Functional imaging: New approaches based on bioelectric measurements in focal epilepsy

Electrophysiological measurements (EEG, MEG) provide unique information due to high temporal resolution compared to other functional imaging methods.

While MEG is usually limited to interictal recordings (irritative zone), (long-term) surface-EEG allows inferences on the epileptogenic (seizure generating) zone by additional ictal measurements. However, MEG has a proven value in the presurgical evaluation.

Different approaches of inverse analysis (dipole or multiple source analysis, distributed source models) are applied to the hypothesis on the center of an activated cortical area. Anatomical considerations result from superposition of individual 3D-MRI. However, no inverse approach can be used as a trustful estimate of the extent of the activated area.

EEG source analysis depends markedly on the head-model applied. The estimation of conductivity values characterizing tissues within the volume conductor, e.g. bone, is based on rough values in adults and may not apply to children. Magnetoencephalographic (MEG) analysis is less influenced by head models.

MEG signals are generated exclusively by sources with tangential orientation (e.g. fissural pyramidal neurons), while EEG signals are dominated by radial activities. This clearly favors combined EEG and MEG recordings to gain complementary information.

Interictal spike analysis is influenced by the signal-to-noise ratio (SNR). The scatter of single spike dipoles may reflect either influence of background activity or different maxima within an irritative zone. Averaging similar spikes improves SNR and allows the analysis of onset activity. We introduce a new approach by displaying scatterplots of dipoles modeling multiple, re-averaged subsets of “similar“ single spikes.

In addition to the identification of epileptic foci, EEG and MEG are applicable to the identification of eloquent areas by analyzing event related activities (sensory-motor, motor, visual and auditive system).

Non-invasive EEG and MEG provides useful clinical information on physiological and abnormal cortical activities in childhood epilepsy.