Changes in intracortical excitability after successful epilepsy surgery

Interictal activity from epileptic foci may have remote effects as demonstrable by assessing brain perfusion, metabolism and excitability. So far, the effect of surgical removal of the epileptic focus on cortical networks has only rarely been addressed. This study aims at an assessment of changes in intracortical inhibition and excitability in patients undergoing successful epilepsy surgery. Twenty-two patients (12 females, 10 males, mean age 37.8 years) with identical pre- and postsurgical antiepileptic medication were investigated using focal transcranial magnetic stimulation. Motor thresholds (MT), contralateral cortical silent periods (CSP) and amplitudes of motor evoked potentials (MEP) using paired pulse paradigms were investigated both, at the focal and non-focal hemisphere before and at least 3 months after successful epilepsy surgery. The postsurgical mean MT when stimulating at the focal hemisphere was significantly higher than at the non-focal hemisphere (p=0.01). Postsurgically, the mean duration of the CSP at 120% and at 150% of MT of the non-focal hemisphere was significantly shorter than presurgically (p=0.04), and the mean MEP amplitude following paired-pulse stimulation with an interstimulus interval of 10 ms of the non-focal hemisphere was significantly lower than presurgically (p=0.03). In summary, both parameters representing inhibition and facilitation changed following epilepsy surgery; effects were statistically significant on the non-focal hemisphere. Transcranial magnetic stimulation thus gave evidence of remote effects of an epileptic focus and its surgical removal. Extended changes in excitability due to the presence or absence of an epileptic focus may be related to widespread functional impairments in patients with focal epilepsy.