Changes in interhemispheric inhibition following successful epilepsy surgery: a TMS study.

Epileptic foci can influence cortical excitability, brain perfusion and metabolism not only directly in the focus or perifocally, but also in remote areas. Effects of successful epilepsy surgery on cortical networks and changes in excitability have rarely been addressed. We report a study on changes in interhemispheric inhibition following successful surgical removal of an epileptic focus. Eighteen patients (11 females, 7 males, mean age 34.2 years) were enrolled in this transcranial magnetic stimulation (TMS) study. All patients were seizure free after surgery and had identical antiepileptic medication pre- and postsurgically. Investigations were performed before and at least 3 months after surgery. Motor thresholds (MT) and motor evoked potentials (MEPs) of interhemispheric paired pulse paradigms were investigated on both hemispheres. Resection of the epileptic focus resulted in a significant change in interhemispheric inhibition (IHI). The ability of the non-focal hemisphere to inhibit the motor cortex (M1) of the focal hemisphere significantly increased (p=0.02) and normalized to the level of the other hemisphere. In summary, this TMS study suggests that an epileptic focus can modulate interhemispheric inhibitory interactions between the motor cortices. A decreased susceptibility of M1 of the focal hemisphere or alterations in the non-focal hemispheric inhibitory output may be underlying mechanisms. These findings may contribute to a better understanding of widespread functional impairments in focal epilepsy.