Resident/ PhD student Ghent University Hospital Gent, Belgium
Rationale: Transcutaneous Vagus Nerve Stimulation (tVNS) is a non-invasive neurostimulation modality that targets the cutaneous receptive field of the auricular branch of the vagus nerve. This technique has been studied as a potential treatment for several neuropsychiatric disorders with inconsistent results. The goal of this study is to provide more insight into the underlying effect of tVNS on brain neurophysiology. By combining transcranial magnetic stimulation (TMS) with electromyography (EMG) and electroencephalography (EEG), we can study the modulation of Motor Evoked Potentials (MEPs) and TMS Evoked Potentials (TEPs). As these MEPs and TEPS are reproducible within subjects, they can be useful to study the effect of neuromodulatory interventions, like tVNS. Methods: In this prospective cross-over study, 15 healthy male subjects underwent two sessions, at least one week apart. During each session, tVNS or sham stimulation was delivered at the maximum tolerated amplitude during one hour. The resting motor threshold, MEPs and TEPs were measured before and after the intervention. For these measurements, 120 single TMS pulses, 120 paired TMS pulses with a short interstimulus interval and 120 paired TMS pulses with a long interstimulus interval were delivered over the motor hotspot. To assess the effect of tVNS on MEP morphology, we measured the average latency and amplitude of all MEPS for each condition and each pulse type. Intracortical inhibition after paired pulses was assessed by expressing the amplitude of the paired pulse response as a percentage of the amplitude of the single pulse response. MEP latency, amplitude and percentage were then compared at the single subject level before and after each intervention. Statistical analysis was conducted by means of a two-way repeated measures ANOVA. TMS-EEG data was preprocessed offline using the TESA toolbox in Matlab. A cluster-based permutation statistical analysis was conducted. Results: The resting motor threshold showed no significant change after real tVNS compared to sham stimulation. MEP data analysis showed no significant change in mean amplitude after real tVNS compared to sham stimulation for both single pulses and paired pulses. Only a trend could be found towards decreased amplitude of MEPS evoked by short interval paired pulses after real tVNS. Intracortical inhibition produced by paired pulses also showed no statistically significant change. Regarding onset latency, a significant effect of timing, but not intervention was found, showing an increase in latency for both interventions. TMS-EEG data analysis is currently ongoing. Conclusions: This prospective cross-over study is the first study to investigate the neuromodulatory effects of tVNS by means of both TMS-EMG and TMS-EEG. Active tVNS, as compared to sham stimulation, did not modify cortical excitability measured by single and paired pulse TMS-EMG. Unexpectedly, a significant increase in latency was found for both interventions, suggesting a possible effect of repeated measurements or timing throughout the experiment. Funding: Please list any funding that was received in support of this abstract.: This research is supported by a grant of the ‘Geneeskundige Stichting Koningin Elisabeth’ (GSKE). Ann Mertens is supported by an “Aspirant” grant of the ‘Fonds voor Wetenschappelijk Onderzoek’ (FWO) Flanders.
