An Unusual Communication between the Radial and the Ulnar Nerves

Communication between the terminal nerves of the brachial plexus has been often reported, that is, the median and musculocutaneous nerves in the upper arm [1, 2], median and ulnar nerves (Martin-Gruber and Marinacci anastomoses) in the forearm [3-5], ulnar and median nerves (Riche-Cannieu anastomosis) in the hand [6]. All of these communications were connections between nerves derived from the anterior division of the brachial plexus. However, in literatures, it could not be found to describe communication from radial to ulnar nerves, which are connections between nerves derived from the anterior and posterior divisions.

Bergmann et al. (1988) [7] described an anomalous branch of the ulnar nerve innervating the medial head of the triceps brachii muscle. These innervations are uncommon, because the nerve derived from the anterior division distributes the territories of the nerve derived from the posterior division.

If these two anomalies, which are communication from radial to ulnar nerves and the ulnar nerve innervating the medial head of the triceps brachii muscle, appeared in same cadaver, a correlation of the two variations can be deduced. The purpose of this paper is to present a detailed report of unusual communication between the radial and the ulnar nerves and uncommon innervation of the ulnar nerve.

An unusual unilateral communicating branch was found in formalin-fixed Korean cadaver of a 59-year-old male during dissection in a gross anatomy course. The brachial plexus containing the communicating branch were extracted en bloc. The extracted specimens were immersed in Guanidine-HCl (0.2 M) for two weeks and then treated several times with an ultrasonic cleaner for an hour to soften the connective tissue around the nerve bundles. The processed brachial plexus were dissected carefully removing the connective tissue around the nerve bundles to expose the running of the nerve fibers under a surgical microscope (Carl Zeiss OPMI-FC, ×7.5), and to confirm the spinal root compositions of the communicating branch [8] (Lee 2007). Applying the routine histological preparation, numbers of the nerve fiber were counted to estimate the correlation of the two variant nerve branches.

The unusual communicating branch from the radial to the ulnar nerves was found unilaterally in the axilla region on the right side. The spinal root origins of this communicating branch were largely C7 and some C8 (Fig. 1). In the same cadaver, another variant was observed unilaterally on the right side. The branches of the ulnar nerve innervated the medial side of the medial head of the triceps brachii muscle (Fig. 2). Numbers of nerve fibers of the communicating branch and the main distributing branch of the ulnar nerve to the triceps brachii muscle were 523 and 525, respectively (Fig. 3). Thus two branches had similar quantities of the nerve fibers.

Fig. 1
Photograph showing the communicating branch between the radial and the ulnar nerves and its spinal nerve components in the posterior aspect (right side). The communicating branch (arrow) arises as short distance from the origin point of the radial nerve (A). After separating nerve fascicles, the components of the communicating branch are confirmed that it derives from mainly C7 and some C8 (B). Ax: axillary nerve, Ul: ulnar nerve, Rd: radial nerve, Td: thoracodorsal nerve, LS: lower subscapular nerve, Mc: Musculocutaneous nerve, Md: median nerve.

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Fig. 2
The branches of the ulnar nerve innervating the triceps brachii muscle. The branches (arrows) of the ulnar nerve innervate the medial two-third of the medial head (med.h) of the triceps brachii muscle. Ul: ulnar nerve, BA: brachial artery, Mc: musculocutaneous nerve, Bra.m.: brachialis muscle.

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Fig. 3
Histologic comparison of the communicating branch between the radial to the ulnar nerves (A) and main distributing branches to the medial head of the triceps brachii muscle (B). Numbers of nerve fibers of communicating branch and main distributing branch of the ulnar nerve to the triceps brachii muscle are 523 and 525, respectively.

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The communicating branch was believed to occur when the nerve fibers that run into the different route (nerve) return to the original route (nerve), such as in Martin-Gruber anastomosis, in which the ulnar nerve components that are mixed with the median nerve return to their original innervating territories. Therefore this result implies that nerve fibers moving from the radial to the ulnar nerves may innervate the original distributing territories of the radial nerve.

Bergman et al. (1988) [7] described a muscular branch of the ulnar nerve innervating the medial head of the triceps brachii muscle. However, the authors could not explain the "occurring cause of the variation". We believe that the communicating branch certainly shows the "occurring cause" in the present study. The spinal root compositions of this communicating branch were largely C7 and some C8, and the triceps brachii muscle was normally innervated by C7 and C8 [9]. Even though terminal innervation of the communicating branch was not observed in the arm, numbers of nerve fibers of the communicating branch and the main distributing branch of the ulnar nerve to the triceps brachii muscle were similar. Thus it is thought that the communicating branch from the radial to the ulnar nerves innervated the original distributing territories of the radial nerve. Some articles have described the variant sensory distribution of the dorsum of the hand [7, 10, 11]. It is known that the sensory nerve innervating the muscle may convey proprioceptive impulse from the muscle. If the communicating branch in this result was sensory components, the ulnar dominant sensory distribution of the dorsum of the hand may have appeared.

As another possibility of the occurrence of this variant communication, the communicating branch may be driven from the radial muscular branch to the medial head of triceps brachii muscle. This muscular branch runs generally close to the ulnar nerve. A part of this branch may be fused to the ulnar nerve in their distributing territories. This muscular branch is called as 'ulnar collateral nerve'. Therefore, this unusual communication could appear in running process of the neighboring nerves. The "occurring cause" may present in the proximal or distal portion of the related nerves of various communications of several regions.