ROD AND CONE FUNCTION IN THE NOUGARET FORM OF STATIONARY NIGHT BLINDNESS

Background: Recently, a mutation (Gly38Asp) was identified in the alph a subunit of rod transducin in members of the Nougaret pedigree affect ed with dominantly inherited stationary night blindness. Objective: To evaluate retinal function in patients with the Gly38Asp gene defect. Design: Ocular examinations, including specialized measures of rod and cone function. Setting: A clinical research facility in Boston, Mass. Patients: A father (aged 48 years) and son (aged 25 years) with the G ly38Asp mutation. Main Outcome Measures: Psychophysical thresholds to white and narrowband lights and full-field electroretinographic (ERG) responses. Results: Both patients showed dark-adapted thresholds to wh ite light that were elevated approximately 2 log-units across the reti na. Spectral sensitivity testing revealed thresholds that seemed to be governed mostly by rods. Although both patients' dark-adapted ERG res ponses to a dim blue flash were nondetectable, their dark-adapted ERGs to a white flash showed an a-wave with cone and rod components and a b-wave amplitude larger than what could have been generated by cone fu nction alone. Rod ERGs to bright blue flashes had subnormal, but detec table, amplitudes that seemed to result from a profound reduction in s ensitivity. The patients also showed loss of a cone subcomponent in th e dark-adapted response to a red flash. The abnormal dark-adapted ERG responses of the patients could be simulated in the ERG responses of n ormal subjects tested with blue, white, and red flashes presented in t he presence of a mesopic background. Conclusions: Although the Nougare t form of stationary night blindness has been cited as a prototype of absent rod function with normal cone function, our findings, based on the genealogically and genotypically documented descendants of Jean No ugaret, show that rod function is present, although subnormal, and tha t there is slight impairment of cone function. The data also suggest t hat these abnormalities can be simulated by light-adapting the normal retina, compatible with the proposal that the rod transducin encoded b y the mutant gene is constitutively active and that the night blindnes s results from partial desensitization of rods caused by the constitut ive activity.