An Unusual Formation of Median Nerve with Absence of Musculocutaneous Nerve

Abstract

The Musculocutaneous nerve arises from the lateral cord of the brachial plexus, passes inferiorly and laterally, and then pierces through the coracobrachialis .After supplying it descends between the biceps and the brachialis, giving branches to both the muscles and continues as the lateral cutaneous nerve of the forearm. Variations in the origin, course, branching pattern, termination and connections of the musculocutaneous nerve (MCN) are not uncommon. Though its absence has been described previously but its real prevalence is unknown. A case of absence of the musculocutaneous nerve was observed during the dissection of the left arm of a male cadaver. The medial root of median nerve was found to be bifurcated and the area of innervation was supplied by lateral root of median nerve. From this, three branches emerged: one to the coracobrachialis muscle, another to the biceps brachii muscle and the third to the brachialis muscle. The knowledge of such Anatomical variations would be of value to the practicing neurosurgeons, orthopaedic surgeons, anaesthetist and interventional radiologists, during surgical procedures, including brachial plexus block and in diagnostic clinical neurophysiology to avoid unexpected complication. The musculocutaneous nerve arises from the lateral cord of the brachial plexus, opposite the lower border of the pectoralis major, its fibres being derived from C5, C6 and C7. It penetrates through coracobrachialis muscle and passes obliquely between the biceps brachii and the brachialis, to the lateral side of the arm. A little above the elbow it pierces the deep fascia lateral to the tendon of the biceps brachii and continue into the forearm as the lateral cutaneous nerve of the forearm and supplies the lateral margin of the skin of the forearm. In its course through the arm it innervates the coracobrachialis, biceps brachii, and the greater part of the brachialis.  The branch to the coracobrachialis is given off from the nerve close to its origin, and in some instances as a separate filament from the lateral cord of the plexus; it is derived from the seventh, cervical nerve.  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. After branching to form the musculocutaneous nerve, the lateral cord continues as a lateral root of the median nerve, joining the medial root at the level of the distal margin of the pectoralis minor muscle in front of the axillary artery. The median nerve does not have any muscular branches in the arm. In this case during routine dissection we observed the absence of the musculocutaneous nerve from the lateral cord of brachial plexus with innervation of the anterior compartment of the muscles of the arm by the lateral root of the median nerve. Such variations may be present clinically or may be observed during surgery. Since there is a high incidence of variations, they are important for neurologist, orthopaedicians and traumatologist and will assist clinicians and the surgeons by pointing out anatomical anomalies associated with the origin of musculocutaneous nerve and variations in the origin of muscular branches to the anterior compartment muscles of the upper arm. So, a detailed study was done to observe the variations of the musculocutaneous nerve .