August, 2016: How is opposition possible with a complete median nerve laceration?
FREE FULL TEXT: Ahadi et al: Prevalence of ulnar-to-median nerve motor fiber anastomosis (Riché-Cannieu communicating branch) in hand: An electrophysiological study Med J Islam Repub Iran 30; 2016: 324
Riche and Cannieu both published articles in 1897 describing a nerve crossing between the deep branch of the ulnar nerve and the motor branch of the median nerve in the first web. The Riche Cannieu communication (RCC) could account for paradoxical findings of thenar muscle function or absence of function with median or ulnar nerve lacerations at the wrist. Also it could be mistaken for a lower cervical root or brachial plexus problem.
This area at the base of the first web is rarely exposed at surgery, and cadaveric dissection of these fine nerve fibers deep in the first web is difficult. When anatomists do identify fibers, they have no way of knowing whether the fibers are for motor or sensory function. So the Riche-Cannieu connection is a black box: the anatomists see fibers but do not know their function, and examiners can test for functioning and nonfunctioning muscles after an injury but do not know what the anatomic particulars are.
To shed light on this enigma, investigators in Iran studied 23 patients with less than one-month-old complete median nerve lacerations in the distal third of the forearm. They placed needle electrodes directly into the abductor pollicis brevis (APB) and opponens pollicis (OP) muscles and then stimulated the ulnar and median nerves at the wrist, first one, then the other.
In absence of an RCC, stimulation of either nerve should not evoke a response in the ABP or OP. (They did not test the FPB because its deep location prevented accurate needle electrode placement.)
Before including a patient in the study, the investigators took pains to ensure that the median nerve was completely cut and that the ulnar nerve was uninjured. They also excluded patients with evidence of peripheral neuropathy, injuries to the thenar and palmar areas, burns, and electrical shock injuries.
The average age of the 23 patients was 29 years +/- 6. Nineteen were men. Stabs accounted for 8 injuries, glass for 15.
RESULTS: Stimulating the ulnar nerve led to a motor action potential of the APB in 19/23 (83%) of hands and to a motor action potential of the OP in 14/23 (61%) of hands. Every hand that had an OP action potential also had an APB action potential.
DISCUSSION: The authors note that since this was an electrophysiologic study, the presence or size of a motor action potential cannot indicate the number of neurons in the RCC or in the number of muscle fibers so innervated. They also note that they could not be entirely confident that their needle electrodes were in the muscles of interest, particularly in instances where the thenar muscles were already atrophic.
COMMENT: The paper provides good electrophysiologic proof of the existence of an RCC in 5 out of 6 hands. In other words, the absence of an RCC should be considered the anomaly, not the presence of an RCC. Our clinical experience tells us that patients with a complete median nerve laceration typically have no opposition, even though they have an 83% chance of having an RCC. This tells me than in most instances, the RCC constitutes only a few fibers, so that the unlar nerve’s contribution to opposition is minor and not identifiable clinically. There are, however, instances where a patient does retain partial or complete opposition in face of a complete median nerve laceration. Here, the RCC is presumably robust, and ulnar nerve innervation explains the preservation of opposition. Isn’t anatomy fun!
To keep the take home point simple, I only need to remember that 5 out of 6 hands have electrophysiologic evidence of an RCC. Since the APB does everything that the OP does and more, I doubt that it is possible to isolate OP function on physical examination. Therefore remembering that some RCCs innervate the OP as well as the APB provides me no useful clinical information. Bottom line: the RCC supplies some ulnar nerve fibers to the APB in 5 out of 6 hands.