A unique variation in the course of the musculocutaneous nerve

Abstract

Department of Anatomy, St. George’s University School of Medicine, St. George’s, Grenada, West IndiesThe musculocutaneous nerve, derived from the lat-eral cord, is one of the terminal branches of thebrachial plexus. The classical pathway of this nerveis that it penetrates the coracobrachialis muscle,passes between the biceps brachii and the brachia-lis muscles, and terminates as the lateral antebra-chial cutaneous nerve. The lateral cord is largelycomposed of fibers originating from the C5 and C6ventral rami, thus accounting for the presence ofthese fibers in the musculocutaneous nerve(Standring, 2008).Multiple studies have been conducted assessingthe complexity and variation of the brachial plexus’formation and branching pattern (Pandey and Shu-kla, 2007; Loukas et al., 2008; Aggarwal et al.,2009, 2010; Johnson et al., 2010). Knowledge ofthese variations is critical to surgeons, radiologistsand other clinicians working in the axillary region.In this study, we document the unusual path ofthe musculocutaneous nerve through the short headof the biceps brachii. Although variations of the ori-gin and course of the musculocutaneous nerve havebeen previously documented (Saeed and Rufai,2003; Abu-Hijleh, 2005; Bhattarai and Poudel,2009), and variations of the biceps muscle are com-mon (Asvat et al., 1993; Nakatani et al., 1998;Rodriguez-Vazquez et al., 1999; Rodriguez-Nieden-fuhr et al., 2003; Nayak et al., 2006; Paval andMathew, 2006); to the best of our knowledge thepassage of a sole musculocutaneous nerve throughthe proximal belly of the short head of the biceps isnot in the reported literature.The axilla was completely dissected in a malecadaver at St. George’s University School of Medi-cine. Observation revealed that the right musculo-cutaneous nerve originated from the lateral cord ofthe brachial plexus, passed deep to the coracobra-chialis where it sent fibers to the brachialis. Thenerve then separated the proximal belly of theshort head of the biceps brachii, sent a muscularbranch to the long head, and terminated as the lat-eral antebrachial cutaneous nerve. The point ofpassage through the biceps brachii was largeenough to separate the short head into two distinctfasciculata (Fig. 1).Previous studies have been conducted to quan-tify brachial plexus variations (Kerr, 1918; Choi etal., 2002; Guerri-Guttenberg and Ingolotti, 2009;Fetty et al., 2010). Variations of the musculocuta-neous nerve range from complete absence (Ihunwoet al., 1997; Sud and Sharma, 2000; Prasada Raoet al., 2001; Jahanshahi et al., 2003; Krishnamur-thy, 2007; Guerri-Guttenberg and Ingolotti, 2009)to duplication (Abu-Hijleh, 2005), thus depicting anarea of high variability. In 2005 Abu-Hijleh pre-sented a case in which the musculocutaneous nervepierced the biceps brachii; however, there alsoexisted a second musculocutaneous nerve. In thisunique case, one musculocutaneous nerve wasobserved unilaterally on the right side. This studypresents a unique variation in the course of thenerve, not yet defined in the classification systems.Compression or entrapment of this nerve would beclinically significant as there is no second nerve tocompensate for losses.Passage of this nerve through the biceps brachiimay place it at a high risk for compression. Muscu-locutaneous entrapment has been documented inthe coracobrachialis, a significantly smaller muscle(Papanikolaou et al., 2005), as well as between thebiceps brachii and brachialis muscles (DeFrancoand Schickendantz, 2008). It is plausible forentrapment syndromes to arise in our case sincethe musculocutaneous nerve pierced through thebiceps brachii itself.Daily activity involving the upper limb or hypertro-phy of the biceps brachii may result in compressionof the musculocutaneous nerve (Fattal et al., 1998;Simonetti, 1999). Compression within this musclemay result in variable Wallerian and/or retrogradedemyleination (Papanikolaou et al., 2005) manifest-ing motor deficits in both the long head of the bicepsand the brachialis. This would result in impairment offlexion at the glenohumeral, humeroulnar joints,supination at the radioulnar joint, and a sensory def-