Electrophysiological study of anomalous innervation of intrinsic hand muscles.

The collision technique was used to study the incidence of anastomosis between median and ulnar nerves in the forearm (Martin-Gruber anastomosis). A motor anastomosis from median to ulnar nerve was found in 57 (17%) of 328 unselected subjects. The anomaly was bilateral in 39 subjects (68%) and unilateral in 18, nine on the right and nine on the left. Both hypothenar and thenar muscles were tested in 77 of 96 hands with the anastomosis (right and left sides combined). Both muscle groups were innervated by the communicating fibers in 63 hands (82%). Anomalous innervation was limited to hypothenar muscles in 13 and to thenar muscles in one hand. In contrast to the high incidence of the anastomosis from median to ulnar nerve, crossover from ulnar to median nerve in the forearm was found in none of 303 subjects tested.