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Journal of the Anatomical Society of India

Bilaterally Symmetrical Dual Origin of Muculocutaneous Nerve

Author(s): Avinash Abhaya, Bhardwaj R., Prakash R.

Vol. 55, No. 2 (2006-07 - 2006-12)

Avinash Abhaya, Bhardwaj R.(1) and Prakash R.(1)
King George Medical University, Lucknow and (1)UCMS and G.T.B. Hospital, Delhi

Abstract:

Variations of the arrangement and distribution of the lateral cord and its branches in the infraclavicular part of the brachial plexus are common and are of significance to the neurologists, surgeons, anesthetists and the anatomists. The present report describes a rare variation of the musculocutaneous nerve having a bilaterally symmetrical dual origin, observed during routine dissection of a 33-year-old Indian male cadaver. The musculocutaneous nerve and its twigs should be identified and protected, keeping in mind the variations in anatomy while planning any exploration in the region of axilla and upper arm. The clinical importance of the variation is discussed.

Key words: Musculocutaneous nerve, Median nerve, Anatomical variations.

Introduction:

Variations of the brachial plexus regarding its origin, level of junction or separation of cords, composition of fiber bundle, pre-fixation, post-fixation, relations with subclavian and axillary artery and absence or communication between its branches are common and are being reported by several authors (Kerr, 1918; Linell, 1921; Miller, 1934; Iwamotto et al, 1990; Nakatani et al, 1997; Sud and Sharma, 2000; Chauhan and Roy, 2002; Choi et al, 2002; Abhaya et al, 2003). Brachial plexus is a complex of nerves originating in the neck and axilla and is formed by the union of the ventral rami of fifth, sixth, seventh, eighth cervical, and the first thoracic spinal nerves which then unite, divide and unite again to form three trunks (upper, middle, and lower), three cords (medial, lateral, and posterior) and the nerves of the upper extremities. The lateral cord contains the fibers from C5, C6 and C7 and also from C4 if these join the plexus, while the medial cord from C8 and T1. The lateral and medial cord represents the anterior divisions of the brachial plexus and their branches supply the anterior muscles of the limb. The posterior cord is formed by the union of all the posterior divisions and so receives fibers from all the nerves entering in the plexus (Hollinshed, 1979).

Normally, the lateral cord gives its first branch, the lateral pectoral nerve to supply the pectoralis major muscle and then it divide into musculocutaneous and the lateral root of the median nerve. The musculocutaneous nerve (C 5, 6, 7) pierces the coracobrachialis muscle and than passes obliquely down to the lateral side of the arm between the biceps brachii and brachialis muscle, pierces the deep fascia lateral to the tendon of the biceps brachii near elbow and is continued as the lateral cutaneous nerve of the forearm. In its course through the arm it supplies the coracobrachialis, biceps brachii and the greater part of the brachialis muscle. The branch to the coracobrachialis is given off from the musculocutaneous nerve close to its origin, and in some instances as a separate filament from the lateral cord of brachial plexus. The branches to the biceps brachii and brachialis are given off after the musculocutaneous has pierced the coracobrachialis; that supplying the brachialis gives a filament to the elbow joint. The nerve also sends a small branch to the bone, which enters the nutrient foramen with the accompanying artery.

The median nerve (C5-T1) is formed anterior or anterolateral to the third part of the axillary artery by the union of its medial root from the medial cord and lateral root from the lateral cord of the brachial plexus. The median nerve passes in the arm at first lateral to brachial artery and near the insertion of coracobrachialis it crosses in front of (rarely behind) the artery, descending medial to it in the cubital fossa, where it passes posterior to the bicipital aponeurosis and anterior to brachialis muscle. (Williams et al, 1995).

Case Report

During routine dissection of the axilla and arm region of a 33 yr old Indian male cadaver in the department of Anatomy, University College of Medical Sciences, Delhi it is observed in the infraclavicular part of brachial plexus that the musculocutaneous nerve is having a dual origin and the variation of its origin, course and distribution is symmetrical bilaterally. The higher origin is reduced to a thin nerve, arising normally from the lateral cord of brachial plexus and supplies only the coracobrachialis muscle (Fig 1 and 2), while the lower origin is of normal usual thickness is separated from the lateral side of the median nerve, supplies the biceps brachii and the brachialis muscles and then become continuous as the lateral cutaneous nerve of the forearm after piercing the deep fascia lateral to the tendon of biceps brachii (Fig 1 and 2). The first branch of the lateral cord, the lateral pectoral nerve arises normally bilaterally just below the outer border of the first rib to supply the pectoralis major muscle. Bilaterally the relations of all the three cords of the brachial plexus with the second part of the axillary artery and the further course, branching and termination of the musculocutaneous and median nerve in the forearm and hand follow the normal usual pattern. No communication is observed between both the origins of the musculocutaneous nerve and in between the musculocutaneous and the median nerve. No other vascular or muscular variation is observed in both the upper limbs.

