Abstracts

Rationale: Magnetoencephalographic (MEG) data has been used in determining the location of epileptic foci in patients with demonstrable lesion-related partial epilepsies. A single cluster of equivalent current dipole (ECD) correlated highly with ictal onset zone on intracranial EEG. On the other hand, epileptogenic localization is less elucidated in patients with scattered ECD and/or MRI-negative epilepsy. Vagus nerve stimulation has been beneficial to such patients considered unsuitable candidates for resective surgery. Recent study showed some patients who underwent prior VNS for intractable epilepsy, some of whom with lack of response to VNS underwent subsequent intracranial epilepsy surgery via the further evaluation of epileptogenic localization. This study evaluated (1) the feasibility of magnetoencephalography (MEG) in vagus nerve stimulation (VNS) and (2) the dynamic changes of epileptic spikes using equivalent current dipole (ECD and gradient magnetic-field topography (GMFT) before and after VNS.Methods: Seven patients were studied by MEG using the MaxFilter (Elekta-Neuromag) for elimination of magnetic noise during VNS. MEG was performed with 306-channel whole-head type system which included 204 channels of planar gradiometers. We classified distributions of ECDs into clusters and scatters as previously reported (Iida K et al., 2005). We have recently developed GMFT for MEG to visualize the dynamic change of gradient magnetic fields for interictal epileptic discharges (Hashizume A et al., 2007). GMFT evaluated predominant distributions (anterior, A/ posterior, P) and then spreading patterns of the hemisphere(s) (unilateral, U/ bilateral, B) for pre-and post-VNS MEG spikes.Results: In all 7 patients, interpretable MEG data were obtained using the MaxFilter. Before VNS, all patients had multiple ECD clusters or scatters in the bilateral hemispheres. MEG after VNS revealed the similar distributions of the ECDs in 6 patients and a single cluster in 1 patient. The number of total MEG spikes decreased without statistical significance (p=0.16) after VNS. GMFT showed the proportion (%) of AB spikes was not markedly changed but that of AU spikes was significantly increased after VNS. The proportions of PB and PU spikes were significantly decreased after VNS.Conclusions: This study demonstrates the feasibility of MEG in VNS patients. VNS produced dynamic changes in epileptic spikes which were revealed by GMFT analysis.