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. 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.
CSNB2A, or type 2A, is associated with myopia which ranges from mild to severe. Residual rod function is diminished but not completely absent as suggested by the presence of small b-waves. Cone function is impacted to some degree as well. Nystagmus and strabismus are inconsistent findings. Retinal pigmentation is usually normal in the X-linked forms. Visual acuity ranges from 20/30 to 20/200. Night blindness is less severe in this form than in another X-linked CSNB (CSNB1A; 310500). Mild dyschromatopsia is present in some patients but this is primarily a disease of rods.