IL 60007-1098. (847) 434-4000. Fax: (847) 434-8000. 〈http://www.aap.org/default.htm〉. [email protected]

American Association on Mental Retardation, 444 North Capitol Street, NW, Suite 846, Washington, D.C. 20001-1512. (202) 387-1968 or (800) 424-3688. Fax: (202) 387-2193. 〈http://www.aamr.org〉.

American Speech-Language Hearing Association, 10801 Rockville Pike, Rockville, MD 20852. (800) 638-8255. 〈http://www.asha.org/〉. [email protected]

Association of Retarded Citizens of the United States, PO Box 6109, 2501 Avenue J, Arlington, TX 76011.

National Association for Down Syndrome, PO Box 4542, Oak Brook, IL 60522. (630) 325-9112. 〈http://www.nads.org/〉.

National Down Syndrome Congress, 7000 Peachtree-Dunwoody Road, NE, Lake Ridge 400 Office Park, Building #5, Suite 100, Atlanta, GA 30328-1655. (800) 232-6372 or (770) 604-9500. 〈http://www.ndsccenter.org/〉. [email protected]

National Down Syndrome Society, 666 Broadway, New York, NY 10012. (212) 460-9330 or (800) 221-4602. Fax: (212) 979-2873. 〈http://www.ndss.org/〉.

OTHER

Administration on Developmental Disabilities (Dept of Health and Human Services). 〈http://www.acf.dhhs.gov/programs/add/index.htm〉.

Arc. 〈http://www.thearc.org/faqs/down.html〉.

Children With Disabilities. 〈http://www.childrenwithdisabilities.ncjrs.org/〉.

Disability Solutions. 〈http://www.disabilitysolutions.org/〉.

National Council on Disability. 〈http://www.ncd.gov/〉.

Resources for Children with Special Needs. 〈http://www.childrenwithdisabilities.ncjrs.org/〉.

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Down Syndrome

Down Syndrome

Causes and symptoms

Diagnosis and prevention

Resources

Down syndrome is the most common cause of mental retardation. It is caused by the presence of an extra chromosome. Chromosomes contain sequences of deoxyribonucleic acid (DNA) called genes, which represent the genetic information that exists within a cell. Twenty-three distinctive pairs of chromosomes (46 in total), are located within the nucleus (a region of the cell that is bounded by a sepcialized membrane, and which houses the genetic material). When a sperm cell fertilizes an egg cell, the newly created zygote normally receives 23 chromosomes from each parent. The contribution of genetic information from each parent is what makes each baby a distinctive blend of both parental characteristics. But, in Down syndrome, a mistake during division of the sperm or egg cell produces a cell with an extra chromosome 21. This event occurs during cell division and is referred to as nondisjunction, or the failure of all chromosomes to separately properly resulting in retention of one of the chromosomes in one of the two new daughter cells. This event is also called trisomy 21 and accounts for approximately 95% of all Down syndrome patients.

In a very rare number of Down syndrome cases, the original egg and sperm cells begins with the correct number of chromosomes but shortly after fertilization during the phase where cells are dividing rapidly, a single cell can divide abnormally, creating a line of cells with an extra chromosome 21. This is called a cell line mosaicism. The individual with this type of Down syndrome has two types of cells: some with 46 chromosomes (the normal number), and some with 47 chromosomes (causing Down syndrome symptomatology). Individuals who are mosaic for trisomy 21 typically have less severe signs and symptoms of the disorder.

Another relatively rare genetic abnormality that can cause Down syndrome is called a chromosome translocation. This is an event that unlike the numerical abnormality causing trisomy 21, there is a structural

abnormality. Exchange of material from two different chromosomes during the production of sex cells can take place such that there is a whole chromosome 21 attached to another chromosome but the chromosome number is normal. These types of translocations, involving chromosome 21, occur in about 34% of cases of Down syndrome.

Down syndrome occurs in about one in every 800 liveborns. It affects an equal number of male and female babies. The majority of cases of Down syndrome occur due to a nondisjunction event that occurs in the maternal sex cells. As the maternal age increases, the risk of having a Down syndrome baby increases significantly. For example, at younger ages, the risk is about one in 1, 250. By the time the woman is 35 years old, the risk increases to one in 385; by age 40 the risk increases to one in 100; and by age 45 the risk is one in 25. There is no known maternal age-related increased risk if down syndrome results from either a cell line mosaicism or translocation.

Causes and symptoms

While Down syndrome is a chromosomal disorder, a baby is usually identified at birth through observation of a set of common physical characteristics. Babies with Down syndrome tend to be overly quiet, less responsive, with weak, floppy muscles. Furthermore, a number of physical signs may be present. These include a flat appearing face; smaller than normal head; flat bridge of the nose; smaller than normal, low-set nose; small mouth, with a protruding tongue; upward slanting eyes; extra folds of skin located at the inside corner of each eye, near the nose (epicanthal folds); small, outwardly rotated ears; small, wide

KEY TERMS

Chromosomes The structures that carry genetic information in the form of DNA. Chromosomes are located within every cell and are responsible for directing the development and functioning of all the cells in the body. The normal number of chromosomes in humans is 46 (23 pairs).

Developmental delay A condition where an individual has a lower-than-normal IQ, and thus is developmentally delayed.

Egg cell The females reproductive sex cell.

Embryo A stage in development after fertilization.

Karyotype The specific chromosomal makeup of a particular cell.

Mosaic A term referring to a genetic situation in which a different cells do not have the exact same composition of chromosomes. In Down syndrome, this may mean that some of the individuals cells have the normal 46 number of chromosomes, while other cells have an abnormal number, or 47 chromosomes.

Nondisjunction A genetic term referring to an event that takes place during cell division in which one of the newly created cells has 24 chromosomes and the other cell has 22, rather than the normal 23.

Sperm Substance secreted by the testes during sexual intercourse. Sperm includes spermatozoon, the mature male cell which is propelled by a tail and has the ability to fertilize the female egg.

Translocation A genetic term referring to a situation during cell division in which a piece of one chromosome breaks off and sticks to another chromosome.

Trisomy The condition of having three identical chromosomes, instead of the normal two.

Zygote The cell resulting from the fusion of male sperm and the female egg. Normally the zygote has double the chromosome number of either gamete, and gives rise to a new embryo.

hands; an unusual, deep crease across the center of the palm (simian crease); malformed fifth finger; wide space between the big and the second toes; unusual creases on the soles of the feet; and, later in childhood, shorter than normal height.

Other types of defects often accompany Down syndrome. About one third of all children with Down syndrome have heart defects. These heart defects are characteristic of Down syndrome, including abnormal openings (or holes) in the walls which separate the hearts chambers (atrial septal defect, ventricular septal defect). These defects result in abnormal patterns of blood flow within the heart, resulting in inefficient oxygen delivery.

Malformations of the gastrointestinal tract are present in about 57% of children with Down syndrome. The most common malformation is a narrowed, obstructed duodenum (the part of the intestine into which the stomach empties). This disorder, called duodenal atresia, interferes with the babys milk or formula leaving the stomach and entering the intestine for digestion. The baby often vomits forcibly after feeding, and cannot gain weight appropriately until the defect is surgically repaired.

Developmental milestones in a child with Down syndrome are delayed. Due to weak, floppy muscles (hypotonia), babies learn to sit up, crawl, and walk much later than their normal peers. Talking is also delayed. The extent of delayed brain development is considered to be mild-to-moderate. Most people with Down syndrome can learn to perform regular tasks and can have relatively easy jobs (with supervision).

As people with Down syndrome age, they face an increased risk of developing Alzheimer disease, a degenerative disease that affects the brain. This occurs several decades earlier than the risk of developing Alzheimer disease in the general population. As people with Down syndrome age, they also have an increased chance of developing a number of other illnesses, including cataracts, thyroid problems, diabetes, leukemia, and seizure disorders.

There is no cure for Down syndrome. However, some of the clinical manifestations can be treated. For example, heart defects and duodenal atresia can often be corrected with surgical repair. In general, Down syndrome people tend to be easy going and good natured. The prognosis in Down syndrome is variable, depending on the types of complications (heart defects, susceptibility to infections, leukemia) of each affected individual. The severity of the developmental delay also varies. Without the presence of heart defects, about 90% of children with Down syndrome survive past their teenage years. In fact, people with Down syndrome can live until they are 50 years old.

Diagnosis and prevention

Down syndrome can be diagnosed at birth, when the characteristic physical signs of Down syndrome are noted and chromosome analysis can also be performed to confirm the diagnosis and determine the recurrence risks.

