Abstracts

Rationale: Magnetoencephalography (MEG) has become a valuable tool in the presurgical evaluation of patients with refractory partial epilepsy, especially in patients who do not display focal epileptogenic lesion on the brain MRI. These patients often require intracranial electroencephalographic (ICEEG) monitoring utilizing either subdural or depth electrodes. In this study, we aim to analyze the interictal and ictal findings on pre-operative MEG and compare to the ICEEG recordings, and determine the role of MEG in the seizure outcome. Methods: 14 consecutive children (age 4-17 years) with histopathologically proven FCD underwent preoperative MEG and subsequent ICEEG evaluation (2008- 2012). Chronic invasive ICEEG monitoring was performed using stereoelectroencephalography (SEEG) or a combination of subdural and depth electrodes. Ictal and interictal findings were analyzed on the ICEEG data. MEG clusters were co-registered (MRILab and CURRY softwares) to the pre-operative and post-op brain MRI as well to the ICEEG electrodes (high resolution CT). A cluster was defined in this study as at least 5 dipoles separated by less than 1 cm. All patients underwent resective epilepsy surgery, had FCD (ascertained per ILAE classification by Neuropathologist) and had at least one year follow up after resection. Seizure outcome was classified using ILAE Classification. Results: Of 14 patients, 11 had MEG clusters and were analyzed further. 3 patients were excluded (1 with scatter and 2 with single spikes). 4 patients (36%) were MRI negative. Age at surgery range from 10 years to 17 years. 4 patients underwent subdural grid implantation, and 7 SEEG. The implantation was bilateral in 5 patients. In all patients the primary irritative zone identified with MEG was concordant with the ictal onset zone as defied by ICEEG recordings. The primary irritative zone identified with MEG was concordant with the irritative zone defined by ICEEG recordings in 10 of the 11 patients analyzed in this study. Localization of the surgery was: frontal (4 patients-36%), fronto-temporal (4 patients-36%), temporal (2 patients-18%), parietal (1 patient- 9%). Pathology revealed FCD type II in 6 patients (54%) and FCD type I in 5 patients (46%). Of 5 patients (46%) that were seizure free (type 1), 4 had lesion on MRI and 2 had ictal MEG recordings concordant with ICEEG data. However, of 6 patients that were not seizure free, 3 had a negative MRI. Conclusions: A localizing MEG study increases the chances that the epileptogenic zone will be sampled with intracranial electrodes and should guide the strategy of electrode implantation. Importantly the presence of concordance between ictal MEG and subsequent ICEEG is associated with a higher probability of favorable surgical outcome following resective epilepsy surgery in this cohort of pediatric patients with FCD and refractory focal epilepsy.