Communication between musculocutaneous and median nerves: a case report

INTRODUCTION

Variations in the formation, branching and communication between the branches of the brachial plexus is a common phenomenon and it has several clinical and surgical implications.^2-6 The musculocutaneous, median and ulnar nerves after arising from the brachial plexus, pass through the flexor compartment of arm without receiving any communicating branch from neighbouring nerves.⁷ Although communications between the nerves in the arm are rare, the communication between the Median Nerve (MN) and Musculocutaneous Nerve (MCN) have been documented earlier. When the lateral root of the median nerve carries the fibers of the MCN it leaves as a communicating branch from the median nerve and joins the MN in the lower third of arm.⁶ Normally MCN passes through the Coracobrachialis Muscle (CBM) and innervates it as well as the brachialis and the biceps brachii muscles and later continues as the lateral cutaneous nerve of the forearm without exhibiting any communication with the MN or other nerves.^8,9 MCN may run behind the CBM or adhere for some distance to the MN and pass behind the biceps brachii muscle. The aim of this paper is to report a rare finding where MCN did not pierce CBM and gave communicating branch to MN in the middle of the arm on the right side. This would enable the surgeons to have better knowledge of the field during surgery to avoid neurological damages.

CASE REPORT

During routine dissection of right upper limb in an adult male cadaver, it was observed that MCN was originating normally from lateral cord of brachial plexus but did not pierce the coracobrachialis muscle and was medial to it. MCN gave a branch to coracobrachialis, biceps brachii. Before giving a muscular branch to brachialis MCN gave a communicating branch to MN in the middle of the arm (Figure 1). The communicating branch had an oblique course between the two nerves. Further course and branches of two nerves in arm, forearm and hand was normal. The course and branches of two nerves were normal on left side.

DISCUSSION

In the earlier studies done on communications between MCN and MN, a maximum of two communications have been reported.^4,10 In the present case only one communication is seen, which is a rare case finding. Le Minor² has classified the variations of MCN and MN into five types. In type I, there are no communication between the MCN and MN. In type II, the fibers of medial root of MN pass through MCN and join the median nerve in the middle of the arm. In type III, the lateral root of the MN from the lateral cord runs in the MCN and leaves it after a distance to join the main trunk of MN. In type IV, the fibers of the MCN unite with the lateral root of the MN. After some distance, the MCN arises from the MN. In type V, the MCN is absent. The fibers of the MCN run within the MN along its course. In this type the MCN does not pierce the CBM. Veinreratos and Anagnostopolou³ studied 79 cadavers and found communications between MCN and MN in 22 cadavers.³ They reported the following three types of communications between MCN and MN, in relation to CBM. In type I: the communication was proximal to the entrance of the MCN into the CBM (9/22); in type II: the communication was distal to the CBM (10/22) and in type III: the nerve as well as the communicating branch did not pierce the muscle (3/22). In a recent study by Choi et al., communications between MCN and MN have been broadly classified into three types.⁴ In type I: the MCN and MN were fused; in type II: there was one connecting branch between the MCN and MN and in type III: two connecting branches were present between MCN and MN. So, the present case coincides with type II category of Choi D classification. Studies by Nakatasi et al., revealed three variations in which the musculocutaneous nerve did not pierce the coracobrachialis.¹⁰ Tsikaras et al. revealed that MCN arise from the MN unilaterally in a male cadaver.¹¹ Chaitra R¹² also reported one case on right side; MCN descended without piercing the coracobrachialis muscle and supplied muscles of the flexor compartment of arm.

In the context that ontogeny recapitulates phylogeny; it is possible that the variation seen in the present study is the result of developmental anomaly. In human being forelimb muscles develops from mesenchyme of paraxial mesoderm in the fifth week of intrauterine life.¹³ Regional expression of five Hox D (Hox D 1 to Hox D 5) genes is responsible for upper limb development.¹⁴ The motor axons arrive at the base of limb bud; they mix to form brachial plexus in upper limb. The growth cones of axons continue in the limb bud.¹³ As the guidance of the developing axons is regulated by the expression of chemo-attractants and chemo-repulsants in a highly coordinated site specific fashion any alterations in signaling between mesenchymal cells and neuronal growth cones can lead to significant variations.¹⁵ Studies of comparative anatomy have observed the existence of such connections in monkeys and in some apes; the connections may represent the primitive nerve supply of the anterior arm muscles.⁵

These variations also have clinical importance especially in post-traumatic evaluations and exploratory innervations of the arm for peripheral nerve repair. The knowledge of the variations of this communication between the MCN and MN in the middle of the arm is important in the anterior approach for the fracture of the humerus. Clinical implication of this could be that injury of MCN proximal to the anastomotic branch between musculocutaneous and median nerve may lead to unexpected presentation of weakness of flexors of forearm and thenar muscles.¹⁶

Significant variations in nerve patterns may also occur circulatory factors at the time of fusion of brachial plexus cords.¹⁵ The presence of such nerve communications are not just confined to man, studies on comparative anatomy have reported the existence of such connections in monkeys and in some apes. Thus suggesting that communications may represent the primitive nerve supply of anterior arm muscles.⁵

CONCLUSION

Communication branches between musculocutaneous nerve and median nerve or between median nerve and ulnar nerve have been reported.¹⁷ In diagnostic clinical neurophysiology, variations in connections between musculocutaneous nerve and median nerve may have significance. These variations have clinical importance in post-traumatic evaluations and exploratory interventions of the arm for peripheral repair. The knowledge of the possible communications is also important in the anterior approach for the fracture of the humerus and regional nerve blocks. It is important to be aware of this variation while planning the surgery in the region of the arm, as these nerves are more liable to be injured during the operation. Any compression on the communicating branch may give rise to varying pattern of weakness that may impede difficulty in diagnosis for neurologists.

Funding: No funding sources

Conflict of interest: None declared

Ethical approval: Not required

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