Dog with acute non weight bearing lameness; marked triceps atrophy and inability to extend elbow; most likely diagnosis?

Dog with acute non weight bearing lameness; marked triceps atrophy and inability to extend elbow; most likely diagnosis?

• A. Radial nerve injury
• B. Brachial plexus avulsion
• C. Median nerve injury
• D. Ulnar nerve injury

Answer: (A)

Radial nerve injury best explains these findings. The radial nerve innervates the triceps brachii, the muscle that extends the elbow. When this nerve is damaged, the triceps can’t contract, so the elbow cannot be actively extended, leading to a non‑weight bearing stance. Denervation also causes marked atrophy of the triceps over time. The radial nerve also supplies the extensor muscles of the carpus and digits, so extension of those joints is affected as well, reinforcing the abnormal gait. In contrast, injuries to the median or ulnar nerves affect other muscle groups (flexors of the forearm and intrinsic paw muscles) and wouldn’t produce an inability to extend the elbow. A brachial plexus avulsion would cause a broader loss of motor function and sensation, not a focal triceps atrophy with elbow extension failure.

Radial nerve injury best explains these findings. The radial nerve innervates the triceps brachii, the muscle that extends the elbow. When this nerve is damaged, the triceps can’t contract, so the elbow cannot be actively extended, leading to a non‑weight bearing stance. Denervation also causes marked atrophy of the triceps over time. The radial nerve also supplies the extensor muscles of the carpus and digits, so extension of those joints is affected as well, reinforcing the abnormal gait.

In contrast, injuries to the median or ulnar nerves affect other muscle groups (flexors of the forearm and intrinsic paw muscles) and wouldn’t produce an inability to extend the elbow. A brachial plexus avulsion would cause a broader loss of motor function and sensation, not a focal triceps atrophy with elbow extension failure.