Abstracts

Rationale: Magnetoencephalography (MEG) is a useful diagnostic tool for presurgical evaluation of medically intractable epilepsy. MEG has increasingly been applied for pediatric epilepsy. MEG source localization has improved the likelihood of seizure onset zone (SOZ) sampling for presurgical evaluation with intracranial EEG (ICEEG). There are several challenges in localization or even lateralization of SOZ with scalp-EEG recordings in the pediatric population: 1) extratemporal epilepsy is more common than temporal lobe epilepsy; 2) etiology of epilepsy may be genetic or other disorders of brain development i.e., cortical dysplasia. This study was designed to investigate whether the ictal MEG onset source localization is superior to interictal MEG source localization to define the SOZ more precisely.Methods: We identified 20 patients diagnosed with intractable epilepsy who had both interictal discharges and at least one of their habitual seizures during MEG recordings between October 2007 and April 2011. Patients were then screened for additional inclusion criteria: completed non-invasive presurgical evaluation, underwent surgical resection, and had a post-surgical follow up period of at least 6 months. MEG was recorded with sampling rates of 600 Hz and 4000 Hz for 10 and 2 minutes respectively, for total of at least 40 minutes. Scalp-EEG was also simultaneously recorded with the same settings. Continuous head localization (CHL) was applied. The source localization analyses were applied to the MEG signals, both at the beginning of ictal onset and interictal MEG discharges using multiple algorithms, i.e., ECD, MUSIC, sLORETA, SAM(gnull2null). The ictal MEG onsets were defined with both visual inspection and power spectrum using FFT. All MEG source localizations were compared with the onset and propagation of ICEEG as well as surgical outcome.Results: Nine patients met all inclusion criteria. The comparison of source localization between ictal MEG onset and interictal MEG discharge onset was made. Five of the 9 patients (56 %) had ictal MEG onset source localization and interictal MEG discharge source localizations in the same lobe and same hemisphere with a little different localization. The source of ictal MEG onset was closer to the SOZ defined by ICEEG recordings. The other 4 patients (44 %) had bilateral independent interictal MEG discharges. In 3 of them, the discharge lateralization ratios were higher in the same hemisphere where the ictal MEG onset source was localized. The surgical outcome was excellent (Engel class I: 5, II: 1) in the patients with complete excision of the ictal MEG onset.Conclusions: Although the capture of seizures during MEG recording is challenging, the source localization for ictal MEG onset proved to be a useful tool for presurgical evaluation in patients with medically intractable epilepsy, especially in pediatrics, where seizures are frequently not localized or lateralized with other methodologies, including interictal MEG source localizations.