Noninvasive vagus nerve stimulation and the trigeminal autonomic reflex: An fMRI study.

Objective: The trigeminal autonomic reflex is a physiologic reflex that plays a crucial role in primary headache and particularly in trigeminal autonomic cephalalgias, such as cluster headache. Previous studies have shown that this reflex can be modulated by the vagus nerve, leading to an inhibition of the parasympathetic output of the reflex in healthy participants. The aim of the present study was to characterize neural correlates of the modulatory effect of noninvasive vagus nerve stimulation (nVNS) on the trigeminal autonomic reflex.

Methods: Twenty-one healthy participants were included in a 2-day, randomized, single-blind, within-subject design. The reflex was activated inside the MRI scanner using kinetic oscillation stimulation placed in the left nostril, resulting in an increase in lacrimation. After the first fMRI session, the participants received either sham vagus nerve stimulation or nVNS outside the scanner and underwent a subsequent fMRI session.

Results: nVNS prompted an increase in activation of the left pontine nucleus and a decreased activation of the right parahippocampal gyrus. Psychophysiologic interaction analyses revealed an increased functional connectivity between the left pontine nucleus and the right hypothalamus and a decreased functional connectivity between the right parahippocampal gyrus and the bilateral spinal trigeminal nuclei (sTN).

Conclusions: These findings indicate a complex network involved in the modulatory effect of nVNS including the hypothalamus, the sTN, the pontine nucleus, and the parahippocampal gyrus.