Utility of temporally-extended signal space separation algorithm for magnetic noise from vagal nerve stimulators

Objective: To evaluate the utility of a temporally-extended signal space separation algorithm (tSSS) for patients with vagal nerve stimulator (VNS). Methods: We evaluated median nerve somatosensory evoked responses (SER) of magnetoencephalography (MEG) in 27 VNS patients (48 sides) with/without tSSS processing. We classified SER dipoles as 'acceptable' if: (A) the location of the dipole was in the expected location in the central sulcus, and (B) the goodness of fit value (GOF) was greater than 80%. We evaluated (1) the number of sides which produced acceptable dipoles in each dataset (i.e. with/without tSSS processing), and in cases where the both data produced reliable dipoles, (2) compared their GOFs and the 95% confidence volumes (CV) (mm(3)). Statistical differences in the GOF and CV between with/without tSSS conditions were determined by paired t test. Results: Only 11 (23%) responses had reliable dipoles without tSSS processing, while all 48 (100%) had acceptable dipoles under tSSS processing. Additionally, the latter group had significantly higher GOF (increased by 7% on average) and lower CV (mean decrease of 200 mm(3)) than the former (p < 0.01). Conclusions: Processing with tSSS quantitatively improves dipole fitting of known sources in VNS patients. Significance: This algorithm permits satisfactory MEG testing in the relatively commonly encountered epilepsy patient with VNS. (C) 2012 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.