The Riche-Cannieu anastomosis is a neural connection between the deep branch of the ulnar nerve and the recurrent motor branch of the median nerve to the thenar. Rarely, it provides complete or nearly complete ulnar innervation of the thenar muscles (all-ulnar hand). This may lead to confusing clinical and electrodiagnostic findings in patients with median or ulnar nerve pathology, or during workup for suspected nerve lesions. Two patients referred for electrodiagnostic assessment of carpal

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... el syndrome and one patient with suspected peripheral neuropathy presented with absent or very low compound motor action potential from the thenar after median nerve stimulation. This occurred in spite of normal thenar muscles' bulk and function, in one hand (one subject) or both hands (two subjects). Ulnar nerve stimulation at wrist and elbow while recording over abductor pollicis brevis, first and second lumbrical muscles and needle electromyography allowed us to recognize Riche-Cannieu anastomosis and avoid diagnostic errors. The rare innervation variant of all-ulnar hand should be considered in less typical presentations of hand disorders, especially when there is a discrepancy between the clinical findings and the expected electrodiagnostic changes.