A patient sustains a mid-shaft humeral fracture and develops wrist drop. On examination, brachioradialis reflex is absent but the triceps reflex is intact. At which specific level of the radial nerve is the injury most likely?

• A In the axilla, proximal to all branches
• B At the lateral epicondyle, distal to the origin of brachioradialis branch
• C In the posterior compartment of the arm proximal to triceps branch
• D In the spiral (radial) groove of the humerus, distal to the branch to triceps and proximal to the branch to brachioradialis ✓

Explanation

The radial nerve gives branches to the triceps (long, medial, and lateral heads) in the axilla and upper arm proximal to the spiral groove. The branch to brachioradialis and extensor carpi radialis longus arises just proximal to or at the lateral epicondyle, AFTER the nerve exits the spiral groove. Therefore, an injury in the spiral groove spares triceps (its branch already given off) but damages brachioradialis and all extensors distal to that level, causing absent brachioradialis reflex and wrist drop. Axillary injury would also cause triceps weakness and loss of triceps reflex.

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

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Written and medically reviewed by the StethoPrep medical team.