Abstracts

Rationale: Mesial temporal lobe epilepsy (MTLE) is often medically refractory but can be surgically amenable. Presurgical evaluation with extracranial recordings is not always sufficient to localize a seizure focus. Intracranial evaluation is particularly challenging in patients with normal MRI. Foramen ovale electrodes (FOEs) are less invasive than conventional intracranial electrodes. Added to scalp electrodes, FOEs provide a window into mesial temporal activity. They may enable patients with equivocal scalp recordings and no MRI evidence of mesial temporal sclerosis (MTS) to bypass further intracranial evaluation and undergo surgery. FOEs can also reveal bilateral independent seizure onsets not suspected on scalp recordings. This study compares the ictal FOE recordings of MTS and nonlesional surgery patients. Seizure-free MTS patients served as the gold standard for mesial temporal patterns of seizure onset and propagation against which patterns of nonlesional patients were compared. Methods: We retrospectively reviewed 118 patients with refractory complex partial seizures and suspected MTLE evaluated with 3 to 5 contact FOEs at UCSD. Two groups were selected for further analysis: patients with 1) no MRI evidence of lesion (n=38) and 2) MRI evidence of MTS who remained seizure-free (Engel Class I) after temporal resection (n=23). Follow up was at least 1 year (range 1-11 yrs, mean 5 yrs). FOE-recorded seizures were analyzed for discharge pattern at onset, propagation, duration at epileptogenic foci, and seizure duration. Results: Of 38 nonlesional patients, FOE data excluded 23 from consideration of mesial temporal resections (bilateral seizure onsets in 13, onset not mesial temporal in 10). Of 15 patients determined to be surgical candidates by FOE recordings, 2 declined surgery and 13 underwent tailored temporal lobectomies, 11 of which were included in the analysis (data from 2 patients was unavailable). No consistent differences in electrographic patterns at onset, propagation, duration at epileptogenic foci, and seizure duration, were observed between nonlesional surgery patients and the seizure-free MTS group. Outcome of nonlesional patients was Class I in 10 patients and Class II in 1 patient. Conclusions: With minimal invasiveness, presurgical evaluation with FOEs provides valuable localizing information in nonlesional patients with suspected MTLE. The utility is three-fold: patients with mesial temporal seizure onsets are identified as surgical candidates, patients with bilateral independent onsets are excluded from surgery, obviating further invasive evaluation, and patients with non-mesial temporal onsets (lateral temporal or extratemporal) are selected for further evaluation. FOE recordings of nonlesional patients identified as surgical candidates reveal seizure patterns and propagation similar to those of MTS patients. Their good surgical outcomes demonstrate that surgery succeeds in selected nonlesional patients and evaluation with FOEs is a viable alternative to conventional invasive recordings.