Negative electroretinograms in retinitis pigmentosa.

PURPOSE Patients with typical clinical features of retinitis pigmentosa were found to have the atypical electroretinographic finding of a negative waveform to a bright flash in the dark-adapted state. The full-field electroretinogram (ERG) was studied in seven such patients to understand better the pathophysiology. METHODS Rod ERGs were isolated using blue and red flash stimuli in the dark-adapted state. The rod ERG was assumed to be the sum of two major components, P3 and P2. A family of delayed Gaussian functions fitted to the rod a-wave intensity series was used to estimate the P3 component. The P2 component was derived by subtracting the estimated P3 component from the rod-isolated ERG. Long duration stimuli were used to elicit "on" and "off" components of the light-adapted cone ERG. Oscillatory potentials were isolated by digitally filtering cone ERGs to white flash stimuli. RESULTS The estimated rod P3 component was reduced in amplitude in all patients. The derived P2 component of the rod ERG was present but abnormally reduced relative to the P3 component. Cone waveforms had decreased a-waves, "on" and "off" components. Many of the patients had a disproportionate reduction of the "on" compared to the "off" component. Photopic oscillatory potentials were either reduced in amplitude and delayed in timing or not detectable. CONCLUSIONS The ERG findings in this subset of RP patients indicate there is dysfunction not only at the level of the photoreceptor outer segment but also at or proximal to the photoreceptor terminal region.