Understanding changes in the b-wave of the ERG caused by heterogeneous receptor damage.

PURPOSE To understand better the relationship between heterogeneous receptor damage and the changes in the b-wave of the rod ERG. METHODS A computational model of the b-wave is used to simulate b-waves from retinas with two regions (a healthier and a more affected region) differing in area and sensitivity. The peak-to-peak amplitudes of the simulated b-waves are fitted with a Naka-Rushton equation, and the parameters log K and Vmax are estimated. Insights gained from these simulations are tested against rod ERGs and rod visual fields from patients with autosomal dominant retinitis pigmentosa (n = 11) and cone-rod dystrophy (n = 17). RESULTS In the simulated retinas, Vmax decreases when the more affected region of receptors is less sensitive than the healthier region by more than 0.5 log unit. However, the relative change in Vmax does not match the relative area of the more affected region unless this region is depressed by 2.0 log units or more. As the more affected region loses sensitivity, log K at first increases but then decreases and approaches the log K of the healthier region for losses greater than 2.0 log units or so relative to the healthier region. For the patients' data, the simulations predict the general relationships observed between the summary statistics of the visual fields (e.g., area of field and mean of log sensitivity changes in the field) and the changes in log K and Vmax. CONCLUSION A decrease in Vmax indicates that regions of the patient's retina have lost 0.5 log unit or more of sensitivity. An increase in log K indicates that either the healthiest part of the patient's retina is abnormal by at most delta log K, a large part of the retina has lost considerable sensitivity (i.e., 0.5 to 2.0 log units) but is still contributing to the response, or both.