Keeping the upper limb at 90 degree abducted position the distance from the tip of the coracoid process is recorded on both the sides. The higher origin of the musculocutaneous nerve arise from the lateral side of the lateral cord at 42 mm (Rt) while 44 mm (Lt) and enter into the coracobrachialis muscle from its medial side. The lateral and medial root of the median nerve unite to form the median nerve at 64 mm (Rt) and 65 mm (Lt) lying anterolateral to the axillary artery. The lower origin of the musculocutaneous nerve is separated from the lateral side of median nerve at 73 mm (Rt) and 75 mm (Lt). The nerve to biceps brachii originate from the lower origin of musculocutaneous nerve at 101 mm (Rt) and 106 mm (Lt) and enter into the biceps brachii at 128 mm (Rt) and 153 mm (Lt) from the undersurface of the muscle from its medial side. The branch to the brachialis muscle splits from the lower origin of musculocutaneous nerve at 211 mm (Rt) and 205 mm (Lt) and enter into muscle at 230 mm (Rt) and at 240 mm (Lt). The lower musculocutaneous continue to run along the lateral border of the forearm and pierces the deep fascia lateral to the tendon of the biceps brachii at 324 mm (Rt) and 325 mm (Lt) to become the lateral cutaneous nerve of the forearm.

Fig. 1: Photograph of the dissected left axilla and upper arm showing the dual origin of the musculocutaneous nerve. The higher origin (MC-1, Black arrow head) is reduced to a thin nerve supplying only the coracobrachialis muscle (CB), while the lower origin (MC-II, Black arrow head) split from the lateral side of the median nerve (MN) and give the nerve (NBB) to biceps brachii muscle (BB) and continue and give branches to the brachialis (BR) as shown as NBR in the figure and than later become continuous as lateral cutaneous nerve of the forearm (LCF) after piercing the deep fascia near elbow and lateral to the tendon of the biceps muscle. (LR), lateral root and (MR), medial root of the median nerve join in front of the axillary artery (AA) to form the median nerve. (UN) Ulnar nerve.

Fig. 2: Photograph of the dissected right axilla and upper arm showing the symmetrical variation of the dual origin, course and distribution of the musculocutaneous nerve.

Discussion

The musculocutaneous nerve ordinarily enters coracobrachialis muscle from its medial aspect approximately 5 cm. distal to the tip of coracoid process but is shown to have frequent variations. It may run behind the coracobrachialis muscle or adhere for some distance to the median nerve and pass behind the biceps or may be accompanied by fibers from the median nerve as it transits coracobrachialis (Kaus and Wotowicz, 1995; Williams et al, 1995; Venieratos and Anagnostopoulou, 1998; Sevki, 2001); less frequently the reverse occurs, the median nerve sending a branch to the musculocutaneous nerve. Occasionally it supplies the pronator teres and may replace the radial branch to the dorsal surface of the thumb (Williams et al, 1995). Rarely the lateral cord pierces the coracobrachialis muscle and then divides into musculocutaneous and the lateral root of median nerve (Le Minor, 1990, Abhaya et al, 2003).

During shoulder reconstruction surgery it is important to identify or palpate the musculocutaneous nerve, as it is vulnerable to injury from the retractors placed under the coracoid muscle. The operative management by coracoid graft transfers in the recurrent dislocations of the shoulder and frequent shoulder arthroscopies could be source of lesions to the structures piercing the muscle. (Flatow et al, 1989, Laburthe-Tolra, 1994-95). The lesion of the musculocutaneous nerve produces weakness of elbow flexion and supination and loss of sensation on the lateral aspect of forearm.

The brachial plexus lesions may occur following trauma, traction, compression of nerves, shoulder dislocation, intraoperative nerve damages, traumatic delivery in infants and malposition of the patient during general anesthesia (Cooper-Daniel et al. 1988; Schwartzman, 1991; Miller 1993; Mcllveen 1994), resulting in complete or partial palsy of the musculature innervated. By surgery of the lesions in continuity and nerve grafting in cases with complete interruption, recovery can be achieved. If only one suitable root is present than the supraclavicular nerve and lateral cord (musculocutaneous nerve, lateral pectoral nerve and lateral root of median nerve) are given priority (Alnot et al. 1981). The priority of the restoration of function is also an important consideration. Elbow flexion should be given the first priority, followed by wrist extension, finger flexion and shoulder abduction in the order. (Sedal-Laurent 1988).

The knowledge of the variations of the course and distribution of the lateral cord of brachial plexus is important while performing neurotization of brachial plexus lesions, shoulder arthroscopy by anterior glenohumeral portal and shoulder reconstructive surgery so that these structures can be identified and protected.

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