Abstracts

Rationale: Confounding features of scalp EEG (sEEG) ictal patterns (IPs) and interictal epileptiform discharges (IEDs), magnetic resonance (MR) imaging and other nonelectrographic data in temporal lobe epilepsy (TLE) requires the use of extraoperatvie electrocorticography (eECoG) for further clarification of the ictogenic source. We assessed both electrographic and MR imaging features in cases requiring lateralization by such means. Methods: An institutional archival review identified adults with a putative TLE who required eECoG with bitemporal electrode placement. According to ictal patterns on sEEG, these patients were categorized into two main groups: I, unilateral (UL) IPs and II, bilateral (BL) IPs. Each group was further subdivided according to IED findings (Table). Patients who proceeded to surgery after eECoG were grouped according to findings on sEEG: I, exclusively concordant ipsilateral (IP) with discordant nonelectrographic data; II, IL preponderant (>75% of IPs); III, contralateral (CL) preponderant (>75% of IPs). The presence of medial temporal sclerosis (MTS) in each group was studied with regard to outcome of eECoG and resection. A favorable seizure outcome was accepted as Engel class II B and above. Descriptive statistics were applied to test for correlation between each of eECoG, sEEG and the presence of MTS and postsurgical outcome. Results: Twenty-nine patients (14F, 15M) with a mean age of 37y (16-59y) and a mean age at epilepsy onset of 16y (0-32y) were accrued. In the surgery group, the mean age at surgery was 33y (14-52y) and the mean postoperative followup was 37m (7-67m). A unilateral IP on sEEG was found in 22 patients (Group I) and a bilateral IP in 7 (Group II). In Group I, 14/22 (64%) had correctly lateralized IEDs. Of those showing bilateral (6) and generalized (1) IEDs, 5 cases showed a preponderance (>75%) on the epileptogenic side raising the correlative value to 86%. In Group II, IEDs suggested bilateral epileptogenicity in 5/7 (71%) with the remainder showing no IEDs. Lateralization according to eECoG provided reliable outcome prediction (p<0.001) for resection (Table). MTS was identified in 10/19 (53%) of the surgery cohort and showed no association with lateralizing IPs or IEDs. Of the 19 cases undergoing resection, 14 (74%) had a favorable outcome including 10 patients who achieved seizure-freedom.