Surg Radiol Anat (2010) 32:891–894 DOI 10.1007/s00276-010-0683-8
ANATOMIC VARIATIONS
Anatomical variation in formation of brachial plexus and its branching Anjali Aggarwal • Nidhi Puri • Aditya K. Aggarwal K. Harjeet • Daisy Sahni
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Received: 7 March 2010 / Accepted: 17 May 2010 / Published online: 3 June 2010 Ó Springer-Verlag 2010
Abstract Variant brachial plexus formation with two trunks and two cords is uncommon and has clinical implications as it may result in failure of regional brachial or axillary block. During routine anatomical dissection, unilateral variation in the formation of brachial plexus accompanied by unusual positional relationship with axillary artery was discovered in the left upper extremity of a 52-year-old Indian male cadaver. Brachial plexus showed two trunks formed by ventral rami of C5, C6 and C7, C8, T1 spinal nerves, respectively, which first split and then reunited in an unusual manner to form two cords: posterior and lateral instead of three. Medial cord was absent. The branching pattern of the brachial plexus also showed important variations. Second part of axillary artery was found lying inferomedial to brachial plexus instead of passing between medial and lateral cords. Transverse cervical artery was found to be coursing between two trunks instead of passing superficial to brachial plexus. Median nerve was observed to be formed from a single root, instead Electronic supplementary material The online version of this article (doi:10.1007/s00276-010-0683-8) contains supplementary material, which is available to authorized users. A. Aggarwal K. Harjeet D. Sahni Department of Anatomy, Post Graduate Institute of Medical Education and Research, Chandigarh 160012, India A. K. Aggarwal Department of Orthopaedic Surgery, Post Graduate Institute of Medical Education and Research, Chandigarh 160012, India N. Puri Department of Anatomy, MMIMSR, Mullana, India A. Aggarwal (&) # 123-C Type V, Sector 24-A, Chandigarh 160023, India e-mail:
[email protected];
[email protected]
of usual two roots. Embryologically, this rare variation may be due to the development of axillary artery from ninth segmental artery instead of usual seventh cervical intersegmental artery. Such rare variation is clinically important as this knowledge may help the anesthesiologists and the surgeons to avoid any inadvertent damage to nerves and axillary artery during blocks and surgical interventions. Keywords Brachial plexus Variation Median nerve Axillary artery
Introduction Complex nature of the brachial plexus and its intimate relationship with axillary artery has always been an area of interest not only for the anatomists but also for the clinicians as well. Normally, ventral rami of C5 and C6 spinal nerves unite to form upper trunk, C8 and T1 ventral rami unite to form lower trunk and C7 continues as middle trunk of brachial plexus. Anterior and posterior divisions of the trunks unite in a systematic manner to form three cords, i.e. lateral, medial and posterior cords, named according to their position relative to second part of axillary artery [13]. Variations in the formation of trunk, cords and their subsequent branches have been well documented in literature [4, 9]. However, variation in the brachial plexus at all levels right from the formation of trunks to the origin of terminal branches along with its aberrant relationship with axillary artery stem is very rare. Purpose of the present case report was to highlight the rare variant formation of brachial plexus, its variant relationship with axillary artery and its relevance in clinical situations especially during the surgical interventions. Embryological basis of this variation is also suggested.
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Case report The brachial plexus with variations in its formation and its relationship with the axillary artery was found in the left upper extremity of a 52-year-old embalmed male cadaver during routine dissection for undergraduate teaching (Figs. 1, 2). In right upper extremity, brachial plexus and axillary artery showed normal anatomy. Ventral rami of C5 and C6 spinal nerves united to constitute upper trunk (UT), which bifurcated unequally into a thicker posterior (PD) and a thinner anterior divisions (AD). Posterior division gave off the suprascapular nerve. Ventral rami of C7, C8 and T1 spinal nerves unified into a single common flat lower trunk (LT) which was almost double the thickness of upper trunk. Lower trunk was divided into two sub-trunks; for the purpose of description we named them as LT1 and LT2, respectively. Both sub-trunks namely, LT1 and LT2 divided into an anterior and a posterior division (AD and PD). Anterior division of upper trunk while descending anterior to posterior division of LT2 was joined on its deeper aspect by anterior division of LT2 and subsequently by anterior division of LT1 to constitute a unified cord named as lateral cord (LC). Posterior divisions of upper trunk, LT1 and LT2 merged together to form the posterior cord (PC). Posterior cord was placed in a posterolateral plane relative to the lateral cord. Medial and lateral pectoral nerves emanated from anterior aspect of the lateral cord. After giving rise to the medial cutaneous nerve of arm from its medial aspect, lateral cord divided into four terminal branches namely: musculocutaneous nerve from the lateral aspect, ulnar nerve and medial cutaneous nerve of forearm from the medial aspect and median nerve lying in intermediate
Fig. 1 Dissection of the left side of neck and axilla. Axn axillary nerve, Ax.art axillary artery, LC lateral cord, LT lower trunk, LT1 lower subtrunk1, LT2 lower subtrunk2, Lsn lower subscapular nerve, Mcn musculocutaneous nerve, Mcna medial cutaneous nerve of arm, Mcnf medial cutaneous nerve of forearm, Mn median nerve, PC posterior cord, Rn radial nerve, Sn suprascapular nerve, S.art suprascapular artery, Tdn thoracodorsal nerve, Tr.cer.art transverse cervical artery, Un ulnar nerve, Usn upper subscapular nerve, UT upper trunk
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position. Median nerve was formed from a single root originating from the lateral cord. Posterior cord terminated into radial and axillary nerves. Thoracodorsal nerve emanated from the radial nerve. Lower subscapular nerve arose from the thoracodorsal nerve, whereas axillary nerve gave off upper subscapular nerve. Second part of axillary artery was found lying inferomedial to the cords of brachial plexus and further distally it was seen coursing laterally after passing deep to the terminal branches of lateral cord. In posterior triangle, transverse cervical artery after arising from the subclavian artery passed laterally lying superficial to the unified lower trunk. Further, it plunged into the gap between two trunks and then coursed laterally lying deep to the upper trunk. Suprascapular artery traversed laterally superficial to LT1 and LT2 and deep to anterior division of the upper trunk and further laterally superficial to posterior division of upper trunk. Branches of axillary artery did not demonstrate any variation from the standard described anatomy. Axillary vein was placed medial to axillary artery.
