thoracic outlet

**Anatomy – three spaces of compression** 1. **Interscalene triangle**: between the anterior and middle scalene muscles and the first rib. Contains the brachial plexus roots/trunks and the subclavian ARTERY (the subclavian VEIN passes anterior to the anterior scalene, not through the triangle). 2. **Costoclavicular space**: between the clavicle (anterior/superior) and the first rib (posterior/inferior). Contains the subclavian vein, artery, and plexus. Narrows with shoulder depression/bracing. 3. **Retropectoralis minor (subcoracoid) space**: deep to the pectoralis minor tendon near the coracoid process. Neurovascular bundle can be compressed with hyperabduction.

**Etiology of thoracic outlet syndrome (TOS)** - Congenital: cervical rib (from C7 costal process; ~0.5–1% of the population), fibrous band from C7 to first rib, anomalous scalene muscles, elongated C7 transverse process. - Acquired: trauma (whiplash, clavicle fracture with callus), repetitive overhead activity (swimmers, baseball pitchers, weightlifters), poor posture, hypertrophied scalenes, tumors (Pancoast tumor can mimic). - More common in young to middle-aged women (neurogenic type).

**Types** - **Neurogenic TOS (~90–95%)**: compression of the lower trunk of the brachial plexus (C8–T1), often by a cervical rib or fibrous band. Pain and paresthesias in the ulnar nerve distribution (medial forearm, 4th–5th digits), weakness and atrophy of hand intrinsics (thenar wasting – "Gilliatt-Sumner hand"). Can mimic ulnar neuropathy or C8 radiculopathy. - **Venous TOS (~3–5%)**: subclavian vein compression or thrombosis → arm swelling, cyanosis, venous distension. **Paget-Schroetter syndrome** = effort thrombosis of the axillary/subclavian vein in young active patients after repetitive arm use. - **Arterial TOS (<1–2%)**: subclavian artery compression → post-stenotic dilatation/aneurysm, thrombus, distal embolization → pallor, coolness, claudication, pulselessness, digital ischemia. Often associated with a cervical rib.

**Diagnosis** - Clinical: provocative maneuvers – Adson test (radial pulse loss with neck extension/rotation toward the affected side and deep inspiration), Roos test (EAST – elevated arm stress test; 3 min of hand opening/closing with arms abducted and externally rotated), Wright (hyperabduction) test. Poor specificity (false positives common). - Imaging: chest/cervical X-ray (cervical rib, long C7 transverse process), CT/MR angiography or venography, duplex ultrasound for vascular types, MRI/MR neurography for the plexus. Nerve conduction studies/EMG help exclude carpal tunnel and ulnar neuropathy (reduced median sensory/medial antebrachial cutaneous responses).

**Treatment** - Neurogenic: conservative first – physical therapy, posture correction, scalene stretching/strengthening; surgery (first rib resection ± scalenectomy) if refractory. - Venous: anticoagulation ± catheter-directed thrombolysis, followed by first rib resection. - Arterial: surgical decompression (first rib/cervical rib resection) with arterial repair/bypass as needed.

**Boards pearls** - Lower trunk (C8–T1) involvement → ulnar-type sensory loss and intrinsic hand weakness; distinguishes from carpal tunnel (median nerve). - Cervical rib → TOS, often compressing the lower trunk and subclavian artery. - Subclavian artery and brachial plexus pass between anterior and middle scalenes; subclavian vein passes anterior to the anterior scalene. - Pancoast tumor (superior sulcus) can invade the lower plexus → Horner syndrome + ulnar-type arm pain + hand weakness; keep in the differential (history of smoking, apical lung mass). - Klumpke palsy (lower trunk injury, C8–T1) from hyperabduction/traction of the arm presents with a "claw hand" – similar territory to neurogenic TOS. - Differentiate from Erb palsy (upper trunk, C5–C6; "waiter's tip").