Interpretation of MEG spike source localization in frontal lobe epilepsy with multiple independent spike foci

Magnetic source imaging using a whole-head MEG system provides a more accurate localization of epileptic focus than other routinely used noninvasive methods such as scalp video EEG and magnetic resonance imaging (MRI) [1-3]. However, MEG source localization, as estimated by the single dipole and spherical model, may not fully describe an epileptic region that includes extensive or multiple epileptic spike sources [4]. To localize the epileptogenic zone (EGZ), careful interpretation of MEG spike sources (MEG-SSs) is needed, particularly for patients with multiple independent spike foci (MISF). We report on three patients with MISF, revealed by their EEG and MEG, who underwent surgery for frontal lobe epilepsy and became seizure free for follow-up periods of 5-28 months. MEG findings were compared with the results of MRI, FDG, PET, intracranial invasive video EEG recording (IVEEG) using subdural grid electrodes, and surgery. We discuss here: (1) whether MEG-SSs can delineate the EGZ in a case with MISF, (2) whether MEGSSs can distinguish between the EGZ and the irritative zone (IZ).