Transcutaneous auricular VNS eases constipation, pain in patients with IBS-C

The use of transcutaneous auricular vagus nerve stimulation (taVNS) is effective in improving constipation and abdominal pain, as well as anxiety and depression, in patients with constipation-predominant irritable bowel syndrome (IBS-C), reports a study.

“The alleviation of IBS-C symptoms by taVNS may be attributed to its integrative effects on rectal functions mediated through vagal, cholinergic, and multiomics mechanisms,” the researchers said.

This single-centre, randomized controlled trial was conducted at the First Affiliated Hospital of USTC in Anhui, China, between September 2023 and May 2024. Forty patients randomly assigned to taVNS (n=20) or sham-taVNS (n=20) completed the 4-week study.

The researchers assessed the participants’ IBS-C‒related symptoms and mental health using questionnaires at baseline and at week 4. They also examined the levels of serum acetylcholine and nitric oxide, anorectal manometry, and heart rate variability before and after the therapy.

Finally, faecal samples from each group were examined to compare the gut microbiota, short-chain fatty acids, and gut microbiota-derived tryptophan metabolites among patients with IBS-C.

Intention-to-treat analysis revealed no differences in all parameters between the two groups at baseline. [Am J Gastroenterol 2025;120:2139-2153]

TaVNS led to significant improvements in visual analogue scale score (p<0.001), IBS Severity Scoring System score (p<0.001), weekly frequency of spontaneous bowel movements (SBM; p<0.001), weekly frequency of complete SBM (p=0.004), Bristol Stool Form Scale score (p<0.001), Hamilton Anxiety Scale score (p<0.001), Hamilton Depression Scale score (p<0.001), and IBS Quality of Life score (p<0.001).

Rectal sensation

In addition, taVNS enhanced the rectal sensation in IBS-C patients, including the threshold volume for initial sensation (p=0.033), urge to defecate (p=0.022), and rectoanal inhibitory reflex (p=0.002). It also increased serum levels of acetylcholine (p=0.005) and reduced nitric oxide (p=0.016) while improving vagal activity (p<0.001) as determined by spectral analysis of heart rate variability.

Adverse events (AEs) occurred in three patients in the taVNS group and two in the sham group, but these AEs were manageable.

Moreover, taVNS significantly increased the level of the genus Bifidobacterium (p=0.038) and levels of acetic (p=0.003), butyric (p=0.011), and propionic (p=0.005) acids. It also reduced tryptophan metabolism content, including 3-hydroxyanthranilic acid (p=0.007), anthranilic acid (p=0.026), and L-tryptophan (p=0.002).

“This improvement in IBS symptoms likely contributed to the observed alleviation of anxiety and depression … consistent with prior research,” the researchers said. [J Affect Disord 2016;195:172-179]

The effectiveness of taVNS in reducing abdominal pain in patients with functional abdominal pain disorders and IBS in adolescents has also been shown in previous studies. [Lancet Gastroenterol Hepatol 2017;2:727-737; Clin Gastroenterol Hepatol 2020;18:1987-1994.e2]

Mechanisms

Visceral hypersensitivity and delayed gut transit play important roles in the pathophysiology of IBS. [Gut 2023;72:590-599; World J Gastroenterol 2023;29:4120-4135; Am J Gastroenterol 2022;117:1491-1501]

“Our previous research demonstrated that deep, slow breathing significantly enhanced the threshold volume for initial sensation, urge to defecate, and maximum tolerable volume in patients with IBS-C, potentially through the augmentation of vagal activity,” the researchers said. [Front Neurosci 2022;16:1034547]

“In this trial, we found that taVNS had a similar effect on improving rectal sensation in patients with IBS-C, akin to prior studies using transcutaneous electrical acustimulation,” they added.