Abstracts

Rationale: Epileptogenic zone localization is performed with scalp EEG recording, intracranial EEG(iEEG) and structural and functional imaging. MEG, with its temporal resolution, is a useful tool in presurgical evaluation. We evaluated MEG sensitivity in subsets of epilepsy patients, comparing MEG with MRI, iEEG, and histologically confirmed epileptogenic zone localization. Methods: 25 patients (15 males, age 13-47 years) with complete datasets were studied. 9 had mesial temporal sclerosis (MTS) defined by focal onset scalp ictal EEG findings and MTS on MRI. 8 had neocortical epilepsy (5 temporal, 3 extra-temporal) as defined by ictal EEG and structural MRI or FDG-PET abnormalities. 8 had ictal EEG suggestive of temporal lobe epilepsy (TLE) but normal MRI. Seventeen had iEEG and 3 intra-operative electrocorticography (ECog); 2 of these had non-localizing invasive monitoring. 23 went on to resective surgery. MEG data was acquired with CTF 275 channel system (digitization rate 600Hz, digitally filtered at 1-70Hz, minimum 40 minutes). Single equivalent current dipole (ECD) method was used to estimate the location of the signal source. Results: 22 patients (88%) had interictal MEG spikes recorded with reliable dipole fits. 2/3 without MEG spikes had MRI defined MTS; one had hippocampal asymmetry. 15/17 patients with well defined MRI abnormalities had MEG spikes; 11 co-localized with MRI lesions. 3/4 with discordant MEG and MRI findings had concordant MEG and iEEG, 1 had good and 2 had poor seizure outcomes following surgical resection. All 3 were extra-temporal cases. In 1 case, MEG was discordant (MEG left temporal, MRI and iEEG right temporal), with good outcome following right temporal resection. 8 patients had ambiguous or normal MRI; 6 had MEG spikes that co-localized with surface and invasive EEG; 1 patient had no MEG spikes. Overall, 14/18 patients showed concordance between MEG and iEEG or ECog. 18 patients had histopathologic lesions following resection (9 MTS, 3 cavernoma, 3 tumor, 2 traumatic gliosis, 1 hamartoma ). 16 had MEG spikes; 2 with MTS had none. Dipole sources for 15 with MEG spikes clustered within or close to the lesion. 1 with discordant findings had post-traumatic right frontal gliosis and right temporal MEG dipole with poor outcome. Conclusions: Sensitivity of MEG in our series was higher than previously reported possibly due to differences in patient population and analysis methods. As reported previously, MEG spike yield is higher in neocortical versus MTLE. In our series, MEG was useful in localization in MRI lesional and non-lesional patients and was validated in 94% of pathologically proven cases