Achromatopsia

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.