CSNB

CSNB stands for congenital stationary night blindness, a group of inherited retinal disorders characterized by lifelong difficulty seeing in low light while daytime vision remains relatively intact. The condition is non-progressive and typically presents with a normal-appearing fundus. CSNB reflects a defect in signal transmission from photoreceptors to bipolar cells, most notably in the ON-bipolar cell pathway.

There are two main forms: complete CSNB (cCSNB) and incomplete CSNB (iCSNB). In cCSNB, the ON-bipolar pathway is markedly disrupted, leading to characteristic electrophysiological findings. Dark-adapted electroretinography (ERG) shows absent or severely reduced b-waves with a relatively preserved a-wave, reflecting intact photoreceptors but impaired transmission to bipolar cells. Photopic (cone) responses can also be affected but may be less severely impaired. In iCSNB, there is partial disruption of the ON pathway, and ERG findings show more preserved cone-driven responses with a reduced or abnormal b-wave. Clinically, both forms often feature nyctalopia from birth, nystagmus in infancy, and refractive errors such as myopia; visual acuity can be normal or near-normal in many individuals, and fundus examination is usually normal.

Genetically, cCSNB is commonly X-linked (mutations in NYX) or autosomal (GRM6, TRPM1, GPR179). iCSNB is typically autosomal recessive and has been associated with genes such as GRM6, CACNA2D4, and CABP4, among others. Diagnosis relies on clinical history, ophthalmic examination, and distinctive ERG patterns, complemented by genetic testing for precise gene identification.

There is no cure; management focuses on supporting vision and providing genetic counseling. Patients may benefit from low-vision aids, adaptive strategies, and regular follow-up. The condition is generally non-progressive, with stable visual function over time.