|Year : 2020 | Volume
| Issue : 2 | Page : 159-161
Unilateral absence of musculocutaneous nerve with bilateral variation in the formation of the median nerve
Department of Anatomy, College of Medicine, Taibah University, Madinah, Saudi Arabia
|Date of Submission||19-Oct-2019|
|Date of Decision||18-Jun-2020|
|Date of Acceptance||13-Jul-2020|
|Date of Web Publication||9-Oct-2020|
Dr. Shabina Anjum
Department of Anatomy, College of Medicine, Taibah University, Universities Road, PO Box 344, Madinah. 42353
Source of Support: None, Conflict of Interest: None
Anatomical variations of the brachial plexus and its nerves are common. In this case, during a routine cadaver dissection, we encountered different variations of the brachial plexus. On the right side, there was an absence of musculocutaneous nerve (MCN) and the two roots of the median nerve (MN) united in the upper arm despite the axilla. Flexor muscles of the right arm, i.e., coracobrachialis, biceps brachii and brachialis, were innervated directly by the two different branches from the lateral cord. On the left side, the MCN was present and distributed as usual, but the MN received additional contributions from the lateral cord. Variations in the peripheral nerves are usually prone to iatrogenic injuries and brachial plexus block failure. Precise knowledge of such variations helps clinicians in the correct interpretation of unusual clinical findings, nerve conduction tests and imaging. This case report also shows the importance of vigilant anatomical dissection.
Keywords: Anatomical variations, brachial plexus, cadaver dissection, median nerve, musculocutaneous nerve
|How to cite this article:|
Anjum S. Unilateral absence of musculocutaneous nerve with bilateral variation in the formation of the median nerve. Niger J Basic Clin Sci 2020;17:159-61
|How to cite this URL:|
Anjum S. Unilateral absence of musculocutaneous nerve with bilateral variation in the formation of the median nerve. Niger J Basic Clin Sci [serial online] 2020 [cited 2021 May 14];17:159-61. Available from: https://www.njbcs.net/text.asp?2020/17/2/159/297604
| Introduction|| |
Brachial plexus is an intricate network of nerve fibres formed at the root of the neck, which is divided into roots, trunks and cords. The intricacies in the formation of brachial plexus make it more liable for variations, and these variations are not rare. These variations can be diagnosed with the newer imaging tools such as computed tomography and magnetic resonance imaging, but they are better visualised during anatomical dissections. Knowledge of such variations is important for the anatomists, radiologists, orthopaedists and surgeons as many surgical procedures of the upper limbs are associated with injuries to the branches of brachial plexus, including the musculocutaneous nerve (MCN) and the median nerve (MN).,
In this case report, we present a case of unilateral absence of the MCN and bilateral asymmetrical variation in the formation of the MN. The aim of the present study was to report a case and emphasise the importance of careful dissection to the budding medical graduates and its relevance in the clinical practice.
| Case Report|| |
The axilla and upper limb of a 60-year-old formalin-fixed female cadaver were dissected carefully with the basic dissection instruments. The cadaver is placed in the supine position and an anterior chest wall cutaneous incision was given to expose the origin of the pectoralis major muscle; after that, anterior midline skin incision was made from the mid-third of the clavicle to the cubital fossa. The lateral and medial skin flaps were reflected, superficial veins and cutaneous nerves were removed and the muscles of the flexor compartment were exposed. The clavicular and sternal attachments of the pectoralis major muscle were removed, and the axilla was explored. The axillary artery was identified, and dissection of the cords and branches of brachial plexus was done to trace its course and distribution. During the dissection, the unilateral absence of the MCN in the right arm and unusual formation of the MN in both the upper limbs were observed. Normally, MCN enters the arm by piercing the coracobrachialis muscle and provides innervation to the muscles of the anterior compartment of the arm. However, in the right axilla, we noted the absence of the MCN and innervation to coracobrachialis was provided by a direct twig from the lateral cord, and one distal branch of the lateral cord at the lower border of the latissimus dorsi muscle (for better description, we named it as flexor compartment nerve [FCN]) was further divided into two muscular branches. The upper branch provides innervation to the biceps brachii [Figure 1], whereas the lower branch innervates the brachialis muscle and continues downward as lateral cutaneous nerve of the forearm [Figure 2]. The MN was formed distally at the upper part of the arm 5 cm distal to the lower border of the latissimus dorsi muscle and anterior to the brachial artery [Figure 1].
