Abstracts

Rationale: Recent data suggests that scalp EEG monitoring may be sufficient for presurgical ictal localization in patients with intractable epilepsy and with mesial temporal sclerosis (MTS) on MRI. However the MTS region may not be the sole epileptogenic region. EEG evaluation with foramen ovale electrodes (FOEs) provides a minimally invasive and more sensitive way to record ictal discharges from mesial temporal structures. FOE monitoring can also eliminate the need for more invasive forms of intracranial EEG recording. In this study, we reviewed consecutive cases of presurgical video EEG patients undergoing FOE monitoring and compared the conclusions after phase 1 (scalp) and phase II (scalp and FOE) video EEG. Methods: Fifty patients that underwent evaluation with FOEs were retrospectively reviewed. The cases were subdivided into three groups: Group A - MTS established by MRI (n=25), Group B - potentially epileptogenic brain lesions other than MTS (n=13), and Group C - no MRI lesions (n= 12). We quantified the number of cases where discordant findings were discovered by FOE monitoring . Discordance was defined as seizure onset from regions other than the area of structural brain lesion. In non lesional patients, discordance was defined as FOE seizure onset differing from seizure onset suggested by clinical and electrographic features during scalp EEG monitoring. Surgical outcome was categorized using Engel s Classification of Postoperative Outcome with Class1a and 1b considered a good surgical outcome. Results: Six of the 25 patients with MTS (Group A) showed seizure onset localization on FOE monitoring that were discordant with MRI findings. The group included cases with bitemporal, extratemporal, and multifocal ictal onsets. All 13 of the 19 concordant MTS patients had tailored temporal lobectomies with good surgical outcomes. Five of the 13 patients with other brain lesions (Group B) had findings on FOE monitoring that were discordant with expected ictal localization. Of the 8 concordant patients, 6 went to surgery and 4 had good surgical outcomes. Finally, 1 of the 12 patients with nonlesional epilepsy (Group C) had discordant findings consisting of bitemporal seizure onsets. Conclusions: EEG monitoring with FOEs provides a minimally invasive method of recording ictal discharges from mesial temporal structures. These data suggest that up to 24% of patients with MTS can have seizure onsets from areas other than the region of MRI abnormality and that these onsets can be detected by FOEs. 19 of the total 21 patients receiving FOE monitoring and epilepsy surgery had good surgical outcomes in this series. Nonlesional patients in this group were not always good surgical candidates for other reasons. FOE monitoring can also be useful in patients with seizure semiology suggestive of mesial temporal onset but with other potentially epileptogenic brain lesions. It may demonstrate mesial temporal onset and rule out the need for more invasive EEG monitoring. FOE monitoring is a helpful, minimally invasive tool in the surgical evaluation of patients with and without MTS on brain MRI.