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Color Blindness

Color blindness

Definition

Color blindness is an abnormal condition characterized by the inability to clearly distinguish different colors of the spectrum. The difficulties can range from mild to severe. It is a misleading term because people with color blindness are not blind. Rather, they tend to see colors in a limited range of hues; a rare few may not see colors at all.

Description

Normal color vision requires the use of specialized receptor cells called cones, which are located in the retina of the eye. There are three types of cones, red, blue, and green, which enable people to see a wide spectrum of colors. An abnormality, or deficiency, of any of the types of cones will result in abnormal color vision.

There are three basic variants of color blindness. Red/green color blindness is the most common deficiency, affecting 8 percent of Caucasian males and 0.5 percent of Caucasian females. The prevalence varies with culture.

Blue color blindness is an inability to distinguish both blue and yellow, which are seen as white or gray. It is quite rare and has equal prevalence in males and females. It is common for young children to have blue/green confusion that becomes less pronounced in adulthood. Blue color deficiency often appears in people who have physical disorders such as liver disease or diabetes mellitus .

A total inability to distinguish colors (achromatopsia) is exceedingly rare. These affected individuals view the world in shades of gray. They frequently have poor visual acuity and are extremely sensitive to light (photophobia), which causes them to squint in ordinary light.

Demographics

Researchers studying red/green color blindness in the United Kingdom reported an average prevalence of only 4.7 percent in one group. Only 1 percent of Eskimo males are color blind. Approximately 2.9 percent of boys from Saudi Arabia and 3.7 percent from India were found to have deficient color vision. Red/green color blindness may slightly increase an affected person's chances of contracting leprosy. Pre-term infants exhibit an increased prevalence of blue color blindness. Achromatopsia has a prevalence of about one in 33,000 in the United States and affects males and females equally.

Causes and symptoms

Red/green and blue color blindness appear to be located on at least two different gene locations. The majority of affected individuals are males. Females are carriers but are not normally affected. This indicates that the X chromosome is one of the locations for color blindness. Male offspring of females who carry the altered gene have a 50 percent chance of being color-blind. The rare female that has red/green color blindness, or rarer still, blue color blindness, indicates there is an involvement of another gene. As of 2004, the location of this gene was not yet identified.

Achromatopsia, the complete inability to distinguish color, is an autosomal recessive disease of the retina. Thus, both parents have one copy of the altered gene but do not have the disease. Each of their children has a 25 percent chance of not having the gene, a 50 percent chance of having one altered gene (and, like the parents, being unaffected), and a 25 percent risk of having both the altered gene and the condition. In 1997, the achromatopsia gene was discovered to reside on chromosome 2.

The inability to correctly identify colors is the only sign of color blindness. It is important to note that people with red/green or blue varieties of color blindness use other cues such as color saturation and object shape or location to distinguish colors. They can often distinguish red or green if they can visually compare the colors. However, most have difficulty accurately identifying colors without any other references. Most people with any impairment in color vision learn colors, as do other young children. These individuals often reach adolescence before their visual deficiency is identified.

Color blindness is sometimes acquired. Chronic illnesses that can lead to color blindness include Alzheimer's disease, diabetes mellitus, glaucoma, leukemia, liver disease, chronic alcoholism , macular degeneration, multiple sclerosis, Parkinson's disease, sickle cell anemia , and retinitis pigmentosa. Accidents or strokes that damage the retina or affect particular areas of the brain eye can lead to color blindness. Some medications such as antibiotics , barbiturates, anti-tuberculosis drugs, high blood pressure medications, and several medications used to treat nervous disorders and psychological problems may cause color blindness. Industrial or environmental chemicals such as carbon monoxide, carbon disulfide, fertilizers, styrene, and some containing lead can cause loss of color vision. Occasionally, changes can occur in the affected person's capacity to see colors after age 60.

When to call the doctor

An ophthalmologist should be consulted at the time color blindness is first suspected.

Diagnosis

There are several tests available to identify problems associated with color vision. The most commonly used is the American Optical/Hardy, Rand, and Ritter Pseudoisochromatic Test. It is composed of several discs filled with colored dots of different sizes and colors. A person with normal color vision looking at a test item sees a number that is clearly located somewhere in the center of a circle of variously colored dots. A color-blind person is not able to distinguish the number.

The Ishihara Test is comprised of eight plates that are similar to the American Optical Pseudoisochromatic Test plates. The individual being tested looks for numbers among the various colored dots on each test plate. Some plates distinguish between red/green and blue color blindness. Individuals with normal color vision perceive one number. Those with red/green color deficiency see a different number. Those with blue color vision see yet a different number.

A third analytical tool is the Titmus II Vision Tester Color Perception Test. The subject looks into a stereoscopic machine. The test stimulus most often used in professional offices contains six different designs or numbers on a black background, framed in a yellow border. Titmus II can test one eye at a time. However, its value is limited because it can only identify red/green deficiencies and is not highly accurate.

Treatment

As of 2004 there is no treatment or cure for color blindness. Most color vision deficient persons compensate well for their abnormality and usually rely on color cues and details that are not consciously evident to persons with typical color vision.

Inherited color blindness cannot be prevented. In the case of some types of acquired color deficiency, if the cause of the problem is removed, the condition may improve with time. But for most people with acquired color blindness, the damage is usually permanent.

Prognosis

Color blindness that is inherited is present in both eyes and remains constant over an individual's entire life. Some cases of acquired color vision loss are not severe, may appear in only one eye, and last for only a short time. Other cases tend to become worse with time.

Prevention

There is no way to prevent genetic color blindness. There is no way to prevent acquired color blindness that is associated with Alzheimer's disease, diabetes mellitus, leukemia, liver disease, macular degeneration, multiple sclerosis, Parkinson's disease, sickle cell anemia, and retinitis pigmentosa.

Some forms of acquired color blindness may be prevented. Limiting use of alcohol and drugs such as antibiotics, barbiturates, anti-tuberculosis drugs, high blood pressure medications, and several medications used to treat nervous disorders and psychological problems to levels that are required for therapeutic benefit may limit acquired color blindness.

Parental concerns

Parents can inquire about other family members who have experienced color blindness. If such family members exist, parents can have their children tested for color perception at an early age. Screening for color perception is usually performed in grade school.

KEY TERMS

Achromatopsia The inability to distinguish any colors.

Cones Receptor cells, located in the retina of the eye, that allow the perception of colors.

Photophobia An extreme sensitivity to light.

Retina The inner, light-sensitive layer of the eye containing rods and cones. The retina transforms the image it receives into electrical signals that are sent to the brain via the optic nerve.

Rods Photoreceptors, located in the retina of the eye, that are highly sensitive to low levels of light.

Resources

BOOKS

Color Blindness: A Medical Dictionary, Bibliography, and Annotated Research Guide to Internet Resources. San Diego, CA: Icon Group International, 2003.

Fay, Aaron, and Frederick A. Jokobiec. "Diseases of the Visual System." In Cecil Textbook of Medicine, 22nd ed. Edited by Lee Goldman, et al. Philadelphia: Saunders, 2003, pp. 240619.

Olitsky, Scott, and Leonard B. Nelson. "Disorders of Vision." In Nelson Textbook of Pediatrics, 17th ed. Edited by Richard E. Behrman, et al. Philadelphia: Saunders, 2003, pp. 20879.

Wiggs, Janey L. "Color Vision." In Ophthalmology, edited by Myron Yanoff and Jay S. Duker. St. Louis, MO: Mosby, 2000.

