Abstracts

Rationale: Epileptic spikes are often associated with Blood-Oxygen-Level-Dependent (BOLD) signal changes in a network of brain regions, as measured by spike-related electroencephalography-functional magnetic resonance imaging (EEG-fMRI), potentially reflecting spike onset and propagation. We investigated whether spike sources, determined by magnetoencephalography (MEG) equivalent current dipole (ECD) modeling, spatially correspond to EEG-fMRI BOLD signal changes.Methods: Refractory focal epilepsy patients with a well-defined epileptogenic zone and similar epileptic spikes during EEG-fMRI and EEG-MEG were selected (N=3). Spike-related EEG-fMRI activation clusters (p uncorr<0.001) were determined within and outside the epileptogenic zone. MEG sequential ECD modeling (step size 1ms) was performed before (early dipoles from -25ms – peak) and after (late dipoles from peak – 45ms) the negative peak of 20 EEG-spikes (gof>80%, Cvol<1500 mm3, Q<500 nAm). Euclidian distances were calculated between EEG-fMRI activations and MEG dipoles.Results: The median distance between all MEG dipoles and the closest EEG-fMRI activations was 11 mm (range: 0-28). Early dipoles were significantly closer to EEG-fMRI activations within (mean 7 mm (standard error: 1.1)) compared to outside (mean 11 mm (standard error: 0.9)) the epileptogenic zone in all 3 patients (p<0.05). In two of the three patients, late dipoles were significantly closer (p<0.05) to EEG-fMRI activations outside (mean 12 mm (standard error: 0.9)) compared to inside (mean 17 mm (standard error: 1.7)) the epileptogenic zone.Conclusions: MEG-based electromagnetic and EEG-fMRI-based hemodynamic sources, related to epileptic spikes, showed a close spatial relationship. Early dipoles pointed to EEG-fMRI activations within the epileptogenic zone, reflecting spike onset, while late dipoles were closer to BOLD activations at a distance, interpreted as spike propagation. Both imaging modalities offer complementary information about the interictal spike-related network. Research funded by Belgian American Educational Foundation