At-risk pregnancies are referred for genetic counseling and prenatal diagnosis. Screening tests are available during a pregnancy to determine if the fetus has Down syndrome. During 1417 weeks of pregnancy, a substance called AFP (alpha-fetoprotein) can be measured. AFP is normally found circulating in a pregnant womans blood, but may be unusually high or low with certain disorders. Carrying a baby with Down syndrome often causes AFP to be lower than normal. This information alone, or along with measurements of two other hormones, is considered along with the mothers age to calculate the risk of the baby being born with Down syndrome.

A common method to directly determine whether the fetus has Down syndrome, is to test tissue from the fetus. This is usually done either by amniocentesis, or chorionic villus sampling (CVS). In amniocentesis, a small amount of the fluid in which the baby is floating is withdrawn with a long, thin needle. In chorionic villus sampling, a tiny tube is inserted into the opening of the uterus to retrieve a small sample of the chorionic villus (tissue that surrounds the growing fetus). Chromosome analysis follow both amniocentesis and CVS to determine whether the fetus is affected.

Resources

BOOKS

Brandt, Caroline. He Has Up Syndrome Not Down Syndrome. Philadelphia: PublishAmerica, 2004.

Kumin, Libby. Early Communication Skills for Children With Down Syndrome: A Guide for Parents and Professionals. Bethesda: Woodbine House, 2003.

Soper, Kathryn L. Gifts: How Children with Down Syndrome Have Enriched Our Lives. Charleston: BookSurge Publishing, 2006.

Rosalyn S. Carson-DeWitt

Bryan R. Cobb

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Down Syndrome

DOWN SYNDROME

Down syndrome, named after the physician Langdon Down, is the most common genetic form of mental retardation, occurring in 1 to 1.5 of every 1,000 live births. In approximately 95 percent of cases, Down syndrome results from an extra chromosome on the twenty-first of the twenty-three pairs of human chromosomes. Exactly what causes the addition of the extra chromosomal material associated with Down syndrome is not clear, although risk factors include maternal age and possibly paternal age. People with Down syndrome usually function in the moderate range of mental retardation, with IQs generally ranging from 40 to 55 on average, though IQs can sometimes be higher or lower. People with Down syndrome experience particular deficits in certain aspects of language development, particularly expressive language, articulation, and grammar. Despite these deficits, many individuals with Down syndrome have relatively good social skills.

See also:DEVELOPMENTAL DISABILITIES; MILESTONES OF DEVELOPMENT

Bibliography

Burack, Jake, Robert Hodapp, and Edward Zigler, eds. Handbook of Mental Retardation and Development. New York: Cambridge University Press, 1998.

Rondal, Jean, Juan Perera, Lynn Nadel, and A. Comblain. Down Syndrome: Psychological, Psychobiological, and Socio-Educational Perspectives. London: Whurr Publishers, 1996.

Sigman, Marian. "Developmental Deficits in Children with Down Syndrome." In Helen Tager Flusberg ed., Neurodevelopmental Disorders: Developmental Cognitive Neuroscience. Cambridge, MA: MIT Press, 1999.

Simonoff, E., P. Bolton, and M. Rutter. "Mental Retardation: Genetic Findings, Clinical Implications and Research Agenda." In M. Hertzig and E. Farber eds., Annual Progress in Child Psychiatry and Child Development: 1997. Philadelphia, PA: Brunner/Mazel, 1998.

David W.Evans

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Down syndrome

Down syndrome

Definition

Down syndrome is the most common chromosome disorder and genetic cause of mental retardation. It occurs because of the presence of an extra copy of chromosome 21. For this reason, it is also called trisomy 21.

Description

When a baby is conceived, the sperm cell from the father and the egg cell from the mother undergo a reduction of the total number of chromosomes from 46 to 23. Occasionally an error occurs in this reduction process and instead of passing on 23 chromosomes to the baby, a parent will pass on 24 chromosomes. This event is called nondisjunction and it occurs in 95% of Down syndrome cases. The baby therefore receives an extra chromosome at conception. In Down syndrome, that extra chromosome is chromosome 21. Because of this extra chromosome 21, individuals affected with Down syndrome have 47 instead of 46 chromosomes.

Genetic profile

In approximately one to two percent of Down syndrome cases, the original egg and sperm cells contain the correct number of chromosomes, 23 each. The problem occurs sometime shortly after fertilization—during the phase when cells are dividing rapidly. One cell divides abnormally, creating a line of cells with an extra copy of chromosome 21. This form of genetic disorder is called mosaicism. The individual with this type of Down syndrome has two types of cells: those with 46 chromosomes (the normal number), and those with 47 chromosomes (as occurs in Down syndrome). Individuals affected with this mosaic form of Down syndrome generally have less severe signs and symptoms of the disorder.

Another relatively rare genetic accident that causes Down syndrome is called translocation. During cell division, chromosome 21 somehow breaks. The broken off piece of this chromosome then becomes attached to another chromosome. Each cell still has 46 chromosomes, but the extra piece of chromosome 21 results in the signs and symptoms of Down syndrome. Translocations occur in about 3–4% of cases of Down syndrome.

Once a couple has had one baby with Down syndrome, they are often concerned about the likelihood of future offspring also being born with the disorder. Mothers under the age of 35 with one Down syndrome-affected child have a 1% chance that a second child will also be born with Down syndrome. In mothers 35 and older, the chance of a second child being affected with Down syndrome is approximately the same as for any woman at a similar age. However, when the baby with Down syndrome has the type that results from a translocation, it is possible that one of the two parents is a carrier of a balanced translocation. A carrier has rearranged chromosomal information and can pass it on, but he or she does not have an extra chromosome and therefore is not affected with the disorder. When one parent is a carrier of a translocation, the chance of future offspring having Down syndrome is greatly increased. The specific risk will have to be assessed by a genetic counselor.

Demographics

Down syndrome occurs in about one in every 800 live births. It affects an equal number of male and female babies. The majority of cases of Down syndrome occur due to an extra chromosome 21 within the egg cell supplied by the mother (nondisjunction). As a woman's age (maternal age) increases, the risk of having a Down syndrome baby increases significantly. By the time the woman is age 35, the risk increases to one in 400; by age 40 the risk increases to one in 110; and, by age 45, the risk becomes one in 35. There is no increased risk of either mosaicism or translocation with increased maternal age.

Down syndrome occurs with equal frequency across all ethnic groups and subpopulations.

Signs and symptoms

While Down syndrome is a chromosomal disorder, a baby is usually identified at birth through observation of a set of common physical characteristics. Not all affected babies will exhibit all of the symptoms discussed. There is a large variability in the number and severity of these characteristics from one affected individual to the next. Babies with Down syndrome tend to be overly quiet, less responsive to stimuli, and have weak, floppy muscles. A number of physical signs may also be present. These include: a flat appearing face; a small head; a flat bridge of the nose; a smaller than normal, low-set nose; small mouth, which causes the tongue to stick out and to appear overly large; upward slanting eyes; bright speckles on the iris of the eye (Brushfield spots); extra folds of skin located at the inside corner of each eye and near the nose (epicanthal folds); rounded cheeks; small, misshapen ears; small, wide hands; an unusual deep crease across the center of the palm (simian crease); an inwardly curved little finger; a wide space between the great and the second toes; unusual creases on the soles of the feet; overly flexible joints (sometimes referred to as being double-jointed); and shorter-than-normal stature.

Other types of defects often accompany Down syndrome. Approximately 30–50% of all children with Down syndrome are found to have heart defects. A number of different heart defects are common in Down syndrome. All of these result in abnormal patterns of blood flow within the heart. Abnormal blood flow within the heart often means that less oxygen is sent into circulation throughout the body, which can cause fatigue, a lack of energy, and poor muscle tone.

Malformations of the gastrointestinal tract are present in about 5–7% of children with Down syndrome. The most common malformation is a narrowed, obstructed duodenum (the part of the intestine into which the stomach empties). This disorder, called duodenal atresia, interferes with the baby's milk or formula leaving the stomach and entering the intestine for digestion. The baby often vomits forcibly after feeding, and cannot gain weight appropriately until the defect is repaired.

Another malformation of the gastrointestinal tract seen in patients with Down syndrome is an abnormal connection between the windpipe (trachea) and the digestive tube of the throat (esophagus) called a tracheoesophageal fistula (T-E fistula). This connection interferes with eating and/or breathing because it allows air to enter the digestive system and/or food to enter the airway.

Other medical conditions occurring in patients with Down syndrome include an increased chance of developing infections, especially ear infections and pneumonia; certain kidney disorders; thyroid disease (especially low or hypothyroid); hearing loss; vision impairment requiring glasses (corrective lenses); and a 20 times greater chance than the population as a whole of developing leukemia.

