Magnetoencephalography (MEG) and other neurophysiological investigations

Cortical generators of epileptic and certain physiological activity can be localized noninvasively by magnetoencephalography (MEG). MEG detects weak magnetic fields produced by the postsynaptic currents of pyramidal cortical cells in sulcal walls. Unlike EEG, MEG signals are not distorted by edema or bone defects, and unlike fMRI, abnormal hemodynamics do not alter the MEG. The patient's head is centered inside a helmet housing over a hundred magnetic field sensors. Cortical generators of MEG signals are determined with a useful spatial resolution and an excellent time resolution, which enable tracking of brain activity in successive points of, for example, an epileptic network. MEG sources can be co-registered and visualized on magnetic resonance images (MRI). MEG is highly sensitive for the detection of interictal epileptic discharges, and present techniques allow some degree of head movements enabling ictal recordings also. MEG is also useful for localizing the somatosensory, visual, and language areas before tailored surgery in the vicinity of eloquent cortex. In conjunction with other noninvasive modalities MEG provides nonredundant data in one-third of epilepsy surgery patients. Clinical MEG utilization is mainly focused on presurgical localization of the epileptogenic zone and eloquent cortex in epilepsy surgery candidates, including patients with Landau-Kleffner syndrome. However, MEG is also an excellent noninvasive tool to study the source distribution in childhood epilepsy syndromes and epileptic encephalopathies.