2.6 Thoracic Outlet Syndrome & Peripheral Nerve Entrapment

Thoracic Outlet Syndrome & Upper-Limb Nerve Entrapment

Upper-limb peripheral nerve entrapments are recurrent Domain D topics. Thoracic outlet syndrome (TOS) and its mimics (pronator syndrome, anterior interosseous nerve syndrome, suprascapular nerve entrapment) test the candidate's ability to localize compression by history, provocative exam, and electrodiagnostic pattern.

Thoracic Outlet Syndrome Subtypes

The thoracic outlet is the space between the clavicle and first rib through which the brachial plexus and subclavian vessels pass. Three subtypes differ in anatomy, presentation, and workup:

Provocative maneuvers (Adson, Wright hyperabduction, Roos/elevated arm stress test, Tinel at supraclavicular fossa) reproduce symptoms but limited specificity; they are interpreted with electrodiagnosis and vascular imaging rather than used in isolation.

Neurogenic TOS Management

• Conservative first-line: physical therapy emphasizing postural correction, scapular stabilization, first-rib decompression maneuvers, and nerve-gliding; activity modification.
• Surgical: first-rib resection with anterior scalenectomy reserved for refractory neurogenic cases or for any vascular TOS, which is less amenable to conservative care.
• Electrodiagnostic caveat: NCS are often normal early; only advanced cases show ulnar sensory and medial antebrachial cutaneous amplitude reduction with lower-trunk motor changes.

Median Nerve Entrapments Beyond Carpal Tunnel

• Pronator syndrome: compression of the median nerve at the pronator teres (or ligament of Struthers / sublimis arch). Symptoms resemble carpal tunnel—palmar thenar numbness—but include proximal forearm pain and a negative Phalen sign, and the thenar muscles and lumbricals are affected (unlike isolated carpal tunnel, which spares the thenar in early disease... actually carpal tunnel affects thenar). Distinguishing features: symptoms provoked by resisted elbow flexion/pronation or resisted middle-finger PIP flexion; preserved thenar strength differentiates from advanced CTS.
• Anterior interosseous nerve (AIN) syndrome: pure motor branch of median; presents with pinch weakness (inability to flex the DIP of index and IP of thumb, producing the "OK sign" deficit) and no sensory loss.

Suprascapular Nerve Entrapment

The suprascapular nerve (C5-C6, upper trunk) innervates supraspinatus and infraspinatus. Compression location dictates the pattern:

Suprascapular Notch  ──►  Supraspinatus + Infraspinatus Weakness (abduction & external rotation)
       │
Spinoglenoid Notch  ──►  Infraspinatus Weakness Only (external rotation; supraspinatus spared)

A ganglion cyst at the spinoglenoid notch (posterior shoulder, common in overhead athletes) selectively compresses the nerve after the supraspinatus branch, causing isolated infraspinatus atrophy and weak external rotation with preserved abduction.

Electrodiagnostic Differentiation

Localization rests on NCS and needle EMG: TOS affects the ulnar sensory and lower-trunk-derived muscles; pronator syndrome shows median sensory involvement with proximal forearm pain; AIN syndrome shows median motor branch involvement with normal sensory studies; suprascapular entrapment shows suprascapular nerve motor changes with preserved deltoid and rhomboid (ruling out C5 radiculopathy).

Ulnar Nerve & Cubital Tunnel Syndrome

The ulnar nerve (C8-T1, medial cord) is vulnerable at the cubital tunnel (most common) and Guyon canal at the wrist. Cubital tunnel syndrome produces paresthesias in the little finger and ulnar half of the ring finger, intrinsic weakness (grip, pinch), and Wartenberg sign (small finger abduction). Provocative testing includes elbow flexion with ulnar nerve compression and Tinel at the cubital tunnel. Management begins conservative: night splinting in extension, avoiding prolonged elbow flexion, ergonomic modification; surgical decompression/transposition for refractory cases.

Radial Nerve Entrapment

Posterior interosseous nerve (PIN) syndrome—a pure motor branch of the radial nerve—causes finger and thumb extension weakness with preserved wrist extension (the ECRL is supplied proximal to the PIN branch), and no sensory loss. Radial tunnel syndrome refers to pain (often at the mobile wad / extensor origin) without motor loss, a controversial clinical entity that must be distinguished from lateral epicondylitis (which is tender at the ECRB origin and provoked by resisted wrist extension).

Differentiating TOS From Cervical Radiculopathy & Pancoast

Lower-trunk TOS resembles C8-T1 radiculopathy, but radiculopathy produces dermatomal pain extending proximally into the neck/shoulder, reproduces with cervical Spurling maneuvers, and shows denervation in a myotomal (not peripheral-nerve) pattern on EMG. A Pancoast tumor (apical lung) invades the lower trunk and sympathetic chain, producing TOS-like pain plus Horner syndrome (ptosis, miosis, anhidrosis)—an apical-lung mass on imaging excludes simple TOS and demands oncologic workup. This is a high-yield trap: any suspected neurogenic TOS warrants imaging to exclude an apical mass before attributing symptoms to a cervical rib or fibrous band.

Electrodiagnostic & Imaging Workup

NCS in neurogenic TOS may show reduced ulnar sensory and medial antebrachial cutaneous amplitudes; EMG may show chronic neurogenic changes in C8-T1-innervated muscles. Cervical spine imaging excludes radiculopathy; chest/apical imaging excludes Pancoast tumor. Arterial/venous studies are obtained for the corresponding vascular TOS subtypes. The physiatrist synthesizes these to localize and stage treatment.