Intermittent photic stimulation affects motor cortex excitability in photosensitive idiopathic generalized epilepsy

To assess the effect of intermittent photic stimulation (IPS) at a common activating frequency, i.e. 20Hz, on motor cortex excitability by means of transcranial magnetic stimulation (TMS) in photosensitive patients with idiopathic generalized epilepsy (IGE).

We studied 15 photosensitive IGE patients showing a photoparoxysmal response (PPR) to IPS at 20Hz. Nineteen normal subjects of similar age and sex acted as controls. After the resting motor threshold (rMT) was measured, we studied the corticomotor excitability in two conditions randomly delivered, during IPS (5s) at 20Hz and without IPS. Motor evoked potentials (MEPs) were recorded from the right first dorsal interosseous muscle (FDI). We determined the cortical silent period (cSP), the short-latency intracortical inhibition (SICI) at the interstimulus interval (ISI) of 3 and 4ms and the intracortical facilitation (ICF) at ISIs of 12 and 14ms. Data were analyzed by means of rmANOVAs.

IPS at 20Hz is significantly shortening the cSP in normal subjects, while no significant changes were detected in patients. The rMT was significantly higher in patients than controls, as expected by the concurrent antiepileptic treatment. Other corticomotor excitability measures were unaffected.

We confirm that IPS has a weak influence on the motor cortical output in patients with IGE and PPR. The loss of the normal shortening of the cSP, otherwise present in healthy subjects in response to IPS, may have a possible protective nature.