Variability in the spatiotemporal propagation of interictal spikes on magnetoencephalography in temporal lobe epilepsy

Magnetoencephalography (MEG) is clinically used to help localize interictal spikes in discrete brain areas through the equivalent current dipole (ECD) method for patients with refractory epilepsy. The propagation of interictal spikes in adjacent regions was shown to be correlated with worse surgical outcomes in patients with temporal lobe epilepsy (TLE). Yet, the ECD method does not account for the temporal dynamics of spike activity, making it challenging to define the margins of the interictal network. Furthermore, the ECD results can be affected by states of arousal or sleep that alter background MEG activity. In this study, we developed a module in the custom-built Multi-Modal Analysis and Visualization Tool (MMVT) that analyzes the spatiotemporal dynamics of interictal spikes and removes normal variations associated with sleep. We analyzed MEG data from seven TLE patients (median age 32 years) and characterized their interictal spatiotemporal dynamics. In all patients, the onset of interictal spike activity appeared near the determined site of the epileptogenic zone based on corresponding neuroimaging and EEG data. The propagated source activity by 10 ms ranged from near the region of onset to spreading to extratemporal regions, such as the frontal lobe or insula. We demonstrated the feasibility of a novel method to assess the spatiotemporal properties of interictal spikes in patients with medically refractory TLE and to better delineate their onset and propagation patterns on MEG analysis. In doing so, we illustrate how variable the spike propagation distribution and speed may be within a 10 ms window. This visualization of the time component of the interictal networks offers an advantage over the currently used ECD method in characterizing the epileptic network and may help better guide surgical planning and provide for improved interventional outcomes in patients with TLE.