Colorblindness-Achromatopsia 3
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. At least 4 mutations cause achromatopsia and this one accounts for about half of cases with complete color blindness. It is particularly common among Pingelapese islanders in Micronesia where many parents are related to each other.
Clinical Correlations
This is a congenital, nonprogressive disorder with symptoms often notable at birth or soon thereafter. These include nystagmus (dancing of the eyes), extreme sensitivity to light, cataracts in some individuals and, of course, no perception of color. Nearsightedness is often present, at least in some populations. Vision is often better in dim light which is why it is sometimes called ‘day blindness’.
Genetics
Inheritance
This is an autosomal recessive condition in which two clinically normal parents each contribute the mutation to their child. Achromatopsia 3 requires the presence of two mutations. Each child born to such parents has a 25% risk of inheriting this disease.
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
The diagnosis is usually made by an ophthalmologist and no systemic abnormalities are associated. Most individuals are legally blind with 20/200 or worse vision but complete blindness does not occur. Glasses for nearsightedness and astigmatism should be worn and these should be darkly tinted for daytime use. Red-colored contact lenses, low vision aids and vocational training can be helpful. Children especially should have periodic examinations to ensure that their glasses are optimized. Life expectancy is normal.
Web Resources
Printer Friendly Version: Ctrl/Cmd+P