An ultrasound diagnostic aid in hand surgery: observation on a supernumerary muscle in the wrist.

The authors report on a case of dynamic compression of the ulnar nerve in the wrist by a supernumerary hypothenar muscle in a twenty-six-year-old female patient. For eight months, she had been suffering from acroparesthesias in the territory of the ulnar nerve with pain upon effort irradiating into the forearm. The initial clinical examination was rather non-conclusive and the electromyogram found no anomaly. Faced with this dynamic symptomatology, a provisional ultrasonography was performed, revealing a picture of apparent muscular appearance, confirmed on the MRI. Surgical exploration also confirmed the presence of this muscle located between the ulnar artery at the front and the ulnar nerve, which it was pressing against, at the back. It was a supernumerary fascicle of the flexor digiti minimi brevis for which was performed a complete surgical removal. At three months from neurolysis of the ulnar nerve and removal of the muscle, the preoperative symptoms had completely disappeared. This observation reminds us of the primordial role that imaging plays in detecting ulnar nerve compression at the wrist. Although the precision of an MRI as regards the description of supernumerary muscle of the wrist is not discussed, this case emphasizes the interest of ultrasonography.