Genetic Muscular Dystrophy
Genetic Muscular Dystrophy
Muscular dystrophy, or MD, is actually the name of nine groups of inherited disorders that affect muscle strength and the connections between
muscles and nerves. Some begin in childhood while others affect mostly adults; some lead to early death while others cause only mild disabilities. What they have in common is progressive weakening of the skeletal muscles. Some forms of MD also affect the muscular tissue of the heart.
The nine types of MD vary in the age of onset, major symptoms, and course.
- Duchenne muscular dystrophy. Named for Guillaume Duchenne (1806–1875), the French doctor who first described it in the 1860s, Duchenne muscular dystrophy is the most common form, accounting for half of all cases of MD. It is gender-linked, affecting mostly boys, and is usually diagnosed in childhood. Muscle wasting begins in the muscles of the legs and pelvis, then moves to the arms and neck. Boys with Duchenne MD have frequent falls, breathing difficulties and heart problems, and are usually confined to a wheel-chair by age twelve. Most patients die before age twenty.
- Becker muscular dystrophy. This is a milder form of MD but affects the same muscles as Duchenne muscular dystrophy. Patients are generally diagnosed at age eleven but generally remain independent longer than patients with Duchenne MD and live into middle age.
- Congenital MD. This is a group of muscular dystrophies that affects both boys and girls and is usually diagnosed before age two. Some children with these disorders die in infancy while others live into adulthood.
- Emery-Dreifuss MD. This form of MD primarily affects boys; symptoms appear at any point from age ten to twenty-five. Contractures in the ankles, knees, and other joints occur before the muscles start to weaken. Almost all patients develop heart problems by age thirty.
- Facioscapulohumeral MD. This form of muscular dystrophy affects the muscles of the face, shoulders, and upper arms before other parts of the body. Symptoms may begin at any age from the late teens through age forty. Patients often develop a pouting or masklike expression as the muscles of the face deteriorate.
- Limb-girdle MD. This form of MD affects both boys and girls. Muscle weakness is typically noticed first around the hips before spreading to the shoulders, legs, and neck. People with this form of MD develop a waddling gait, fall frequently, and are unable to run.
- Distal MD. This form of the disease does not appear until patients are forty to sixty years old. It affects the muscles of the hands, forearms, lower legs, and feet.
- Myotonic MD. This is the most common form of adult-onset MD, appearing between twenty and thirty years of age. It affects both men and women. Also known as Steinert's disease, this form of MD is characterized by stiffness; the patient cannot relax the muscles voluntarily after tightening then. In addition to weakening the patient's muscles, myotonic MD affects the central nervous system and other organs, including the heart, adrenal glands and thyroid, the eyes, and the digestive tract. People with myo-tonic MD may also develop cataracts in the eyes.
- Oculopharyngeal MD. This form of muscular dystrophy typically begins when the patient is in his or her forties or fifties. It also affects both men and women. It is most common in people of French Canadian descent and some Hispanics living in New Mexico. Muscle weakness first affects the eyelids and throat muscles, causing difficulty swallowing as well as interfering with vision. Most patients will eventually lose the ability to walk.
The demographics of muscular dystrophy are different for different forms of the disease. Duchenne MD is the most common, occurring in about one in every 3,500 male babies. Becker MD is the second most common, with a rate of one in every 30,000 male births. The other forms of MD are very rare; limb-girdle dystrophy, for example, occurs in only 1 percent of patients diagnosed with muscular dystrophy, while Emery-Dreifuss MD affects only one boy in every 100,000. Oculopharyngeal MD is most common among French Canadians and some Hispanics, while distal MD is more common in Sweden than in other countries.
As has been noted, some forms of MD affect only males because the genetic mutation responsible for the absence or defective quality of a protein essential to maintaining muscle tissue is located on the X chromo-some. Females may be carriers of the mutation and may have mild symptoms of the disorder. Other types of muscular dystrophy have different inheritance patterns and can affect women as well as men.
