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Night Blindness, Congenital Stationary, CSNB1C
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 and without RPE changes, 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. The photopic ERG is usually 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 (CSNB1C) the b-wave responses are severely deficient (no scotopic response) and a-waves seem to be normal. Some reduction in central acuity is common. High myopia may be present together with nystagmus and strabismus. ERG responses suggest loss of ON bipolar cell function similar to that found in patients with GRM6 mutations (CSNB1B; 257270).
No systemic disease is associated with congenital stationary night blindness.
CSNB1C, or type 1C, is one of four congenital nightblindness disorders with autosomal recessive inheritance. It results from mutations in the TRPM1 (15q13-q14) gene which encodes for a calcium ion channel protein, part of the GRM6 signaling cascade.
No treatment beyond correction of the refractive error is available but tinted lenses are sometimes used to enhance vision. Refractive errors need to be corrected and low vision aids can be helpful for those with some loss of central acuity.