Diurnal rhythm in the human rod ERG: retinitis pigmentosa

Rod ERGs were measured at three times of day over an extensive range of retinal illuminances in six light-entrained patients with autosomal recessive or isolate forms of retinitis pigmentosa and at five times of day in six light-entrained normal volunteers. B-wave amplitude versus retinal illuminance functions from each time of day were described by determining the parameters of the best-fit Naka-Rushton function. Results from normal subjects showed that rod ERG threshold (defined as the log retinal illuminance necessary to elicit a 2.0 microV response) was elevated 1 1/2 hr after daily light-onset due to both an increase in log k (semi-saturation constant) and a decrease in log Vmax (maximum rod amplitude). The magnitude of the threshold elevation 1 1/2 hr after light-onset was comparable in patients with retinitis pigmentosa and normal subjects. Whereas thresholds returned to pre-light exposure levels rapidly during the light-phase of the daily cycle in normal subjects, thresholds continued to rise in patients with retinitis pigmentosa due primarily to a further increase in log k. These findings are consistent with abnormal rod photoreceptor disc renewal mechanisms in retinitis pigmentosa.