Colorblindness-Achromatopsia 5
Patient Information
Characteristics
Background and History
The light sensitive cells in the retina are called rods (useful in dim vision) and cones (used for color vision). Gene mutations can impact either or both types. Those that cause a dysfunction in cones result in defective color vision of various types, sometimes called colorblindness.
This is a form of inherited colorblindness in which no color is perceived by most patients. At least 4 mutations cause achromatopsia and this one accounts for only a small percentage of cases with complete color blindness.
Clinical Correlations
Most individuals have poor vision and lack of color vision from birth. They frequently also manifest ‘dancing eyes’ or nystagmus. They are usually highly sensitive to light and vision may actually be better in more dim light. Some patients are markedly nearsighted. The disorder may be milder in some individuals who have some residual color perception and less sensitivity to light. Electrical testing of retinal function (ERG) shows that the cones do not respond but rod function seems to be normal. Usually the condition is stable but patients with milder disease early in life can experience some loss of vision later.
Genetics
Inheritance
This is an autosomal recessive condition that requires two mutations in the same gene to be present. The parents who are not colorblind each have to contribute one mutation to their child before the disease is manifest. The risk is 25% that each of their children will be colorblind.
Pedigree
Autosomal recessive
In order for autosomal recessive disorders to be expressed, offspring generally must inherit two mutations, one from each carrier parent. Carriers with only one mutation, such as the parents, do not have clinical disease. Note that carrier parents can expect that 1 in 4 children (25%) will inherit both mutations and have the disorder, 2 in 4 children (50%) will be carriers like their parents, while 1 in 4 children (25%) inherit neither mutation.
In order for autosomal recessive disorders to be expressed, offspring generally must inherit two mutations, one from each carrier parent. Carriers with only one mutation, such as the parents, do not have clinical disease. Note that carrier parents can expect that 1 in 4 children (25%) will inherit both mutations and have the disorder, 2 in 4 children (50%) will be carriers like their parents, while 1 in 4 children (25%) inherit neither mutation.
Diagnosis and Prognosis
There are no systemic abnormalities in this disorder and lifespan is normal. The diagnosis is usually made by an ophthalmologist based on the symptoms and perhaps the family history. The ERG test can be helpful in making the diagnosis. Dark glasses or red tinted contact lenses can be used for comfort and may result in some vision improvement in daylight. Low vision aids and vocational training can also be beneficial.
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