Abstracts

Rationale: Ictal and interictal surface EEG recordings are obtained in all patients with medically refractory epilepsy undergoing phase 1 presurgical evaluation. The goal of this study is to identify surface patterns associated with strictly mesial temporal onset and excellent postsurgical outcomes. We report surface EEG findings in a "pure culture" of patients with mesial temporal lobe epilepsy (mTLE), defined as seizure freedom following selective amygdalohippocampectomy (SAH). Methods: Retrospective review of 16 patients who are seizure free (Engel Class I)> one year postoperatively following SAH. Demographic and imaging data abstracted from chart review are summarized in table 2. Background surface EEG and interictal epileptiform discharge (IIED) findings were obtained from summative neurophysiology reports of presurgical video-EEG monitoring. IIED were classified as unilateral (80-100% concordant with surgical side), predominantly unilateral (>60<80% from surgical side), or bilateral (<60% from surgical side). Original ictal recordings were reviewed by 2 board certified EEG readers. For each patient the three best EEG examples were selected for analysis. If more than one ictal pattern was reported, up to three additional ictal recordings were reviewed. Reviewers categorized frequency, spatial, and temporal characteristics of the ictal recordings. Localization was determined using predefined criteria (table 1). Ictal recordings were also classified according to the system proposed by Ebersole (1). Results: Baseline EEG recordings showed diffuse background slowing in 6/16 subjects, and focal slowing congruent with the side of ultimate surgery in 11/16. IIEDs were unilateral on the side of surgery in 15/16, predominantly unilateral and concordant in 1/16, and bilateral in 0/16. The average time from clinical onset to EEG onset was -3 seconds (range -50 to 40.5), and 11/16 patients showed a lateralized ≧ 5Hz seizure discharge within 30 seconds of seizure onset. The initial ictal frequency was delta in 4/16 and theta in 12/16. The best ultimate EEG localizations were anterior temporal (1), mid-anterior temporal (9), midtemporal (1), temporal diffuse (2), frontotemporal (1), frontocentrotemporal (1), non-localized (1). Ebersole Classes were IA (9), Class IIB (4), IIA (1), and non-classified (2). Conclusions: Previous reports of surface EEG patterns in patients with mTLE have identified cases based on intracranial EEG studies that are subject to sampling error, or on seizure free outcome following anterior temporal lobectomy, which could include cases with a non-mesial temporal epileptogenic zone. We found that a variety of ictal patterns can be identified in patients with definite mTLE, defined as seizure freedom following SAH. Similar to previous reports (1,2)we found that “classic” surface EEG patterns for mTLE were not highly sensitive. Defining patterns associated with mesial temporal onset and postoperative seizure freedom may help in the selection of optimal candidates, and highlight patients in need of confirmatory studies. 1. Ebersole JS and Pacia SV Epilepsia 37(4)386-99,1996 2. Risinger MW et al Neurology 40:74-9,1989