Rod ERGs in retinitis pigmentosa and cone-rod degeneration.

Rod amplitudes as a function of retinal illuminance were analyzed by determining the parameters of the best fit Naka-Rushton function for 15 normal subjects, 25 patients with rod-cone degeneration, and 10 patients with cone-rod degeneration. Rod b-wave implicit times were analyzed by determining the best linear fit to log retinal illuminance. Retinal illuminances necessary for a criterion b-wave implicit time and for a half-maximum amplitude response (log k) were highly correlated indices of rod sensitivity. Both measures were normal or near normal in patients of all ages with cone-rod degeneration, but significantly elevated in patients with rod-cone degeneration. These results suggest that the underlying mechanisms of rod degeneration are fundamentally different in these two forms of hereditary retinal degeneration.