Abnormal response of motor cortex to photic stimulation in idiopathic generalized epilepsy

Intermittent photic stimulation (IPS) shortens the cortical silent period (CSP) elicited by transcranial magnetic stimulation (TMS) over the primary motor hand area (M1(HAND)). This response is absent in healthy individuals with a photoparoxysmal response (PPR). Here we combined TMS of the M1(HAND) with IPS to examine whether patients with idiopathic generalized epilepsy (IGE) exhibit an abnormal cortical response pattern to IPS.

In 13 PPR-positive and 12 PPR-negative patients with IGE and in 13 PPR-negative healthy controls, we used focal TMS to the M1(HAND) to study how cortical excitability is changed by concurrent IPS at 50 Hz.

IPS at 50 Hz reduced the duration of the CSP in healthy PPR-negative individuals, whereas IPS had no effect on the CSP in PPR-positive and PPR-negative patients with generalized epilepsy. The failure of IPS to shorten the CSP was independent of antiepileptic medication. Single-pulse or paired-pulse TMS only without concurrent IPS showed a higher motor threshold in PPR-positive patients with epilepsy, presumably caused by antiepileptic medication. No additional differences in cortical excitability were found among groups.

Because the CSP is mediated by intracortical GABAergic mechanisms, our results indicate that IGEs are associated with an altered responsiveness of GABAergic inhibitory circuits in the M1(HAND). This electrophysiological trait is independent of photosensitivity. Excitability changes at the cortical or thalamic level may mediate this abnormal cortical response pattern in patients with IGE.