PERIODICALS

Abadi, R. V. "Effects of Color Blindness." Ophthalmic and Physiological Optics 24, no. 3 (2004): 25257.

Atchison, D. A., et al. "Traffic Signal Color Recognition Is a Problem for Both Protan and Deutan Color-vision Deficients." Human Factors 45, no. 3 (2003): 495503.

Dick, F., et al. "Is Color Vision Impairment Associated with Cognitive Impairment in Solvent Exposed Workers?" Occupational and Environmental Medicine 61, no. 1 (2004): 7678.

Tagarelli, A., et al. "Color Blindness in Everyday Life and Car Driving." Acta Ophthalmology Scandinavia 52, no. 4 (2004): 43642.

ORGANIZATIONS

American Academy of Family Physicians. 11400 Tomahawk Creek Parkway, Leawood, KS 662112672. Web site: <www.aafp.org/>.

American Academy of Pediatrics. 141 Northwest Point Boulevard, Elk Grove Village, IL 600071098. Web site: <www.aap.org/default.htm>.

WEB SITES

"Color Blindness: More Prevalent Among Males." Howard Hughes Medical Institute. Available online at <www.hhmi.org/senses/b130.html> (accessed November 16, 2004).

Rutherford, Kim. "What Is Color Blindness?" KidsHealth for Kids. Available online at <http://kidshealth.org/kid/talk/qa/color_blind.html>(accessed November 16, 2004).

Waggoner, Terrace L. Ishihara Test for Color Blindness. Available online at <www.toledo-bend.com/colorblind/Ishihara.html>(accessed November 16, 2004).

L. Fleming Fallon Jr., MD, DrPH

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Color Blindness

Color Blindness

Definition

Color blindness is an abnormal condition characterized by the inability to clearly distinguish different colors of the spectrum. The difficulties can be mild to severe. It is a misleading term because people with color blindness are not blind. Rather, they tend to see colors in a limited range of hues; a rare few may not see colors at all.

Description

Normal color vision requires the use of specialized receptor cells called cones, which are located in the retina of the eye. There are three types of cones, termed red, blue, and green, which enable people to see a wide spectrum of colors. An abnormality, or deficiency, of any of the types of cones will result in abnormal color vision.

There are three basic variants of color blindness. Red/green color blindness is the most common deficiency, affecting 8% of Caucasian males and 0.5% of Caucasian females. The prevalence varies with culture.

Blue color blindness is an inability to distinguish both blue and yellow, which are seen as white or gray. It is quite rare and has equal prevalence in males and females. It is common for young children to have blue/green confusion that becomes less pronounced in adulthood. Blue color deficiency often appears in people who have physical disorders such as liver disease or diabetes mellitus.

A total inability to distinguish colors (achromatopsia) is exceedingly rare. These affected individuals view the world in shades of gray. They frequently have poor visual acuity and are extremely sensitive to light (photophobia), which causes them to squint in ordinary light.

Researchers studying red/green color blindness in the United Kingdom reported an average prevalence of only 4.7% in one group. Only 1% of Eskimo males are color blind. Approximately 2.9% of boys from Saudi Arabia and 3.7% from India were found to have deficient color vision. Red/green color blindness may slightly increase an affected person's chances of contracting leprosy. Pre-term infants exhibit an increased prevalence of blue color blindness. Achromatopsia has a prevalence of about 1 in 33,000 in the United States and affects males and females equally.

Causes and symptoms

Red/green and blue color blindness appear to be located on at least two different gene locations. The majority of affected individuals are males. Females are carriers, but are not normally affected. This indicates that the X chromosome is one of the locations for color blindness. Male offspring of females who carry the altered gene have a 50-50 chance of being colorblind. The rare female that has red/green color blindness, or rarer still, blue color blindness, indicates there is an involvement of another gene. As of 2001, the location of this gene has not been identified.

Achromatopsia, the complete inability to distinguish color, is an autosomal recessive disease of the retina. This means that both parents have one copy of the altered gene but do not have the disease. Each of their children has a 25% chance of not having the gene, a 50% chance of having one altered gene (and, like the parents, being unaffected), and a 25% risk of having both the altered gene and the condition. In 1997, the achromatopsia gene was located on chromosome 2.

The inability to correctly identify colors is the only sign of color blindness. It is important to note that people with red/green or blue varieties of color blindness use other cues such as color saturation and object shape or location to distinguish colors. They can often distinguish red or green if they can visually compare the colors. However, most have difficulty accurately identifying colors without any other references. Most people with any impairment in color vision learn colors, as do other young children. These individuals often reach adolescence before their visual deficiency is identified.

Color blindness is sometimes acquired. Chronic illnesses that can lead to color blindness include Alzheimer's disease, diabetes mellitus, glaucoma, leukemia, liver disease, chronic alcoholism, macular degeneration, multiple sclerosis, Parkinson's disease, sickle cell anemia, and retinitis pigmentosa. Accidents or strokes that damage the retina or affect particular areas of the brain eye can lead to color blindness. Some medications such as antibiotics, barbiturates, anti-tuberculosis drugs, high blood pressure medications, and several medications used to treat nervous disorders and psychological problems may cause color blindness. Industrial or environmental chemicals such as carbon monoxide, carbon disulfide, fertilizers, styrene, and some containing lead can cause loss of color vision. Occasionally, changes can occur in the affected person's capacity to see colors after age 60.

Diagnosis

There are several tests available to identify problems associated with color vision. The most commonly used is the American Optical/Hardy, Rand, and Ritter Pseudoisochromatic test. It is composed of several discs filled with colored dots of different sizes and colors. A person with normal color vision looking at a test item sees a number that is clearly located somewhere in the center of a circle of variously colored dots. A color-blind person is not able to distinguish the number.

The Ishihara test is comprised of eight plates that are similar to the American Optical Pseudoisochromatic test plates. The individual being tested looks for numbers among the various colored dots on each test plate. Some plates distinguish between red/green and blue color blindness. Individuals with normal color vision perceive one number. Those with red/green color deficiency see a different number. Those with blue color vision see yet a different number.

A third analytical tool is the Titmus II Vision Tester Color Perception test. The subject looks into a stereoscopic machine. The test stimulus most often used in professional offices contains six different designs or numbers on a black background, framed in a yellow border. Titmus II can test one eye at a time. However, its value is limited because it can only identify red/green deficiencies and is not highly accurate.

Treatment

There is no treatment or cure for color blindness. Most color vision deficient persons compensate well for their abnormality and usually rely on color cues and details that are not consciously evident to persons with typical color vision.

Inherited color blindness cannot be prevented. In the case of some types of acquired color deficiency, if the cause of the problem is removed, the condition may improve with time. But for most people with acquired color blindness, the damage is usually permanent.

Prognosis

Color blindness that is inherited is present in both eyes and remains constant over an individual's entire life. Some cases of acquired color vision loss are not severe, may appear in only one eye, and last for only a short time. Other cases tend to be progressive, becoming worse with time.

Resources

BOOKS

Wiggs, Janey L. "Color Vision." In Ophthalmology, edited by Myron Yanoff and Jay S. Duker. St. Louis: Mosby, 2000.

ORGANIZATIONS

Achromatopsia Network. c/o Frances Futterman, PO Box 214, Berkeley, CA 94701-0214. http://www.achromat.org/how_to_join.html.