Development in a baby and child affected with Down syndrome occurs at a much slower than normal rate. Because of weak, floppy muscles (hypotonia), babies learn to sit up, crawl, and walk much later than their unaffected peers. Talking is also quite delayed. The level of mental retardation is considered to be mild-to-moderate in Down syndrome. The degree of mental retardation varies a great deal from one child to the next. While it is impossible to predict the severity of Down syndrome at birth, with proper education, children who have Down syndrome are capable of learning. Most children affected with Down syndrome can read and write and are placed in special education classes in school. The majority of individuals with Down syndrome become semi-independent adults, meaning that they can take care of their own needs with some assistance.

As people with Down syndrome age, they face an increased chance of developing the brain disease called Alzheimer's (sometimes referred to as dementia or senility). Most people have a 12% chance of developing Alzheimer disease , but almost all people with Down syndrome will have either Alzheimer disease or a similar type of dementia by the age of 50. Alzheimer disease causes the brain to shrink and to break down. The number of brain cells decreases, and abnormal deposits and structural arrangements occur. This process results in a loss of brain functioning. People with Alzheimer's have strikingly faulty memories. Over time, people with Alzheimer disease will lapse into an increasingly unresponsive state.

As people with Down syndrome age, they also have an increased chance of developing a number of other illnesses, including cataracts, thyroid problems, diabetes, and seizure disorders.

Diagnosis

Diagnosis is usually suspected at birth, when the characteristic physical signs of Down syndrome are noted. Once this suspicion has been raised, genetic testing (chromosome analysis) can be undertaken in order to verify the presence of the disorder. This testing is usually done on a blood sample, although chromosome analysis can also be done on other types of tissue, including the skin. The cells to be studied are prepared in a laboratory. Chemical stain is added to make the characteristics of the cells and the chromosomes stand out. Chemicals are added to prompt the cells to go through normal development, up to the point where the chromosomes are most visible, prior to cell division. At this point, they are examined under a microscope and photographed. The photograph is used to sort the different sizes and shapes of chromosomes into pairs. In most cases of Down syndrome, one extra chromosome 21 will be revealed. The final result of such testing, with the photographed chromosomes paired and organized by shape and size, is called the individual's karyotype . An individual with Down syndrome will have a 47 XX+21 karyotype if they are female and a 47 XY+21 karyotype if they are male.

Women who become pregnant after the age of 35 are offered prenatal tests to determine whether or not their developing baby is affected with Down syndrome. A genetic counselor meets with these families to inform them of the risks and to discuss the types of tests available to make a diagnosis prior to delivery. Because there is a slight risk of miscarriage following some prenatal tests, all testing is optional, and couples need to decide whether or not they desire to take this risk in order to learn the status of their unborn baby.

Screening tests are used to estimate the chance that an individual woman will have a baby with Down syndrome. A test called the maternal serum alpha-fetoprotein test (MSAFP) is offered to all pregnant women under the age of 35. If the mother decides to have this test, it is performed between 15 and 22 weeks of pregnancy. The MSAFP screen measures a protein and two hormones that are normally found in maternal blood during pregnancy. A specific pattern of these hormones and protein can indicate an increased risk for having a baby born with Down syndrome. However, this is only a risk and MSAFP cannot diagnose Down syndrome directly. Women found to have an increased risk of their babies being affected with Down syndrome are offered amniocentesis . The MSAFP test can detect up to 60% of all babies who will be born with Down syndrome.

Ultrasound screening for Down syndrome is also available. This is generally performed in the midtrimester of pregnancy. Abnormal growth patterns characteristic of Down syndrome such as growth retardation, heart defects, duodenal atresia, T-E fistula, shorter than normal long-bone lengths, and extra folds of skin along the back of the neck of the developing fetus may all be observed via ultrasonic imaging.

The only way to definitively establish (with about 99% accuracy) the presence or absence of Down syndrome in a developing baby is to test tissue during the pregnancy itself. This is usually done either by amniocentesis, or chorionic villus sampling (CVS). All women under the age of 35 who show a high risk for having a baby affected with Down syndrome via an MSAFP screen and all mothers over the age of 35 are offered either CVS or amniocentesis. In CVS, a tiny tube is inserted into the opening of the uterus to retrieve a small sample of the placenta (the organ that attaches the growing baby to the mother via the umbilical cord, and provides oxygen and nutrition). In amniocentesis, a small amount of the fluid in which the baby is floating is withdrawn with a long, thin needle. CVS may be performed as early as 10 to 12 weeks into a pregnancy. Amniocentesis is generally not performed until at least the fifteenth week. Both CVS and amniocentesis carry small risks of miscarriage. Approximately 1% of women miscarry after undergoing CVS testing, while approximately one-half of one percent miscarry after undergoing amniocentesis. Both amniocentesis and CVS allow the baby's own karyotype to be determined.

Approximately 75% of all babies diagnosed prenatally as affected with Down syndrome do not survive to term and spontaneously miscarry. In addition, these prenatal tests can only diagnose Down syndrome, not the severity of the symptoms that the unborn child will experience. For this reason, a couple might use this information to begin to prepare for the arrival of a baby with Down syndrome, to terminate the pregnancy, or in the case of miscarriage or termination, decide whether to consider adoption as an alternative.

Treatment and management

No treatment is available to cure Down syndrome. Treatment is directed at addressing the individual concerns of a particular patient. For example, heart defects may require surgical repair, as will duodenal atresia and T-E fistula. Many Down syndrome patients will need to wear glasses to correct vision. Patients with hearing impairment benefit from hearing aids.

While some decades ago all children with Down syndrome were quickly placed into institutions for lifelong care, research shows very clearly that the best out-look for children with Down syndrome is a normal family life in their own home. This requires careful support and education of the parents and the siblings. It is a life-changing event to learn that a new baby has a permanent condition that will affect essentially all aspects of his or her development. Some community groups help families deal with the emotional effects of raising a child with Down syndrome. Schools are required to provide services to children with Down syndrome, sometimes in separate special education classrooms, and sometimes in regular classrooms (this is called mainstreaming or inclusion).

As of May 2000, the genetic sequence for chromosome 21 was fully determined, which opens the door to new approaches to the treatment of Down syndrome through the development of gene-specific therapies.

Prognosis

The prognosis for an individual with Down syndrome is quite variable, depending on the types of complications (heart defects, susceptibility to infections, development of leukemia, etc.). The severity of the retardation can also vary significantly. Without the presence of heart defects, about 90% of children with Down syndrome live into their teens. People with Down syndrome appear to go through the normal physical changes of aging more rapidly, however. The average age of death for an individual with Down syndrome is about 50 to 55 years.

Still, the prognosis for a baby born with Down syndrome is better than ever before. Because of modern medical treatments, including antibiotics to treat infections, and surgery to treat heart defects and duodenal atresia, life expectancy has greatly increased. Community and family support allows people with Down syndrome to have rich, meaningful relationships. Because of educational programs, some people with Down syndrome are able to hold jobs.

As of early 2001, there has only been one report of a male affected with Down syndrome becoming a father. Approximately 60% of women with Down syndrome are fully capable of having children. The risk of a woman with trisomy 21 having a child affected with Down syndrome is 50%.

Resources

BOOKS

Pueschel, Siegfried M. A Parent's Guide to Down Syndrome: Toward a Brighter Future. Revised ed. New York: Paul H. Brookes Publishing Co., 2000.

Selikowitz, Mark. Down Syndrome: The Facts. 2nd ed. London: Oxford University Press, 1997.

Stray-Gunderson, K. Babies with Down Syndrome: A New Parents' Guide. Kensington: Woodbine House, 1986.

PERIODICALS

Carlson, Tucker, and Jason Cowley. "When a Life is Worth Living: Down's Syndrome Children." The Times (29 November 1996): 18+.

Cohen, William, ed. "Health Care Guidelines for Individuals with Down Syndrome: 1999 Revision." Down Syndrome Quarterly (September 1999).

Hattori, M., A. Fujiyama, D. Taylor, H. Watanabe, et al. "The DNA sequence of human chromosome 21." Nature (18 May 2000): 311–19.

ORGANIZATIONS

National Down Syndrome Congress. 7000 Peachtree-Dunwoody Rd., Bldg 5, Suite 100, Atlanta, GA 30328-1662. (770) 604-9500 or (800) 232-6372. Fax: (770) 604-9898. [email protected] <http://www.ndsccenter.org>.

National Down Syndrome Society. 666 Broadway, New York, NY 10012-2317. (212) 460-9330 or (800) 221-4602. Fax: (212) 979-2873. <http://[email protected]>.

WEBSITES

Down Syndrome Health Issues. <http://www.ds-health.com/>. (15 February 2001).

