|Year : 2012 | Volume
| Issue : 1 | Page : 34-36
Bilateral absence of musculocutaneous nerve
Mathada V Ravishankar, Pruful S Jevoor, Lohit Shaha
Department of Anatomy, J.N. Medical College, K.L.E. University, Belgaum, Karnataka, India
|Date of Web Publication||21-May-2012|
Mathada V Ravishankar
Department of Anatomy, J. N. Medical College, K.L.E. University, Belgaum - 590 010, Karnataka
Source of Support: None, Conflict of Interest: None
Brachial plexus is an important group of spinal nerve plexus that supplies the muscles of the upper limb via the ventral rami of the Cervical 5 - Thoracic 1 fibers of the spinal nerves. It is not uncommon to notice the variations during cadaveric dissections in many regions of the body, at different levels, such as, roots, trunks, division, cords, communications, and branches as reported in the literature. Although the nerve supply of the body musculature takes place in the fetal life itself, its course, branching pattern, innervations, and communication can show variable patterns as the fetal development progresses. One such anomaly was noticed during our routine cadaveric dissection in the Department of Anatomy, Jawaharlal Nehru Medical College, Belgaum, showing bilateral absence of the musculocutaneous nerve, which obviously drew the attention of the students of medicine, physiotherapy, and learning clinicians as well.
Keywords: Brachial plexus, electromyography, musculocutaneous nerve
|How to cite this article:|
Ravishankar MV, Jevoor PS, Shaha L. Bilateral absence of musculocutaneous nerve. J Sci Soc 2012;39:34-6
| Introduction|| |
The peripheral nervous system (PNS) arises from the neural tube and involves the neural crest cells and ectodermal placodes, which are the two groups of cells from outside the neural tube. The PNS develops as an integrated system, essentially in a cranial-to-caudal sequence. Each spinal nerve splits into branches, as the dorsal and ventral rami shortly after its exit from the spinal cord. The limb muscles are innervated by the branches of the ventral rami of the spinal nerves arising from Cervical 5 - Thoracic 1 segments, and Lumbar 4 - Sacral 3 segments for the lower limb muscles. The motor neurons that innervate the limbs perform an intricate feat of path finding to reach their target muscles. The ventral rami axons destined to reach the limbs apparently travel to the base of the developing limb bud, by growing along the permissive pathways. The growth cones of these axons take a predetermined path to reach their functional destinations. Once the axons arrive at the base of the developing limb bud, they unite orderly and emerge out to form the pattern called as the brachial plexus. It is the decision-making region for axons to provide the innervation of muscles during their development. The motor and sensory nerves innervate the body wall and limbs in a pattern that is based on segmental organization established by somites. 
The brachial plexus is a complex structure; when damaged it requires meticulous clinical history-taking and examination. The assessment of an individual nerve function can be obtained by nerve conduction studies and electromyography, which assess the latency of muscle contraction when the nerve is artificially stimulated. This helps in the clinical assessment of nerve involvement in the injury. There are many reports widely stating the variations in the musculocutaneous nerve. ,
| Case Report|| |
During routine dissection of a 55-year-old male cadaver, in the Department of Anatomy, J.N.M.C, Belgaum, on reflection of the skin, the anterior compartment of the arm revealed the following findings:
In photo legend one (Right arm)
On the right side, the lateral cord was situated on the lateral aspect of the axillary artery and showed the missing musculocutaneous nerve innervating the coracobrachialis muscle in the anterior compartment of the arm [Figure 1].
|Figure 1: On the right side, the lateral cord was situated on the lateral aspect of the axillary artery and showed the missing musculocutaneous nerve innervating the coracobrachialis muscle in the anterior compartment of the arm|
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In photo legend two (Left arm)
On the left side also the same pattern was recognized. This suggested the bilateral absence of the musculocutaneous nerve from the lateral cord of the brachial plexus [Figure 2].
|Figure 2: On the left side also the same pattern was recognized. This suggested the bilateral absence of the musculocutaneous nerve from the lateral cord of the brachial plexus|
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On further dissection the bilateral course of the median nerve was traced to show the muscular branches supplying the muscles of anterior compartment of the arm. We ultimately concluded our observation by stating that there was bilateral absence of the musculocutaneous nerve, and its compensatory fibers were supplied through the distributing fibers of the median nerve. The lateral cutaneous nerve of the forearm was derived from the median nerve in both the limbs.
| Discussion|| |
Structural variation is a common scenario during routine anatomical dissections, but it depends on the skill and the inquisitiveness of the student who dissects. Several times the most common variations are ignored or sometimes it may not be possible to approach it practically in a keen and distinctive manner.
