Effects of Visceral Manual Therapy on Autonomic Nervous System Modulation in Individuals with Bruxism: A Randomized Controlled Trial

Background/Objectives: Bruxism has been associated with dysregulation of the autonomic nervous system (ANS). Visceral manual therapy (VMT) has demonstrated beneficial effects on vagal tone and modulation of the ANS, although its potential impact in adults with bruxism has not yet been examined. This study aimed to evaluate the immediate and short-term effects of VMT in individuals with bruxism. Methods: A single-blind randomized controlled trial was conducted including 24 individuals diagnosed with “probable bruxism.” Participants received two sessions of either VMT or a sham placebo technique. Outcome measures included heart rate variability (HRV), both normal-to-normal intervals (HRV-SDNN) and the root mean square of successive normal-to-normal intervals (HRV-RMSSD), as well as muscle tone and stiffness of the masseter muscles, and pressure pain thresholds (PPTs) at C4, masseter, and temporalis muscles. Measurements were made at: T1 (baseline), T2 (post- first intervention), T3 (pre- second intervention; week 2), T4 (post- second intervention; week 2), and T5 (4-week follow-up). Results: A significant time*group interaction was observed for HRV-SDNN (p = 0.04, η² = 0.12). No significant changes were found for muscle tone or stiffness, whereas PPTs significantly improved at C4 after the second intervention (p = 0.049, η² = 0.16) and at the left temporalis muscle after the first session (p = 0.01, η² = 0.07). Conclusion: The findings suggest that two sessions of VMT may help to modulate the ANS, as assessed with the HRV. However, no changes were observed for masticatory muscles viscoelastic properties or pressure pain sensitivity. Further research with larger sample sizes and long-term follow-up is needed to clarify the clinical relevance of VMT in the management of bruxism.