Abstracts

Rationale: Magnetoencephalography (MEG) is employed by an estimated twenty percent of epilepsy surgery programs worldwide for interictal epileptogenic source localization, however it remains an expensive and a controversial modality, unvalidated due to limited available data, especially in pediatric surgical epilepsy patients. This study retrospectively examines a large series of pediatric patients, who all had had intracranial monitoring and resection, in whom MEG was used to aid decision-making as to the feasibility of surgery, and to guide intracranial implant placement.Methods: Retrospective review of reports and notes in the electronic medical record (EMR) of 116 patients resulted in a study database of 60 cases, after the exclusion criteria were applied. EMR charts were analyzed by an epileptologist. Localization data for the available modalities - long-term scalp video-EEG montiroring (LTM), imaging (MRI), intracranial EEG recordings (iEEG) ictal and interictal data, and intraoperative monitoring (IOM) data, in a few cases where it was both available and definitively localizing, and MEG - were reviewed and summarized into categories - (0) no localization achieved (1) a hemispheric lateralization achieved (2) a multilobar hemispheric localization achieved (3) a lobar localization achieved. Concordance in ictal source localization between the modalities was defined as - (0) no concordance (1) concordant lateralization (2) concordant regional localization (3) concordant lobar localization. The records were also stratified by surgery location, and by the presence or absence of MRI abnormalities.Results: 1. LTM localization was strongly inferior to either iEEG or MEG localization, while iEEG and MEG localization were not statistically different 2. iEEG and MEG lobar localization was highly concordant with p=0.002 3. None of the localizing modalities alone showed a powerful correlation with outcome, using the Fisher exact test. It is concluded that p-values were unacceptably high to make any conclusions about the connection between localization modality agreement and outcomes.Conclusions: There are two endpoints in this analysis - seizure localization by various modalities, and localization vs postsurgical outcome. Multiple previous studies have shown that successful ictal localization and the clinical outcome are closely correlated. However we found this connection equivocal. With respect to successful localization, iEEG is the golden standard, and this study shows consistently the concordance of iEEG, a perisurgical invasive modality, and MEG, a presurgical noninvasive one, as demonstrated statistically. These findings, given a rather large sample size, allow us to claim with some caveats - that MEG is indeed a valid and valuable pre-surgical evaluation tool in pediatric patients with intractable epilepsy, and ought to be used for lateralization and lobar localization prior to implant placement, especially in the patients in whom scalp EEG (LTM) findings were equivocal.