Dipole localization for identification of neuronal generators in independent neighboring interictal EEG spike foci

Purpose: We evaluated dipole localizations of independent neighboring inter ictal spike foci using scalp electroencephalogram (EEG) to identify neurona l generators of epileptic discharges. Methods: Three pediatric patients with extratemporal lobe epilepsy who had two independent neighboring interictal spike foci on scalp EEG were studied . prolonged video EEG was digitally recorded from 19 scalp electrodes, whos e positions were registered using a three-dimensional digitizer. Interictal spikes were visually selected based on negative phase reversals on bipolar montages. We analyzed the dipole position and moment of each spike using a single moving dipole and three-shell spherical head model. The dipoles wer e overlaid onto magnetic resonance (MR) images and divided into two groups based on two spike foci. Results: The dipoles of the two groups were oriented either tangentially or radially to the scalp in close proximity to each other. The dipoles orient ed radially were located underneath the electrode with a negative peak; tho se oriented tangentially were between electrodes with a negative and positi ve peak. The positions of tangential dipoles were more concentrated than th ose of radial dipoles. The epileptogenic regions corresponded to the dipole localizations. Surgical excisions were performed based on the results of e lectrocorticography. After surgery, two patients were seizure free. and one had rare seizures (follow-up period, 13-31 months). Conclusions: We showed that dipoles in close proximity but with different o rientations projected two negative maxima on scalp EEG in three patients wi th extratemporal localization-related epilepsy. Equivalent current dipole a nalysis of individual interictal spikes can provide useful information abou t the epileptogenic zone in these patients.