The relationship between rod perimetric thresholds and full-field rod ERGs in retinitis pigmentosa

Dark-adapted static perimetric fields and rod full-field electroretinograms (ERGs) were measured in normal subjects and 15 patients with retinitis pigmentosa. Psychophysical thresholds for short-wavelength stimuli were obtained from 76 locations across the retina on a modified Octopus 201 perimeter; threshold differences to short- and long-wavelength stimuli were used to determine whether rods were mediating threshold at each retinal location. Full-field rod ERG peak-to-peak amplitude as a function of retinal illuminance was analyzed by determining the parameters of the best-fit Naka-Rushton equation. Rod ERG thresholds and mean rod perimetric thresholds were significantly correlated with a slope of approximately 1.0. Log k was positively correlated with threshold elevation in the region of peak field sensitivity and log Vmax was positively correlated with log rod visual field area. These results demonstrate correspondence between parameters derived from rod visual fields and parameters derived from the Naka-Rushton analysis of the full-field rod ERG.