Abstracts

Rationale: Post-operative seizure outcomes in drug-resistant epilepsy (DRE) are more favorable in patients with lesions on MRI compared to non-lesional patients. Both mesial-temporal seizure onsets and accurate localization of the epileptogenic network using intracranial EEG (iEEG) are associated with better surgical outcomes. A substantial proportion of patients with DRE with clinical and video-EEG features of temporal lobe epilepsy (TLE) have no MRI evidence of lesions. We sought to determine surgical outcomes and intracranial EEG (iEEG) findings in this patient population. Methods: We identified 51 patients who underwent evaluations for epilepsy surgery between 2002 to 2018 at the Medical College of Wisconsin who met the following criteria: (a) No lesional basis (including hippocampal atrophy) for epilepsy on MRI, (b) Seizure semiology and/or scalp video EEG findings suggestive of TLE, (c) Completed an iEEG study. We reviewed iEEG findings in these patients and their relationship to seizure outcomes following surgery. Outcomes reported here are based on the last follow-up (mean = 6.12 years). Results: Seizure onset zones (SOZs): Ictal data were captured during iEEG in 48/51 patients (94%), while no seizures were captured in 3 patients. SOZs identified by iEEG involved the anteromesial-temporal regions in 22/48 (45.8%) patients (13 mesial-temporal and 9 anterior-temporal/temporal pole). SOZs included the mid to posterior temporal neocortex in 4/48 (8.3%) patients, were bi-temporal in 4/48 (8.3%), temporal-plus-extratemporal in 9/48 (18.8%), and extratemporal in 9/48 (18.8%) patients. Seizure outcomes: Forty patients (78%) had resections. Of these, 33/40 (82.5%) had Engel I outcomes, 4/40 (10%) had class II, 1/40 (2.5%) had class III, and 2/40 (5%) had class IV outcomes. Resections were not pursued in 11 patients because of either overlap of SOZs with functional cortices (3), bitemporal onset seizures (4), poor localization on iEEG (2), or extensive multilobar epileptogenic networks (2). Seizure outcomes in relation to SOZs: All patients with unilateral SOZs within the anterior temporal neocortex (~ 3 cm from temporal pole) or in mesial temporal regions underwent resections. In this group, Engel class I outcomes were observed in 18/22 (81.9%), class II in 3/22 (13.6%), and class IV in 1/22 (4.5%) patients. In patients with SOZs outside the antero-mesial temporal regions, 18/26 (69%) underwent resections. In this group, Engel class I outcomes were reported in 15/18 (83.32%), class II in 1/18 (5.56%), class III in 1/18 (5.56%), and class IV in 1/18 (5.56%) patients. There was no significant difference in seizure outcomes between these two groups. However, patients who had SOZs outside the antero-mesial temporal regions were significantly less likely to proceed to surgery (p=005). Conclusions: Over one-third of non-lesional DRE patients with clinical and video-EEG features of TLE had SOZs that were outside the standard anterior temporal lobectomy extent (i.e., mid-to-posterior temporal, temporal-plus-extratemporal, or extratemporal) based on ictal iEEG recordings. In this sub-group of patients, when findings from iEEG studies are used to stratify risk-benefit and guide patient selection for surgery, seizure outcomes are comparable to those in patients with antero-mesial temporal SOZs. Our results caution against proceeding to ablative procedures or responsive neurostimulation (RNS) therapy in this patient population without an iEEG study to verify SOZs. Funding: No funding