Laterality-dependent biological effects of manual acupuncture stimulation of the auricular vagus nerve in women with fibromyalgia: a randomized, sham-controlled trial

BACKGROUND: Auricular vagus nerve stimulation (aVNS) has emerged as a noninvasive neuromodulatory strategy with the potential to modulate central sensitization and inflammatory pathways. However, its role in fibromyalgia (FM) remains insufficiently explored. OBJECTIVE: To investigate whether stimulation laterality (left vs right auricular branch of the vagus nerve) differentially influences clinical and biological outcomes in women with FM. METHODS: In this randomized, double-blind, sham-controlled trial, 51 women with FM were allocated to sham stimulation, right-sided aVNS (aVNS-R), or left-sided aVNS (aVNS-L). Participants underwent weekly sessions for 4 weeks and were followed for 12 weeks. Pain intensity was the primary outcome. Secondary outcomes included psychological symptoms, sleep, functional status, quality of life, and circulating biomarkers (pro- and anti-inflammatory cytokines, brain-derived neurotrophic factor [BDNF]). RESULTS: While no significant between-group differences were observed in pain intensity, left-sided stimulation (aVNS-L) was associated with a modest but significant reduction in global symptom severity. Importantly, aVNS-L produced consistent immunomodulatory effects, including decreased IL-1β and TNF-α levels, and increased IL-4, IL-10, and BDNF concentrations. CONCLUSIONS: This exploratory trial suggests that stimulation laterality may shape the biological response to aVNS in FM. Although clinical pain relief was not superior to sham, left-sided stimulation promoted an anti-inflammatory profile and enhanced neuroplasticity markers. These findings support further investigation of aVNS laterality as a targeted neuromodulatory approach for FM. TRIAL REGISTRATION: Brazilian Clinical Trials Registry RBR-10d3crcf.