[Riche-Cannieu anastomosis and a paradoxical preservation of thenar muscles in carpal tunnel syndrome: a case report].

: A 72-year old woman had been suffered from a dysesthesia in the left median nerve distribution, followed by a dysesthesia in the right ulnar distribution. Neurological examination revealed weakness in the right intrinsic hand muscles with the ipsilateral thenar and hyothenar atrophy. Paradoxical preservation of the left thenar muscles was a clinical challenge. Nerve conduction studies disclosed bilateral carpal tunnel syndrome and the right cubital tunnel syndrome. In needle electromyography, however, the left abductor pollicis brevis and opponens pollicis muscles had normal motor unit potentials without denervation activity. The needle recording of the left abductor pollicis brevis muscle showed a good motor response with a negative deflection by the left ulnar nerve stimulation, indicating an ulnar to median nerve innervation, i.e., Riche-Cannieu anastomosis. A Riche-Cannieu anastomosis in a setting of a median or ulnar nerve injury can produce confusing clinical and electrodiagnostic findings. We reviewed clinical findings, electrophysiological data, and the impact of a Riche-Cannieu anastomosis in median nerve injury.