The basic cause of the various forms of MD is a genetic mutation that affects the production of proteins necessary to maintain the structure of muscle cells. In the case of Duchenne MD and Becker MD, the affected gene is called the DMD gene and is located on the X chromosome. The gene is responsible for the production of a protein called dystrophin, which is needed to maintain the cell membrane of muscle cells. In Duchenne MD, there is no dystrophin produced; in Becker MD, there is some dystrophin, but not enough to protect the muscle fibers from gradually weakening.
In Emery-Dreifuss MD, the defective gene on the X chromosome is called the EMD gene. It is responsible for producing a protein called emerin, which is essential for the functioning of skeletal and heart muscles. Myotonic dystrophy is caused by a defect in a gene on chromosome 19. The defect interferes with the production of a protein called myosin, which is involved in the movement of muscles.
Although there are some variations among the nine types of muscular dystrophy, the most common symptoms of the disease include:
- Muscle weakness that gets worse over time
- Loss of balance and coordination
- Wasting (loss of muscle tissue, replaced by fat in some cases)
- Scoliosis (curvature of the spine)
- Difficulty walking
- Frequent falls
- Clumsiness; eventual loss of the ability to write
- Gradual weakening of the heart and breathing muscles
- Eventual difficulty in swallowing
Since the symptoms of MD appear in different patients at different ages, depending on the specific type of disease that the patient has, diagnosis begins with a physical examination and taking a history of the patient's symptoms. Next, the patient may be sent to a neurologist (a specialist in diagnosing and treating disorders of the nervous system) for tests of the reflexes, balance, and other functions in order to rule out exposures to toxins or other diseases that may be causing the patient's symptoms.
The third step is a series of laboratory and imaging tests:
- Blood and urine tests. These are used to measure the levels of various enzymes in the patient's body fluids that result from the breakdown of muscle tissue or that leak out of damaged muscle. A blood test can also be performed for genetic testing.
- Exercise tests. These can be carried out at the patient's bedside. They measure the strength of the muscles, particularly the muscles involved in breathing.
- Ultrasound imaging. Ultrasound imaging can detect abnormalities in the patient's muscles even in early stages of MD. Magnetic resonance imaging (MRI) can also be used to look for changes in muscle tissue and to monitor the progression of the disease.
- Muscle biopsy. Taking a sample of the patient's muscle tissue allows he doctor to distinguish between MD and other diseases that affect the muscles and nerves.
- Electromyography. Electromyography is a technique in which the doctor inserts a thin electrode through the skin into a muscle. As the patient tightens and relaxes the muscle, the electrode measures the patterns of electrical activity in the muscle.
- Electrocardiogram (ECG). This test is done to evaluate the functioning of the patient's heart.
- Pulmonary function test. The doctor may order this test for patients whose breathing muscles are showing signs of weakness. It measures the patient's lung capacity.
There is no cure for muscular dystrophy. Treatment consists of keeping the patient mobile and comfortable as long as possible. Specific therapies may include medications to delay the loss of muscle tissue and function or to relax tightened muscles; surgery to minimize problems caused by contractures or scoliosis; physical therapy to keep the muscles as strong and flexible as possible; and occupational therapy to teach the patient the skills that are necessary for self-care. Some patients with breathing difficulties may be placed on an artificial respirator, while those with heart problems may have a pacemaker implanted.
Most patients with MD eventually require assistive devices, from canes, braces, and walkers to wheelchairs and motorized scooters. These devices can help to keep patients independent as well as slow the development of contractures. It is important for patients with MD to stay as active as possible, as bed rest or inactivity can speed up the deterioration of the muscles.
The prognosis of muscular dystrophy depends on the specific type, although all types are incurable. Patients with Duchenne MD rarely live beyond age twenty, while those with congenital MD may die in childhood, and patients with one of the adult-onset dystrophies may live into their sixties. Modern medical and surgical care allows most patients a better quality of life than they would otherwise have, but the outcome is still premature death, usually from heart failure.