American Academy of Ophthalmology. PO Box 7424, San Francisco, CA 94120-7424. (415) 561-8500. http://www.eyenet.org.

International Colour Vision Society: Forschungsstelle fuer Experimentelle Ophthalmologie. Roentgenweg 11, Tuebingen, D-72076. Germany http://orlab.optom.unsw.edu.au/ICVS.

National Society to Prevent Blindness. 500 East Remington Rd., Schaumburg, IL 60173. (708) 843-2020 or (800) 331-2020. http://www.preventblindness.org.

OTHER

"Breaking the Code of Color." Seeing, Hearing and Smelling the World. http://www.hhmi.org/senses/b/b130.htm.

"Color Blindness." Geocities. http://www.geocities.com/Heartland/8833/coloreye.html.

"Medical Encyclopedia: Colorblind." MEDLINEplus. http://medlineplus.adam.com/ency/article/001002sym.htm.

University of Manchester. http://www.umist.ac.uk/UMIST_OVS/welcome.html.

University of Nevada-Reno. http://www.delamare.unr.edu/cb/.

KEY TERMS

Achromatopsia The inability to distinguish any colors.

Cones Receptor cells that allow the perception of colors.

Photophobia An extreme sensitivity to light.

Retina The light-sensitive layer of tissue in the back of the eye that receives and transmits visual signals to the brain through the optic nerve.

Rod Photoreceptor that is highly sensitive to low levels of light and transmits images in shades of gray.

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color blindness

color blindness, visual defect resulting in the inability to distinguish colors. About 8% of men and 0.5% of women experience some difficulty in color perception. Color blindness is usually an inherited sex-linked characteristic, transmitted through, but recessive in, females. Acquired color blindness results from certain degenerative diseases of the eyes. Most of those with defective color vision are only partially color-blind to red and green, i.e., they have a limited ability to distinguish reddish and greenish shades. Those who are completely color-blind to red and green see both colors as a shade of yellow. Completely color-blind individuals can recognize only black, white, and shades of gray. Color blindness is usually not related to visual acuity; it is significant, therefore, only when persons who suffer from it seek employment in occupations where color recognition is important, such as airline pilots, railroad engineers, and others who must recognize red and green traffic signals. Tests for color blindness include identifying partially concealed figures or patterns from a mass of colored dots and matching skeins of wool or enameled chips of various colors.

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Color Blindness

COLOR BLINDNESS

DEFINITION


Color blindness is a condition in which people have mild to severe difficulty identifying colors. Color blind people may not be able to recognize various shades of colors and, in some cases, cannot recognize colors at all.

DESCRIPTION


Normal color vision requires the use of special cells in the retina (the innermost lining) of the eye called cones. There are three types of conesblue, green, and redwhich allow an individual to recognize a large spectrum of colors. Cones sometimes do not function normally. When that happens, a person has trouble recognizing colors.

Color Blindness: Words to Know

Cone cells:
Cone cells are special cells in the retina and are responsible for color vision.
Retina:
The retina is the innermost lining of the eye, containing light sensitive nerve tissue composed of rod and cone cells.

The three basic types of color blindness are as follows:

  • Red/green color blindness. Red/green color blindness is the most common form of the disorder. It affects about 8 percent of all Caucasian (white) males and 0.5 percent of all Caucasian females. People with this disorder can distinguish red from green if the two colors are next to each other, but they cannot identify red or green by itself. For example, they can pick out red or green from a package of colored pencils, but if handed a red pencil, they could not identify that the pencil was red.
  • Blue color blindness. Blue color blindness is rare. People with this disorder cannot distinguish blue or yellow. Both colors are seen as white or gray. The disorder occurs with equal frequency in men and women and usually accompanies certain other physical disorders, such as liver disease or diabetes (see diabetes mellitus entry).
  • Total color blindness. Total color blindness is called achromatopsia (pronounced a-KRO-muh-tope-see-uh). This disorder is the rarest of all forms of color blindness. People with this disorder see everything as white, black, or some shade of gray. The disorder affects about 1 person in 33,000 in the United States. It is caused by hereditary factors. Achromatopsia is usually accompanied by other vision problems, such as extreme sensitivity to light.

CAUSES


Most cases of color blindness are inherited, with males being affected far more often than females. Color blindness can also be acquired in other ways. These include:

  • Chronic (long-term) illnesses, such as Alzheimer's disease (see Alzheimer's disease entry), diabetes, glaucoma (see glaucoma entry), leukemia (see leukemia entry), liver disease, chronic alcoholism (see alcoholism entry), multiple sclerosis (see multiple sclerosis entry), and retinitis pigmentosa, a disease of the retina.
  • Traumas, such as those caused by accidents or strokes (see stroke entry).
  • Medications, such as antibiotics and drugs used to treat tuberculosis (see tuberculosis entry), high blood pressure, and nervous disorders.
  • Industrial toxins (poisons), including carbon monoxide, carbon disulfide, fertilizers, and chemicals that include lead.
  • Aging, people are at higher risk after the age of sixty.

SYMPTOMS


The symptom of color blindness is the long-term inability to distinguish colors or to see colors at all.

DIAGNOSIS


A variety of tests can be used to diagnose color blindness. The most common of these tests is the American Optical/Hardy, Rand, and Ritter (AO/H.R.R.) Pseudoisochromatic (pronounced SOO-doe-I-so-kro-MAT-ik) Test. This test includes the use of a plate covered with spots of one color (green or red, for example). In the middle of the plate is a figure, such as a number or letter, made of spots of a different color. A person with normal color vision can see the figure against the background. A color blind person cannot.

A similar test, the Ishihara test, uses eight test plates similar to those used in the AO/H.R.R. test. The person looks for numbers made up of dots of various colors on each plate.

A third test is the Titmus II Vision Tester Color Perception Test. In this test, a person looks into a viewing device at a series of figures on a black background framed by a yellow border. The test can easily be performed in a doctor's office. It is not considered to be a very accurate test, however, and can only test for red/green color blindness.

JOHN DALTON AND DALTONISM

The first person to describe color blindness was the English chemist and physicist John Dalton (17661844). Dalton is famous because he was the first modern scientist to develop the atomic theory. However, Dalton was interested in many topics besides atoms. For example, he was keenly interested in meteorology (the study of the weather) and kept daily weather records for fifty-seven years. His records, published as Meteorological Observations and Essays, are among the most complete in all of scientific history. The first scientific paper Dalton ever wrote was about color blindness. He probably became interested in the subject because he, as well as his brother, was color blind. In honor of his research, the condition of color blindness is still sometimes called daltonism.

TREATMENT


There is no cure or treatment for color blindness. Most people with the disorder learn to live with the problem and learn how to adjust to it.

PROGNOSIS


Hereditary forms of color blindness do not change during a person's lifetime. In cases where color blindness was not inherited, the disorder may gradually become more or less severe over time.

PREVENTION


Hereditary color blindness cannot be prevented. Acquired color blindness can be prevented if all possible causes of the disorder can be avoided.

FOR MORE INFORMATION


Books

D'Alonzo, T. L. Your Eyes! A Comprehensive Look at the Understanding and Treatment of Vision Problems. Clifton Heights, PA: Avanti, 1992.

Rosenthal, Odeda, and Robert H. Phillips. Coping with Color-Blindness. Garden City Park, NY: Avery Publishing Group, 1997.

Organizations

Achromatopsia Network. c/o Frances Futterman, PO Box 214, Berkeley, CA 947010214. http://www.achromat.org.

