Eye specialists using standardised electrophysiological criteria for the first time have suggested that these are more reliable indicators of genotype in X linked congenital stationary night blindness (CSNBX) than those used formerly.

Just three measures separated 20 affected males from 11 British families with CSNBX, according to whether they had mutations in NYX or CACNA1F genes. NYX mutations resulted in absent scotopic oscillating potentials, CACNA1F mutations in subnormal OFF response, and each mutation had different wave forms and amplitudes in 30 Hz flicker tests. Other indicators of eye function were not specific enough.

Three of the families had CACNA1F mutations, each with a different mutation, one of which was a novel nonsense mutation in exon 7. The others were a nonsense mutation in exon 24 and a base pair (bp) deletion in exon 9. All were expected to produce a truncated protein product. The eight remaining families had five NYX mutations: a splicing mutation; a missense and a nonsense mutation predicting truncated protein product; a 15 bp in frame deletion; and a 335 bp deletion.

Affected males were prospectively clinically evaluated for visual disorders and by psychophysiological and electrophysiological testing in parallel with genotyping.

Until now, non-standardised testing has subdivided CSNBX phenotype into “complete” and “incomplete” forms, apparently associated with NYX and CACNA1F mutations, respectively. However this distinction has proved unsatisfactory, in the light of reports of a functional rod pathway in patients with NYX mutations and of complete and incomplete forms of the condition in the same family.