Thoracic outlet syndrome compressing the lower trunk of the brachial plexus (C8, T1) typically produces which pattern of weakness and sensory loss?

• A Weakness of shoulder abductors and sensory loss over lateral arm
• B Weakness of wrist extension and sensory loss over dorsum of hand
• C Weakness of intrinsic hand muscles and sensory loss over medial forearm and hand ✓
• D Weakness of elbow flexion and sensory loss over thumb and index finger

Explanation

The lower trunk (C8, T1) of the brachial plexus gives rise to the medial cord, which contributes to the ulnar nerve (all intrinsic hand muscles) and the medial root of the median nerve (thenar muscles), as well as the medial cutaneous nerves of arm and forearm. Compression at the thoracic outlet (e.g., by a cervical rib or scalene muscle) therefore causes weakness of intrinsic hand muscles with wasting, and sensory loss along the medial forearm (C8-T1 dermatome) and medial hand. This must be distinguished from ulnar nerve entrapment, which does not involve the medial forearm.

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

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Written and medically reviewed by the StethoPrep medical team.