Characteristics and clinical impact of stimulation-evoked seizures during extraoperative cortical mapping

Extraoperative electrocortical stimulation mapping (ESM) is used to identify functional cortex prior to epilepsy surgery, with the goal of preserving function postoperatively. Although attempts are made to avoid stimulation-evoked seizure activity, the clinical impact of these events with regard to safety, clinical utility, functional outcome, and even disruption to the procedure itself is unknown. We conducted a retrospective review of 57 patients with pharmacoresistant focal epilepsy who underwent intracranial electrode implantation and ESM. Stimulation-evoked seizures (afterdischarges associated with clinical signs or symptoms) occurred in 19 patients (33%). Mapping sessions were disrupted for 11 of these patients (i.e., 19% of the full sample and 58% of the subgroup of patients with stimulation-evoked seizures). Patients who had ESM disruption were no less likely than patients without ESM disruption to be seizure-free at one year (p=0.63) and two years (p=0.57) postoperatively. Among 23 patients who underwent language assessment pre- and postoperatively, 4 (17%) had evoked seizures that disrupted language mapping; these patients were no more likely to show postoperative language declines relative to those who had no ESM disruption (p=0.26). Results suggest that evoked seizures occur frequently during ESM and can disrupt the procedure; however, these events do not appear to adversely affect postoperative outcomes. Nevertheless, attempts should be made to limit stimulation-evoked seizures in order to reduce patient discomfort, increase efficiency, and maximize the utility of ESM.