Lobar localization information in epilepsy patients: MEG--a useful tool in routine presurgical diagnosis.

Epilepsy surgery is an established therapy for pharmacoresistant focal epilepsy. This study investigated the contribution of routinely used magnetoencepahlography (MEG) in addition to long term video-EEG-monitoring in presurgical evaluation. The distribution of localization results to anatomical lobes was compared with special focus to MEG spike localization results in cases without or with ambiguous EEG findings. A total of 105 consecutive patients with intractable focal epilepsy and epilepsy surgery after investigation by video-EEG-monitoring and MEG were included. The percentages of monolobar results were analysed and compared, especially with respect to the resection lobe. Postoperative outcome was used for further validation. No spikes were recorded on MEG in 30% (32 of 105). In cases with a diagnostic finding by the respective method, MEG localized in 82% (60 of 73 patients) within one anatomical lobe. Ictal EEG localized within one lobe in 72% (66 of 92 patients), interictal EEG in 60% (59 of 98 patients). In 25 of 105 patients (24%) no clear localization within one lobe was found either in interictal or in ictal EEG. In 11 of these cases MEG localized within the resection lobe. Six patients of these became seizure free, the other five had at least 50% reduction of their seizure rate 1 year after surgery. In summary MEG is a useful tool in the routine workup for epilepsy surgery contributing information to focus hypothesis in addition to video-EEG.

Spatial relationship of source localizations in patients with focal epilepsy: Comparison of MEG and EEG with a three spherical shells and a boundary element volume conductor model.

Epilepsy surgery is an option for patients with pharmacoresistant focal epilepsies, but it requires a precise focus localization procedure. Magnetoencephalography (MEG) and electroencephalography (EEG) can be used for analysis of interictal activity. The aim of this prospective study was to compare clusters of source localization results with MEG and EEG using a three spherical shells (3SS) and a boundary element method (BEM) volume conductor model. The study was closed when 100 patients met the inclusion criteria. Simultaneous MEG and EEG were recorded during presurgical evaluation. Epileptiform signals were analyzed using an equivalent current dipole model. Centroids of source localizations from MEG, EEG, 3SS, and BEM in their respective combinations were compared. In a 3SS model, MEG source localizations were 5.6 mm inferior to those obtained by EEG, while in a BEM model MEG source localizations were 6.3 mm anterior and 4.8 mm superior. The mean scattering of source localizations between both volume conductor models was 19.5 mm for EEG and 9.6 mm for MEG. For MEG no systematic difference between BEM and 3SS source localizations was found. For EEG, source localizations with BEM were 5.9 mm posterior and 11.7 mm inferior to those determined using 3SS. No differences were found between the 46 temporal and the 54 extratemporal lobe epilepsy patients. The observed systematic differences of source localizations of epileptic spikes due to the applied source signal modality and volume conductor model should be considered in presurgical evaluation when only one source signal and volume conductor model is available.