Down Syndrome Information Network. <http://www.downsyndrome.net/>. (15 February 2001).

Down Syndrome WWW Page. <http://www.nas.com/downsyn/>. (15 February 2001).

Recommended Down Syndrome Sites on the Internet. <http://www.ds-health.com/ds_sites.htm#natl>. (15 February 2001).

Paul A. Johnson

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Down Syndrome

Down syndrome

Definition

Down syndrome is the most common chromosome disorder and genetic cause of mental retardation. It occurs because of the presence of an extra copy of chromosome 21. For this reason, it is also called trisomy 21.

Description

Chromosomes are the units of genetic information that exist within every cell of the body. Twenty-three distinctive pairs, or 46 total chromosomes, are located within the nucleus (central structure) of each cell. When the sperm cells from the father and egg cell from the mother are formed, they both undergo a reduction of their total number of chromosomes from 46 to 23. This process is called meiosis. When a baby is conceived by the combination of one sperm cell with one egg cell, the baby receives 23 chromosomes from each parent, for a total of 46 chromosomes. Occasionally, an error occurs in the reduction process. Instead of passing on 23 chromosomes to the baby, a parent will pass on 24 chromosomes. This event is called non-disjunction and occurs in 95% of Down syndrome cases. The baby, therefore, receives an extra chromosome at conception. In Down syndrome, this is an extra chromosome 21. The total number of chromosomes in individuals affected with Down syndrome is 47 instead of 46.

In approximately 1-2% of Down syndrome cases, the original egg and sperm cells cells contain the correct number of chromosomes, 23 each. The problem occurs sometime shortly after fertilization; during the phase where cells are dividing rapidly. One cell divides abnormally, creating a line of cells with an extra copy of chromosome 21. This form of genetic disorder is called mosaicism. An individual with this type of Down syndrome has two types of cells: those with 46 chromosomes (the normal number), and those with 47 chromosomes (as occurs in Down syndrome). Individuals affected with this mosaic form of Down syndrome generally have less severe signs and symptoms of the disorder.

Another relatively rare genetic accident that causes Down syndrome is called translocation. During cell division , chromosome 21 somehow breaks. The broken off piece of this chromosome then becomes attached to another chromosome. Each cell still has 46 chromosomes, but the extra piece of chromosome 21 results in the signs and symptoms of Down syndrome. Translocations occur in about 3 to 4% of cases of Down syndrome.

Genetic profile

Down syndrome is usually the result of an extra copy of chromosome 21 (trisomy 21). As described earlier, Down syndrome may occur because of: nondisjunction within the sperm, or more commonly, the egg cell; genetic mosaicism, which occurs after conception; or translocation, which also occurs after conception.

Once a couple has had one baby with Down syndrome, they are often concerned about the likelihood of future offspring also being born with the disorder. Women who are 34 and younger have a recurrence rate of 1% for having another Down syndrome child. Women who are 35 and older are at increased risk. The specific risk varies and increases as their age increases. When a baby with Down syndrome has the type that results from a translocation, it is possible that one of the two parents is a carrier of a balanced translocation. When one parent is a carrier of a balanced translocation, the chance of future offspring having Down syndrome is greatly increased. Specific risks must be assessed by a genetic counselor.

Demographics

The incidence of Down syndrome is about 1 in 800 live births. It affects an equal number of male and female babies. The majority of cases of Down syndrome occur due to an extra chromosome 21 within the egg cell supplied by the mother (nondisjunction). As a woman's age (maternal age) increases, the risk of having a Down syndrome baby significantly increases. By the time the woman is age 35, the risk increases to one in 400; by age 40 the risk increases to one in 110; and, by age 45 the risk becomes one in 35. There is no increased risk of either mosaicism or translocation with increased maternal age.

Causes and symptoms

While Down syndrome is a chromosomal disorder, a baby is usually identified at birth through observation of a set of common physical characteristics. It is important to remember that not all children with Down syndrome will exhibit all of the features discussed. There is great variability in both the number and severity of characteristics. Babies with Down syndrome tend to be overly quiet, less responsive to stimuli, and have weak, floppy muscles. A number of physical signs may also be present. These include: a flat appearing face; brighter sparkles in the iris of the eye (Brushfield spots); a small head; a flat bridge of the nose; a smaller than normal, low-set nose; small mouth. This causes the tongue to stick out and to appear overly large. They have upward slanting eyes; extra folds of skin located at the inside corner of each eye, near the nose (epicanthal folds); rounded cheeks; and small, misshapen ears. They tend to have small, wide hands; an unusual deep crease across the center of the palm (simian crease); a curved little finger; a wide space between the great and the second toes; unusual creases on the soles of the feet; overly-flexible joints (sometimes referred to as being double-jointed); and, a shorter than normal length.

Other types of problems often accompany Down syndrome. Approximately 30 to 50% of all children with Down syndrome are found to have heart defects.

Malformations of the gastrointestinal tract are present in about 5 to 7% of children with Down syndrome. The most common malformation is a narrowed, obstructed duodenum (the part of the intestine into which the stomach empties). This disorder, called duodenal atresia, interferes with the baby's milk or formula leaving the stomach and entering the intestine for digestion. The baby often vomits forcibly after feeding, and cannot gain weight appropriately until the defect is repaired. Another relatively common problem is a tracheo-esophageal fistula. This is an abnormal connection between the trachea (windpipe) and esophagus that interferes with both eating and breathing.

Other medical conditions occurring in persons with Down syndrome include an increased chance of developing infections, especially ear infections and pneumonia ; certain kidney disorders; thyroid disease (especially low or hypothyroidism); hearing loss ; vision impairment requiring glasses (corrective lenses); and a 20 times greater chance than the population as a whole of developing leukemia.

Development in a baby and child affected with Down syndrome occurs at a much slower than normal rate. Because of weak, floppy muscles (hypotonia), babies learn to sit up, crawl, and walk much later than their unaffected peers. Talking is also quite delayed. The level of mental retardation is considered to be mild-to-moderate in people with Down syndrome. The degree of mental retardation varies greatly among children with Down syndrome. While it is impossible to predict the severity of Down syndrome at birth, with proper education, children who have Down syndrome are capable of learning. Most children with Down syndrome can read and write and are placed in special education classes at school. The majority of individuals with Down syndrome become semi-independent, meaning that they can take care of their own needs with some assistance. Many hold non-complex jobs.

As people with Down syndrome age, they face an increased chance of developing the brain disease called Alzheimer's (sometimes referred to as dementia or senility). People without Down syndrome have a lifetime risk of 12% for developing Alzheimer's disease . In contrast, by age 40-50, almost all persons with Down syndrome will also develop Alzheimer's disease.

As people with Down syndrome age, they also have an increased chance of developing a number of other illnesses, including cataracts , thyroid problems, diabetes, and seizure disorders.

Diagnosis

Diagnosis is usually suspected at birth, when the characteristic physical signs of Down syndrome are noted. Once this suspicion has been raised, genetic testing (chromosome analysis) can be undertaken in order to verify the presence of the disorder. This testing is usually done on a blood sample, although chromosome analysis can also be performed on other types of tissue, including the skin. The cells to be studied are prepared in a laboratory. Chemical stain is added to make the characteristics of the cells and the chromosomes stand out. Chemicals are added to prompt the cells to go through normal development, up to the point where the chromosomes are most visible, prior to cell division. At this point, they are examined under a microscope and photographed. The photograph is used to sort the different sizes and shapes of chromosomes into pairs. In most cases of Down syndrome, one extra chromosome 21 will be revealed. The final result of such testing, with the photographed chromosomes paired and organized by shape and size, is called the individual's karyotype. A female with Down syndrome will have a 47, XX+21 karyotype; a male with Down syndrome will have a 47 XY+21 karyotype.

Women who are over the age of 35 are offered prenatal tests to determine if their developing baby is affected with Down syndrome. A genetic counselor meets with these families to inform them of the risks and to discuss the tests that are available to make a diagnosis prior to delivery. Because there is a slight risk of miscarriage following some prenatal tests, all testing is optional. Couples must decide whether or not they desire to take this risk to learn the status of their unborn baby. Couples must also decide if they wish to know whether or not the baby the mother carries has Down syndrome. Some couples choose not to have the test because they are certain they would not choose an abortion if they find theirs is a Down syndrome baby.

Ultrasound is now available for prenatal screening. Some abnormalities associated with Down syndrome, including intrauterine growth retardation, may be detected using ultrasound. The use of ultrasound as a screening test for Down syndrome is limited by the difficulty of producing reliable sonographic images of critical fetal structures.