Prevent Blindness America. 500 East Remington Road, Schaumburg, IL 60173. (847) 8432020; (800) 3312020. http://www.preventblindness.org.

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Color Blindness

Color Blindness

How Does Color Blindness Happen?

How Is Color Blindness Diagnosed and Treated?

Resource

Color blindness is a condition in which a person has a defect in the eye that causes an inability to identify various colors and shades.

KEYWORDS

for searching the Internet and other reference sources

Chroma

Color saturation

Hue

Ophthalmology

Vision

Red means stop. Green means go. It is one of the earliest lessons a child learns. But for more than 10 million people in the United States, this is not as simple as it sounds. These people usually are called color blind, although it is more accurate to say that they have poor color vision.

How Does Color Blindness Happen?

Color blindness almost always is inherited from the mothers genes. It affects boys most often, as girls usually have additional genetic material that overrides the vision problem. About 1 in 12 males has some degree of color blindness, whereas only about 1 in 100 females has it. People with color blindness often have no other vision problems, but color blindness is sometimes a result of other eye diseases and vision problems.

Eight million colors

The human eye can identify more than 8 million shades of colors. But the ability to distinguish among the colors begins with the three primary colors* of light: red, green, and blue. Just as a person can mix the color brown by coloring the same area with red and green crayons, the eye sees various colors by combining primary colors.

* primary colors
are sets of colors that can be mixed to create all other colors. There are two kinds of primary colors: subtractive or colorant primaries (red, blue, and yellow), which refer to pigments like crayons; and additive or light primaries (red, blue, and green), which refer to light.

As light passes through the eye, it focuses the image on the retina. The retina contains layers of cells at the inside rear of the eyeball and acts a little like the photographic film in a camera. The retina contains millions of receptors called rods that help see light and cones that help see light and colors. When light strikes the rods and cones, chemicals are released.

Red and green

People with poor color vision have cones that do not function properly, because they do not release some of the chemicals when they are struck by light. As a result, these people see only certain colors and shades. The most common form of color blindness is difficulty in seeing the colors red and green properly, or the same way most people see them. The condition can range from mild to severe. Sometimes a person simply cannot see the colors as vividly as a person who has normal color vision. Other times, there are areas that seem to lack color and to appear in shades of gray. Rarely does color blindness mean that people see everything in shades of gray, as in black-and-white photographs, movies, or television shows.

How Is Color Blindness Diagnosed and Treated?

The first signs of poor color vision may be noticed in school, when a child starts to learn to identify colors. A simple vision test can determine if the problem is color blindness. An image made up of dots is shown to the child. It may be a number (4, for example, as in the illustration at right) made up of green dots on a background of yellow and orange dots. If the child cannot see the green numeral 4 distinctly because it appears to blend in with the background, he may have color blindness.

There is no treatment or cure for color blindness. People are often taught to recognize colors in other ways. For example, traffic lights usually have the red light on top and green on the bottom.

See also

Blindness

Resource

Lighthouse International, 111 East 59 Street, New York, NY 10022. The Lighthouse International website posts fact sheets about vision problems, color contrast, and partial sight.

Telephone 800-829-0500 or 212-821-9713 (TTY)
http://www.lighthouse.org/color_contrast.htm

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Color Blindness

Color Blindness

Definition

The term color blindness describes a deficiency in discriminating various colors. It is a misnomer because most color-blind people do, in fact, see colors. The deficiency is a lack of perceptual sensitivity to certain colors. A rare few may not see colors at all.

Description

Normal color vision requires the use of special cells, called cones. They are wavelength receptors located at the back of the eye on the retina. Most of us are trichromats, which means that we have three types of cones, commonly called red, green, and blue cones. They are long, medium, and short wavelength receptors, respectively. The interplay among these cones enables us to see a large spectrum of colors. A defect in any of these types of cones will result in deficient color vision. Most color-deficient individuals are dichromats. They are not entirely blind to color, rather they get some colors confused with each other. For example, they may see certain colors (like red and green) as very similar, whereas people without the deficiency would easily be able to differentiate these colors.

The following are three basic types of color deficiency:

  • Protanopia and deuteranopia (commonly called red/green color blindness). Red/green color blindness is the most common deficiency, affecting about 10% of Caucasian males and 0.5% of females. People with protanopia have fewer red cones; blue-green and red-purple appear gray to them. Deuteranopes have fewer green cones; green and purple-red appear gray to them.
  • Tritanopia (commonly called Blue color blindness). People with tritanopia have fewer blue cones; blue and yellow appear as white or gray to them. Such people are very rare and have poor blue and/or yellow perception. As many females as males have this deficiency. It usually appears in people who have physical disorders, such as liver disease or diabetes mellitus.
  • Achromatopsia (commonly called total color blindness). Total color blindness—vision only in black, white, and shades of gray—can be caused by monochromacy (a retina that has only one type of receptor) or from acquired brain damage. Monochromacy is a very rare hereditary disorder. It affects one person in 33,000 in the United States, males and females equally. They usually have poor visual acuity and extreme sensitivity to light. Their vision is significantly impaired and they protect their light-sensitive eyes by squinting in even ordinary light.

Causes and symptoms

The key symptom of color blindness is the long-term inability to distinguish colors or notice some colors entirely. Most cases of color blindness (in particular red/green) are inherited, and affect males almost exclusively.

Color blindness can be acquired by the following:

  • Chronic illness. Illnesses that can lead to color blindness are: Alzheimer's disease, diabetes, glaucoma, leukemia, liver diseases, chronic alcoholism, macular degeneration, multiple sclerosis, Parkinson's disease, sickle cell anemia, and retinitis pigmentosa.
  • Trauma. Accidents or strokes that damage the eye can lead to color blindness.
  • Medications. Some frequently used medications may cause color blindness. Some antibiotics, barbiturates, anti-tubercular drugs, high blood pressure medications, and a number of medications used to treat nervous disorders and psychological problems may lead to color blindness.
  • Industrial toxins. Strong chemicals can cause color vision loss. Some include carbon monoxide, carbon disulfide, fertilizers, styrene, and lead-based chemicals.
  • Aging. After age 60, changes occur in people's capacity to discriminate colors.

Diagnosis

Some of the tests available to detect color vision in the general public include:

  • American Optical/Hardy, Rand, and Ritter (AO/H.R.R.) Pseudoisochromatic test. This is the test used most often to detect color blindness. A person with full color vision looking at a sample plate from this test would see a number, composed of blobs of one color, clearly located somewhere in the center of a circle of blobs of another color. A colorblind person is not able to distinguish the number.
  • Ishihara test. The Ishihara test is made up of eight test plates similar to the AO/H.R.R. pseudoisochromatic test plates. The person being tested looks for numbers made up of various colored dots on each test plate.
  • Titmus II Vision Tester Color Perception test. During this test, a person looks into a stereoscopic machine. The chin rests on a base, and the image comes on only when the forehead touches a pad on the top of the unit. Either a series of plates, or only one plate, can be used to test for color vision. The one most often used in doctors' offices is one that has six samples on it. Six different designs or numbers are on a black background, framed in a yellow border. While Titmus II can test one eye at a time, its value is limited because it only tests for red/green deficiencies and is not highly accurate.

