Sparing of the second lumbrical in a Riche-Cannieu anastomosis: the nearly all-ulnar hand.

The Riche-Cannieu anastomosis (RCA) is an anatomic variant wherein a deep branch of the distal ulnar nerve innervates muscles of the thenar eminence. A few cases in the literature describe an "all-ulnar hand" when the muscles of the thenar eminence get innervation solely from the ulnar nerve without any contribution from the median nerve. Clinically, patients with ulnar mononeuropathies in the setting of an RCA would also have weakness/atrophy of thenar muscles. Conversely, patients with median mononeuropathies would have sparing of these muscles. This case highlights RCA in the setting of carpal tunnel syndrome and documents the persistent innervation of the second lumbrical by the median nerve. No anatomic dissections have disclosed innervation of the second lumbrical by the deep ulnar nerve, negating the concept of the all-ulnar hand.