Electrophysiological study of forearm sensory fiber crossover in Martin-Gruber anastomosis

Although anatomical studies have shown that a crossover of sensory fibers is not rare in forearm Martin-Gruber median-ulnar anastomosis (MGA), it has been electrophysiologically described only in rare subjects. Using a near-nerve needle technique, the possibility of electrophysiologically detecting a forearm median-ulnar crossover of sensory fibers was investigated in 24 arms of 21 subjects with unilateral or bilateral MGA, by stimulating the fifth digit of the hand and recording along the median nerve. Small-amplitude elbow responses were found in the median nerve in 10 of the 24 arms but, in 9, the responses disappeared after lidocaine block of the ulnar nerve distal to the elbow sulcus, indicating their volume-conducted origin. In one subject with carpal tunnel syndrome and a subclinical ulnar neuropathy at the elbow, the elbow response was not affected by the ulnar block, thus confirming the presence of a sensory anastomosis in the forearm. In another subject with MGA, a clear-cut sensory response was recorded in the median nerve at the elbow by stimulating the fifth digit of the right hand but no anesthetic block was performed, because ulnar responses were absent above the elbow sulcus due to a severe lesion at the elbow. Thus, use of a near-nerve recording technique facilitates recognition of median-ulnar crossover of sensory fibers to the fifth digit, which is, however, uncommon.