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Achromatopsia- Rod Monochromatism
Nature of the Genetic Difficulty
The rod monochromatism is the most common form of achromatopsia. It can be can be caused by more than one gene mutation. So far, mutations or variants of any one of these four genes, CNGA3, CNGB3, GNAT2 and recently PDE6C, have been linked to each causing rod monochromatism.
Rod monochromatism is an autosomal recessive condition. Acquiring rod monochromatism requires that we have inherited a “abnormal” gene from each parent. If we inherit one “normal” and one “affected” gene, we would be a carrier, because in this situation one “normal” gene alone is enough to instruct our body how to produce the appropriate chemicals for cone development or function. If, however, we inherit a gene causing rod monochromatism from each parent, we then lack the instruction code to make the necessary proteins required for functional cone cells and would have rod monochromatism.
Basic Facts of Disease
Congenital achromatopsia is a rare hereditary vision disorder which affects 1 person in 33,000 in the U. S. The incidence varies in different parts of the world.
There are complete rod monochromats, incomplete rod monochromats, and blue cone monochromats. Complete rod monochromats have the most severely impaired vision of all achromats.
Persons who have achromatopsia (sometimes called achromatopia) do not have normal "cone vision."
The risk of a rod monochromat having an affected child is extremely low since those with the condition and those who are carriers are rare in society. However, every child of a rod monochromat will be a carrier of the condition. The typical inheritance of rod monochromatism (RM) involves two asymptomatic parent carriers, each providing one gene.
Except for the following two very rare situations, rod monochromats do not have offspring with rod monochromatism. In the extremely rare case of two individuals with the same form of rod monochromatism having children, 100% of offsprings would have the condition. The other rare option would be a rod monochromat having children with a carrier of the same type of RM. All offspring then would either have the condition or be carriers.
In both forms of congenital achromatopsia, the signs of achromatopsia may not present until age three to six months of age. The first signs are usually nystagmus, a shaky pendular movement of the eyes and squinting or other signs of aversion to bright light.
The diagnosis of rod monochromatism and blue cone monochromatism is based on medical history, color vision testing, electroretinogram testing, and a lack of significant retinal signs in young patients. Mild pigmentary retinal changes or “bulls eye” pigmentation may occur in older patients.
In the past, early diagnosis was complicated since infants could not perform the color vision testing to confirm the diagnosis. Today, genetic testing may aid in diagnosis of some of the most common forms of rod monochromatism.
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