Median--ulnar nerve communications and anomalous innervation of the intrinsic hand muscles: an electrophysiological study.

One hundred (100) left arms were examined for median-to-ulnar or ulnar-to-median nerve anastomoses using surface electrodes. Muscles innervated by the anastomosis were determined using a concentric needle electrode. We found a motor median-to-ulnar nerve anastomosis in the forearm in 32% of the cases (Martin-Gruber anastomosis, MGA). No case of motor ulnar-to-median nerve anastomosis in the forearm could be found. The MGA mainly innervated muscles, which are normally supplied by the ulnar nerve. The high incidence of MGA in this study shows that all clinically relevant motor median-to-ulnar nerve communications in the forearm can be detected using surface electrodes in routine examinations. Stimulus spread must be looked for and eliminated by the methods described.