Brachial plexus, being a complex structural organization, shows its natural tendency of variable patterns in its gross presentations. Practically it may or may not be significant, but certainly it draws the attention of learners at any level. The musculocutaneous nerve is a dominant nerve supplying the flexor group of muscles in the anterior compartment of the arm. Literature shows that there is sufficient evidence of its variations, in its presentation, course, distribution, and its accompanied structural variations.
Variations in the course and branching pattern and connections of the musculocutaneous nerve were noticed in 6.25% of the cases unilaterally on the right side, without statistically significant difference by gender. In 3.125% of the cases, the musculocutaneous nerve, after piercing the coracobrachialis, terminated by joining the median nerve at the distal part of the arm, which suggested right arm susceptibility.  A unilateral left arm was found with the absence of the musculocutaneous nerve, the motor branches to the coracobrachialis were contributed from the lateral cord. They also found additional findings related to roots contributing to a long thoracic nerve, from Cervical 6 to 7 only.  It was interesting to note that the absence of the musculocutaneous nerve was often accompanied by the unusual pattern of other structural presentations in the upper limb of the same cadaver, which was suggestive of a versatile pattern in its presentations. ,
It is also important to dissect the communicating fibres between the branches of Brachial plexus nerves. We come across such communications in many regions during cadaveric dissections, and it needs a meticulous operation. They normally show a fixed pattern of communication between the branches. In a study carried out on 50 cadavers, it was observed that unusual communications were noted between the musculocutaneous and median nerves in 6% of the population, out of which 4.4% was in males and 20% in females. 
The dominance of the median nerve is appreciated where the muscles of the flexor compartment of the arm, including the biceps, coracobrachialis, and brachialis, are supplied through the branches contributed by the median nerve. Interestingly in the same cadaver they have also noted the absence of the piercing musculocutaneous nerve in the coracobrachialis muscle. 
The ulnar nerve was formed from the lateral and posterior cords; the entire medial cord continued downward as the contributing medial root of the median nerve. Here the median nerve was formed by the contribution of the lateral cord alone, where the contributing fibres from the medial cord were missing, without any unusual finding with respect to the musculocutaneous nerve in the arm. 
The musculocutaneous nerve is an important motor supply to the flexor muscles of the arm, its injury or compression occurs frequently due to causative factors like sports injury, vigorous muscular activity, trauma, and so on. A correct surgical action plan is necessary, and the injury or compression of the nerve must be investigated through electromagnetic testing. Often musculocutaneous neuropathy may mimic a cervical nerve root compression or lesion of the brachial plexus,  which is an important observation and needs keen evaluation, from the neurosurgical point of view.
Literature describes the wide frequency of musaculocutaneous nerve variations at different levels.
Our case obtained special attention as it was presented with a typical mirror image pattern of its bilateral absence in the arm.
The musculocutaneous nerve is a subject of considerable variations and its clinical importance is stressed during flap dissections, post-traumatic evaluation of the arm or peripheral nerve repair, and even for peripheral nerve stimulation, in the practice of anesthesia. It is also important to differentiate the overlapping signs and symptoms of cervical root compression versus musculocutaneous nerve compression. Surgically it is important to handle the lesions of the musculocutaneous nerve and median nerve, which may impose difficulty in diagnosis due to their unusual structural presentations. From the students' learning point of view, it is important to make them aware of such natural cadaveric variations when learning dissections. Such natural variations in any part of the human body cannot be replaced by artificial or sophisticated student learning models.
| Acknowledgment|| |
The first author initiated and conceived the study, the second author helped in the literature review, the third author supported in writing the dissection part and the documentation work.
| References|| |
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[Figure 1], [Figure 2]