color blindness (redirected from Anomalous trichromacy)

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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blindness /blind·ness/ (blīnd´nes) lack or loss of ability to see; lack of perception of visual stimuli.

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blue blindness , blue-yellow blindness popular names for imperfect perception of blue and yellow tints; see tritanopia and tetartanopia .

color blindness 

1. popular name for color vision deficiency.

2. see monochromatic vision.

complete color blindness  monochromatic vision.

day blindness  hemeralopia.

flight blindness  amaurosis fugax due to high centrifugal forces encountered in aviation.

green blindness  imperfect perception of green tints; see deuteranopia and protanopia.

legal blindness  that defined by law, usually, maximal visual acuity in the better eye after correction of 20/200 with a total diameter of the visual field in that eye of 20 degrees.

letter blindness  alexia characterized by inability to recognize individual letters.

music blindness  musical alexia.

night blindness  failure or imperfection of vision at night or in dim light.

object blindness , psychic blindness visual agnosia.

red blindness  popular name for protanopia.

red-green blindness  popular name for any imperfect perception of red and green tints, including all the most common types of color vision deficiency. See deuteranomaly, deuteranopia, protanomaly, and protanopia.

snow blindness  dimness of vision, usually temporary, due to glare of sun upon snow.

text blindness  alexia.

total color blindness  monochromatic vision.

word blindness  alexia.

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color blindness

n.

Deficiency of color perception, whether hereditary or acquired, partial or complete.

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color blindness

Etymology: L, color; AS blint

an abnormal condition characterized by an inability to distinguish colors of the spectrum clearly. In most cases it is not a blindness but a weakness in perceiving colors distinctly. There are two forms of color blindness: Daltonism, the more common form, is characterized by an inability to distinguish reds from greens. It is an inherited, sex-linked disorder. Total color blindness, or achromatic vision, is characterized by an inability to perceive any color at all. Only white, gray, and black are seen. It may be the result of a defect in or absence of the cones in the retina. Also spelled colour blindness.

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color blindness,

n See blindness, color.

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color blindness

Colorblind The partial or, rarely, complete inability to distinguish colors, which affects up to 10% of men; most CB is X-linked–90% occurs in ♂ and is tested by the Ishihara pseudoisochromic charts