|Figure 1: The right axilla and upper arm showing the absence of musculocutaneous nerve and its replacement from the branch of the lateral cord (named as FCN) and formation of the median nerve in the upper arm anterior to the brachial artery. LC: Lateral cord of the brachial plexus, LR: Lateral root of the median nerve, MR: Medial root of the median nerve, MN: Median nerve, FCN: Flexor compartment nerve, 1: Muscular branch to the biceps brachii from FCN, 2: Muscular branch to brachialis from FCN, AAr: Axillary artery, BAr: Brachial artery|
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|Figure 2: The right arm and cubital fossa showing the muscular branch of brachialis from FCN and continuing as the lateral cutaneous nerve of the forearm. FCN: Flexor compartment nerve, MN: Median nerve, 1: Muscular branch to the biceps brachii from FCN, 2: Muscular branch to the brachialis muscle from FCN, Arrow heads: Continuation of the muscular branch of the brachialis muscle into the LCNF, LCNF: Lateral cutaneous nerve of the forearm|
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In the left axilla, the MCN was present and distributed as usual. However, the variation was noted in the formation of the MN, which received contributions from the lateral cord twice, one in the axilla (LR-1) and again in the proximal 1/3rd of the arm (LR-2) [Figure 3]. The LR-1 ran downwards and medially to unite with the medial root to form MNa on the medial aspect of the 3rd part of the axillary artery. The additional root came again from the lateral cord after its division into the MCN and LR-2. Interestingly, the circumference of LR-2 (12 mm) was more than that of the MCN (9 mm). The LR-2 ran downwards and crossed the brachial artery from lateral to medial to join with the MNa in the upper arm and continued as the MNb which maintained its medial relation with the brachial artery throughout its course in the arm.
|Figure 3: The left axilla and arm showing the formation of the median nerve by three roots (MR + LR1 + LR2). The additional root from the lateral cord (LR2) is joining in the arm medial to the brachial artery. LC: Lateral cord of the brachial plexus, MC: Medial cord of the brachial plexus, AAr: Axillary artery, BAr: Brachial artery, MR: Medial root of the median nerve, LR1: Lateral root of the median nerve, LR2: Additional lateral root of the median nerve from the lateral cord, MNa: Median nerve in the axilla, MNb: Median nerve after joining with the additional lateral root, MCN: Musculocutaneous nerve|
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| Discussion|| |
The absence of MCN was reported in a range of 5%–15% in different studies., The absence of MCN and its complete replacement by MN was observed by Parchand and Patil and Gümüsburun et al., while Srimani et al. and Zhang et al. have found that only biceps brachii and brachialis received nerve supply from the MN and the coracobrachialis received innervation from the lateral root of the MN or directly from the lateral cord. However, in our case, all the muscles of the anterior compartment were supplied directly by the branches of the lateral cord although we could not label it as the MCN as it neither pierced the coracobrachialis nor followed its usual course. This is an extremely rare observation, and search of relevant literature revealed only two reported cases showing similar findings where the complete replacement of MCN was noted by the branches of lateral cord., Moreover, the formation of MN, in the upper arm anterior to the brachial artery, makes this case even more unusual. Le Minor gave a broad classification about the variations of the MCN and MN, and, according to him, these variations can be classified into five types. Despite this broad categorization, we could not categorize this case under any type of Le Minor classification. Therefore, we believe that variations of the MCN, and the MN, need to be explored more and modified according to the innervation of the anterior compartment of the arm.
On the left side, the MN is formed by three roots instead of two roots. A large-scale cadaveric study conducted by Mat Taib et al. to explore the variations of the MN, revealed that the most common variation in the formation of MN was by three roots, in which two roots came from the lateral cord. In our case also, the additional root of MN came from the lateral cord of the brachial plexus. Interestingly, in our cadaver, the additional lateral root was of significant thickness, which might have compressed the axillary artery and could have led to vascular symptoms in the upper limb. Thus, the precise knowledge of these variations is indeed necessary for the clinicians, especially in the interpretation of unusual clinical findings.
To conclude, this is a unique case of unilateral absence of the MCN and bilateral asymmetrical variation in the MN formation in one cadaver. The localisation of the nerves of the brachial plexus is important for many surgical and diagnostic procedures. For example, in brachial plexus block, the MN is localised by the pulsation of the axillary artery in the axillary fossa and the MCN is identified by palpating the coracobrachialis muscle., Hence, it is important to report the variations in the formation, relation and distribution of the brachial plexus to enrich the knowledge and statistics of variations so that unexpected complications during various surgical procedures can be avoided. Lastly, this case report also emphasises the importance of vigilant anatomical dissection.
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Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3]