A counselor needs to inform a woman that her risk of having a baby with Down syndrome increases with her increasing age. Two types of testing are available during a pregnancy to determine if the baby being carried has Down syndrome.

Screening tests are used to estimate the chance that an individual woman will have a baby with Down syndrome. All pregnant women under the age of 35 are offered a maternal serum alpha-fetoprotein (MSAFP) screen. This test is normally performed at 15-22 weeks of pregnancy. The MSAFP screen measures a protein and two hormones normally found in maternal blood during pregnancy. A specific pattern of these hormones and protein can give a pregnant woman an increased risk for having a baby born with Down syndrome. Carrying a baby with Down syndrome often causes MSAFP to be lower than normal. Test results reveal only an increased risk and cannot diagnose a baby born with Down syndrome. The MSAFP test can detect up to 60% of all babies who will be born with Down syndrome. Women with an increased risk are offered amniocentesis .

The only way to definitively (with about 99% accuracy) establish the presence or absence of Down syndrome in a developing baby is to test tissue from the pregnancy itself. This is usually done either by amniocentesis or chorionic villus sampling (CVS). In chorionic villus sampling, usually performed at 10-12 weeks of pregnancy, a tiny tube is inserted into the opening of the uterus to retrieve a small sample of the placenta (the organ which attaches the growing baby to the mother via the umbilical cord, and provides oxygen and nutrition ). In amniocentesis, a small amount of the fluid in which the baby is floating is withdrawn with a long, thin needle. Both amniocentesis and CVS allow the baby's own karyotype to be determined. Both tests carry a small risk of causing miscarriage. The risk from CVS is 1% and the risk from amniocentesis is 0.5%. This small risk must be considered when deciding to perform these tests. If test information is positive for Down syndrome, a couple must then decide how to use this information to begin to prepare for the arrival of a baby with Down syndrome, to consider adoption for the baby or to terminate the pregnancy. It must be noted that while the results of prenatal tests can diagnose Down syndrome, they are unable to predict the severity of symptoms.

Treatment

No treatment is available to cure Down syndrome. Treatment is directed at addressing the individual concerns of a particular person. For example, heart defects will many times require surgical repair, as will duodenal atresia or a tracheo-esophageal fistula. Many persons with Down syndrome will need to wear glasses to correct vision. Persons with hearing impairment benefit from hearing aids .

While some decades ago, all Down syndrome children were quickly placed into institutions for lifelong care, research shows very clearly that the best outlook for children with Down syndrome is a normal family life in their own homes. This requires careful support and education of both parents and siblings. It is a life-changing event to learn that a new baby has a permanent condition that will affect essentially all aspects of development. Some community groups exist to help families deal with the emotional effects of this new information, and to help plan for the baby's future. Schools are required to provide services for children with Down syndrome, sometimes in separate special education classrooms, and sometimes in regular classrooms (this is called mainstreaming or inclusion).

As of 2001, the genetic sequence for chromosome 21 was fully determined, which may open the door to new approaches to the treatment of Down syndrome through the development of gene-specific therapies.

Prognosis

The prognosis in Down syndrome is quite variable, depending on the types of complications (heart defects, susceptibility to infections, development of leukemia) of each individual baby. The severity of the retardation can also significantly vary. Without the presence of heart defects, about 90% of children with Down syndrome live into their teens. People with Down syndrome appear to go through the normal physical changes of aging more rapidly, however. The average age of death for an individual with Down syndrome is about 50 to 55 years.

The prognosis for a baby born with Down syndrome continues to improve. Due to modern medical treatments, including antibiotics to treat infections, and surgery to correct heart defects, duodenal atresia and tracheoesophageal fistula, life expectancy has greatly increased. Community and family support allows people with


KEY TERMS


Cell —A fundamental unit of living tissue. The specialized nature of organs and tissues in a human body reflects the specialized structure and function of its constituent cells.

Chromosome —The structures that carry genetic information. Chromosomes are located within every cell and are responsible for directing the development and functioning of all the cells in the body. The normal number is 46 (23 pairs).

Genetic counseling —A communication process by which personal genetic risk information is translated into practical information for families. Genetic counselors are health care professionals with specialized training and experience in the areas of medical genetics and counseling.

Karyotype —The specific chromosomal makeup of a particular cell.

Mental retardation —A condition where an individual has a significantly lower-than-normal IQ, and thus is developmentally delayed.

Mosaic —A term referring to a genetic situation in which all of an individual's cells do not have the same composition of chromosomes. In Down syndrome, this may mean that some of an individual's cells have a normal 46 chromosomes, while other cells have 47 chromosomes.

Nondisjunction —A genetic term referring to an event that takes place during cell division in which a genetic accident causes an egg or sperm cell to have 24 chromosomes, rather than the normal 23.

Translocation —A genetic term referring to a situation during cell division in which a piece of one chromosome breaks off and sticks to another chromosome.

Trisomy —The condition of having three identical chromosomes, instead of the normal two.


Down syndrome to have rich, meaningful relationships. Because of educational programs, some people with Down syndrome are able to hold jobs.

Most men with Down syndrome appear to be sterile (meaning that they are unable to have offspring). There has been at least one report of a male with Down syndrome who fathered a child. Some women with Down syndrome, however, are capable of having babies. Approximately 40% of women with Down syndrome are unable to become pregnant. The risk of a woman with trisomy 21 to have a child with Down syndrome is approximately 50%.

Health care team roles

Obstetricians, nurse practitioners, or family doctors often make an initial recommendation that a woman be screened for Down syndrome. Depending on the test, a physician, phlebotomist or ultrasonographer may obtain sample materials. A laboratory technician will process sample materials. A genetic counselor, physician or other person with specialized training usually provides test results to a couple or woman. Counselors must be available to assist a woman or couple to make appropriate decisions about their baby. Pediatricians, family doctors, internists, and geriatric physicians provide care throughout the life of an individual with Down syndrome. Other health professionals, including surgeons, eye specialists and hearing experts, provide services as needed.

Prevention

As of 2001, there is no known way to prevent Down syndrome.

Resources

BOOKS

Girod, Christina M. Down Syndrome. Farmington Hills, MI: Lucent Books, 2001.

Jones, Kenneth L. "Down syndrome." In Smith's Recognizable Patterns of Human Malformation, 5th ed. Richard E. Behrman et al., Philadelphia: Saunders, 1997, 8-14.

Miller, Jon F, Mark Leddy and Lewis A. Leavitt. Improving the Communication of People With Down Syndrome. Baltimore, MD: Paul H Brookes Publishers, 1999.

Pueschel, Sigfried M. A Parent's Guide to Down Syndrome: Toward a Brighter Future, 2nd ed. Baltimore, MD: Paul H Brookes Publishers, 2000.

Van Allen, Margot and Judith C. Hill. "Congenital anomalies." In Cecil Textbook of Medicine, 21st ed. Goldman, Lee and Bennett, J. Claude. Philadelphia: W.B. Saunders, 2000, 150-153.

Weeks, Daniel J. Perceptual-Motor Behavior in Down Syndrome. Champaign, IL: Human Kinetics Publishing, 2000.

PERIODICALS

Kaiser, A. P., P. P. Hester, and A. S. McDuffie. "Supporting communication in young children with developmental disabilities." Mental Retardation and Developmental Disability Research Review 7, no. 2 (2001): 143-150.

Maymon, R., E. Dreazen, I. Buckovsky, Z. Weinraub, and A. Herman. "Does a 'notched' nuchal translucency indicate Down syndrome fetuses or other adverse pregnancy out-come?" Prenatal Diagnosis 21, no. 5 (2001): 403-408.

Palisano R. J., S. D. Walter, D. J. Russell, P. L. Rosenbaum, M. Gemus, B. E. Galuppi, and L. Cunningham. "Gross motor function of children with down syndrome: creation of motor growth curves." Archives of Physical Medicine and Rehabilitation 82, no. 4 (2001): 494-500.

Savulescu, J. "Resources, Down's syndrome, and cardiac surgery." British Medical Journal 322, no. 7291 (2001): 875-876.

Van Riper, M., and W. I. Cohen. "Caring for children with Down syndrome and their families." Journal of Pediatric Health Care 15, no. 3 (2001): 123-131.

Varela, A. M., L. B. Sardinha, and K. H. Pitetti. "Effects of an aerobic rowing training regimen in young adults with Down syndrome." American Journal of Mental Retardation 106, no. 2 (2001): 135-144.

