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.

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 (1766–1844), 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 spectrum’s 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.

Alzheimer’s 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, Parkinson’s 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 people’s 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 person’s 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 doctor’s 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: It’s 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

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. 2406–19.

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. 2087–9.

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): 252–57.

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): 495–503.

Dick, F., et al. "Is Color Vision Impairment Associated with Cognitive Impairment in Solvent Exposed Workers?" Occupational and Environmental Medicine 61, no. 1 (2004): 76–78.

Tagarelli, A., et al. "Color Blindness in Everyday Life and Car Driving." Acta Ophthalmology Scandinavia 52, no. 4 (2004): 436–42.

ORGANIZATIONS

American Academy of Family Physicians. 11400 Tomahawk Creek Parkway, Leawood, KS 66211–2672. Web site: <www.aafp.org/>.

American Academy of Pediatrics. 141 Northwest Point Boulevard, Elk Grove Village, IL 60007–1098. 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

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

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