There is no known way to prevent all the various types of MD. Genetic counseling is advised, however, when there is a family history of muscular dystrophy. Women may have no symptoms but still carry the gene for the disorder, and Duchenne muscular dystrophy can be detected by genetic tests during pregnancy.
As of 2008, a drug known as MYO-029 was being tested in clinical trials as a possible treatment for Duchenne MD. Researchers think that this drug might help damaged muscle to repair itself, and thus slow down or even stop the gradual loss of muscle tissue. Another approach that has been tried in treating MD is gene therapy, in which the defective DMD gene could be replaced by a normal gene. New strategies for replacing the defective gene are the target of current research.
Research into the causes of and possible treatments for MD received a major boost in December 2001, when President George W. Bush signed into law the Muscular Dystrophy Community Assistance, Research, and Education (MD CARE) Amendments Act of 2001. The act led to the establishment of six muscular dystrophy research centers at university medical centers and children's hospitals across the United States.
SEE ALSO Cataracts; Scoliosis
WORDS TO KNOW
Contracture: Shortening or tightening of the muscles surrounding certain joints that limits the movement of the joints.
Dystrophin: A protein found in muscle whose absence or defectiveness is one of the causes of muscular dystrophy.
Gene therapy: The insertion of normal genes into a person's cells or tissues in order to treat a disease by replacing a harmful mutation or nonfunctioning gene.
Myosin: A protein involved in muscle movement.
Nucleotides: The basic structural units of DNA and RNA.
Scoliosis: Curvature of the spine from side to side.
Triplet: In genetics, a unit of three nucleotides that starts or stops the production of a specific protein. Triplets are also called codons.
Wasting: Loss of lean muscle tissue.
Abramovitz, Melissa. Muscular Dystrophy. Detroit: Lucent Books, 2008. Johanson, Paula. Muscular Dystrophy. New York: Rosen, 2008.
Winheld, Josh. Worth the Ride: My Journey with Duchenne Muscular Dystrophy. Beach Haven, NJ: Little Treasure Books, 2008.
Orr, Matthew, and Reed Abelson. “Living Longer with Duchenne Muscular Dystrophy.” New York Times, February 2008. Available online at
http://video.on.nytimes.com/?fr_story=66493aa1fe16d32d6a255cdf200 d1ae7dff4de30 (accessed June 14, 2008). This is a six-minute video about a patient in the final stages of MD. It focuses on current strategies for helping patients live with the disease in the absence of a cure.
Genetics Home Reference. Duchenne and Becker Muscular Dystrophy. Available online at http://ghr.nlm.nih.gov/condition=duchenneandbeckermusculardystrophy (updated December 2007; accessed June 15, 2008).
Genetics Home Reference. Emery-Dreifuss Muscular Dystrophy. Available online at http://ghr.nlm.nih.gov/condition=emerydreifussmusculardystrophy (updated June 2006; accessed June 15, 2008).
Mayo Clinic. Muscular Dystrophy. Available online at http://www.mayoclinic.com/health/muscular-dystrophy/DS00200 (updated December 8, 2007; accessed June 15, 2008).
National Institute of Neurological Disorders and Stroke (NINDS). Muscular Dystrophy: Hope through Research. Available online at http://www.ninds.nih. gov/disorders/md/detail_md.htm (updated February 12, 2008; accessed June 15, 2008).
Senator Paul D. Wellstone Muscular Dystrophy Cooperative Research Centers.
Home Page. Available online at http://wellstonemdcenters.nih.gov/ (accessed June 15, 2008). This is the home page of the Web site for the six muscular dystrophy research centers established by the MD CARE Act of 2001.
TeensHealth. Muscular Dystrophy. Available online at http://www.kidshealth.org/teen/diseases_conditions/bones/muscular_dystrophy.html (updated January 2007; accessed June 15, 2008).
"Genetic Muscular Dystrophy." UXL Encyclopedia of Diseases and Disorders. . Encyclopedia.com. (January 22, 2019). https://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/genetic-muscular-dystrophy
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