ORGANIZATIONS

American Academy of Neurology, 1080 Montreal Avenue, St. Paul, Minnesota 55116. (651) 695-1940. Fax: (651) 695-2791. <http://www.aan.com/>. [email protected]

American Academy of Pediatrics, 141 Northwest Point Boulevard, Elk Grove Village, IL 60007-1098. (847) 434-4000. Fax: (847) 434-8000. <http://www.aap.org/default.htm>. [email protected]

American Association on Mental Retardation, 444 North Capitol Street, NW, Suite 846, Washington, D.C. 20001-1512. (202) 387-1968 or (800) 424-3688. Fax: (202) 387-2193. <http://www.aamr.org>.

American Speech-Language Hearing Association, 10801 Rockville Pike, Rockville, MD 20852. (800) 638-8255. <http://www.asha.org/>. [email protected]

Association of Retarded Citizens of the United States, PO Box 6109, 2501 Avenue J, Arlington, TX 76011.

National Association for Down Syndrome, PO Box 4542, Oak Brook, IL 60522. (630) 325-9112. <http://www.nads.org/>.

National Down Syndrome Congress, 7000 Peachtree-Dunwoody Road, NE, Lake Ridge 400 Office Park, Building #5, Suite 100, Atlanta, GA 30328-1655. (800) 232-6372 or (770) 604-9500. <http://www.ndsccenter.org/>. [email protected]

National Down Syndrome Society, 666 Broadway, New York, NY 10012. (212) 460-9330 or (800) 221-4602. Fax:(212) 979-2873. <http://www.ndss.org/>.

OTHER

Administration on Developmental Disabilities (Dept of Health and Human Services). <http://www.acf.dhhs.gov/programs/add/index.htm>.

Arc. <http://www.thearc.org/faqs/down.html>.

Children With Disabilities. <http://www.childrenwithdisabilities.ncjrs.org/>.

Disability Solutions. <http://www.disabilitysolutions.org/>.

National Council on Disability. <http://www.ncd.gov/>.

Resources for Children with Special Needs. <http://www.childrenwithdisabilities.ncjrs.org/>.

L. Fleming Fallon, Jr., MD, DrPH

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Down Syndrome

Down syndrome

Down syndrome is the most common cause of mental retardation. It can be caused by the presence of an extra chromosome . Chromosomes contain sequences of DNA called genes, which represent the genetic information that exists within almost every cell of the body. Twenty-three distinctive pairs, or 46 total chromosomes, are located within the nucleus (central DNA-containing structure) of each cell. When a sperm cell fertilizes an egg cell, the newly created zygote receives 23 chromosomes from each parent, for a total of 46 chromosomes. An aberration that occurs during division of the sperm or egg cell can cause this cell to have an extra chromosome for a total of 24 chromosomes instead of 23. This event occurs during cell division and is referred to as nondisjunction, or the failure of all chromosomes to separately properly resulting in retention of one of the chromosomes in one of the two new daughter cells and the loss of a chromosome in the other. Loss of a chromosome usually means that the embryo is nonviable. An embryo with an extra chromosome in all its cells (for a total of 47 chromosomes) is usually inconsistent with life. If the extra chromosome is number 21, and the fetus survives to term, the baby with have Down syndrome. This form of Down Syndrome is also called Trisomy 21 and accounts for approximately 95% of all Down syndrome patients.

In a very rare number of Down syndrome cases, the original egg and sperm cells begins with the correct number of chromosomes but shortly after fertilization during the phase where cells are dividing rapidly, a single cell can divide abnormally, creating a line of cells with an extra chromosome 21. This is called a cell line mosaicism. The individual with this type of Down syndrome has two types of cells: some with 46 chromosomes (the normal number), and some with 47 chromosomes (causing Down syndrome symptomatology). Individuals who are mosaic for Trisomy 21 typically have less severe signs and symptoms of the disorder.

Another relatively rare genetic abnormality that can cause Down syndrome is called a chromosome translocation. This is an event that unlike the numerical abnormality causing Trisomy 21, there is a structural abnormality. Exchange of material from two different chromosomes during the production of sex cells can take place such that there is a whole chromosome 21 attached to another chromosome but the chromosome number is normal. These types of translocations, involving chromosome 21, occur in about 3–4% of cases of Down syndrome.

Down syndrome occurs in about one in every 800 liveborns. It affects an equal number of male and female babies. The majority of cases of Down syndrome occur due to a nondisjunction event that occurs in the maternal sex cells. As the maternal age increases, the risk of having a Down syndrome baby increases significantly. For example, at younger ages, the risk is about one in 1,250. By the time the woman is 35 years old, the risk increases to one in 385; by age 40 the risk increases to one in 100; and by age 45 the risk is one in 25. There is no known maternal age-related increased risk if down syndrome results from either a cell line mosaicism or translocation.


Causes and symptoms

While Down syndrome is a chromosomal disorder, a baby is usually identified at birth through observation of a set of common physical characteristics. Babies with Down syndrome tend to be overly quiet, less responsive, with weak, floppy muscles. Furthermore, a number of physical signs may be present. These include:

  • flat appearing face
  • small head
  • flat bridge of the nose
  • smaller than normal, low-set nose
  • small mouth, with a protruding tongue
  • upward slanting eyes
  • extra folds of skin located at the inside corner of each eye , near the nose (called epicanthal folds)
  • small, outwardly rotated ears
  • small, wide hands
  • an unusual, deep crease across the center of the palm (called a simian crease)
  • a malformed fifth finger
  • a wide space between the big and the second toes
  • unusual creases on the soles of the feet
  • shorter than normal height

Other types of defects often accompany Down syndrome. About one third of all children with Down syndrome have heart defects. These heart defects are characteristic of Down syndrome, including abnormal openings (or holes) in the walls which separate the heart's chambers (atrial septal defect, ventricular septal defect). These defects result in abnormal patterns of blood flow within the heart, resulting in inefficient oxygen delivery.

Malformations of the gastrointestinal tract are present in about 5–7% of children with Down syndrome. The most common malformation is a narrowed, obstructed duodenum (the part of the intestine into which the stomach empties). This disorder, called duodenal atresia, interferes with the baby's milk or formula leaving the stomach and entering the intestine for digestion. The baby often vomits forcibly after feeding, and cannot gain weight appropriately until the defect is surgically repaired.

Other medical conditions occurring in patients with Down syndrome include an increased chance of developing infections, especially ear infections and pneumonia ; certain kidney disorders; thyroid disease ; hearing loss; vision impairment requiring glasses (corrective lenses); and a 15-fold increased risk for developing leukemia .

Developmental milestones in a child with Down syndrome are delayed. Due to weak, floppy muscles (hypotonia), babies learn to sit up, crawl, and walk much later than their normal peers. Talking is also delayed. The extent of delayed brain development is considered to be mild-to-moderate. Most people with Down syndrome can learn to perform regular tasks and can have relatively easy, jobs (with supervision).

As people with Down syndrome age, they face an increased risk of developing Alzheimer disease , a degenerative disease that affects the brain. This occurs several decades earlier than the risk of developing Alzheimers disease in the general population. As people with Down syndrome age, they also have an increased chance of developing a number of other illnesses, including cataracts, thyroid problems, diabetes, leukemia, and seizure disorders.


Treatment

There is no cure for Down syndrome. However, some of the clinical manifestations can be treated. For example, heart defects and duodenal atresia can often be corrected with surgical repair. It was common only a few decades ago to enforce involuntary sterilization of individuals with Down syndrome. Additionally, it used to be common for these patients to be institutionalized. Today, involuntary sterilizations are illegal and most patients reside with their families. Many community support groups exist to help families deal with the emotional effects and to help plan for the affected individuals future. In general, Down syndrome people tend to be easy going and good natured.


Prognosis

The prognosis in Down syndrome is variable, depending on the types of complications (heart defects, susceptibility to infections, leukemia) of each affected individual. The severity of the developmental delay also varies. Without the presence of heart defects, about 90% of children with Down syndrome survive past their teenage years. In fact, people with Down syndrome can live until they are 50 years old.

The prognosis for a baby born with Down syndrome has improved compared to previous years. Modern medical treatments, including antibiotics to treat infections, and surgery to treat heart defects and duodenal atresia has greatly increased their life expectancy.


Diagnosis and prevention

Down syndrome can be diagnosed at birth, when the characteristic physical signs of Down syndrome are noted and chromosome analysis can also be performed to confirm the diagnosis and determine the recurrence risks.

At-risk pregnancies are referred for genetic counseling and prenatal diagnosis. Screening tests are available during a pregnancy to determine if the fetus has Down syndrome. During 14–17 weeks of pregnancy, a substance called AFP (alpha-fetoprotein) can be measured. AFP is normally found circulating in a pregnant woman's blood, but may be unusually high or low with certain disorders. Carrying a baby with Down syndrome often causes AFP to be lower than normal. This information alone, or along with measurements of two other hormones , is considered along with the mother's age to calculate the risk of the baby being born with Down syndrome.

