Compression of the lower trunk of the brachial plexus (C8–T1) by a cervical rib produces the characteristic neurological and vascular features of thoracic outlet syndrome. Which muscles show wasting in neurogenic TOS?

• A Deltoid and supraspinatus — shoulder abduction weakness
• B Biceps brachii and brachioradialis — elbow flexion weakness
• C Hypothenar muscles and interossei with wasting of the ulnar side of the hand; also involvement of thenar muscles (APB) ✓
• D Extensor digitorum communis and extensor carpi radialis — wrist/finger extension weakness

Explanation

Compression of the lower trunk (C8–T1) in thoracic outlet syndrome affects the ulnar nerve component (C8–T1 via medial cord) and the median nerve T1 contribution. Classic neurogenic TOS (Gilliatt–Sumner hand) shows wasting predominantly of the hypothenar eminence and interossei (ulnar-innervated), but also of the abductor pollicis brevis (median nerve, T1). This combined thenar and hypothenar wasting with lower trunk distribution distinguishes TOS from isolated ulnar or median nerve compression. Deltoid is C5–C6 (upper trunk); biceps is C5–C6; EDC is C7 (middle trunk).

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.