Postreceptoral contribution to macular dysfunction in retinitis pigmentosa.

PURPOSE To understand the sites of macular dysfunction in retinitis pigmentosa by evaluating focal electroretinogram (FERG) fundamental and second harmonic components, which are dominated by the activity of the outer and inner retina, respectively. METHODS FERGs were recorded in response to a uniform field (9 degrees x 9 degrees) flickered sinusoidally at either 8 Hz or 32 Hz. The fundamental components of the responses to both the 8-Hz and 32-Hz stimuli and the second harmonic of the response to the 8-Hz stimulus were measured in their amplitudes and phases. Normal subjects (n = 17), as well as patients with typical retinitis pigmentosa (n = 22), cone dystrophy (CD, n = 7) and X-linked congenital retinoschisis (XLR, n = 5) served as subjects. The fundamental (32-Hz)-second harmonic (8 Hz) amplitude ratio was taken as an index of the relative function of outer-inner layers of the macula. RESULTS Compared to controls, patients with retinitis pigmentosa showed losses of both FERG fundamental and second harmonic and an increase of the fundamental-second harmonic ratio. Patients with CD also showed losses of both fundamental and second harmonic, but the fundamental-second harmonic ratio was normal. Patients with XLR had a selective loss of the second harmonic, resulting in an increased fundamental-second harmonic ratio. On average, the fundamental-second harmonic ratio of patients with retinitis pigmentosa tended to increase with age. CONCLUSIONS The results suggest that in retinitis pigmentosa, both receptoral and postreceptoral sites contribute to macular dysfunction. This differs from CD and XLR, where losses appear more selective for the outer and the inner retina. Postreceptoral losses in retinitis pigmentosa may become larger with increasing age.