Discussion In this case report, variation in the formation of brachial plexus was observed right from the level of trunk. It was marked by the formation of two trunks instead of three. We report a union of ventral roots of C7, C8 and T1 spinal nerves to form lower trunk with absent middle trunk. Such uncommon variation has also been reported by Matejeik [5] (3 cases) and Yang [14] (1 case). Fusion of fibers of C7 spinal nerves into upper trunk has more commonly been reported than that into lower trunk [8, 10, 12]. Uysal [12] reported absent lower trunk in 9% and absent upper trunk in 1% cases. In our case the lower trunk was formed by fusion of middle and lower trunks. Subdivision of lower trunk into two sub-trunks and their further bifurcation into anterior and posterior divisions was another unusual feature of the present study which has not been reported in literature so far. In this case report, three anterior divisions merged into a single cord instead of forming lateral and medial cords. Only a few studies have reported fusion of lateral and medial cords in the literature [4, 9, 13]. Kerr [4] found three cases in which lateral and medial cords fused into a single cord placed anterior to the axillary artery. Fused cord and posterior cord were found anterior and posterior to the artery, respectively, whereas in our case fused and posterior cords were found lateral and posterolateral to the artery, respectively. Yang et al. [14] reported formation of brachial plexus from two trunks and two cords in one case but subdivision of fused lower trunk was not observed. In the study by Pandey et al. [9], medial and lateral cords were united by a communicating branch rather
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Fig. 2 Schematic diagram of left brachial plexus. For abbreviations see Fig. 1
than proper fusion as seen in the present study. Complete fusion as seen in our case was reported by Yang et al. [14]. The three cords of brachial plexus have received their names according to their relationship with the second part of axillary artery [3, 13]. Only two cords were noticed in this case report namely, posterior and lateral cords. Medial cord was absent. Suprascapular nerve in our case was found to be arising from posterior division of upper trunk. A few studies have also reported variant origin of suprascapular nerve from posterior division of upper trunk instead of upper trunk [1, 2]. In this case report, lateral cord was found to be terminating into musculocutaneous, median and ulnar nerves representing terminal branches of lateral and medial cords. Normally axillary artery is clasped on its medial and lateral aspects by medial and lateral roots of the median nerve, respectively. In present study, the axillary arterial stem failed to pass through the brachial plexus. Thus, the characteristic loop of median nerve which is formed by branches from lateral and medial cords was not seen as it arose from the single cord. Instead, it was formed from a single root which might be attributed to union of two cords. Such feature of single root median nerve has also been reported in the literature [14]. In this case report, posterior cord gave rise to two terminal branches namely, radial and axillary nerves. Thoracodorsal nerve arose from radial nerve. Upper subscapular nerve originated from the axillary nerve, whereas lower subscapular nerve arose from thoracodorsal nerve. A few studies have reported the origin of upper and lower subscapular nerves from axillary nerve [2, 4, 11]. Kerr [4] has described the origin of thoracodorsal nerve from the radial nerve. Miller [6] and Mu¨ller [7] explained relationship between axillary artery and brachial plexus as a combination of
embryological and evolutionary process. Mu¨ller [7] hypothesized that axillary artery was formed by anastomosis of local vessels and alteration in the level of penetration of brachial plexus by the artery might depend upon its embryological origin from the intersegmental artery. Usually axillary artery is considered to be the continuation of seventh intersegmental branch of dorsal aorta which penetrates the plexus between middle and lower trunks. Sometimes the axillary artery is derived from 6th, 8th or 9th intersegmental branch of dorsal aorta, thus resulting in the altered relationship of axillary artery and brachial plexus [3, 6]. In this case report, arterial stem lays caudal to the brachial plexus. Such caudal positioning of arterial stem in relation to brachial plexus has been reported in a few studies [6, 14]. Probable source of origin of axillary artery in the present study was from 9th segmental artery and a part of the seventh persisted as a branch of it and gave rise to transverse cervical artery which thus traversed through the gap between two trunks. This assumption is based on the hypothesis described by Miller [6]. In his study, main stem of the axillary artery was arising from 9th segmental artery in one case. Remnant of seventh segmental artery branched off from it which passed through the median loop and developed into deep brachial and humeral circumflex arteries. Generally, the transverse cervical artery, a branch of subclavian artery passes laterally superficial to the brachial plexus. In our case, it was found coursing between upper and lower trunks. Until now there is no experimentally proved explanation of exact origin of arterial stem. This variant passage of transverse cervical artery between upper and lower trunk has not been reported in the literature so far. Thus, the knowledge of relationship of the brachial plexus with the axillary artery is paramount to ensure safe and successful regional anesthesia of upper extremity.
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Variant formation of brachial plexus with superficial position of axillary artery may lead to injury during surgery in upper axilla. Hence, such variations are of clinical importance to the anesthesiologists and the surgeons. Supra- or infraclavicular brachial plexus block may inadvertently damage the artery. Acknowledgments The authors wish to thank Mr. Vijay Bakshi senior artist of Department of Anatomy for drawing the illustration.
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