Electrophysiologic evaluation of entrapment neuropathy caused by scar formation in recurrent motor branch of the median nerve

A unique case of mononeuropathy was encountered in a 20-year-old man with isolated involvement of the recurrent motor branch of the right median nerve following a skiing accident one year previously. There was also isolated involvement of the right thenar muscles innervated by the median nerve. Sensation was intact in the right hand and fingers. Conventional conduction studies in the right median nerve yielded no abnormal findings except a reduction in the amplitude of the muscle action potential. However, insertion of a needle electrode into the abductor pollicis brevis muscle revealed some delayed motor unit potentials following electrical stimulation at the wrist. We performed surgery and found the right recurrent motor branch to be entrapped by fibrous scar tissue 3 to 4 mm distal to branching from the median nerve. Direct recordings made from the nerve trunk using a tungsten microelectrode with supramaximal stimulation at the elbow revealed reduced amplitude of the compound nerve action potential distal to the site of entrapment. We concluded that scar tissue resulting from the previous injury had compressed the recurrent motor branch, causing this unique neuropathy.