The Martin-Gruber anastomosis (MGA) is a variant connecting median and ulnar nerves found in about 20% of healthy individuals. It has important diagnostic implications for electrodiagnosticians since it can yield unusual findings on nerve conduction studies including: ulnar pseudo-conduction block, larger proximal response with median motor studies, erroneously fast measured conduction velocity in the median nerve, and an initial positive deflection with median nerve stimulation at the elbow but not the wrist. But less has been written about the beneficial or protective effects of the anomaly.
We present the case of a 15 year old male who was cutting brush with a co-worker, both of whom were using “brush cutters” (essentially a circular saw on the end of a pole). His co-worker mistakenly cut through the patient’s medial elbow, severing the ulnar nerve and the triceps tendon. He had primary repair within days of the injury. He presented for electrodiagnostic and surgical assessment 18 months later due to persistent weakness in grip strength and consideration of nerve transfer from the anterior interosseous nerve (branch to pronator quadratus) to the ulnar nerve.
On examination, weakness was noted in flexor digitorum profundus (FDP) to D4 and D5 at 2/5; abductor digiti minimi (ADM) at 4/5; and first dorsal interosseous (FDI) at 4+/5. Sensation in the affected ulnar nerve territory was 20% of contralateral. Needle EMG demonstrated reinnervation in FDP, and early reinnervation with nascent motor unit action potentials (MUAPs) in ADM. But the FDI demonstrated good recruitment of large MUAPs. Ulnar motor responses demonstrated small responses to ADM with wrist (3.7 mV) and elbow (3.1 mV) stimulation, and 10.6 mV responses to FDI with wrist stimulation, but only 0.2 mV responses with elbow stimulation. Stimulating the median nerve at the elbow produced a 10.6 mV response from FDI. It was concluded that the patient had a severe nerve ulnar neuropathy with partial recovery, but had preserved interosseous strength primarily via the MGA.
We propose that the MGA has important protective effects in the setting of severe ulnar neuropathy at the elbow and may offer an evolutionary advantage. Moreover, it is important to recognize this anomaly in such patients by recording from the FDI (the most commonly affected muscle). Finally, one may be less likely consider a nerve transfer from AIN to ulnar nerve in these cases, since there is essentially already such a connection naturally.
