MRCEM Primary Question Bank

C5 nerve root injury produces weakness of shoulder abduction (deltoid) and lateral rotation (infraspinatus), with loss of the regimental badge area sensation. The C5 root primarily supplies the deltoid via the axillary nerve and the biceps via the musculocutaneous nerve, explaining the absent biceps reflex. Preserved elbow extension (C7 via radial nerve) and wrist movements confirm the lesion is isolated to C5.

C4 primarily supplies the diaphragm and trapezius. C6 would additionally affect brachioradialis and wrist extension. C7 would affect elbow extension and wrist flexion. C8 would affect the intrinsic hand muscles. In motorcycle accidents, traction injuries to the upper brachial plexus (Erb-Duchenne pattern, C5-C6) are common when the rider is thrown and the head is forced away from the shoulder, stretching the upper roots.

Remember: Regimental badge sensory loss with absent biceps reflex and weak shoulder abduction is the classic C5 triad - think upper brachial plexus injury in motorcycle RTCs.

Ulnar nerve injury at Guyon's canal produces a claw hand with interosseous weakness but preserved FDP function to the ring and little fingers. The key distinguishing feature is that DIP flexion of these digits is intact - the FDP to the ring and little fingers is supplied by the ulnar nerve proximal to the wrist. If the injury were at the elbow, FDP function would be lost, producing a less obvious claw deformity (the ulnar paradox).

Sensory loss over the medial one and a half fingers with intact dorsal hand sensation (the dorsal cutaneous branch arises proximal to Guyon's canal) further localises the lesion. C8/T1 root or lower trunk lesions would affect additional muscles including thenar muscles and long finger flexors. Median nerve injury at the wrist would cause thenar wasting and lateral three and a half finger sensory loss.

Weakened wrist flexion suggests partial FCU involvement, which can occur with proximal extension of the injury or concurrent trauma.

Remember: The ulnar paradox - a more distal ulnar nerve lesion produces a worse claw deformity because intact FDP allows unopposed MCP hyperextension.

The carpal tunnel contains the median nerve and nine flexor tendons, including the flexor pollicis longus. A dorsally displaced (Colles') fracture can acutely compromise the carpal tunnel, causing traumatic carpal tunnel syndrome. The tunnel is bounded by the carpal bones posteriorly and the flexor retinaculum anteriorly. Contents include the median nerve, four FDS tendons, four FDP tendons, and the FPL tendon.

The ulnar nerve and artery pass through Guyon's canal, not the carpal tunnel. The radial nerve does not pass through either tunnel. The anterior interosseous nerve branches from the median nerve in the forearm and does not traverse the carpal tunnel. The posterior interosseous nerve is on the extensor aspect.

The FCR tendon runs in its own compartment within the flexor retinaculum and is technically not within the carpal tunnel itself. Post-fracture swelling and haematoma increase carpal tunnel pressure, and urgent decompression may be needed if symptoms are acute and severe.

Remember: Nine tendons plus one nerve in the carpal tunnel - the median nerve is most vulnerable to compression because it is the only neural structure in this rigid space.