CSNB1

CSNB1 stands for congenital stationary night blindness type 1, a hereditary retinal disorder characterized by lifelong difficulties seeing in low light while daytime vision remains relatively normal. It represents the complete form of congenital stationary night blindness and is contrasted with CSNB2, which is associated with other genes and often a different functional pattern. CSNB1 is typically inherited in an X-linked recessive manner.

Genetically, CSNB1 is most commonly caused by loss-of-function mutations in the NYX gene, located on the X chromosome and encoding the protein nyctalopin. Nyctalopin is involved in signaling from photoreceptors to ON-bipolar cells in the retina. Disruption of this signaling pathway leads to impaired rod-driven vision and abnormal ON-bipolar cell transmission.

Pathophysiology centers on malfunction of the ON-bipolar cell pathway, which normally relays signals from photoreceptors in response to light. In CSNB1, the disruption results in severely reduced or absent rod-mediated responses, while some residual cone pathway function may be variably affected. The condition is non-progressive, meaning visual function does not typically deteriorate over time.

Clinical features typically appear in early life and include nystagmus and markedly reduced night vision. Visual acuity can range from normal to mildly reduced, and refractive errors such as myopia are common. Color vision is usually preserved, and everyday activities are affected mainly by reduced low-light vision.

Diagnosis relies on characteristic electrophysiological findings. Full-field electroretinography shows a markedly reduced or absent scotopic (dark-adapted) b-wave with a preserved a-wave, reflecting ON-bipolar cell dysfunction. Genetic testing confirms NYX mutations. Family history may reveal affected male relatives due to X-linked inheritance.

Management is supportive, focusing on low-vision aids, safety measures, and genetic counseling. Research into gene therapies and retinal circuit restoration is ongoing.