Martin-Gruber anastomosis with anomalous superficial radial innervation to ulnar dorsum of hand: a pitfall when common variants coexist.

The Martin-Gruber anastomosis (MGA) is the most common anatomic variation in the upper extremity. Anomalous superficial radial innervation to the ulnar dorsum of the hand is the most common cause of an absent dorsal ulnar cutaneous (DUC) response. The coexistence of these variants introduces a relatively common yet underrecognized potential pitfall in nerve conduction studies (NCS). We performed confirmatory NCS in two cases referred for ulnar neuropathy in the forearm (case 1) and at the elbow (UNE, case 2). Initial NCS in both cases suggested ulnar nerve injury at the forearm and elbow, respectively, based on an apparent conduction block in ulnar motor fibers in the forearm (case 1) and elbow (case 2), and absent DUC responses. Additional NCS documented an MGA in the mid-forearm (case 1) and high proximal forearm (case 2) with anomalous superficial radial innervation to the ulnar dorsum of the hand (both cases). Failure to recognize the coexistence of these two common variants may lead to misdiagnosis of ulnar neuropathy and inappropriate treatment.