Median-ulnar nerve communications and carpal tunnel syndrome

SUMMARY Carpal tunnel syndrome in the presence of anomalous median to ulnar nerve communications in the forearm produces a characteristic change in motor conduction studies. Median nerve stimulation at the elbow evokes a thenar muscle action potential (MAP) with an initial positive deflection not seen on stimulation at the wrist. In 63 patients this change occurred in 16 (25%) and is a useful additional criterion in the diagnosis of carpal tunnel syndrome. The initial positive deflection is due to the volume-conducted MAP from the first dorsal interosseous and some thenar muscles whose motor point lies some distance from the recording electrode over abductor pollicis brevis. The first dorsal interosseous and thenar MAPs resulting from elbow stimulation of those median nerve axons crossing to ulnar nerve in fore-arm, are generated before that from thenar muscles supplied by the axons going through the carpal tunnel. Anomalous innervations of muscles in the ex-tremities occur commonly and involve the median, ulnar, and peroneal nerves. It has been estimated that in the arms of 15-31 % of people, motor axons descend in the median nerve which, in the forearm, cross over to the ulnar nerve, and ulti-mately innervate intrinsic hand muscles (Fig. 1) normally supplied by the ulnar nerve (Mannerfelt, 1966; Wilbourn and Lambert, 1976). This anomaly gives rise to changes in motor conduction studies identifying its presence. When this common anomaly occurs in association with the often seen carpal tunnel syndrome additional changes may occur in evoked muscle potentials (Lambert, 1962; Iyer and Fenichel, 1976). The most frequent of these has received very little attention and yet, in our laboratory, we view it as a useful additional criterion in the diagnosis of carpal tunnel syndrome in the presence of this anomaly. It is the purpose of this study to describe this change, its frequency, and the evidence that it is due to median-ulnar nerve communications. Patients and methods The study was carried out in two phases. In the first, the data from 63 consecutive patients with bilateral carpal tunnel syndrome were evaluated