Transcutaneous Cervical Vagus Nerve Stimulation for Primary Dysmenorrhea

This randomized, sham-controlled, parallel-group clinical trial will investigate the effects of bilateral transcutaneous cervical vagus nerve stimulation (tcVNS) on menstrual pain and related clinical outcomes in women with primary dysmenorrhea. Seventy women aged 18-30 years will be randomly allocated in a 1:1 ratio to either active tcVNS or sham stimulation. The intervention will be administered once daily for 8 minutes during the first three days of menstruation. Outcomes will include menstrual pain intensity, multidimensional pain characteristics, dysmenorrhea-related functional and emotional symptoms, anxiety and depression, menstrual symptoms, analgesic use, and heart rate variability. Participants will be followed for five days during one menstrual cycle.

Inclusion Criteria: * Female participants aged 18-30 years. * History of primary dysmenorrhea for at least 6 months. * Menstrual pain intensity of ≥4 on the 0-10 Numerical Rating Scale during the first 24-48 hours of menstruation in at least two of the previous three menstrual cycles. * Regular menstrual cycles of 21-35 days. * Menstrual duration of 3-7 days. * At least 2 years since menarche. * Ability to understand and perform the study procedures. * Provision of written informed consent. Exclusion Criteria: * Diagnosis or history suggestive of secondary dysmenorrhea, including endometriosis, uterine fibroids, ovarian cysts, or pelvic inflammatory disease. * Pregnancy, suspected pregnancy, or breastfeeding. * Presence of a cardiac pacemaker, implantable cardioverter-defibrillator, or other active electronic implant. * Cardiac arrhythmia. * History of epilepsy, unexplained syncope, or serious neurological disease. * Wound, infection, dermatological disorder, or metal implant in the cervical stimulation area. * Regular use of beta-blockers, antiarrhythmic drugs, or medications known to substantially affect heart rate variability.