Neurogenic thoracic outlet syndrome (NTOS) is a complex disorder characterized by compression of the brachial plexus, resulting in pain, weakness, and sensory changes in the upper extremity. While anterior scalene muscle (ASM) is most commonly implicated, a rare aberrant presence of subclavius posticus hypertrophy because of its unique location in the costoclavicular space is reported as cause of NTOS(1). This is a case wherein successful treatment of NTOS was achieved by targeting subclavius muscle hypertrophy.

Written informed consent was obtained from the patient for publication of this case report. All identifying information has been removed. This report did not require approval from the University Health Network Research Ethics Board (UHN REB), as single-patient case reports are considered exempt from review under institutional policy.

A 42-year-old female presented with gradual-onset neuropathic symptoms in both upper extremities, predominantly affecting the right side, involving the ulnar forearms and hands. Symptoms were attributed to prolonged poor ergonomics during the COVID-19 pandemic. EMG revealed bilateral carpal tunnel syndrome and mild ulnar nerve entrapment at the elbows. Cervical MRI showed a mild C6/7 disc bulge without compression. Thoracic outlet syndrome (TOS) testing was positive for Roos (sensory changes) and Adson’s (radial pulse reduction on the right), with negative Wright and Phalen’s, and positive Tinel’s at the right cubital tunnel. Doppler studies confirmed combined neurovascular compression at the thoracic outlet, rather than isolated distal entrapment. Repeat EMG supported this diagnosis. MRI of the thoracic outlet showed subclavius hypertrophy. The vascular team recommended botulinum toxin type A (BTX-A) injection as a diagnostic and therapeutic trial before considering surgical resection. Ultrasound-guided injection of 40 units into the subclavius improved symptoms by 80% for two months. A subsequent injection targeting both the subclavius and anterior scalene yielded greater relief lasting four months. Physical therapy was integrated into management, and surgical intervention remains deferred due to sustained improvement with conservative measures.

Thoracic outlet syndrome (TOS), first described in 1927, involves neurovascular compression often due to hypertrophy of muscles like the anterior scalene or subclavius. While surgery, including rib or muscle resection, is an option, it carries morbidity. BTX-A injections achieve symptom relief in 64–69% of patients for up to three months and may be effective for subclavius-related TOS. Ultrasound-guided BTX-A is a safe, non-surgical option before considering operative intervention.