A common method to directly determine whether the fetus has Down syndrome, is to test tissue from the fetus. This is usually done either by amniocentesis , or chorionic villus sampling (CVS) . In amniocentesis, a small amount of the fluid in which the baby is floating is withdrawn with a long, thin needle. In chorionic villus sampling, a tiny tube is inserted into the opening of the uterus to retrieve a small sample of the chorionic villus (tissue that surrounds the growing fetus). Chromosome analysis follow both amniocentesis and CVS to determine whether the fetus is affected.

Once a couple has had one baby with Down syndrome, they are often concerned about the likelihood of future offspring also being born with the disorder. In most cases, it is unlikely that the risk is greater than other woman at a similar age. However, when the baby with Down syndrome has the type that results from a translocation, it is possible that one of the two parents is a carrier of that defect. When one parent is a carrier of a particular type of translocation, the chance of future offspring having Down syndrome is increased. The specific risks can be estimated by a genetic counselor.


Resources

books

Nussbaum, R.L., Roderick R. McInnes, and Huntington F. Willard. Genetics in Medicine. Philadelphia: Saunders, 2001.

Rimoin, David, L. Emery and Rimoin's Principles and Practice of Medical Genetics. London; New York: Churchill Livingstone, 2002.

periodicals

"Medical and Surgical Care for Children with Down Syndrome." The Exceptional Parent 25 no. 11 (November 1995): 78+.


Rosalyn S. Carson-DeWitt

Bryan R. Cobb

KEY TERMS


. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chromosomes

—The structures that carry genetic information in the form of DNA. Chromosomes are located within every cell and are responsible for directing the development and functioning of all the cells in the body. The normal number of chromosomes in humans is 46 (23 pairs).

Developmental delay

—A condition where an individual has a lower-than-normal IQ, and thus is developmentally delayed.

Egg cell

—The female's reproductive sex cell.

Embryo

—A stage in development after fertilization.

Karyotype

—The specific chromosomal makeup of a particular cell.

Mosaic

—A term referring to a genetic situation in which an different cells do not have the exact same composition of chromosomes. In Down syndrome, this may mean that some of the individual's cells have the normal 46 number of chromosomes, while other cells have an abnormal number, or 47 chromosomes.

Nondisjunction

—A genetic term referring to an event that takes place during cell division in which one of the newly created cells has 24 chromosomes and the other cell has 22, rather than the normal 23.

Sperm

—Substance secreted by the testes during sexual intercourse. Sperm includes spermatozoon, the mature male cell which is propelled by a tail and has the ability to fertilize the female egg.

Translocation

—A genetic term referring to a situation during cell division in which a piece of one chromosome breaks off and sticks to another chromosome.

Trisomy

—The condition of having three identical chromosomes, instead of the normal two.

Zygote

—The cell resulting from the fusion of male sperm and the female egg. Normally the zygote has double the chromosome number of either gamete, and gives rise to a new embryo.

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Down Syndrome

Down syndrome

Definition

Down syndrome is the most common chromosome disorder and genetic cause of mental retardation. It occurs because of the presence of an extra copy of chromosome 21. For this reason, it is also called trisomy 21.

Description

When a baby is conceived, the sperm cell from the father and the egg cell from the mother undergo a reduction of the total number of chromosomes from 46 to 23. Occasionally an error occurs in this reduction process and instead of passing on 23 chromosomes to the baby, a parent will pass on 24 chromosomes. This event is called nondisjunction and it occurs in 95% of Down syndrome cases. The baby therefore receives an extra chromosome at conception. In Down syndrome, that extra chromosome is chromosome 21. Because of this extra chromosome 21, individuals affected with Down syndrome have 47 instead of 46 chromosomes.

Genetic profile

In approximately one to two percent of Down syndrome cases, the original egg and sperm cells contain the correct number of chromosomes, 23 each. The problem occurs sometime shortly after fertilization—during the phase when cells are dividing rapidly. One cell divides abnormally, creating a line of cells with an extra copy of chromosome 21. This form of genetic disorder is called mosaicism. The individual with this type of Down syndrome has two types of cells: those with 46 chromosomes (the normal number), and those with 47 chromosomes (as occurs in Down syndrome). Individuals affected with this mosaic form of Down syndrome generally have less severe signs and symptoms of the disorder.

Another relatively rare genetic accident that causes Down syndrome is called translocation. During cell division, chromosome 21 somehow breaks. The broken off piece of this chromosome then becomes attached to another chromosome. Each cell still has 46 chromosomes, but the extra piece of chromosome 21 results in the signs and symptoms of Down syndrome. Translocations occur in about 3–4% of cases of Down syndrome.

Once a couple has had one baby with Down syndrome, they are often concerned about the likelihood of future offspring also being born with the disorder. Mothers under the age of 35 with one Down syndrome-affected child have a 1% chance that a second child will also be born with Down syndrome. In mothers 35 and older, the chance of a second child being affected with Down syndrome is approximately the same as for any woman at a similar age. However, when the baby with Down syndrome has the type that results from a translocation, it is possible that one of the two parents is a carrier of a balanced translocation. A carrier has rearranged chromosomal information and can pass it on, but he or she does not have an extra chromosome and therefore is not affected with the disorder. When one parent is a carrier of a translocation, the chance of future offspring having Down syndrome is greatly increased. The specific risk will have to be assessed by a genetic counselor.

Demographics

Down syndrome occurs in about one in every 800 live births. It affects an equal number of male and female babies. The majority of cases of Down syndrome occur due to an extra chromosome 21 within the egg cell supplied by the mother (nondisjunction). As a woman's age (maternal age) increases, the risk of having a Down syndrome baby increases significantly. By the time the woman is age 35, the risk increases to one in 400; by age 40 the risk increases to one in 110; and, by age 45, the risk becomes one in 35. There is no increased risk of either mosaicism or translocation with increased maternal age.

Down syndrome occurs with equal frequency across all ethnic groups and subpopulations.

Signs and symptoms

While Down syndrome is a chromosomal disorder, a baby is usually identified at birth through observation of a set of common physical characteristics. Not all affected babies will exhibit all of the symptoms discussed. There is a large variability in the number and severity of these characteristics from one affected individual to the next. Babies with Down syndrome tend to be overly quiet, less responsive to stimuli, and have weak, floppy muscles. A number of physical signs may also be present. These include: a flat appearing face; a small head; a flat bridge of the nose; a smaller than normal, low-set nose; small mouth, which causes the tongue to stick out and to appear overly large; upward slanting eyes; bright speckles on the iris of the eye (Brushfield spots); extra folds of skin located at the inside corner of each eye near the nose (epicanthal folds); rounded cheeks; small, misshapen ears; small, wide hands; an unusual deep crease across the center of the palm (simian crease); an inwardly curved little finger; a wide space between the great and the second toes; unusual creases on the soles of the feet; overly flexible joints (sometimes referred to as being double-jointed); and shorter-than-normal stature.

Other types of defects often accompany Down syndrome. Approximately 30–50% of all children with Down syndrome are found to have heart defects. A number of different heart defects are common in Down syndrome. All of these result in abnormal patterns of blood flow within the heart. Abnormal blood flow within the heart often means that less oxygen is sent into circulation throughout the body, which can cause fatigue, a lack of energy, and poor muscle tone.

Malformations of the gastrointestinal tract are present in about 5–7% of children with Down syndrome. The most common malformation is a narrowed, obstructed duodenum (the part of the intestine into which the stomach empties). This disorder, called duodenal atresia, interferes with the baby's milk or formula leaving the stomach and entering the intestine for digestion. The baby often vomits forcibly after feeding, and cannot gain weight appropriately until the defect is repaired.

Another malformation of the gastrointestinal tract that is seen in patients with Down syndrome is an abnormal connection between the windpipe (trachea) and the digestive tube of the throat (esophagus) called a tracheoesophageal fistula (T-E fistula). This connection interferes with eating and/or breathing because it allows air to enter the digestive system and/or food to enter the airway.

Other medical conditions occurring in patients with Down syndrome include an increased chance of developing infections, especially ear infections and pneumonia; certain kidney disorders; thyroid disease (especially low or hypothyroid); hearing loss; vision impairment requiring glasses (corrective lenses); and a 20 times greater chance than the population as a whole of developing leukemia.

Development in a baby and child affected with Down syndrome occurs at a much slower than normal rate. Because of weak, floppy muscles (hypotonia), babies learn to sit up, crawl, and walk much later than their unaffected peers. Talking is also quite delayed. The level of mental retardation is considered to be mild-to-moderate in Down syndrome. The degree of mental retardation varies a great deal from one child to the next. While it is impossible to predict the severity of Down syndrome at birth, with proper education, children who have Down syndrome are capable of learning. Most children affected with Down syndrome can read and write and are placed in special education classes in school. The majority of individuals with Down syndrome become semi-independent adults, meaning that they can take care of their own needs with some assistance.

