MEG in epilepsy and pre-surgical functional mapping

Localization of epileptic discharges and pre-surgical functional brain mapping are the most common clinical applications of magnetoencephalography (MEG). According to the European Union-funded EPILEPSY project survey, performed in 2014, MEG source localization was used as a part of the presurgical diagnostic workup in 7 out of 25 centers (28%) (Mouthaan et al. (2016) Epilepsia 57:770-776) indicating that the majority of "MEG centers" provide clinical services (Bagic et al. (2009) J Clin Neurophysiol 26:290-293). MEG is also utilized for pre-surgical functional brain mapping, that is, for accurate localization of "eloquent" cortex, used for planning surgical procedures near healthy functional brain areas. For example, somatosensory evoked fields to median nerve stimulation lead to an accurate, within a few millimeters, identification of the central sulcus, which may not be identifiable in anatomical MRI alone. In addition, MEG analysis of event-related fields or event-related (de)synchronization in response to language tasks provides more than 80% sensitivity and specificity in language lateralization compared to intracarotid amobarbital procedures. Therefore, MEG is a noninvasive alternative for presurgical determination of the language-dominant hemisphere. In this chapter, the current status of clinical MEG in epilepsy and pre-surgical mapping is reviewed.