A long-distance cyclist presents with weakness of finger abduction and adduction of the right hand with numbness over the palmar aspect of the little and ring fingers. Sensation over the dorsum of these fingers is completely normal. The lesion is localised to:

• A The cubital tunnel behind the medial epicondyle
• B Guyon canal, distal to the origin of the dorsal sensory branch ✓
• C The deep motor branch within the hypothenar muscles only
• D The lower trunk of the brachial plexus

Explanation

Handlebar compression at Guyon canal affects the ulnar nerve after it enters the wrist. The dorsal cutaneous branch and the palmar cutaneous branch arise several centimetres proximal to the canal, so dorsal sensation is characteristically spared, which distinguishes this from a cubital tunnel lesion where the dorsum of the hand is affected. A pure deep motor branch lesion would spare all cutaneous sensation, and a lower trunk palsy would involve median territory as well.

Reference: Campbell's Operative Orthopaedics, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.