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 10 mutant genes are responsible with phenotypes so similar that genotyping is usually necessary to distinguish them. All are caused by defects in visual signal transduction within rod photoreceptors or in 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.
CSNB1A, or type 1A, is associated with myopia which ranges from mild to severe. Rod function is completely absent. Nystagmus and strabismus are inconsistent findings. Visual acuity ranges from 20/30 to 20/200. Retinal pigmentation is usually normal in the X-linked forms. Night blindness is more severe in this form than in another X-linked CSNB, type 2A (300071).
