Five of 65 patients referred for electrodiagnosis because of clinical evidence of carpal tunnel syndrome were found to have near normal latency on proximal stimulation of the median nerve, although the distal motor latency was prolonged. In one patient, the proximal latency was actually shorter than the distal latency. The failure of the proximal latency to be prolonged in proportion to the distal latency results in a spuriously high apparent conduction velocity in the forearm segment of the nerve. This value may even exceed the conduction velocity of the corresponding nerve segment in the unaffected arm. Stimulation studies on the ulnar nerve reveal that this disparity is the result of some of the median nerve fibres destined for the thenar muscles taking an aberrant course through the ulnar nerve and thus escaping compression at the wrist. A median-ulnar communication in the forearm, the 'Martin-Gruber' anastomosis, may occur in up to 15% of the population. The presence of the Martin-Gruber anastomosis in patients with carpal tunnel syndrome results in a partial or total sparing of thenar muscles from denervation and the paradoxical recording of normal proximal latencies in the median nerve when the distal latency is prolonged.
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