A 47-yr-old physiatrist was demonstrating right ulnar nerve conduction studies to residents when the responses displayed in Figure 1 were recorded.1 Review of systems was remarkable only for intermittent tingling of the small finger, worsened by prolonged bicycle rides. Physical examination revealed no weakness in any of the upper limb muscles. Muscle stretch reflexes were 2+ and symmetrical. The tracings demonstrate recordings from the abductor digiti minimi (top four traces) and first dorsal interosseous (bottom four traces), while stimulating the ulnar nerve at the wrist, below elbow, above elbow, and axilla. Concern was raised by the residents that this might demonstrate an ulnar neuropathy in the distal forearm because the recording from the first dorsal interosseous dropped from the wrist to below elbow by almost 50% in amplitude and area. The residents were ready to make a surgical referral.
Differential diagnosis included ulnar neuropathy in the forearm (which is very rare) or Martin-Gruber anastomosis (which is quite common). Subsequent nerve conduction studies stimulating the median nerve at the elbow while recording at the first dorsal interosseous demonstrated an initially negative response, with an amplitude of 3.8 mV, consistent with a crossover in the forearm. The larger amplitudes seen with stimulation in the axilla likely represents co-stimulation of the median nerve, which is very close to the ulnar nerve in the axilla. Studies on the contralateral limb revealed essentially the same results.
The physician/patient quickly recognized that this was consistent with Martin-Gruber anastomosis and fortunately saved himself an operation for ulnar neuropathy in the forearm. His numbness has since resolved on its own.
1. Gutman L: AAEM minimonograph #2: Important anomalous innervations of the extremities. Muscle Nerve