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 Encyclopedia - Achromatopsia

What causes achromatopsia?
What are the signs of achromatopsia?
How is achromatopsia managed?

Achromatopsia is a congenital eye defect that results in severe color blindness, poor detail vision, and photophobia, which is sensitivity to light. A hereditary condition, it occurs in 1 of 33,000 people in the United States, and it is more likely to occur in men than women. Congenital achromatopsia is not progressive, and it does not lead to blindness.

What causes achromatopsia?

The cells that make up the retina are responsible for the ability to see detail, brightness, and color. There are two types of photoreceptor cells in the cornea -- rods and cones. The rods specialize in work at low light levels, and the cones provide sharp vision, color, and contrast discrimination. People with achromatopsia have defective cone cells and must rely on their rod photoreceptors for vision.

In normal eyes, there are about 6 million cone photoreceptors, located mainly in the macula at the center of the retina. These cells are primarily responsible for sharp, straight-ahead vision and also for the ability to distinguish colors. There are 100 million rod receptors, located mostly at the periphery of the retina. The rods are more sensitive to light than cones are, but rods are not able to differentiate among colors, nor can they perceive shades of gray, black, and white. (See Anatomy of the Eye.)

There are different variations in the severity of symptoms among individuals with achromatopsia. The rarest and most severe is called complete rod monochromatism, where there is a total lack of cone function. People with this disorder are extremely sensitive to light, even in normally lit rooms. They also have symptoms of poor visual acuity and nystagmus, which is involuntary movement of the eyes.

Other less severe variations of the disorder are known as incomplete rod monochromatism and blue cone monochromatism. The type depends on which cones are affected.

What are the signs of achromatopsia?

The main symptom of achromatopsia is photophobia, which is extreme sensitivity to light. The sun blinds people with achromatopsia, known as achromats, when they are outside, and some people are so sensitive to light that they are even uncomfortable indoors in a normally lit room. Because rods are sensitive to light but do not provide color or detail, people with achromatopsia are totally or partially color deficient, meaning they can see no or little color. They also have poor visual acuity and are limited in their ability to see at a distance.

Achromatopsia is hard to diagnose because the backs of the eyes appear normal when examined. The eye care professional bases the diagnosis on the patient's symptoms and on an electroretinogram, which must be administered in a special clinic.

How is achromatopsia managed?

Although there is no cure for achromatopsia, symptoms are manageable. Optical aids, sunglasses, and lowered illumination may be helpful to those with achromatopsia. In full sunlight or in very bright light indoors, achromats use very dark tinted lenses because their retinas do not possess the photoreceptors necessary for seeing well in such settings. Tinted contact lenses cut glare and allow achromats to see more clearly. Low vision rehabilitation also may be helpful.

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