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Journal of the Anatomical Society of India

Inter-Communications Between Median And Musculocutaneous Nerves With Dual Innervation of Brachialis Muscle-A Case Report

Author(s): Arora, J; Kapur, V; Suri, R.K., Khan, R.Q.

Vol. 52, No. 1 (2003-01 - 2003-12)

Department of Anatomy, Maulana Azad Medical College, N. Delhi, INDIA

Abstract

Inter-communications between peripheral nerves deserve special attention in view of their clinical significance. Thepresent article pertains to a case displaying two sites of communications between median and musculocutaneous nerves in the left arm ofa 60-years old male cadaver. The proximal communicating trunk coursed between musculocutaneous and median nerves. It was given offbefore the musculocutaneous nerve pierced the coracobrachialis muscle. The distal communicating trunk carried fibres from median tomusculocutaneous nerve and joined the latter after it had supplied coracobrachialis and biceps brachii. From the site of union of the distalcommunicating branch and the musculocutaneous nerve, branches were given off to the brachialis muscle.

Key words: Median nerve, communications, musculocutaneous nerve, brachialis.

Introduction:

Connections at the arm level between the musculocutaneous (MCN) and median nerves (MN) have been reported by many authors (Turner, 1864; Le Minor, 1990; Choi et al, 2002). The most frequent of these variations consists of the presence of a communicating branch that bifurcates from the MCN and goes distally to join the MN. (Venieratos and Anagnastopoulou, 1998). The opposite situation namely the presence of a communicating branch from MN to MCN is rarely encountered (Ferner, 1938; Venieratos and Anagnostopoulou, 1998).

In the present case, we recorded two communications between MN and MCN. Such incidence of two communicating branches between MN and MCN is uncommonly reported in literature (Vallois, 1922; Yang et al, 1995) and presents great variability. Interestingly, the brachialis muscle was innervated by nerve fibres from the site of union of the distal communicating branch and the MCN.

In the present article we have discussed the topography, morphology and clinical aspects of these variations.

Case Report:

The present obsevation was encountered during routine dissection of the left side of the upper limb of a 60 year old male cadaver. The MCN and MN had inter-communications at two sites. The proximal communicating trunk was 2.5cm in length and was given off by the MCN before the latter pierced the coracobrachialis (CB) muscle. This trunk coursed distally to join the MN 4.6 cms from the coracoid process. The length of distal communicating branch was 10.7cm. it emerged from the MN, 12.4 cms from the coracoid process, and coursed distally to join the MCN after the latter had pierced the CB and had supplied branches to biceps brachii. From the site of union of the distal communicating branch and the MCN, branches were given off to the brachialis muscle. The MCN continued distally as the lateral cutaneous nerve of the forearm. (Photograph 1, 2 & 3)

Discussion:

The communications between MN and MCN have been classified by earlier workers (Le Minor, 1990; Kosugi et al, 1986; Venieratos and Anagnastopoulou, 1998). In the most recent observations reocrded by Choi et al, 2002 such communications have been broadly classified into 3 patterns. In pattern I, the MCN and MN were fused. In pattern 2, There was one connecting branch between the MCN & MN. In pattern 3, two connecting branches were present between the MCN and MN. The incidence of this variant was 6.8%. The pattern of communication recorded in the present case can be placed in type 3 of classification advocated by Choi et al, 2002, since there were two communicating branches between MN and MCN. In most of these cases reported by Choi et al 2002, the communicating branches were from MCN to MN. However, in our case, the proximal communicating branch emerged from MCN to MN while the distal communicating branch was given off in the opposite direction i.e. from MN to MCN and coursed distally to join the latter in the middle one third of the arm. Such connections from the MN to the MCN, in the opposite direction are rarely found (Vallois, 1922; Hirasawa, 1931; Ferner, 1938; Kosugi et al, 1992; Venieratos & Anagnostopoulou, 1998).

The incidence of 2 communicating branches between MN and MCN has been reported as the most uncommon pattern with great variability owing to the number of possibilities of origin, length and direction of the supplementary branches. (Choi et al, 2002) The presence of such communications may be attributed to random factors influencing the mechanism of formation of limb muscles and the peripheral nerves during embryonic life. Significant variations in nerve patterns may be a result of altered signalling between mesenchymal cells and neuronal growth cones (Sanes et al, 2000) or circulatory factors at the time of fusion of brachial plexus cords (Kosugi et al, 1986). 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. (Miller, 1934.)

If the surgeon finds it necessary to isolate and trace the MN and MCN distally, it is essential to be alert to communications that may occur between them. The clinical relevance of such variations might be correlated to entrapment syndromes. Entrapment of MCN is rare and has its origin either in physical activity (Falsenthal et al, 1984) or in violent passive movements of arm and forearm (Kim & Goodrich, 1984). If this situation coexists with anastomosis to MN, it may give rise to symptoms of MN neuropathy (Wertsch et al, 1982). This knowledge may prove useful for clinicians in order to avoid an unnecessary Carpal tunnel release (Venieratos & Anagnostopoulou, 1998).

The distal communicating branch from MN to MCN coursed between the biceps and brachialis muscles. Presumably, there may be compression of the distal communicating branch at this site during vigorous exercise with flexion of the arm or direct injury. Such compression may be the anatomic basis for nerve compression syndromes of unexplained etiology.

In the present case, the brachialis muscle was supplied by branches from the site of union of the MCN and the distal communicating branch from the MN. Branches to the brachialis muscle from connection between MN and MCN are rarely reported (Kosugi et al, 1992; Vallois, 1922; Yang et al, 1995). Such an innervation is of clinical significance since there is an increased chance of denervating the brachialis muscle during an anterior surgical approach to the humerus, that includes longitudinally bissecting the muscle (Mahakkanukrauh & Somsoarp, 2002). The clinical implication of such a variant distal communication between MCN & MN may mislead the physician in cases of nerve injury in this region of the arm since in such cases flexion at the elbow joint would still be possible due to the presence of innervation from the MN to brachialis.

Variations that were observed in the present study involving the MN and MCN are important in repairs for trauma to the shoulder and the understanding of MN & MCN dysfunction (Sunderland, 1978; Flatow et al, 1989; Sonck et al, 1991). Hence, we conclude that variations in the anatomical connections between MN and MCN should be considered during surgical procedures and management of disorders of the upper limb.

References:

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Fig-1

Schematic diagram of inter-communications between MN and MCN. Note the branches to brachialis from the site of union of distal communicating branch and MCN. MN- median nerve, MR & LR-medial and lateral root of MN, P- proximal communicating branch, MCN-musculocutaneous nerve, D–distal communicating branch, S -site of union of D & MCN, bbc - branches to biceps brachii, bbr-branches to brachialis, UN -ulnar nerve, CB -coracobrachialis, BB - biceps brachii, CP - coracoid process, L - lateral cutaneous nerve of forearm, TBc tendon of biceps:

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Fig-2

Photograph of the dissected left upper arm showing proximal communication between MN and MCN before the latter pierced the CB. MN - median nerve, MR -medial root, LR - lateral root, MCN - musculocutaneous nerve, P - proximal communicating branch, UN -ulnar nerve, CB - coracobrachialis.

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Fig-3

Photograph of the dissected left upper arm showing distal communication and branches to brachialis muscle. MN - median nerve,MCN - musculocutaneous nerve, D- distal communicating branch, S - site of union of D & MCN, bbc - branches to biceps brachii, bbr- branches to brachialis, UN - ulnar nerve, CB - coracobrachialis, BB - biceps brachii, CP-coracoid process, L- lateral cutaneous nerve of forearm.

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