Abstracts

Rationale: Vagus nerve stimulators (VNS) alike other implanted devices, cause limitations when performing magnetoencephalography (MEG). Recording of physiological brain signals is hampered by artefacts caused by movement of these devices for example during breathing. Conventional artefact rejection methods are unable to remove these artefacts, especially when these are caused by objects in proximity of the sensor array. A recently developed method of artefact removal called spatiotemporal signal space separation (tSSS, Elekta Neuromag Oy) has been proposed to remove artefacts generated by metallic objects in movement close to the sensor array. Basically, this method suppresses such artefacts in two steps. First, it separates brain-related signals from those generated by external interference. This procedure uses the conventional signal space separation method, based on sensor geometry and Maxwell’s equations. Then, the artefacts generated by nearby sources such as those associated with the VNS device are extracted using statistical analysis in the time domain, and projected out from the MEG signal (Taulu S & Simola J, 2006). In a multicenter trial we studied the feasibility and added value of MEG with the application of the tSSS method in the presurgical re-evaluation of refractory epilepsy patients implanted with VNS who were non-responders. Methods: Nine patients (mean age:34 y;7M) treated with VNS underwent a 1-hour MEG (supine position, eyes-closed rest) with the 306-channel Elekta Neuromag system using a light-weight magnetically shielded room (MaxShield, Elekta Neuromag Oy). All patients were VNS non-responders experiencing <30% seizure reduction since VNS implantation. During acquisition VNS was switched off. tSSS software was applied off-line (correlation coefficient: 0.8). Signal to Noise Ratio (SNR) of the tSSS-filtered data was evaluated at the maximum of the interictal peaks. Interictal epileptiform discharges (IED) were visually detected by 2 independent investigators. Equivalent current dipoles (ECD, g/% >80%) were fitted on MR images of individual patients. The added value for the presurgical evaluation of MEG results was evaluated by monitoring the patients’ management with and without the MEG result. Results: In 7/9 patients filtered data showed sufficient SNR (background brain activity in the pico Tesla range). In 6/7 patients IED were detected. IED were explained by ECD that clustered within one lobe in 4/6 patients. MEG results confirmed the earlier presurgical findings in 2/6 patients (confirmed by invasive EEG monitoring in 1) and changed patient management in 4/6 patients by adding further information. Conclusions: This study demonstrates the feasibility of MEG in epilepsy patients implanted with VNS. Moreover, it suggests that MEG represents an added value in the presurgical re-evaluation of these patients by changing management in half of the implanted patients.