Introduction: Temporal lobectomy or multiple hippocampal transections (MHT) can be effective for mesial temporal lobe epilepsy, but patients without mesial temporal sclerosis may not be candidates for these procedures due to extra-hippocampal seizure onset. We report the result of invasive depth electrode analysis to determine site of seizure onset.
Methods: Thirty-two patients with semiology and video-EEG findings consistent with mesial temporal epilepsy but without localizing changes on MRI or neuropsychological analysis were included. Each patient underwent implantation of depth electrodes into the amygdala, hippocampal head, hippocampal body, mesial frontal lobe, posterior temporal lobe, posterior cingulate gyrus, anterior insula, and posterior insula. Electrodes were placed bilaterally if there was evidence of bilateral seizure activity. All patients subsequently underwent inpatient monitoring for 7-14 days followed by surgical treatment based on results of the evaluation.
Results: Twenty-five patients (78%) had seizure onset within the hippocampus: fifteen (47%) exclusively from one hippocampus, three (9%) from both hippocampi independently, eight (25%) from hippocampus and temporal tip, two (6%) from temporal tip but not hippocampus, and four (12%) from extratemporal sources. There were no cases with ictal onset from the posterior cingulate or insula. Patients underwent temporal lobectomy (11 patients), removal of temporal tip/amygdala with MHT (8 patients), MHT alone (5 patients), removal of temporal tip and amygdala with hippocampal sparing (2 patients), removal of posterior temporal neocortical seizure focus (1 patient), or no surgery (5 patients). All patients who underwent surgery had improvement in seizure control at last follow-up.
Conclusions: Nonlesional temporal lobe epilepsy usually arises from the mesial structures even when imaging and neuropsychological analysis are normal, but semiology and scalp video EEG are not sufficient to localize seizure onset. Depth electrode evaluation can be helpful to identify seizure onset and determine optimal therapy.
Patient Care: This technique will allow for identification of seizure focus in nonlesional temporal lobe epilepsy patients, and may enable some patients who would otherwise not be candidates for surgery to undergo resection of epileptic tissue.
Learning Objectives: By the conclusion of this session, participants should be able to: 1) Describe the importance of identifying the epileptic focus in nonlesional temporal lobe epilepsy, 2) Discuss, in small groups, the technique of depth electrode implantantion for localization of seizure focus, 3) Identify epilepsy patients for whom depth electrodes would be appropriate for surgical planning.