Three-dimensional localization of subclinical ictal activity by magnetoencephalography: Correlation with invasive monitoring

BACKGROUND Although magnetoencephalography (MEG) provides accurate information on the spatial distribution and temporal patterns of the 'interictal' epileptic activities, it is interictal in nature and therefore also prone to all the problems associated with interictal data. METHODS We investigated the subclinical 'ictal' epileptic activity with a 37-channel, large-array biomagnetometer and mapped the data onto a three-dimensional image in a patient with intractable frontal lobe epilepsy. Dipole source localization was calculated based on magnetic fields for both the interictal and subclinical ictal activities. RESULTS The current dipoles of the interictal MEG spikes (MEG irritative zone) were revealed to be scattered in the left anterior frontal lobe, whereas that of the subclinical ictal onset (MEG subclinical ictal onset zone) was surrounded by the interictal dipole cluster. The dipole source localization of the propagating activities was not calculated with a single dipole model. The MEG subclinical ictal onset zone correlated well with the ictal onset zone subsequently recorded by invasive subdural electrophysiological monitoring. After multiple subpial transection of the deduced epileptogenic area, a dramatic reduction of the seizures occurred. CONCLUSION These results illustrate the potential of MEG for localizing the epileptogenic foci with high spatial and temporal resolution.