Simultaneous recording of epileptiform discharges by MEG and subdural electrodes in temporal lobe epilepsy.

Spontaneous epileptiform discharges were recorded by whole head magnetoencephalography (MEG) and subdural electrodes simultaneously from two patients with medically intractable temporal lobe epilepsy. In one patient whose epileptiform discharges emerged from the lateral temporal lobe, simultaneously recorded MEG could estimate equivalent current dipole reliably near the tumor. The amplitude of the dipole was in proportion not only to the amplitude of epileptiform discharge but also to the number of subdural electrodes involved. In the other patient, MEG detected only a small proportion of epileptiform discharges, even when they were recorded by subdural electrodes from the mesial temporal lobe. It is concluded that the amplitude and the depth of epileptiform discharges would largely affect the sensitivity of dipole localization by MEG.