Defective visual inhibition in photosensitive idiopathic generalized epilepsy

To assess the visual system excitability of photosensitive patients with idiopathic generalized epilepsy (IGE) with the paired-pulse flash-evoked potential (paired F-VEP) technique.

We studied 19 photosensitive patients with IGE (16 women) showing a photoparoxysmal electroencephalographic (EEG) response (PPR). Twenty-two normal subjects of similar age and sex acted as controls (17 women). We recorded F-VEPs from occipital and central electrodes. Stimuli were single flashes, intermingled to flash pairs at the interstimulus interval (ISI) of 333, 125, 62.5, 50, 33, and 16.5 msec (i.e., at the internal frequency of 3, 8, 16, 20, 30, and 60 Hz). Recordings were done both with closed and open eyes. The single F-VEP was split into a "main complex" and a "late response," which were measured separately. As to paired stimuli, the "test" F-VEP emerged from electronic subtraction of the single F-VEP to the paired F-VEP. Grouped data were analyzed by means of nonparametric analyses of variance (ANOVAs).

In patients, the single F-VEP showed some enhanced components in its early "main complex." Then, the "test" F-VEP behaved differently than controls, particularly if recorded with closed eyes, when the normal inhibition was abolished at given ISIs, corresponding to an internal frequency of 16-30 Hz. In patients with a posterior PPR, impaired inhibition was evident over the occipital region only, but in those with a widespread PPR, it also involved the central areas.

The paired F-VEP technique documents a defective inhibition in the visual system of photosensitive patients with IGE, whose features and timing likely underlie the PPR origin.