Regional cone dysfunction in retinitis pigmentosa evaluated by flicker ERGs: relationship with perimetric sensitivity loss

PURPOSE: To evaluate regional cone dysfunction in retinitis pigmentosa (RP) by recording focal electroretinograms (FERGs) from the central and paracentral retinal regions and to correlate the FERG with perimetric sensitivity losses. METHODS: Twenty-three typical patients with RP (age, 18-65 years; visual acuity, 20/100 to 20/20; kinetic visual field by size II/4e, 20-40 degrees ) and eight age-matched control subjects were evaluated. FERGs were recorded in response to either a central (eccentricity, 0-2.25 degrees ) or a paracentral annular (2.25-9 degrees ) field, presented on a light-adapting background. Fields' luminances (mean: 80 cd/m(2)) were sinusoidally modulated at different temporal frequencies (TFs; 10.3, 14, 21, 32, 41, and 52 Hz). Amplitude and phase of the responses' fundamental harmonic (1F) were measured. Perimetric sensitivity was measured by a visual field perimeter. For each patient, mean sensitivity losses were calculated for both the central (0-2.25 degrees ) and paracentral (2.25-9 degrees ) regions. RESULTS: On average, central and paracentral FERGs of patients with RP were reduced in amplitude (P </= 0.05) compared with control values. Amplitude losses tended to be smaller in the central than the paracentral region and were limited to low-medium TFs (10.3-14 Hz). Paracentral losses were rather invariant with TF. Paracentrally, but not centrally, the FERG phase in patients was delayed on average (P < 0.01), compared with control values. The central FERG phase was delayed only in patients with visual acuities less than 20/40. In individual patients, paracentral 41-Hz amplitude losses were positively correlated with corresponding perimetric losses (r = 0.7, P < 0.005). Both central and paracentral 41-Hz amplitudes displayed high specificity (87.5% and 100%, respectively) with relatively low sensitivity (46.6% and 63.6%, respectively) in predicting perimetric results in corresponding retinal regions. CONCLUSIONS: In RP, central and paracentral FERGs are differently altered as a function of TF, indicating regional differences in the stage and/or pathophysiology of retinal cone dysfunction. FERG abnormalities may predict, to some extent, perimetric results at corresponding retinal regions. The data support the use of the present FERG method to evaluate regional cone dysfunction in different stages of RP.