Treatment

There is no treatment or cure for color blindness. Most color deficient persons compensate well for their defect and may even discover instances in which they can discern details and images that would escape normal-sighted persons. Colorblind people tend to look for outlines, not colors. Consequently, they are not easily confused by camouflage. (Some colorblind people were used in World War II spy planes to spot camouflaged German camps.) Also, their night vision may be much better than average.

Health care team roles

Color blindness can be tested for and diagnosed by a general physician, ophthalmologist, or optometrist. Questions about color blindness may be addressed by nurses or optometry assistants.

Prognosis

Color blindness that is hereditary is present in both eyes and remains constant over time. Some cases of acquired color vision loss are not severe and last for only a short time. Other cases tend to be progressive, becoming worse over time.

Prevention

Hereditary color blindness cannot be prevented. In the case of acquired color blindness, if the cause of the problem is removed, the condition may improve with time. If not, damage may become permanent.

KEY TERMS

Acuity— Acuity is the clarity or sharpness of vision.

Cone cells— Cone cells are special cells in the retina and are responsible for color vision and fine visual discrimination.

Retina— The retina is the innermost lining of the eye, containing light sensitive nerve tissue composed of rod and cone cells.

Stereoscopic— Stereoscopic refers to vision in which things have a three-dimensional appearance.

Resources

BOOKS

Newell, Frank. Ophthalmology Principles and Concepts. Boston, MA: Mosby, 1996.

Rosenthal, Odeda, and Robert H. Phillips. Coping with Color-Blindness. Garden City Park, NY: Avery Publishing Group, 1997.

ORGANIZATIONS

Achromatopsia Network. C/O Frances Futterman, P.O. Box 214, Berkeley, CA 94701-0214. 〈http://www.achromat.org/how_to_join.html〉.

American Academy of Ophthalmology. 1533 Shattuck Avenue, Berkeley, CA 94701. (847) 845-1059. 〈http://www.geocities.com/Heartland/8833/coloreye.html〉 and 〈http://www.zipmall.com/mpm-art-colorbl.html〉.

National Society to Prevent Blindness. 500 East Remington Road, Schaumburg, IL 60173. (708) 843-2020 or (800) 331-2020. 〈http://www.preventblindness.org〉.

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Color Blindness

Color Blindness

Reds and greens

Inherited or acquired defect

Detection through tests

Adapting to a different world

Resources

The condition known as color blindness is a defect in vision that causes problems in distinguishing between certain colors. The condition is usually passed on genetically, and is more common in men than in women. About 7% of all men and about 0.6% of women inherit the condition. Individuals can also acquire the condition through various eye diseases. There is no treatment for color blindness.

Total color blindness is called achromatopsia. This very rare hereditary disorder results in vision that is black, white, and shades of gray. It affects one person in 33, 000 in the United States, with males and females being affected equally. People with achromatopsia usually have poor visual acuity and extreme sensitivity to light. Their vision is significantly impaired, and they protect their light-sensitive eyes by squinting in even ordinary light.

Reds and greens

The first study of color blindness was published in 1794 by British chemist John Dalton (17661844), who was color-deficient himself. The condition Dalton described is not actually any sort of blindness. A small number of people can not distinguish between any color and see all things in shades of gray.

People who are color blind often are not aware they have a problem until they are asked to distinguish between reds and greens. This is the most common problem among individuals who are color blind. Some people who are color blind also have trouble telling the difference between green and yellow.

Color blindness stems from a problem in the cone cells of the retina. Light rays enter the eye in some combination of red, green, or blue. Normal cone cells contain molecules sensitive to one of the color spectrums band of colors. Shortwave cone cells absorb blue, middle-wave cone cells absorb green, and long-wave cone cells absorb red.

Individuals with a color defect do not have a normal complement of these substances, and may be missing one or more of them. Some people who are color blind have trouble distinguishing between reds and greens when the light is dim, but are capable of seeing the difference between the two colors in good light. A less common type of color blindness makes distinguishing between reds and greens difficult regardless of the light quality.

A simple test for color blindness involves the use of cards with dots in different colors. Individuals who are color blind see different numbers or words than those who have a complete range of color vision.

Inherited or acquired defect

Most individuals who are color blind inherit the trait. Men are more likely to be color blind because of the way color blindness is inherited. The gene for the trait is located on the X chromosome. Men have one X chromosome and women have two. If a man inherits the gene for the trait, he will have a color vision defect. If a woman inherits a single gene for the trait, she will not, because the normal gene on her other X chromosome will dominate over the defective gene.

Color blindness is a so-called sex-linked characteristic. This means it is a gene that occurs only on the X chromosome, which is passed to the child by the mother. The Y chromosome, which is passed to the child by the father, does not carry the defective gene. This means that children inherit color blindness only from their mothers. Children can inherit color blindness from a mother who is color blind or from a mother who is a carrier of the gene but is not color blind herself. Daughters of men who are color blind will carry the trait, but sons will not.

A more unusual way to become color blind is through disease. Cataracts, a cloudy layer in the lens or eye capsules are the most common cause of acquired color deficiency. The condition can cause vision to worsen in bright sunlight. Other conditions that may cause acquired color deficiency are retinal and optic nerve diseases.

Alzheimers disease, diabetes, glaucoma, leukemia, liver diseases, chronic alcoholism, macular

KEY TERMS

Cataract Eye disease characterized by the development of a cloudy layer in the lens of the eye.

Chromosomes The structures that carry genetic information in the form of DNA (deoxyribonucleic acid). Chromosomes are located within every cell and are responsible for directing the development and functioning of all the cells in the body.

Molecule A chemical combination of atoms, and the smallest amount of a chemical substance.

Retina An extremely light-sensitive layer of cells at the back part of the eyeball. The image formed by the lens on the retina is carried to the brain by the optic nerve.

degeneration, multiple sclerosis, Parkinsons disease, sickle cell anemia, and retinitis pigmentosa can all lead to color blindness.

Accidents or strokes that damage the eye can lead to color blindness. Some frequently used medications such as antibiotics, barbiturates, antitubercular drugs, high blood pressure medications, and a number of medications used to treat nervous disorders and psychological problems may also lead to color blindness.

Medications such as digitalis, a common medication for heart disease; and quinine, medicine for malaria, can also make color perception change. Alcohol has also been known to change the way people see color.

Strong chemicals, such as those used in industry, can cause loss of color vision. These include carbon monoxide, carbon disulfide, fertilizers, styrene, and lead-based chemicals.

Aging may also play a role in color blindness. After the age of 60 years, changes occur in peoples capacity to see colors.

Detection through tests

Several tests are available to detect color vision in the general public. The American Optical/Hardy, Rand, and Ritter (AO/H.R.R.) pseudoisochromatic test is the test used most often to detect color blindness. A person with full color vision looking at a sample plate from this test would see a number, composed of blobs of one color, clearly located somewhere in the center of a circle of blobs of another color. A color-blind person is not able to distinguish the number.

The Ishihara test is made up of eight test plates similar to the AO/H.R.R. pseudoisochromatic test plates. The person being tested looks for numbers made up of various colored dots on each test plate.

The Titmus II Vision Tester Color Perception test requires a person to look into a stereoscopic machine. The persons chin rests on a base, and the image comes on only when the forehead touches a pad on the top of the unit. Either a series of plates, or only one plate, can be used to test for color vision. The one most often used in doctors offices is one that has six samples on it. Six different designs or numbers are on a black background, framed in a yellow border. While Titmus II can test one eye at a time, its value is limited because it only tests for red/green deficiencies and is not highly accurate.

