Verification of the Median-to-ulnar and Ulnar-to-median Nerve Motor Fiber Anastomosis in the Forearm: an Electrophysiological Study

Objective: To estimate the real occurrence of the motor median-to-ulnar nerve anastomosis in the proximal forearm (Martin-Gruber anastomosis, MGA), as its frequency varies between 6 and 44% in the literature and to investigate the incidence of the ulnar-to-median nerve anastomosis in the distal forearm.

Methods: Compound muscle action potentials (CMAP) recorded over thenar, hypothenar, and first dorsal interosseus muscle on median or ulnar nerve stimulation at wrist and elbow and collision blocks of the median and ulnar nerve were compared in a group of 50 healthy volunteers. Particular precautions were undertaken in order to avoid false positive results due to stimulus spread to the neighboring nerve. Cases of uncertain MGA were classified as either MGA or non-MGA on the basis of posterior probabilities estimated by discriminant analysis.

Results: The estimated frequency of MGA was 54% using the potential comparison method and 46% using the collision technique. An ulnar-to-median nerve anastomosis was not found in any subject.

Conclusions: While the MGA is very common, the ulnar-to-median nerve anastomosis is a rarity. Standard nerve conduction studies of the median nerve with CMAP recordings solely over thenar will detect less than 14% of MGA cases.