A cadaveric study on Musculocutaneous nerve and its variations in south Indian population

Abstract               Musculocutaneous Nerve is a branch of the lateral cord of Brachial plexus. It pierces the Coracobrachialis muscle and runs downward between Biceps brachii and Brachialis. At the lateral side of the arm, just below the elbow, it continues as Lateral cutaneous nerve of forearm. Musculocutaneous nerve supplies Coracobrachialis, Biceps brachii and Brachialis which are muscles of the front of arm. It also provides sensory innervation for the lateral aspect of forearm. A detailed study of Musculocuateous nerve was done in 100 limbs of 50 embalmed human cadavers at the Institute of Anatomy, Madras Medical College, Chennai, India. Musculocutaneous nerve was not piercing the Coracobrachialis muscle in 15% of limbs. In 9%, communication was noted between Musculocutaneous nerve and Median nerve. The Musculocutaneous nerve was absent in 6% of limbs. The Musculocutaneous Nerve has significant variations and this knowledge is important for Neurologists, Anaesthesiologists, Orthopaedicians and Surgeons.

Key Word: Musculocutaneous nerve, Coracobrachialis, Median nerve, Lateral cutaneous nerve of forearm, Nerve Communication.

INTRODUCTION

Musculocutaneous nerve is the nerve of the anterior compartment of arm, supplying Coracobrachialis, two heads of Biceps brachii and most of Brachialis¹. Musculocutaneous nerve (MCN) (C5,C6,C7) arises from the lateral cord of brachial plexus at the lower border of Pectoralis minor¹ . In the upper part of arm, the nerve runs down between the third part of Axillary artery and Coracobrachialis muscle. Then it pierces the Coracobrachialis muscle and descends laterally between Biceps brachii and Brachialis to the lateral side of the arm. Just below the elbow, it pierces the deep fascia, along the lateral margin of Biceps tendon and continues as Lateral cutaneous nerve of forearm². The nerve to Coracobrachialis, carrying C7 fibres³ is given off before the MCN pierces the Coracobrachialis muscle and branches to Biceps brachii and Brachialis leave the nerve after the nerve has pierced the muscle. The nerve to Coracobrachialis may also arise directly from lateral cord. MCN supplies the elbow joint through the nerve to Brachialis and it also gives a small branch to humerus. MCN continues as Lateral cutaneous nerve of forearm and it provides sensory innervation for the skin of lateral part of forearm. Many variations including absence of MCN have been reported in literature. The lateral root of Median nerve arises from the lateral cord and unites with medial root, anterior or lateral to the third part of Axillary artery¹. Since the MCN and lateral root of Median nerve arise from lateral cord of Brachial plexus, communications between these two nerves are common. When the lateral root of Median Nerve is small, MCN connects with Median Nerve in the arm¹. Sometimes the fibres of MCN run along Median Nerve and branches to flexor muscles are given off from Median Nerve. Instead of penetrating the Coracobrachialis, the nerve may pass behind it or between it and short head of Biceps Brachii⁴. The knowledge of MCN and its variations is important for clinicians in the diagnosis and management of peripheral neuropathy and nerve injury. This study was done to provide additional data on MCN in the South Indian population.

MATERIALS AND METHODS

100 upper limbs from 50 embalmed human cadavers were studied at the Institute of Anatomy, Madras Medical College, Chennai during routine undergraduate dissection classes. The limbs were dissected according to the standard dissection manual. In all the limbs the Musculocutaneous nerve was studied with regard to its origin, course and communications. Its relation to Coracobrachialis muscle was specifically noted. The data obtained were recorded and analyzed.

RESULTS

Out of 100 upper limbs, variations were noted in 30 limbs (30%). Among which, the Musculocutaneous nerve not piercing Coracobrachialis was the most common variation. It was noted in 15 limbs (15%) [Figure 1]. The second most common variation was the communication between Musculocutaneous nerve and Median nerve which was noted in 9 % of limbs [Figure 2]. The Musculocutaneous nerve was absent in 6 limbs (6%) [Figure 3].

Table 1: Variations of Musculocutaneous nerve observed in the present study

The MCN did not pierce Coracobrachialis muscle in 15 limbs (15%). In 5 cadavers (10 specimens) this variation was bilateral and in 5 limbs (5 specimens) it was unilateral. In all the 15 limbs MCN arose from lateral cord, ran between Coracobrachialis and Biceps brachii muscles and continued as Lateral cutaneous nerve of forearm [Fig 1]. In 3 limbs, the branch to Coracobrachialis was a branch of lateral cord. In all the limbs Biceps brachii and Brachialis were supplied by MCN.

Table2: Innervation of muscles of Front of arm in 15 limbs where MCN did not pierce Coracobrachialis.

Communication between Musculocutaneous nerve and Median nerve was noted in 9 limbs (9%). All the communications were single and were noted in the arm after MCN pierced Coracobrachialis muscle [Fig 2]. In one specimen MCN joined the Median nerve after piercing Coracobrachialis muscle. Before joining with Median Nerve it provided branches for all muscles of front of arm and Lateral cutaneous nerve of forearm. Musculocutaneous nerve was absent in 6 limbs (6 %). The Lateral cutaneous nerve of forearm was a branch of Median nerve in all these limbs. All the muscles of front of arm were supplied by Median nerve in 4 limbs [Fig 3]. In the other 2 limbs, Coracobrachialis was supplied by a direct branch from lateral cord and Biceps brachii and Brachialis were supplied by Median nerve [Fig 4].

Table 3: Innervations of muscles of front of arm in 6 limbs where MCN was absent

The Lateral cutaneous nerve of forearm was the continuation of MCN in 94 limbs (94%). In 6 upper limbs where the MCN was absent, Lateral cutaneous nerve was a branch from Median Nerve [Fig 3 and Fig 4].

Figure1: Right front of arm showing Musculocutaneous Nerve (MCN) not piercing the Coracobrachialis (CB) muscle. Branch to Coracobrachialis (CB), Branch to Biceps brachii (BB) and branch to Brachialis(Bs) muscles are seen. The MCN continues as Lateral cutaneous nerve of forearm (LCN). Figure 2: Left front of arm showing communication* between Musculocutaneous Nerve (MCN) and Median nerve (MN). The communication is in midarm after the point of entry of MCN into the Coracobrachialis muscle (CB)

Figure3: Left front of arm with absent Musculocutaneous nerve. Coaracobrachialis(CB), Biceps brachii (BB) and Brachialis muscles are supplied by Median nerve (MN).Lateral cutaneous nerve of forearm (LCN) is a separate branch of MN. Figure 4: Left Front of Arm with absent Musculocutaneous nerve. Coaracobrachialis(CB) muscle is supplied by lateral cord (LC) of brachial plexus. Biceps brachii (BB) and Brachialis muscles are supplied by Median nerve (MN).