Rod phototransduction in retinitis pigmentosa. Distinguishing alternative mechanisms of degeneration.

PURPOSE: To test alternative hypotheses concerning the mechanisms of rod degeneration in retinitis pigmentosa (RP).
METHODS: Full-field rod electroretinograms and rod visual fields were measured for 15 patients with RP and a normal control group. The rod a-wave was fitted with a computational model based on known transduction biochemistry. The values of td (the initial delay), S (a sensitivity parameter), and RmP3 (the maximum amplitude) were estimated. Rod b-wave amplitudes were fitted with the Michaelis-Menten equation, and the parameters Kbw (the semisaturation intensity--a sensitivity parameter) and Vmax (the maximum amplitude) were estimated.
RESULTS: The patients all had significantly reduced values of RmP3, indicating rod receptor damage, and a wide range of S values. Three patients had S values in the normal range. Four had abnormal S values but normal thresholds in some locations in their visual field. The remaining patients had abnormal values of S and entirely abnormal visual fields. Three of those had a history of large elevations in Kbw. For all patients, the changes in Kbw and Vmax followed closely the changes in S and RmP3, respectively.
CONCLUSION: Retinitis pigmentosa has a varying initial impact on the activation phase of rod transduction. Available evidence suggests that the activation of transduction is initially normal in most patients with RP. In some patients, RP appears to result in a reduced transduction amplification from birth. In all patients, subsequent degeneration of the rods effects progressive reductions in transduction amplification but no other major functional changes. Outer segment shortening and local dropout of rods appear to have little functional impact.