Color blindness

Encyclopedia of Medicine, Apr 06, 2001 by Lorraine T. Steefel

Color blindness is the word used to describe mild to severe difficulties with identifying various colors and shades of colors. It is a misleading term because colorblind people are not blind. Rather, they tend to confuse some colors, and a rare few may not see colors at all.

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

The following are three basic types of color blindness:

• Red/green color blindness. Red/green color blindness is the most common deficiency, affecting 8% of Caucasian males and 0.5% of females. People with red/green color blindness can often distinguish red or green if they can visually compare the colors. For example, they can pick out red or green from a package of colored pencils. However, if handed a red pencil, they cannot tell what color the pencil is.
• Blue color blindness. Blue color blindness, which is rare, is an inability to distinguish both blue and yellow. Blue and yellow are seen as white or grey. Although as many females as males have this deficiency, it usually appears in people who have physical disorders, such as liver disease or diabetes mellitus. However, it is not uncommon for young boys to have blue/green confusion that becomes less pronounced in adulthood.
• Total 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 (1/33,000) in the United States, males and females 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.

The symptom of color blindness is the long-term inability to distinguish colors or notice some colors entirely. Most cases of color blindness are inherited, affecting 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 see colors.

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 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.

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.

Color blindness that is hereditary is present in both eyes and remains constant through 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 with time.

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.