Clinical Guidance

X-linked congenital stationary night blindness (CSNB) is a congenital non-progressive retinal disorder characterized by defective night vision, reduced visual acuity, myopia, nystagmus, and strabismus that primarily affects males.

Males

Reduced visual acuity. Vision is reduced in all affected males in the range of 20/30 (6/9; log MAR 0.1) to 20/200 (6/60; log MAR 1.0). Defective dark adaptation. Night blindness is a subjective finding. Individuals with NYX X-linked CSNB generally report severe night blindness. Individuals with CACNA1F X-linked CSNB do not uniformly report severe night blindness. Myopia may range from low (-0.25 diopters [D] to -4.75 D) to high (≥ -10.00 D) . A few affected individuals have hyperopia. Nystagmus and strabismus are reported in 50%-70% of affected individuals .

Diagnostic Criteria

Suggestive Findings Males. X-linked congenital stationary night blindness (CSNB) should be suspected in a male proband with the following characteristic clinical and electroretinogram (ERG) findings characteristicec of complete X-linked CSNB or incomplete X-linked CSNB : Characteristic clinical findings: • Reduced visual acuity • Night blindness • Myopia • Nystagmus (not universal) and strabismus (50%-70%) • Normal color vision • Normal fundus examination • Family history consistent with X-linked inheritance Characteristic findings on ERG examination: • ERG is used to assess the changes in electrical activity of the retina in response to light. The b-wave is caused by the depolarization of ON bipolar cells in response to light stimuli and is strictly dependent on synaptic transmission from photoreceptors to ON bipolar cells. • Individuals with X-linked CSNB have reduced scotopic b-wave amplitudes in response to bright flashes after dark adaptation. The resulting ERG waveform is essentially a negative wave (amplitude of the a-wave is larger than the b-wave, not reaching the baseline) , referred to as the Schubert-Bornschein form . • The ERG can define specific retinal dysfunctions and, in general, differentiate the forms of X-linked CSNB, thereby identifying the gene most likely to be involved . Table 1. Electroretinogram Findings in Complete and Incomplete X-Linked Congenital Stationary Night Blindness

Management Guidelines

Evaluations Following Initial Diagnosis

To establish the extent of disease in an individual diagnosed with X-linked congenital stationary night blindness (CSNB), the following evaluations (if not performed as part of the evaluation that led to the diagnosis) are recommended:

Surveillance Guidelines

Regular (yearly) eye examinations are recommended with refraction at a young age to monitor for the development of myopia.

Source: GeneReviews — "X-Linked Congenital Stationary Night Blindness"