Cone and rod ERG phototransduction parameters in retinitis pigmentosa

PURPOSE. To analyze cone and rod phototransduction parameters from ERG a-waves in patients with RP and to determine the relationships among these parameters, age, and mode of inheritance. METHODS. Sets of four white flashes (3.2-4.4 log scotopic troland [scot td-s]) were presented in the dark. The same stimuli were later presented against a rod-saturating background and the generated cone a-waves were subtracted from the dark-adapted responses to produce rod-only a-waves. The rod-only and cone a-waves were fit with computational models. RESULTS. Of 418 consecutive patients with retinitis pigmentosa (RP), cone a-waves were quantifiable in 136 (33%), whereas rod a-waves were quantifiable in 125 (30%). Cone R-max (maximum response) and cone S (sensitivity) parameters were significantly below normal in all RP subgroups. Cone R-max was lower in XlRP than in other forms of inheritance (P < 0.05). Cone S was abnormal in 77.9% of all patients with RP and in 96.8% of those with XlRP. More than 95% of the rod R-max values were abnormal, whereas rod S was abnormal in 61.6% of these patients. CONCLUSIONS. The efficiency of cone phototransduction appears to be affected in all forms of RP, even in some patients in whom the sensitivity of rod phototransduction is normal. In this cross-sectional sample, there was no evidence that transduction efficiency decreased with increasing age of the patient. The X-linked mode of inheritance is associated with greater abnormalities in cone and rod photoreceptor function at a younger age compared with the other modes of inheritance.