Rod and Rod Driven Responses in Congenital Stationary Night Blindness (CSNB)

Abstract

Purpose: : To characterize rod phototransduction and rod driven postreceptoral activity in patients with congenital stationary night blindness (CSNB). Poor night vision and attenuated postreceptoral activity are recognized characteristics of CSNB.

Methods: : ERG responses to a > 4 log unit range of blue ( < 440nm), full-field stimuli were recorded from 22 dark adapted patients with CSNB (median age 9.3 years; median elevation of threshold 1.24 log units). All patients were myopic (median -8.13D; range -1.62D to -13.75D) and half showed paradoxical pupillary constriction to darkness. The rod response parameters, sensitivity (S_ROD) and saturated amplitude (R_ROD), were calculated by fit of the Hood & Birch formulation of the Lamb & Pugh model of the activation of phototransduction to the ensemble of a-waves. Postreceptor sensitivity (log σ) and saturated amplitude (V_MAX) were calculated by fit of the equation [V/V_MAX = I/ (I + σ)] to the b-wave amplitudes. The oscillatory potentials (OP) were evaluated by Discrete Fourier Transform (DFT) and the total energy of the OP (E) and dominant frequency (f peak) were determined. All response parameters were compared to those of 26 emmetropic control subjects and 6 myopes (median -6.79D).

Results: : In CSNB, the mean saturated amplitude of the rod response, R_ROD, was 59% of that in controls (232.98 vs. 391.31µV; t = -7.20; p <0.001) and of the b-wave, V_MAX , was 30% of that in controls (112.05 vs. 378.19µV; t = -18.86; p < 0.001). Rod sensitivity, S_ROD, did not differ significantly between CSNB and controls, but mean log σ values did differ significantly indicating, on average, the sensitivity was 1.61 log unit less in CSNB. As for the OPs, the total power (E) was significantly lower in CSNB than controls. On no ERG parameter did the myopic and non-myopic controls differ.

Conclusions: : Moderate to mild compromise of the activation of rod phototransduction is found in about half of the CSNB patients. In all patients with CSNB, the combined a-wave, b-wave and OP results define anomalous post-transduction circuitry in the CSNB retina.

Keywords: electrophysiology: clinical • photoreceptors • retina