Abstracts

Rationale: Magnetoencephalography (MEG) has been increasingly widely used as a pre-surgical evaluation tool in patients with intractable epilepsy. The Epileptogenic zone (EZ) is defined as the area responsible for initiating the seizure, and the zone which once resected or disconnected will most likely produce seizure freedom. Although MEG most often records only interictal epileptic activity, seizures are sometimes captured during MEG recording. In this study, we looked at the correlation between ictal MEG and ICEEG in patients who subsequently had epilepsy surgery to elucidate the relationship between EZ, ICEEG and ictal MEG findings. Methods: A consecutive series of 26 cases (from 25 patients) with ictal MEG localizable by Single Equivalent Current Dipole (SECD) model and ICEEG using stereo-electroencephalography (SEEG) electrodes (n=16) or subdural/depth electrodes (n=10) followed by epilepsy surgery at Cleveland Clinic Epilepsy Center from 2008-2017 were included in this study. We analyzed the ictal data for the initial 5 seconds from the beginning of the EEG/MEG seizure onset. The localization between ictal (n=26) & interictal (n=23) MEG dipoles, ictal ICEEG onset zone, and the area of resection or laser ablation was compared. Seizure outcomes at last available follow-up (minimum 6 months) was categorized into seizure free (Engel class Ia) or not seizure-free (Ib-IVc). We used chi-square test (or Fisher’s exact test when n<5) to assess the relationship of nominal parameters and seizure outcomes. T-test and Wilcoxon rank sum tests were used to compare the association of continuous variables. Differences were considered significant when P<0.03. Results: Median age at time of ICEEG was 24 (8-56) years, and median duration of epilepsy was 8.5 (1-34) years. 10 patients (40%) were female. 12 cases (48%) had MRI lesions. 21 cases (81%) were diagnosed as extra-temporal lobe epilepsy. Seizure frequency was daily in 14 (54%), weekly in 5 (19%), and monthly in 7 (27%) cases. The surgical outcomes were Engel class I in 10 (38%), II in 3 (12%), III in 4 (15%), and IV in 9 (35%) cases. 8 (31%) patients achieved seizure freedom (Ia) after surgery. Median duration of follow-up was 22 (6-100) months. We found that patients in whom the ictal (P=0.0067) and interictal (P=0.0048) MEG dipoles were completely resected had a significantly higher chance to achieve seizure freedom as compared to the partial or no resection groups respectively. Patients who showed sublobar concordance in location between ictal MEG dipoles and ictal ICEEG onset zone were more likely to become seizure-free compared to patients with less precise concordance (P=0.0039). For prediction of seizure outcome, interictal dipoles were just as good as ictal dipoles (P=0.0402). Conclusions: Our data demonstrates that ICEEG exploration and subsequent epilepsy surgery are more likely to succeed, when guided by the ictal and interictal dipoles. Patients who show concordant localization between the ictal dipoles and ictal ICEEG location have a higher chance to achieve seizure freedom. Our study shows that ictal SECD dipoles have superior value compared to the interictal ones in term of identifying the EZ. Funding: None