OMIM ID:
Retinal Cone Dystrophy 3B
Alternate Names
Defective Genes
Clinical Characteristics
Ocular Features
This is a degenerative disorder in which patients have a progressive deterioration of visual acuity and color vision. Most patients have significant myopia. Visual difficulties begin in early childhood with acuity of 20/100 or worse by the second decade of life. Color vision deficits can be detected in the second decade but nyctalopia occurs later in young adults. Photophobia is a prominent symptom. The ERG shows reduced and delayed cone responses. Rod responses to low intensity flashes are undetectable but increased stimulus intensity leads to an abrupt increase in amplitude, often exceeding the upper limits of normal.
The fundus appears normal in some patients but foveal or parafoveal atrophy, a macular bull’s eye, hyperfluorescence anomalies, and a generalized fine pigmentary retinopathy have been reported. There may be some temporal pallor in the optic nerves. Nystagmus and strabismus may be present.
Systemic Features
No systemic disease has been reported.
Genetics
Inheritance
This is an autosomal recessive disorder resulting from homozygous or compound heterozygous mutations in the KCNV2 gene (9p24.2).
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.