Adapting to a different world

Color blindness generally does not cause a great deal of hardship. However, there is evidence that individuals who are color blind may face higher risks on the road. A German study found that men who were color blind were twice as likely to have rear-end collisions as were men who had normal vision. About 7 million North American drivers can not distinguish easily between red and green lights.

Designers of traffic signals are working to make driving easier for color-deficient motorists. Traffic lights are generally made in a standard format today, with red on top, amber in the middle and green at the bottom. One improvement would be changing the shape of each of the different signals, so that color-deficient drivers could more easily distinguish between stop and go. Another possible change would involve altering the color of brake lights. Experts bemoan the fact that people who are color-deficient can not see the red in brake lights clearly.

There is no cure or treatment for color blindness. However, there is an abundant amount of research concerning the nature of vision in people with normal and limited color discrimination. As researchers become more knowledgeable about the process of sight, correction of color blindness may become a possibility.

See also Eye; Vision disorders.

Resources

BOOKS

Evans, Arlene. Seeing Color: Its My Rainbow, Too. Auburn, CA: CVD Publishing, 2003.

OTHER

Lighthouse International. Effective Color Contrast.

<http://www.lighthouse.org/color_contrast.htm> (accessed October 5, 2006).

Prevent Blindness America. Color Blindness. <http://www.preventblindness.org/eye_problems/color vision.html> (accessed October 5, 2006).

Patricia Braus

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Color blindness

Color blindness

Definition

Color blindness is an abnormal condition characterized by the inability to clearly distinguish different colors of the spectrum. The difficulties can be mild to severe. It is a misleading term because people with color blindness are not blind. Rather, they tend to see colors in a limited range of hues; a rare few may not see colors at all.

Description

Normal color vision requires the use of specialized receptor cells called cones, which are located in the retina of the eye. There are three types of cones, termed red, blue, and green, which enable people to see a wide spectrum of colors. An abnormality, or deficiency, of any of the types of cones will result in abnormal color vision.

There are three basic variants of color blindness. Red/green color blindness (deuteranopia) is the most common deficiency, affecting 8% of Caucasian males and 0.5% of Caucasian females. The prevalence varies with culture.

Blue color blindness (protanopia) is an inability to distinguish both blue and yellow, which are seen as white or gray. Protanopia is quite rare and has equal prevalence in males and females. It is common for young children to have blue/green confusion that becomes less pronounced in adulthood. Blue color deficiency often appears in people who have physical disorders such as liver disease or diabetes mellitus .

A total inability to distinguish colors (achromatopsia) is exceedingly rare. These affected individuals view the world in shades of gray. They frequently have poor visual acuity and are extremely sensitive to light (photophobia), which causes them to squint in ordinary light.

Genetic profile

Red/green and blue color blindness appear to be located on at least two different gene locations. The majority of affected individuals are males. Females are carriers but are not normally affected. This indicates that the X chromosome is one of the locations for color blindness. Male offspring of females who carry the altered gene have a fifty-fifty chance of being color-blind. The rare female that has red/green color blindness, or rarer still, blue color blindness, indicates there is an involvement of another gene. As of 2001, the location of this gene has not been identified.

Achromatopsia, the complete inability to distinguish color, is an autosomal recessive disease of the retina. This means that both parents have one copy of the altered gene but do not have the disease. Each of their children has a 25% chance of not having the gene, a 50% chance of having one altered gene (and, like the parents, being unaffected), and a 25% risk of having both the altered gene and the condition. In 1997, the achromatopsia gene was located on chromosome 2.

Demographics

Researchers studying red/green color blindness in the United Kingdom reported an average prevalence of only 4.7% in one group. Only 1% of Eskimo males are color blind. Approximately 3% of boys from Saudi Arabia and 4% from India were found to have deficient color vision. Red/green color blindness may slightly increase an affected person's chances of contracting leprosy. Pre-term infants exhibit an increased prevalence of blue color blindness. Achromatopsia has a prevalence of about one in 33,000 in the United States and affects males and females equally.

Color blindness is sometimes acquired. Chronic illnesses that can lead to color blindness include Alzheimer disease , diabetes mellitus , glaucoma , leukemia, liver disease, chronic alcoholism , macular degeneration , multiple sclerosis, Parkinson disease , sickle cell anemia , and retinitis pigmentosa . Accidents or strokes that damage the retina or affect particular areas of the brain can lead to color blindness. Some medications such as antibiotics, barbiturates, anti-tuberculosis drugs, high blood pressure medications, and several medications used to treat nervous disorders and psychological problems may cause color blindness. Industrial or environmental chemicals such as carbon monoxide, carbon disulfide, fertilizers, styrene, and some containing lead can cause loss of color vision. Occasionally, changes can occur in the affected person's capacity to see colors after age 60.

Signs and symptoms

The inability to correctly identify colors is the only sign of color blindness. It is important to note that people with red/green or blue varieties of color blindness use other cues such as color saturation and object shape or location to distinguish colors. They can often distinguish red or green if they can visually compare the colors. However, most have difficulty accurately identifying colors without any other references. Most people with any impairment in color vision learn colors, as do other young children. These individuals often reach adolescence before their visual deficiency is identified.

Diagnosis

There are several tests available to identify problems associated with color vision. The most commonly used is the American Optical/Hardy, Rand, and Ritter Pseudoisochromatic test. It is composed of several discs filled with colored dots of different sizes and colors. A person with normal color vision looking at a test item sees a number that is clearly located somewhere in the center of a circle of variously colored dots. A color-blind person is not able to distinguish the number.

The Ishihara test is comprised of eight plates that are similar to the American Optical Pseudoisochromatic test plates. The individual being tested looks for numbers among the various colored dots on each test plate. Some plates distinguish between red/green and blue color blindness. Individuals with normal color vision perceive one number. Those with red/green color deficiency see a different number. Those with blue color vision see yet a different number.

A third analytical tool is the Titmus II Vision Tester Color Perception test. The subject looks into a stereoscopic machine. The test stimulus most often used in professional offices contains six different designs or numbers on a black background, framed in a yellow border. Titmus II can test one eye at a time. However, its value is limited because it can only identify red/green deficiencies and is not highly accurate.

Treatment and management

There is no treatment or cure for color blindness. Most color vision deficient persons compensate well for their abnormality and usually rely on color cues and details that are not consciously evident to persons with typical color vision.

Inherited color blindness cannot be prevented. In the case of some types of acquired color deficiency, if the cause of the problem is removed, the condition may improve with time. But for most people with acquired color blindness, the damage is usually permanent.

Prognosis

Color blindness that is inherited is present in both eyes and remains constant over an individual's entire life. Some cases of acquired color vision loss are not severe, may appear in only one eye, and can last for only a short time. Other cases tend to be progressive, becoming worse with time.

Resources

BOOKS

Rosenthal, Odeda, and Robert H. Phillips. Coping with ColorBlindness. Garden City Park, NY: Avery Publishing Group, 1997.

Sacks, Oliver. The Island of the Colorblind. New York, Knopf, 1997.

Wiggs, Janey L. Color Vision. In: Ophthalmology, edited by Myron Yanoff and Jay S. Duker. St. Louis, Mosby, 2000, pp. 8-10.

PERIODICALS

Arbour, N. C., et al. "Homozygosity Mapping of Achromatopsia to Chromosome 2 Using DNA Pooling." Human Molecular Genetics 1997 May; 6(5): 689-694.

