Abstracts

Rationale: In recent years, the invasive electroencephalography (EEG) monitoring approach to delineate the seizure onset zone (SOZ) using minimal invasive stereotactic-EEG (sEEG) has been preferred over traditional monitoring using surface grid electrodes following a large craniotomy. The challenges of pursuing sEEG include the decision of electrode placement. Electrode coverage must be based on anatomical, electrophysiological and/or clinical semiological evidence during non-invasive testing. Magnetoencephalography (MEG) is one of the non-invasive physiological data that can provide excellent temporal and spatial resolution. One promising technique to better define the epileptic network is functional connectivity, which can be used to identify complex networks of interconnected brain regions. The aim of this study was to test our hypothesis that patients who underwent sEEG monitoring show abnormal pre-surgical MEG functional connectivity, defining the SOZ and able to demonstrate the propagation of seizures. Methods: MEG recordings as part of the pre-surgical evaluation were used for functional connectivity analysis. MEG effective connectivity at sEEG locations was performed using the following steps. First, post-operative CT scans with sEEG positions was co-registered to the pre-surgical MRI. The x,y,z coordinates for each sEEG electrode were extracted and these positions was used define functional nodes in the network. These nodes were then used as virtual sensor (VS) locations to compute the spatial filters using a linearly constrained minimum variance (LCMV) beamformer analysis of the patient’s pre-surgical MEG. Effective connectivity was estimated using phase slope index (PSI). The directed PSI value was used to determine connectivity patterns and the nodes which were maximal drivers and receivers of information. PSI value at each sEEG contact was retrospectively compared with the SOZ identified by invasive sEEG monitoring to determine spatial concordance. Results: An 18 year old female underwent MEG recording as part of presurgical evaluation. The clinical semiology started with a sudden sensation of eyes ‘jiggling’ with witnessed fast phase nystagmus. She sometimes complained of an out of body experience, such as 'she is sitting in her room but sees herself in the kitchen’. MEG functional connectivity revealed the highest degree nodes (2 electrodes in left cingulate) which were adjacent to the SOZ revealed by sEEG. These nodes showed nonlinear interactions between the electrodes within Wernicke’s area. The patient was seizure free for 10 months then discontinued seizure medications by herself. Seizures recurred and she is undergoing another evaluation with sEEG.  Conclusions: Our methods show the potential usefulness of predicting SOZ using pres-surgical MEG functional connectivity, and ensuring that the highest degree of nodes are included in the resection area. Moreover, MEG-VS functional connectivity and whole brain network analysis may be able help delineate the SOZ even prior to the surgical intervention. Funding: N/A