As people with Down syndrome age, they face an increased chance of developing the brain disease called Alzheimer's (sometimes referred to as dementia or senility). Most people have a 12% chance of developing Alzheimer's, but almost all people with Down syndrome will have either Alzheimer disease or a similar type of dementia by the age of 50. Alzheimer disease causes the brain to shrink and to break down. The number of brain cells decreases, and abnormal deposits and structural arrangements occur. This process results in a loss of brain functioning. People with Alzheimer's have strikingly faulty memories. Over time, people with Alzheimer's disease will lapse into an increasingly unresponsive state.

As people with Down syndrome age, they also have an increased chance of developing a number of other illnesses, including cataracts, thyroid problems, diabetes , and seizure disorders.

Diagnosis

Diagnosis is usually suspected at birth, when the characteristic physical signs of Down syndrome are noted. Once this suspicion has been raised, genetic testing (chromosome analysis) can be undertaken in order to verify the presence of the disorder. This testing is usually done on a blood sample, although chromosome analysis can also be done on other types of tissue, including the skin. The cells to be studied are prepared in a laboratory. Chemical stain is added to make the characteristics of the cells and the chromosomes stand out. Chemicals are added to prompt the cells to go through normal development, up to the point where the chromosomes are most visible, prior

to cell division. At this point, they are examined under a microscope and photographed. The photograph is used to sort the different sizes and shapes of chromosomes into pairs. In most cases of Down syndrome, one extra chromosome 21 will be revealed. The final result of such testing, with the photographed chromosomes paired and organized by shape and size, is called the individual's karyotype . An individual with Down syndrome will have a 47 XX+21 karyotype if they are female and a 47 XY+21 karyotype if they are male.

Women who become pregnant after the age of 35 are offered prenatal tests to determine whether or not their developing baby is affected with Down syndrome. A genetic counselor meets with these families to inform them of the risks and to discuss the types of tests available to make a diagnosis prior to delivery. Because there is a slight risk of miscarriage following some prenatal tests, all testing is optional, and couples need to decide whether or not they desire to take this risk in order to learn the status of their unborn baby.

Screening tests are used to estimate the chance that an individual woman will have a baby with Down syndrome. A test called the maternal serum alpha-fetoprotein test (MSAFP) is offered to all pregnant women under the age of 35. If the mother decides to have this test, it is performed between 15 and 22 weeks of pregnancy. The MSAFP screen measures a protein and two hormones that are normally found in maternal blood during pregnancy. A specific pattern of these hormones and protein can indicate an increased risk for having a baby born with Down syndrome. However, this is only a risk and MSAFP cannot diagnose Down syndrome directly. Women found to have an increased risk of their babies being affected with Down syndrome are offered amniocentesis . The MSAFP test can detect up to 60% of all babies who will be born with Down syndrome.

Ultrasound screening for Down syndrome is also available. This is generally performed in the mid-trimester of pregnancy. Abnormal growth patterns characteristic of Down syndrome such as growth retardation, heart defects, duodenal atresia, T-E fistula, shorter than normal long-bone lengths, and extra folds of skin along the back of the neck of the developing fetus may all be observed via ultrasonic imaging.

The only way to definitively establish (with about 99% accuracy) the presence or absence of Down syndrome in a developing baby is to test tissue during the pregnancy itself. This is usually done either by amniocentesis, or chorionic villus sampling (CVS). All women under the age of 35 who show a high risk for having a

baby affected with Down syndrome via an MSAFP screen and all mothers over the age of 35 are offered either CVS or amniocentesis. In CVS, a tiny tube is inserted into the opening of the uterus to retrieve a small sample of the placenta (the organ that attaches the growing baby to the mother via the umbilical cord, and provides oxygen and nutrition). In amniocentesis, a small amount of the fluid in which the baby is floating is withdrawn with a long, thin needle. CVS may be performed as early as 10 to 12 weeks into a pregnancy. Amniocentesis is generally not performed until at least the fifteenth week. Both CVS and amniocentesis carry small risks of miscarriage. Approximately 1% of women miscarry after undergoing CVS testing, while approximately one-half of one percent miscarry after undergoing amniocentesis. Both amniocentesis and CVS allow the baby's own karyotype to be determined.

Approximately 75% of all babies diagnosed prenatally as affected with Down syndrome do not survive to term and spontaneously miscarry. In addition, these prenatal tests can only diagnose Down syndrome, not the severity of the symptoms that the unborn child will experience. For this reason, a couple might use this information to begin to prepare for the arrival of a baby with Down syndrome, to terminate the pregnancy, or in the case of miscarriage or termination, decide whether to consider adoption as an alternative.

Treatment and management

No treatment is available to cure Down syndrome. Treatment is directed at addressing the individual concerns of a particular patient. For example, heart defects may require surgical repair, as will duodenal atresia and T-E fistula. Many Down syndrome patients will need to wear glasses to correct vision. Patients with hearing impairment benefit from hearing aids.

While some decades ago all children with Down syndrome were quickly placed into institutions for lifelong care, research shows very clearly that the best out-look for children with Down syndrome is a normal family life in their own home. This requires careful support and education of the parents and the siblings. It is a life-changing event to learn that a new baby has a permanent condition that will affect essentially all aspects of his or her development. Some community groups help families deal with the emotional effects of raising a child with Down syndrome. Schools are required to provide services to children with Down syndrome, sometimes in separate special education classrooms, and sometimes in regular classrooms (this is called mainstreaming or inclusion).

In May 2000, the genetic sequence for chromosome 21 was fully determined, which opens the door to new approaches to the treatment of Down syndrome through the development of gene-specific therapies.

Prognosis

The prognosis for an individual with Down syndrome is quite variable, depending on the types of complications (heart defects, susceptibility to infections, development of leukemia, etc.). The severity of the retardation can also vary significantly. Without the presence of heart defects, about 90% of children with Down syndrome live into their teens. People with Down syndrome appear to go through the normal physical changes of aging more rapidly, however. The average age of death for an individual with Down syndrome is about 50 to 55 years.

Still, the prognosis for a baby born with Down syndrome is better than ever before. Because of modern medical treatments, including antibiotics to treat infections, and surgery to treat heart defects and duodenal atresia, life expectancy has greatly increased. Community and family support allows people with Down syndrome to have rich, meaningful relationships. Because of educational programs, some people with Down syndrome are able to hold jobs.

There has only been one report of a male affected with Down syndrome becoming a father. Approximately 60% of women with Down syndrome are fully capable of having children. The risk of a woman with trisomy 21 having a child affected with Down syndrome is 50%.

Resources

BOOKS

Pueschel, Siegfried M. A Parent's Guide to Down Syndrome: Toward a Brighter Future. Revised ed. New York: Paul H. Brookes Publishing Co., 2000.

Selikowitz, Mark. Down Syndrome: The Facts. 2nd ed. London: Oxford University Press, 1997.

Stray-Gunderson, K. Babies with Down Syndrome: A New Parents' Guide. Kensington: Woodbine House, 1986.

PERIODICALS

Carlson, Tucker, and Jason Cowley. "When a Life is Worth Living: Down's Syndrome Children." The Times (29 November 1996): 18+.

Cohen, William, ed. "Health Care Guidelines for Individuals with Down Syndrome: 1999 Revision." Down Syndrome Quarterly (September 1999).

Hattori, M., A. Fujiyama, D. Taylor, H. Watanabe, et al. "The DNA sequence of human chromosome 21." Nature (18 May 2000): 311–19.

ORGANIZATIONS

National Down Syndrome Congress. 7000 Peachtree-Dunwoody Rd., Bldg 5, Suite 100, Atlanta, GA 30328-1662. (770) 604-9500 or (800) 232-6372. Fax: (770) 604-9898. [email protected] <http://www.ndsccenter.org>.

National Down Syndrome Society. 666 Broadway, New York, NY 10012-2317. (212) 460-9330 or (800) 221-4602. Fax: (212) 979-2873. <http://[email protected]>.

WEBSITES

Down Syndrome Health Issues. <http://www.ds-health.com/>. (15 February 2001).

Down Syndrome Information Network. <http://www.downsyndrome.net/>. (15 February 2001).

Down Syndrome WWW Page. <http://www.nas.com/downsyn/>. (15 February 2001).

Recommended Down Syndrome Sites on the Internet. <http://www.ds-health.com/ds_sites.htm#natl>. (15 February 2001).

Paul A. Johnson

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