A patient has complete paralysis and wasting of the intrinsic hand muscles (both thenar and hypothenar), loss of sensation over the entire palm and all fingers (including the dorsum of the medial three and a half fingers), but preserved wrist and finger flexor power. Which nerve is injured and where?

• A Combined median and ulnar nerve lesion at the wrist
• B Ulnar nerve lesion at the wrist (Guyon's canal)
• C Lower trunk (C8–T1) lesion of the brachial plexus ✓
• D Medial cord lesion of the brachial plexus

Explanation

The lower trunk (C8–T1) of the brachial plexus carries fibres that will form both the median and ulnar nerves. A lower trunk lesion causes: loss of ALL intrinsic hand muscles (thenar via median; hypothenar and other intrinsics via ulnar), sensory loss over the medial forearm and whole hand (C8, T1 dermatomes), and often Horner syndrome (T1 sympathetic fibres). Crucially, the long flexors (FDP, FPL via anterior interosseous and ulnar nerve branches arising in the forearm) are spared because those fibres originate higher in the plexus from C7–C8 contributions that reorganise into the posterior and medial cords. A pure wrist lesion would spare the forearm muscles and preserve dorsal hand sensation (from the dorsal cutaneous branch of ulnar that arises above the wrist).

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

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Written and medically reviewed by the StethoPrep medical team.