Cortical Excitability and Heart Rate Variability in Response to Short Duration Transauricular Vagus Nerve Stimulation in Healthy Adults

BACKGROUND: Vagus nerve stimulation (VNS) is approved as an adjunctive therapeutic intervention in neurologic conditions, including epilepsy and primary headache disorders. Transauricular VNS (tVNS) is increasingly used as a method for noninvasively activating the vagus nerve. However, the central neurophysiologic effects of tVNS are not well understood.

OBJECTIVE: We evaluated the effects of tVNS on selected paired-pulse transcranial magnetic stimulation (TMS) tests and heart rate variability (HRV) in a group of healthy participants.

MATERIALS AND METHODS: In this cross-over study design, we randomized healthy participants into either the tVNS or no-stimulation groups. Electrophysiologic assessments (short-interval intracortical inhibition [SICI], intracortical facilitation [ICF], and short-latency afferent inhibition [SAI], using TMS, and HRV) were performed before and after one hour of tVNS or no-stimulation. The same participants were allocated to the opposite treatment arm after a one-week wash-out period. tVNS was delivered to the tragus of the left ear using a portable electrical stimulation device with a custom-made clip electrode. An electrode was placed for an hour on the left ear lobe without stimulation as the control intervention.

RESULTS: tVNS increased SICI (reflected by the reduced motor-evoked potential amplitude) but had no significant effect on ICF or SAI. The HRV low-to-high-frequency ratio showed a greater reduction, and the normalized high-frequency power exhibited a greater increase after tVNS than did no-stimulation; these are markers of cardiovagal activity. None of the participants reported pain or discomfort during or after the tVNS. No adverse events were reported.

CONCLUSION: This study establishes enhanced cortical inhibition and cardiovagal activity in response to tVNS and its safety in healthy adult participants.