Temporal aspects of the dark-adapted cone a-wave in retinitis pigmentosa.

Cone electroretinograms were elicited with a full-field red flash after-45 min of dark adaptation from 16 patients with retinitis pigmentosa and from 17 normal subjects. The average latency and implicit time of the cone a-waves recorded from the patients were each significantly delayed (p less than 0.005) compared with the average normal values. In addition, the patient's waveforms typically showed a loss of two oscillations on the rising slope of the come b-wave. These delays and waveform changes could be simulated in normal subjects by reducing the luminance, but not the diameter, of the stimulus. These studies in normal subjects suggest that shortening of outer segments of remaining cones and not simply loss of cones account for the waveforms observed in patients with retinitis pigmentosa. The results also suggest that remaining cones generate an a-wave in the dark that is normal for a reduced cone visual pigment density.