Retinal Cone Dystrophy 3B
Patient Information
Characteristics
Background and History
The retina inside the eye contains rods and cones which receive light and transmit their responses as nerve impulses to the brain. Many genes are involved in this process and hence there are numerous inherited disorders that can result if there are mutations in these genes. The condition described here is primarily a disorder of the cones which enable us to see color.
Clinical Correlations
This is a progressive disease which begins in childhood. Vision difficulties are evident even in early childhood and cause increasing problems throughout life. Nearsightedness occurs in nearly all individuals. Some develop strabismus (crossing of the eyes) and nystagmus (abnormal movements of the eyes). Defects in color vision occur also in children but night blindness may not be a symptom until the second decade of life. Abnormal sensitivity to light is a prominent symptom. The clinical appearance of the retina may show some pigmentary changes.
The disease can be detected using the electroretinogram (ERG) which shows abnormal responses to light in both rods and cones. There are no abnormalities outside of the eye.
Genetics
Inheritance
This is an autosomal recessive condition which requires a change (mutation) in both copies of a specific gene. Both sexes are equally affected. A parent, who may carry one mutated copy, is normal but when both carry the gene change there is a 25% chance that the child from each pregnancy will inherit this disorder.
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
This is an ocular disorder in which an ophthalmologist is most likely to make the diagnosis. A painless diagnostic test known as an ERG is definitive. As there is no systemic disease, patients should have normal longevity. There is no treatment for the retinal disease but visual functioning can be improved with low vision aids, at least in early stages. Tinted lenses can enhance daytime visual comfort.
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