Physiological correlates and predictors of vasovagal responses following dry needling in myofascial pain syndrome: a controlled observational study

Background: Vasovagal responses (VVR) are common transient autonomic reactions to invasive procedures such as injections or needling, characterized by bradycardia, hypotension, and transient loss of consciousness; however, their occurrence and underlying autonomic mechanisms during dry needling (DN) for myofascial pain syndrome (MPS) remain poorly understood. Objectives: This study aimed to determine the frequency of VVR during DN in patients with MPS and to compare autonomic parameters—heart rate variability (HRV), pupil diameter, and skin perfusion—between those with and without VVR. Design: Prospective observational study. Methods: In total, 100 adults with acute cervical MPS underwent a standardized DN procedure. Outcomes included pressure pain threshold (PPT), heart rate (HR), blood pressure (BP), HRV indices [average NN interval (AVNN), standard deviation of NN intervals (SDNN), low-frequency (LF)/high-frequency (HF) ratio], pupil diameter, and skin perfusion, assessed at baseline, 5 min, and 30 min post-intervention. Participants were classified post hoc as VVR positive (VVR+) or VVR negative (VVR−) based on clinical signs of vasovagal reaction. Results: No significant changes in skin perfusion or BP were found over time or between groups ( p > 0.05). Significant main effects of time and Time × Group interactions were observed for PPT ( p < 0.001), HR ( p < 0.001), AVNN ( p < 0.001), SDNN ( p < 0.001), LF/HF ratio ( p < 0.001), and pupil diameter ( p < 0.001). Post hoc analyses revealed that only the VVR+ group showed significant increases in PPT ( p < 0.001), AVNN ( p < 0.001), SDNN ( p < 0.001), and pupil diameter ( p < 0.001), alongside decreases in HR ( p < 0.001) and LF/HF ratio ( p < 0.001) from baseline to post-intervention. The VVR− group showed no significant changes. Conclusion: Patients experiencing VVR during DN demonstrated distinct autonomic modulation with parasympathetic predominance, pupil dilation, and increased pain threshold. These findings suggest individual variability in autonomic reactivity during DN and suggest the need for awareness and monitoring of susceptible patients. Trial registration: The clinical registration number is ISRCTN16484644, date: January 28, 2025.