Temporal frequency deficits in the electroretinogram of the cone system in X-linked retinoschisis

Abstract

We investigated the extent of, and basis for, abnormalities in the flicker electroretinogram (ERG) of the cone system of patients with X-linked retinoschisis (XLRS), a form of hereditary vitreoretinal degeneration. ERGs were recorded from six patients with XLRS and from six visually normal subjects using high-contrast sinusoidal flicker that ranged in temporal frequency from 8 to 96 Hz, and that was presented against a rod-desensitizing adapting field. Compared to the control subjects, the patients with XLRS showed a significant reduction in the amplitude of the ERG response fundamental at temporal frequencies of 32 Hz and higher. In addition, their response phases were at or below the lower limits of normal (representing a phase lag) for temporal frequencies greater than 8 Hz. The higher harmonics of the patients' ERG responses to a low frequency stimulus were attenuated over the same temporal frequency range as was the response fundamental. This finding indicates that a major component of the abnormal temporal filtering responsible for the ERG abnormalities in XLRS occurs beyond the level of the early retinal nonlinearity that generates the harmonic components of the ERG response, and therefore is most likely postreceptoral in origin. Consistent with this interpretation, the ERG waveforms of the XLRS patients showed a significant attenuation of the ON-response component, with a normal OFF response. The overall pattern of results suggests that the marked reduction of ERG response amplitudes and the phase lag at the higher temporal frequencies in XLRS stem, at least in part, from a predominant attenuation of the ON-bipolar cell contribution to the flicker ERG.