No significant modulation of pressure pain sensitivity and cardioception after transauricular vagal nerve stimulation

INTRODUCTION: Transauricular vagal nerve stimulation (taVNS) is gaining interest as a chronic pain treatment due to its convergence with nociceptive pathways. Although pain-modulation by taVNS has been demonstrated, this research often lacks the use of active control or does not simultaneously explore effects on associated interoceptive pathways or vagal activation.

OBJECTIVES: This double-blind, active-controlled, cross-over study aimed to corroborate the pain-modulating effect of taVNS whilst further investigating cardioception and vagal-activation markers.

METHODS: Thirty healthy individuals underwent 20 minutes of continuous 25-Hz electrical stimulation of the left concha cymba (taVNS) or earlobe (active control). Stimulation intensity was individually adjusted at two-thirds between perception and suprapain thresholds. Assessments were obtained before, immediately after, and 30 minutes after stimulation. These included pressure pain sensitivity measures including, pressure pain perception, detection, tolerance thresholds and conditioned pain modulation (CPM) as well as heart rate variability (HRV), pupillometry and cardioception measures.

RESULTS: Although stimulation intensities were higher for control stimulation than taVNS (P < 0.001), participants reported no significant differences in subjective perception of stimulation or in their beliefs regarding the effects of the stimulations on the assessments. Compared to control stimulation, taVNS did not significantly modulate pressure pain sensitivity, cardioception, HRV, or pupillometry. Explorative analysis revealed a negative correlation between taVNS-associated changes in CPM and parasympathetic-HRV (P = 0.005), supporting a potential link between pain modulation and vagal activation.

CONCLUSION: Although the findings offer partial support for the proposed vagal mechanism of taVNS, they highlight the limited understanding of which parameters and when stimulation exerts effects.