A distinctive form of congenital stationary night blindness with cone ON-pathway dysfunction

Abstract

Purpose: To characterize a distinctive form of congenital stationary night blindness (CSNB).

Design: Observational case report.

Participants: A 30-year-old male with a history of night blindness, several members of his family, a patient with "complete" congenital stationary night blindness (CSNB1), and groups of age-similar control subjects.

Methods: Rod-system function was evaluated by measuring psychophysical dark-adapted thresholds, by recording dark-adapted electroretinograms (ERGs), and by fundus reflectometry. Cone-system function was evaluated by recording light-adapted ERGs, including those to sawtooth flicker, and by recording light-adapted visually evoked potentials (VEPs) to luminance increments and decrements.

Main outcome measures: Dark-adapted thresholds, ERGs, rhodopsin double densities, Goldmann visual fields, and VEPs.

Results: The patient's visual acuity, visual fields, and color vision were normal. His peripheral dark-adapted thresholds were rod-mediated but elevated by approximately 3 log units above normal. Rhodopsin double density and bleaching recovery were normal. His dark-adapted maximal-flash ERG showed a "negative" waveform, in which the b-wave was more reduced in amplitude than the a-wave, although the a-wave amplitude was also reduced. The rod photoreceptors contributed to the patient's dark-adapted ERGs, as illustrated by the unequal responses to cone-matched stimuli. The patient's cone-mediated thresholds for long-wavelength stimuli were within the normal range. However, his light-adapted brief-flash b-wave was abnormal in amplitude and implicit time. Selective abnormalities of the ON responses of the cone system were apparent in the patient's reduced b-wave amplitude to rapid-on flicker with a normal response to rapid-off flicker, and his prolonged VEP latencies to increments but not to decrements.

Conclusions: The overall pattern of findings distinguishes this patient from previously described forms of CSNB. The results suggest that two factors likely contribute to the patient's night blindness: (1) a rod phototransduction defect and (2) a postreceptoral defect. The results also indicate dysfunction within the cone ON pathway.