Dobson, V., et al. "Color Vision Measured with Pseudoisochromatic Plates at Five-and-a-Half Years in Eyes of Children from the CRYO-ROP Study." Investigations in Ophthalmology and Visual Science 37 (12) (November 1996): 2467-2474.

Holroyd, E., Hall, D. M. "A Re-Appraisal of Screening for Colour Vision Impairments." Child Care Health Developments 23 (5) (September 1997): 391-398.

Osuobeni, E. P. "Prevalence of Congenital Red-Green Color Vision Defects in Arab Boys from Riyadh, Saudi Arabia." Ophthalmic Epidemiology 3 (3) (December 1996): 167-170.

ORGANIZATIONS

Achromatopsia Network. C/O Frances Futterman, PO Box 214, Berkeley, CA 94701-0214. <http://www.achromat.org/how_to_join.html>.

American Academy of Ophthalmology. PO Box 7424, San Francisco, CA 94120-7424. (415) 561-8500. <http://www.eyenet.org>.

International Colour Vision Society: Forschungsstelle fuer Experimentelle Ophthalmologie. Roentgenweg 11, Tuebingen, D-72076. Germany <http://orlab.optom.unsw.edu.au/ICVS>.

National Society to Prevent Blindness. 500 East Remington Rd., Schaumburg, IL 60173. (708) 843-2020 or (800) 331-2020. <http://www.preventblindness.org>.

WEBSITES

"Breaking the Code of Color." Seeing, Hearing and Smelling the World. <http://www.hhmi.org/senses/b/b130.htm>.

"Color Blindness." Geocities. <http://www.geocities.com/Heartland/8833/coloreye.html>.

"Medical Encyclopedia: Colorblind." MEDLINEplus. <http://medlineplus.adam.com/ency/article/001002sym.htm>.

University of Manchester. <http://www.umist.ac.uk/UMIST_OVS/welcome.html>.

University of Nevada–Reno. <http://www.delamare.unr.edu/cb/>.

L. Fleming Fallon, Jr., MD, DrPH

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Color Blindness

Color blindness

The condition known as color blindness is a defect in vision that causes problems in distinguishing between certain colors. The condition is usually passed on genetically, and is more common in men than in women. About 6% of all men and about 0.6% of women inherit the condition. Individuals can also acquire the condition through various eye diseases. There is no treatment for color blindness.


Reds and greens

The first study of color blindness was published in 1794 by physicist John Dalton, who was color-deficient himself. The condition Dalton described is not actually any sort of blindness. Color blindness does not affect the overall visual acuity of individuals with the condition. A small number of people can not distinguish between any color and see all things in shades of gray.

People who are color blind often are not aware they have a problem until they are asked to distinguish between reds and greens. This is the most common problem among individuals who are color blind. Some people who are color blind also have trouble telling the difference between green and yellow.

Color blindness stems from a problem in the cone cells of the retina. Light rays enter the eye in some combination of red, green, or blue. Normal cone cells contain light-sensitive molecules sensitive to one of the color spectrum's band of colors. Short-wave cone cells absorb blue, middle-wave cone cells absorb green, and long-wave cone cells absorb red.

Individuals with a color defect do not have a normal complement of these substances, and may be missing one or more of them. Some people who are color blind have trouble distinguishing between reds and greens when the light is dim, but are capable of seeing the difference between the two colors in good light. A less common type of color blindness makes distinguishing between reds and greens difficult regardless of the light quality.

A simple test for color blindness involves the use of cards with dots in different colors. Individuals who are color blind see different numbers or words than those who have a complete range of color vision.


Inherited or acquired defect

Most individuals who are color blind inherit the trait. Men are more likely to be color blind because of the way color blindness is inherited. The gene for the trait is located on the X chromosome . Men have one X chromosome and women have two. If a man inherits the gene for the trait, he will have a color vision defect. If a woman inherits a single gene for the trait, she will not, because the normal gene on her other X chromosome will dominate over the defective gene. Women must inherit the defective trait from both parents to be color blind.

Color blindness is a so-called sex-linked characteristic. This means it is a gene that occurs only on the X chromosome, which is passed to the child by the mother. The Y chromosome, which is passed to the child by the father, does not carry the defective gene. This means that children inherit color blindness only from their mothers. Children can inherit color blindness from a mother who is color blind or from a mother who is a carrier of the gene but is not color blind herself. Daughters of men who are color blind will carry the trait, but sons will not.

A more unusual way to become color blind is through disease . Cataracts are the most common cause of acquired color deficiency. In one of the most common eye diseases, cataracts, a cloudy layer in the lens or eye capsule develops. The condition can cause vision to worsen in bright sunlight. Other conditions that may cause acquired color deficiency are retinal and optic nerve diseases.

Medications such as digitalis , a common medication for heart disease, and quinine , medicine for malaria , can also make color perception change. Alcohol has also been known to change the way people see color.


Adapting to a different world

Color blindness generally does not cause a great deal of hardship. However, there is evidence that individuals who are color blind may face higher risks on the road. A German study found that men who were color blind were twice as likely to have rear-end collisions as were men who had normal vision. About seven million North American drivers can not distinguish easily between red and green lights.

Designers of traffic signals are working to make driving easier for color-deficient motorists. Traffic lights are generally made in a standard format today, with red on top, amber in the middle and green at the bottom. One improvement would be changing the shape of each of the different signals, so that color-deficient drivers could more easily distinguish between stop and go. Another possible change would involve altering the color of brake lights. Experts bemoan the fact that people who are color-deficient can not see the red in brake lights clearly.

There is no cure or treatment for color blindness. However, there is an abundant amount of research concerning the nature of vision in people with normal and limited color discrimination. As researchers become more knowledgeable about the process of sight, correction of color blindness may become a possibility.

See also Eye; Vision disorders.

Resources

books

Donn, Anthony. "The Eye and How it Works." The Columbia University College of Physicians and Surgeons Complete Home Medical Guide. 2nd Ed., 1989.

Kunz, Jeffrey R.M., and Asher J. Finkel. "Color Blindness." The American Medical Association Family Medical Guide. New York: Random House, 1987.

periodicals

Chandler, David G. "Diabetes Problems Can Affect Color Vision." Diabetes in the News (May-June 1993): 42.

"Color Blindness Misconceptions." USA Today February 1992., 16.

Mollon, John. "Worlds of Difference." Nature Vol. 356. (April 2, 1992): 378-379.

"Not Seeing Red." The University of California, Berkeley Wellness Letter (August 1993): 2.


Patricia Braus

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cataract

—Eye disease characterized by the development of a cloudy layer in the lens of the eye.

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.

Molecule

—A chemical combination of atoms, and the smallest amount of a chemical substance.

Retina

—An extremely light-sensitive layer of cells at the back part of the eyeball. The image formed by the lens on the retina is carried to the brain by the optic nerve.

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Color Blindness

Color blindness

Definition

Color blindness is an abnormal condition characterized by the inability to clearly distinguish different colors of the spectrum. The difficulties can be mild to severe. It is a misleading term because people with color blindness are not blind. Rather, they tend to see colors in a limited range of hues; a rare few may not see colors at all.

Description

Normal color vision requires the use of specialized receptor cells called cones, which are located in the retina of the eye. There are three types of cones, termed red, blue, and green, which enable people to see a wide spectrum of colors. An abnormality, or deficiency, of any of the types of cones will result in abnormal color vision.

