Rod sensitivity relative to cone sensitivity in retinitis pigmentosa.

The present study evaluates rod sensitivity relative to cone sensitivity in retinitis pigmentosa patients. From perimetric measures of absolute thresholds to long- and short-wavelength stimuli, we find the results can be characterized in three different ways. For one group of patients it appears that only cone function is present in all parts of the visual field. For another group of patients, despite large losses in absolute sensitivity, rod sensitivity still exceeds cone sensitivity by at least the normal factor, suggesting concomitant rod and cone sensitivity losses. For a third group of patients it appears that central retinal thresholds are determined primarily by cones, midperipheral retinal thresholds determined by a composite of rod and cone function, and far- peripheral thresholds determined predominately by rods. The argument is put forward that the different groups of threshold results represent different disease mechanisms rather than different stages in the progression of a single disease process.