Transcutaneous Auricular Vagus Nerve Stimulation: Efficacy, Applications, and Challenges in Mood Disorders and Autonomic Regulation—A Narrative Review

BACKGROUND: Transcutaneous auricular Vagus Nerve Stimulation (taVNS) is a noninvasive technique that activates vagal projections in the brain and brainstem by stimulating the auricular branch of the vagus nerve. taVNS may be a safer alternative to invasive vagal nerve stimulation for treating treatment-resistant mood disorders, chronic pain, inflammation, cardiovascular dysfunction, inflammatory bowel disease, and Crohn's disease. OBJECTIVE: This study aims to systematically review the current evidence on the efficacy and safety of taVNS in treating depressive disorders and its modulatory effects on the autonomic nervous system. METHODS: Relevant primary and secondary sources were identified through a systematic search of the PubMed and Google Scholar databases from 2008 to 2023. The review used the Scale for Assessment of Narrative Review Articles, resulting in a 0.77% intraclass correlation coefficient, 95% CI, and 0.88 inter-rater reliability. RESULTS: taVNS, a new noninvasive neuromodulation method, stimulates the auricular branch of the vagus nerve and regulates the sympathetic and parasympathetic branches of the ANS, increasing norepinephrine secretion, vagus nerve stimulation adaptability, and heart rate variability. Future studies should clarify the mechanisms and address inconsistencies in taVNS parameters. Standardizing treatment regimens can establish taVNS as a viable, noninvasive treatment. CONCLUSION: Our findings suggest that taVNS may be a safer alternative to invasive vagal nerve stimulation for treatment-resistant mood disorders, chronic pain, inflammation, cardiovascular dysfunction, inflammatory bowel disease, and Crohn's disease. However, further empirical research is needed to elucidate the mechanisms and resolve inconsistencies in the stimulation parameters, and larger studies are required to confirm taVNS as an effective noninvasive treatment.