Effects of Transcutaneous Vagus Nerve Stimulation on Gastrointestinal Symptoms and Cardiovascular Autonomic Outcomes: A Systematic Review and Meta-Analysis

Background: Autonomic dysfunction is increasingly recognized as a key mechanism in disorders involving the brain–gut axis and gastrointestinal symptom generation. Transcutaneous vagus nerve stimulation (tVNS) is a noninvasive neuromodulatory technique investigated for its potential effects on autonomic regulation. Methods: A systematic review and meta-analysis were conducted following PRISMA guidelines, with protocol registration in PROSPERO. Randomized controlled trials (RCTs) investigating auricular or cervical tVNS in patients with visceral disorders were included. Continuous outcomes were pooled using mean differences (MDs) or standardized mean differences (SMDs) with 95% confidence intervals (CIs). When multiple publications originated from the same trial population, only independent datasets were considered for quantitative synthesis to avoid double-counting participants. Risk of bias was assessed using the PEDro scale and certainty of evidence with the GRADE approach. Results: Seven RCTs met the inclusion criteria. tVNS demonstrated a small but statistically significant improvement in gastrointestinal symptoms based on change-from-baseline GSRS scores (MD −0.19; 95% CI −0.29 to −0.09; p < 0.001; I2 = 0%). Although statistically significant, the magnitude of this effect was modest, and its clinical relevance remains uncertain. No significant effects were observed on cardiac vagal tone (SMD 0.19; 95% CI −0.53 to 0.90; p = 0.61; I2 = 73%) or systolic blood pressure (MD −1.24 mmHg; 95% CI −6.69 to 4.21; p = 0.66; I2 = 0%). Evidence regarding cardiac autonomic neuropathy (CAN) was limited to a single independent randomized controlled trial, which found no significant differences between tVNS and sham stimulation. Conclusions: tVNS provides modest statistically significant improvements in gastrointestinal symptoms, supporting its role as a symptomatic neuromodulatory intervention. However, the available evidence for this outcome was based on only two studies, and the clinical relevance of the observed effect remains uncertain. No statistically significant pooled effects were observed for the cardiovascular autonomic markers assessed in this review. Evidence regarding CAN was limited to a single independent study. Therefore, the available evidence remains limited and heterogeneous, and further high-quality randomized controlled trials are warranted.