Abstracts

Rationale: Magnetoencephalography (MEG) provides useful and non-redundant information in the evaluation of patients with epilepsy, and in particular in the pre-surgical evaluation of medically intractable epilepsy. Vagus nerve stimulation (VNS) is a common treatment for medically intractable epilepsy. Interpretation of MEG recordings from patients with a VNS is difficult due to the severe magnetic artifacts produced by the VNS. We present a series of eight patients with a VNS who underwent MEG recordings. While the raw MEG recordings were uninterpretable, analysis of the recordings with SAM(g2) identified foci of peak kurtosis and source signal activity that was unaffected by the VNS artifact. This case series demonstrates that SAM(g2) analysis of MEG recordings in patients with a VNS produces interpretable results and expands the use of MEG for the pre-surgical evaluation of epilepsy. Methods: All patients were identified by a search of computerized records at Wake Forest Baptist Medical Center (IRB 15854). Selection criteria included any patient with a VNS who had a MEG recording, covering the dates from March 2006 through February 2012. All patients underwent an MEG recording in the course of an evaluation of their epilepsy. MEG recordings were performed in a CTF Systems Inc. 2005 whole-head MEG system containing 275 first-order axial gradiometers and 29 reference magnetometers. Simultaneous EEG was recorded. MEG recordings co-registered with the patient's own MRI. The total recording duration for each subject was about 40-48 minutes. Results: Epileptiform activity was found on the EEG of 5 of 8 patients. Despite the presence of excessively large amplitude artifacts (> 50 pT peak to peak) due to the VNS implants, SAM(g2) analysis of the MEG data detected foci of high kurtosis in all eight patients and filtered out the VNS-induced artifact from the source signal series data for all patients. Epileptic activity in the same region was compared to other functional and structural studies. Structural abnormalities were identified in 4 cases and MEG foci clustered over these abnormalities in 2 of 4 cases. Additionally, two of the patients underwent invasive monitoring following their MEG recordings, the results of which were consistent with the SAM(g2) analysis. Conclusions: The presented cases demonstrate an expanded use of MEG with the SAM(g2) beamformer, which removed the VNS artifact from the MEG recordings for all patients. In several of the cases presented, the MEG and SAM(g2) results impacted surgical decisions or surgical outcomes. We conclude that this valuable modality provides unique information that is diagnostically useful, and that with appropriate analysis MEG should not be excluded from the evaluation of patients with a vagus nerve stimulator.