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Night Blindness, Congenital Stationary, CSNB1B
Night blindness is a feature of many pigmentary and other retinal disorders, most of which are progressive. However, there is also a group of genetically heterogeneous disorders, with generally stable scotopic defects, known as congenital stationary night blindness (CSNB). At least 9 mutant genes are responsible with phenotypes so similar that genotyping is usually necessary to distinguish them. All are caused by defective photoreceptor-to-bipolar cell signaling with common ERG findings of reduced or absent b-waves and generally normal a-waves. However, the photopic ERG can be abnormal to some degree as well and visual acuity may be subnormal. In the pregenomic era, subtleties of ERG responses were frequently used in an attempt to distinguish different forms of CSNB. Genotyping now enables classification with unprecedented precision.
Congenital stationary night blindness disorders are primarily rod dystrophies presenting early with symptoms of nightblindness and relative sparing of central vision. Nystagmus and photophobia are usually not features. Dyschromatopsia and loss of central acuity can develop later as the cones eventually become dysfunctional as well but these symptoms are much less severe than those seen in cone-rod dystrophies. The amount of pigmentary retinopathy is highly variable.
In this disorder (CSNB1B) the b-wave responses are severely deficient and a-waves seem to be normal. Color vision is normal and refractive errors are unremarkable. Visual acuity ranges from normal to a mild reduction (20/15-20/40). One patient with 20/40 vision has been reported to have bone spicule pigment clumps in the midperiphery. Several patients with subnormal vision have been reported to have nystagmus.
Patients have a distinctive ERG pattern response to scotopic 15-Hz flicker stimuli that suggest that more than two rod neural pathways exist.
No systemic disease is associated with congenital stationary night blindness.
CSNB1B, or type 1B, is one of four CSNB disorders with autosomal recessive inheritance. It is the result of mutations in the GRM6 gene (5q35) which lead to functional loss of the glutamate receptor.
No treatment beyond correction of the refractive error is available but tinted lenses are sometimes used to enhance vision.