Brain Effect of Vagal Nerve Stimulation at Rest and Pain

Overview

Pain is a ubiquitous distressing sensory experience and is the most frequent symptom in numerous gastrointestinal disorders including inflammatory bowel disease (IBD) and irritable bowel syndrome (IBS). Visceral pain is especially difficult to treat with conventional medications and new treatments are needed. Recently, the relationship between autonomic nerve system (ANS) and pain has gathered attention because it could represent an effective treatment target for visceral pain. The parasympathetic nervous system (PNS), one of the two main branches of the ANS, is considered to play an important role for analgesia possibly due to vagal nerve-mediated activation of key brain areas implicated in descending analgesia of pain. Transcutaneous vagal nerve stimulation (tVNS) can non-invasively modulate vagal nerve and be expected as a new method to treat visceral pain. For example, the preliminary study showed that vagal nerve stimulation experimentally modulated cardiac vagal tone (CVT) and prevented the development of acid-induced oesophageal hyperalgesia. Disturbances in ANS function have been reported not only in IBS patients but also in fibromyalgia and chronic pelvic pain syndrome. Many of these disorders have been associated with differences in brain structure and/or function as demonstrated by the use of structural and functional magnetic resonance imaging (fMRI). Of note, the investigators have recently shown that these differences in brain structure and function may be in part attributable to the aforementioned disturbance in ANS function, adding weight to the proposition that autonomic neuromodulation may be efficacious in pain disorders. For instance, in healthy participants the investigators have recently shown, using functional connectivity analysis, that higher resting parasympathetic CVT predicts the engagement of a subcortical functional network that is implicated in descending analgesia, thereby supporting the notion that vagal-mediated analgesia is achieved via descending inhibitory pathways1,4. Thus, tVNS seems a reasonable method to treat pain. However, to date, the precise real-time effect of tVNS on brain function, including during the processing of visceral pain is unknown. Hence, the aims of this study are to investigate the real-time effect of tVNS compared to sham stimulus on brain activity whilst experiencing acute oesophageal pain, using fMRI in double-blind, randomised crossover study of tVNS vs sham stimulation in healthy subjects.

Following informed written consent and screening questionnaires to characterize psychophysiological traits including personality and anxiety/depression inventory, each subject will have a nasogastric tube inserted with the attached distensible balloon positioned in the distal oesophagus. Painful oesophageal stimulation will be achieved by inflation of the balloon to a pain threshold pre-determined in each subject, defined as the point at which subject describes the transition from sensation to pain. The MR-safe tVNS probe will be attached to the neck for cervical vagal nerve-directed tVNS, and subjects will then be positioned in the MRI scanner. Real-time activity of the ANS will be monitored by heart rate variability as described in the previous study, which has been adapted for use in the MR-environment. All subjects will undergo MRI. For each visit, there will be two scanning periods. High-resolution structural imaging will first be required, and subsequently resting fMRI data will be acquired whilst subjects are asked to relax in the scanner, and their baseline autonomic tone is monitored, using validated methods of acquiring ANS data. This will illustrate a brain signature of real-time brain activity mapped to their parasympathetic tone. Subjects will then be randomised to either the active-tVNS paradigm or sham and a second fMRI data acquisition will then be performed so as to acquire brain activity in conjunction with active and sham-tVNS. Following this, painful oesophageal stimulation will be repeated 20 times while active or sham tVNS continues. Following a two-week washout period, subjects will be crossed over and re-examined to receive the intervention they did not receive in visit 1.