Clinical Features and Electrophysiology

CSNB is characterised by night blindness, variable visual loss and usually normal fundi, although some patients have pale or tilted optic discs. CSNB may be inherited as an AD, AR or XL disorder; with XL inheritance being most common. Patients with AD CSNB usually present with nyctalopia and have normal visual acuity [36]; whereas in XL and AR, CSNB presentation is usually in infancy with nystagmus, moderate to high myopia, strabismus, reduced central vision, and in some cases paradoxical pupil responses (pupillary dilatation to bright light) [34].

XL CSNB is further subdivided into the complete and incomplete forms. Patients with complete CSNB are myopic and have more pronounced night blindness. Both complete and incomplete CSNB show a negative type of ERG, in that the photoreceptor derived a-wave in the maximal response is usually normal,but there is selective reduction in the inner nuclear derived b-wave so that it is smaller than the a-wave. In complete CSNB, the rod-specific ERG is more severely affected and is often nonrecordable [2]. Cone ERGs show mild abnormalities reflecting ON- bipolar pathway dysfunction. In contrast, there is always a detectable rod-specific ERG in incomplete CSNB and cone ERGs are much more abnormal than in complete CSNB, reflecting involvement of both ON- and OFF- bipolar pathways.

AR CSNB is phenotypically very similar to XLCSNB, both clinically and on ERG testing. In most families with AD CSNB, affected individuals show attenuated rod responses but normal cone responses on ERG testing, without evidence of a negative waveform on maximal response testing. Inner retinal dysfunction has been reported in a few cases.

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