This randomized, sham-controlled study will evaluate whether transcutaneous auricular vagus nerve stimulation (taVNS), when added to standard physiotherapy, improves pain, shoulder function, range of motion, and quality of life in adults with adhesive capsulitis.

A total of 40 participants aged 18 to 65 years will be randomly assigned to one of two groups. Both groups will receive 15 sessions of standard physiotherapy over 3 weeks. In addition, one group will receive active taVNS for 30 minutes per day, while the other group will receive sham stimulation for the same duration and frequency. Assessments will be performed before treatment and after the 15th treatment session using pain, shoulder disability, quality-of-life, and shoulder range-of-motion measurements.

Adhesive capsulitis is a musculoskeletal condition characterized by shoulder pain and progressive restriction of active and passive range of motion. Although physiotherapy, therapeutic exercise, and manual therapy are commonly used, pain relief and functional recovery may be incomplete in some patients. Therefore, additional non-invasive treatment approaches that may enhance the effects of rehabilitation are clinically relevant.

Transcutaneous auricular vagus nerve stimulation (taVNS) is a non-invasive neuromodulation technique that stimulates the auricular branch of the vagus nerve. It may influence pain modulation, autonomic nervous system regulation, and inflammatory pathways. Previous studies have reported potential benefits of auricular vagus nerve stimulation in several chronic pain conditions; however, evidence regarding its use in adhesive capsulitis remains limited.

The objective of this study is to determine whether active taVNS added to standard physiotherapy provides greater clinical improvement than sham taVNS added to the same physiotherapy program. The main hypothesis is that participants receiving active taVNS will demonstrate greater reductions in pain and shoulder-related disability after treatment. Secondary objectives are to evaluate changes in shoulder range of motion and health-related quality of life and to assess the tolerability of the intervention.

Inclusion Criteria:

• Age between 18 and 65 years.
• Clinical diagnosis of adhesive capsulitis confirmed by physical examination and, when clinically indicated, imaging.
• Shoulder pain and limitation of shoulder range of motion for at least 3 months.
• Provision of written informed consent.
• General health status sufficient to participate in standard physiotherapy and active or sham transcutaneous auricular vagus nerve stimulation.
• Ability to comply with the planned treatment sessions and outcome assessments.

Exclusion Criteria:

• Shoulder stiffness secondary to trauma, surgery, infection, or tumor.
• Rotator cuff tear, advanced glenohumeral osteoarthritis, or inflammatory rheumatic disease.
• Glucocorticoid injection into the affected shoulder within the previous 3 months.
• Surgical intervention involving the affected shoulder within the previous 6 months.
• Active infection or skin lesion in the shoulder region.
• Cervical radiculopathy, brachial plexopathy, stroke, or another neurological disorder affecting the upper extremity.
• Presence of a cardiac pacemaker or another implanted electronic device.
• Pregnancy or breastfeeding.
• Serious systemic disease or cognitive impairment that may prevent compliance with the study protocol.
• Known intolerance or previous adverse reaction to transcutaneous electrical stimulation.

Individual participant data will not be shared. Study findings will be reported only in aggregate form to protect participant confidentiality, and no external IPD-sharing procedure has been established for this study.

Participants and the outcomes assessor are blinded to group allocation. The physiotherapist administering active or sham transcutaneous auricular vagus nerve stimulation is aware of the assigned intervention and is not involved in outcome assessments.