Unilateral Absence of Musculocutaneous Nerve with Variation in Distribution of Median Nerve - A Case Report

The brachial plexus innervates the upper limb. It is mainly formed by the anterior primary rami of C5 through T1 and provides sensory and motor innervation of the upper extremity. The brachial plexus is divided proximo-distally into rami/roots, trunks, divisions, cords, and terminal branches.[1] The MN (Median Nerve) is the main nerve of the anterior compartment of the forearm. The formation of the trunk of the MN takes place in front of the third part of the axillary artery by the union of lateral and medial roots, which arise from the respective cords of the brachial plexus and pass towards the forearm without supplying any muscles of the arm.[1] In the forearm, it supplies the muscles of the anterior compartment of the forearm through its direct and anterior interosseous branches, except for the flexor carpi ulnaris and the medial side of the flexor digitorum profundus. In the hand it supplies the thenar and the first two lumbricals. It also gives cutaneous branches to the lateral part of the palm and lateral three and a half digits. The MCN (Musculocutaneous Nerve) arises from the lateral cord of the brachial plexus and has root values of C5, C6, and C7. The MCN pierces and passes through the coracobrachialis muscle, supplying the muscle in the process. The MCN in the lower part of the arm pierces the deep fascia below the elbow on its lateral side and becomes subcutaneous, where it continues as the lateral cutaneous nerve of the forearm.[2,3] In 3.57–6.6% of cases, there will be an absence of MCN with several unforeseen clinical implications regarding the atypical innervation of the musculature otherwise supplied by the musculocutaneous nerve.[4] Here we present a case report in which there was an absence of MCN in one arm, low fusion of roots of MN below the level of midarm, and the anterior compartment muscles being supplied by the separate branches from the lateral root of MN.