Magnetoencephalography in focal epilepsy.

The introduction of whole-head magnetoencephalographic (MEG) systems facilitating simultaneous recording from the entire brain surface has led to a major breakthrough in the MEG evaluation of epilepsy patients. MEG localizations estimates of the interictal spike zone showed excellent agreement with invasive electrical recordings and were useful to clarify the spatial relationship of the irritative zone and structural lesions. MEG appears to be especially useful for study of patients with neocortical epilepsy, and helped to guide the placement of subdural grid electrodes in patients with nonlesional epilepsies. MEG could differentiate between patients with mesial and lateral temporal seizure onset. Spike propagation in the temporal lobe and the spatio-temporal organization of the interictal spike complex could be studied noninvasively. MEG was useful to delineate essential brain regions before surgical procedures adjacent to the central fissure. MEG appears to be more sensitive than scalp EEG for detection of epileptic discharges arising from the lateral neocortex, whereas only highly synchronized discharges arising from mesial temporal structures could be recorded. A major limitation of MEG has been the recording of seizures because long-term recordings cannot be performed on a routine basis with the available technology. Because MEG and EEG yield both complementary and confirmatory information, combined MEG-EEG recordings in conjunction with advanced source modeling techniques should improve the noninvasive evaluation of epilepsy patients and further reduce the need for invasive procedures.