There are three basic variants of color blindness. Red/green color blindness is the most common deficiency, affecting 8% of Caucasian males and 0.5% of Caucasian females. The prevalence varies with culture.

Blue color blindness is an inability to distinguish both blue and yellow, which are seen as white or gray. It is quite rare and has equal prevalence in males and females. It is common for young children to have blue/green confusion that becomes less pronounced in adulthood. Blue color deficiency often appears in people who have physical disorders such as liver disease or diabetes mellitus.

A total inability to distinguish colors (achromatopsia) is exceedingly rare. These affected individuals view the world in shades of gray. They frequently have poor visual acuity and are extremely sensitive to light (photophobia), which causes them to squint in ordinary light.

Genetic profile

Red/green and blue color blindness appear to be located on at least two different gene locations. The majority of affected individuals are males. Females are carriers but are not normally affected. This indicates that the X chromosome is one of the locations for color blindness. Male offspring of females who carry the altered gene have a fifty-fifty chance of being color blind. The rare female that has red/green color blindness, or rarer still, blue color blindness, indicates there is an involvement of another gene. The location of this gene has not been identified.

Achromatopsia, the complete inability to distinguish color, is an autosomal recessive disease of the retina. This means that both parents have one copy of the altered gene but do not have the disease. Each of their children has a 25% chance of not having the gene, a 50% chance of having one altered gene (and, like the parents, being unaffected), and a 25% risk of having both the altered gene and the condition. In 1997, the achromatopsia gene was located on chromosome 2.

Demographics

Researchers studying red/green color blindness in the United Kingdom reported an average prevalence of only 5% in one group. Only 1% of Eskimo males are color blind. Approximately 3% of boys from Saudi Arabia and 4% from India were found to have deficient color vision. Red/green color blindness may slightly increase an affected person's chances of contracting leprosy. Pre-term infants exhibit an increased prevalence of blue color blindness. Achromatopsia has a prevalence of about one in 33,000 in the United States and affects males and females equally.

Color blindness is sometimes acquired. Chronic illnesses that can lead to color blindness include Alzheimer disease , diabetes mellitus, glaucoma , leukemia, liver disease, chronic alcoholism , macular degeneration, multiple sclerosis , Parkinson disease , sickle cell anemia, and retinitis pigmentosa . Accidents or strokes that damage the retina or affect particular areas of the brain can lead to color blindness. Some medications such as antibiotics, barbiturates, anti-tuberculosis drugs, high blood pressure medications, and several medications used to treat nervous disorders and psychological problems may cause color blindness. Industrial or environmental chemicals such as carbon monoxide, carbon disulfide, fertilizers, styrene, and some containing lead can cause loss of color vision. Occasionally, changes can occur in the affected person's capacity to see colors after age 60.

Signs and symptoms

The inability to correctly identify colors is the only sign of color blindness. It is important to note that people with red/green or blue varieties of color blindness use other cues such as color saturation and object shape or location to distinguish colors. They can often distinguish red or green if they can visually compare the colors. However, most have difficulty accurately identifying colors without any other references. Most people with any impairment in color vision learn colors, as do other young children. These individuals often reach adolescence before their visual deficiency is identified.

Diagnosis

There are several tests available to identify problems associated with color vision. The most commonly used is the American Optical/Hardy, Rand, and Ritter Pseudoisochromatic

test. It is composed of several discs filled with colored dots of different sizes and colors. A person with normal color vision looking at a test item sees a number that is clearly located somewhere in the center of a circle of variously colored dots. A color-blind person is not able to distinguish the number.

The Ishihara test is comprised of eight plates that are similar to the American Optical Pseudoisochromatic test plates. The individual being tested looks for numbers among the various colored dots on each test plate. Some plates distinguish between red/green and blue color blindness. Individuals with normal color vision perceive one number. Those with red/green color deficiency see a different number. Those with blue color vision see yet a different number.

A third analytical tool is the Titmus II Vision Tester Color Perception test. The subject looks into a stereoscopic machine. The test stimulus most often used in professional offices contains six different designs or numbers on a black background, framed in a yellow border. Titmus II can test one eye at a time. However, its value is limited because it can only identify red/green deficiencies and is not highly accurate.

Treatment and management

There is no treatment or cure for color blindness. Most color vision deficient persons compensate well for their abnormality and usually rely on color cues and

details that are not consciously evident to persons with typical color vision.

Inherited color blindness cannot be prevented. In the case of some types of acquired color deficiency, if the cause of the problem is removed, the condition may improve with time. But for most people with acquired color blindness, the damage is usually permanent.

Prognosis

Color blindness that is inherited is present in both eyes and remains constant over an individual's entire life. Some cases of acquired color vision loss are not severe, may appear in only one eye, and can last for only a short time. Other cases tend to be progressive, becoming worse with time.

Resources

BOOKS

Rosenthal, Odeda, and Robert H. Phillips. Coping with Color Blindness. Garden City Park, NY: Avery Publishing Group, 1997.

Sacks, Oliver. The Island of the Color-blind. New York, Knopf, 1997.

Wiggs, Janey L. Color Vision. In: Ophthalmology, edited by Myron Yanoff and Jay S. Duker. St. Louis, Mosby, 2000, pp. 8-10.

PERIODICALS

Arbour, N. C., et al. "Homozygosity Mapping of Achromatopsia to Chromosome 2 Using DNA Pooling." Human Molecular Genetics 6, no. 5 (May 1997): 689-694.

Dobson, V., et al. "Color Vision Measured with Pseudoisochromatic Plates at Five-and-a-Half Years in Eyes of Children from the CRYO-ROP Study." Investigations in Ophthalmology and Visual Science 37, no. 12 (November 1996): 2467-2474.

Holroyd, E., and D. M. Hall. "A Re-Appraisal of Screening for Colour Vision Impairments." Child Care Health Developments 23, no. 5 (September 1997): 391-398.

Osuobeni, E. P. "Prevalence of Congenital Red-Green Color Vision Defects in Arab Boys from Riyadh, Saudi Arabia." Ophthalmic Epidemiology 3, no. 3 (December 1996): 167-170.

ORGANIZATIONS

Achromatopsia Network. C/O Frances Futterman, PO Box 214, Berkeley, CA 94701-0214. <http://www.achromat.org/how_to_join.html>.

American Academy of Ophthalmology. PO Box 7424, San Francisco, CA 94120-7424. (415) 561-8500. <http://www.eyenet.org>.

International Colour Vision Society: Forschungsstelle fuer Experimente lle Ophthalmologie. Roentgenweg 11, Tuebingen, D-72076. Germany <http://orlab.optom.unsw.edu.au/ICVS>.

National Society to Prevent Blindness. 500 East Remington Rd., Schaumburg, IL 60173. (708) 843-2020 or (800) 331-2020. <http://www.preventblindness.org>.

WEBSITES

"Breaking the Code of Color." Seeing, Hearing and Smelling the World. <http://www.hhmi.org/senses/b/b130.htm>.

"Color Blindness." Geocities. <http://www.geocities.com/Heartland/8833/coloreye.html>.

"Medical Encyclopedia: Color-blind." MEDLINEplus. <http://medlineplus.adam.com/ency/article/001002sym.htm>.

University of Manchester. <http://www.umist.ac.uk/UMIST_OVS/welcome.html>.

University of Nevada–Reno. <http://www.delamare.unr.edu/cb/>.

L. Fleming Fallon, Jr., MD, MPH

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