The fundamental and second harmonic of the photopic flicker electroretinogram: temporal frequency-dependent abnormalities in retinitis pigmentosa

Objectives: The flicker electroretinogram (FERG) consists mainly of a linea r (fundamental, 1F) and a non linear (second harmonic, 2F) component. Previ ous results indicate that 2F originates more proximally in the retina than IF, and that retinitis pigmentosa (RP) may affect 2F to a greater extent th an 1F. The aim of this study was to evaluate FERC 1F and 2F abnormalities i n RP as a function of the stimulus temporal frequency (TF). Methods: Twelve patients with typical RP and 10 age-matched controls were e xamined. FERGs were recorded in response to uniform fields (18 degrees) pre sented in the macula on a light-adapting background. Stimuli were flickered sinusoidally at different, closely spaced TFs between 3.7 and 52 Hz. Ampli tudes and phases of the Fourier analysed 1F and 2F components were measured Components' apparent latencies were estimated from the rate at which phase lagged with TF. Results: When compared to controls, mean 1F amplitudes of patients were red uced at both low (3.7-12.6 Hz) and high (14-52 Hz) TFs, with greatest losse s (0.5 log units) around the peaks (3.7 and 41 Hz) of the normal TF functio n. Mean 2F amplitudes were reduced mainly at low TFs, with greatest losses (0.5 log units) at 5-8 Hz. On average, the shape of the 2F, but not 1F ampl itude versus TF function, differed between patients and controls, showing a selective attenuation at low TFs. Mean 1F apparent latencies were delayed at both low and high TFs, with greater delays at low (85 ms) than at high ( 33 ms)TFs. Mean 2F apparent latencies were delayed only at low TFs (58 ms). Conclusions: In RP, 1F and 2F phase delays as well as 2F amplitude losses a re dependent on TF, suggesting that FERC generators' subpopulations in both distal and proximal retina are differentially affected. Analysis of the FE RG TF response is potentially useful to characterize cone system dysfunctio n in different genetic subtypes of RP. (C) 1999 Elsevier Science Ireland Lt d. All rights reserved.