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 or achromatopsia.
This is a form of inherited colorblindness in which little or no color is perceived. At least 4 mutations cause achromatopsia and this one accounts for less than 2% of cases.
Like other types of achromatopsia, patients with type 4 are born with poor vision and little or no color discrimination. Nystagmus (to-and-fro movement of the eyes) soon develops and most individuals are highly sensitive to light (photophobia). The ERG (electroretinogram), an electrical test that measures retinal responses to light, reveals that the cones are nonfunctional while the rods function normally. This helps explain why many patients see better in dim light. This is considered to be a stationary disorder but a few patients have experienced a worsening of vision with age.
This is an autosomal recessive disorder requiring the presence of two mutations for the disease to occur. Both parents who each carry one copy of the mutation transmit can transmit both to their children with a 25% probability so that on average one-fourth of their children will inherit the disorder. The family pattern is a horizontal one since the parents are not affected.
Ophthalmologists generally make the diagnosis of achromatopsia based on the clinical and ERG findings. No systemic abnormalities are associated with this disease and life expectancy is normal. No treatment is available for the primary disease but patients may benefit from low vision aids and vocational training. Darkly tinted glasses or red contact lenses reduce the light sensitivity and may improve vision.