Prognostic significance of interictal epileptiform discharges after epilepsy surgery

The prognostic significance of interictal epileptiform discharges (IED) after epilepsy surgery is uncertain. We reviewed 20 studies (including 2 unpublished data sets) to assess the usefulness of postoperative EEG findings in predicting seizure outcome after resective epilepsy surgery. Patient selection and methodology varied widely among the studies. The published studies included 1,345 patients (temporal resection, n = 751; extratemporal resection, n = 373; unspecified site, n = 221). We defined a favorable outcome as a postoperative seizure status of Engel class I or equivalent. The frequency of postoperative IED ranged from 13% to 68% (mean, 31.5%). Postoperative IED were strongly associated with an unfavorable seizure outcome for the whole cohort (odds ratio, 3.3; 95% confidence interval, 2.5-4.5), for the subgroup of patients who underwent temporal resection (odds ratio, 2.5; 95% confidence interval, 1.6-4.0), and for the extratemporal resection subgroup (odds ratio, 5.6; 95% confidence interval, 3.9-9.3). Postoperative IED had a modest positive predictive value (52%) but an excellent negative predictive value (71%) for unfavorable seizure outcome. Most IED (>90%) were localized to the site of resection and were also influenced by preoperative spike frequency and completeness of resection. Insufficient data preclude any firm conclusions about the value of postoperative IED in predicting seizure outcomes after hemispherotomy or corpus callosotomy.