Focal cone electroretinograms in dominant retinitis pigmentosa with reduced penetrance.

Focal electroretinograms (ERGs) from the green and red cone systems in combination were elicited with a method of rod silent substitution from three young patients with dominant retinitis pigmentosa with reduced penetrance. With this method, two alternating lights of different wavelength, matched in brightness for the rods, were presented to the same retinal area. The brighter light for the green and red cones was designated as the stimulus increment (ls) and the dimmer light as the background (lb). Foveal cone ERGs were normal in amplitude and b-wave implicit time, and midpehpheral cone ERGs were minimally reduced in amplitude but markedly delayed in b-wave implicit time. Minimal reductions in amplitude with marked delays in implicit time could be recorded from normal subjects in the midperiphery when the brightness of lb was reduced more than the brightness of ls while still maintaining rod silent substitution; this was not observed either when the brightnesses of lb and ls were proportionately diminished or when the area of lb and ls was decreased. These findings support the idea that the abnormal midperipheral cone ERGs observed in young patients with this type of retinitis pigmentosa are due in part to a decreased state of adaptation of the cone system and that a reduced amount of visual pigment in the photoreceptors or reduced numbers of photoreceptors cannot, by themselves, be responsible for these abnormal cone ERGs.