Rod and cone dysfunction in carriers of X-linked retinitis pigmentosa

Carriers of X-linked retinitis pigmentosa were studied using electroretinographic and psychophysical procedures. Under both dark- and light-adapted (cone-isolated) conditions, electroretinogram (ERG) a-waves of carriers were reduced in amplitude but normal in implicit time, whereas b-waves were reduced in amplitude and delayed in implicit time. Reductions in b-wave amplitudes of the carriers as a group were equivalent for the rod and cone systems. Luminance-response functions for both dark-adapted and cone-isolated b-waves were fit by the Naka-Rushton equation and demonstrated a selective reduction of Rmax; the semi-saturation constant (K) and the slope parameter (n) were normal. Electroretinograms recorded using the brightest stimulus flashes were most effective at distinguishing carriers from normals. Absolute thresholds of the carriers were elevated significantly across the central 40 degrees of the visual field. As a group, the threshold elevations of the carriers were approximately equal for the rod and cone systems.