MEG source localization in extratemporal epilepsy

Although patients suffering from medically intractable epilepsies originating from other than the temporal lobe are less numerbale than those with temporal lobe epilepsies (TLE), localization of the epileptogenic region in extratemporal epilepsies (ETE) is considered as much more demanding than in TLE [1, 2]. Even if seizure semiology may hint at the origin in specific parts of the brain [3-7], non pathologic imaging data and propagational features with fast spreading of epileptic activity to and from frontal areas may impede identification of the epileptogenic region. In addition to localization of focal epileptic activity, preoperative evaluation of ETE surgery candidates requires delineatin of sensory, motor, and speech related areas and their mutual relationships with the epileptogenic zone. In order to achieve this difficult localizational task, invasive recording from intracranial electrodes is often required, adding considerable strain and risk to presurgical evaluation. Therefore, the introduction of magnetoencephalography (MEG) as a noninvasive electrophysiological method with promising localizing features [8-13] offered a welcome new dimension to epileptic focus localization. The growing sophistication of analysing tools such as spatiotemporal analyses [14, 15], multiple source estimates [16, 17], and detailed evaluation of propagational features [14] has had its share in the improvement of MEG’s clinical relevance in the ETE field, as well as hardware developments. We present examples of MEG localizations in ETE patients to illustrate some aspects of MEG contribution to successful presurgical evaluation in this challenging field.