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Rod and cone function in the Nougaret form of stationary night blindness
M A Sandberg1, B S Pawlyk, J Dan
1Berman-Gund Laboratory for the Study of Retinal Degenerations, Harvard Medical School, Boston, Mass., USA. masandberg@aol.com
Background:
Recently, a mutation (Gly38Asp) was identified in the alpha subunit of rod transducin in members of the Nougaret pedigree affected 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 Gly38Asp 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 white light that were elevated approximately 2 log-units across the retina. Spectral sensitivity testing revealed thresholds that seemed to be governed mostly by rods. Although both patients' dark-adapted ERG responses 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 function alone. Rod ERGs to bright blue flashes had subnormal, but detectable, amplitudes that seemed to result from a profound reduction in sensitivity. The patients also showed loss of a cone subcomponent in the dark-adapted response to a red flash. The abnormal dark-adapted ERG responses of the patients could be simulated in the ERG responses of normal subjects tested with blue, white, and red flashes presented in the presence of a mesopic background.
Conclusions:
Although the Nougaret 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 Nougaret, show that rod function is present, although subnormal, and that there is slight impairment of cone function. The data also suggest that these abnormalities can be simulated by light-adapting the normal retina, compatible with the proposal that the rod transducin encoded by the mutant gene is constitutively active and that the night blindness results from partial desensitization of rods caused by the constitutive activity.