Absence of upper trunk of the brachial plexus

Abstract

The brachial plexus is a complicated plexus supplying the upper limb. The brachial plexus is of great practical importance to the surgeon. It is encountered during operations upon the root of the neck, and hence it is in danger. Variations in the formation of the brachial plexus are common; and knowledge of the variation of the brachial plexus may be useful for surgeons, for improved guidance during supraclavicular block procedures, and for surgical approaches for brachial plexus. Here we report a case in which the superior trunk of the brachial plexus was found to be absent on the right side during a study on the cadaver-neck specimens in the Department of Anatomy.

INTRODUCTION

The brachial plexus is a major and complicated plexus at the root of the neck.[1] It begins in the lateral cervical region (posterior triangle) and extends into the axilla in the angle between the clavicle and the lower posterior border of the sternocleidomastoid.[2] This allows the nerve fibers derived from different segments of the spinal cord to be arranged and distributed efficiently in different nerve trunks to the various parts of the upper limb.[1] It emerges between the scaleni anterior and medius, superior to the third part of the subclavian artery, and is covered by platysma, deep fascia, and skin, through which it is palpable.[2]

The brachial plexus is formed by the ventral rami of C5, 6, 7, 8, and T1;[1,3–5] and small twigs from the ventral rami of C4 and T2.[2,4] These constitute the roots of the plexus, which lie in the lower part of the neck, between the scalenus anterior and scalenus medius muscles.[4,5] The ventral rami of the C5 and C6 unite to form the superior trunk, those of the C7 continue as the middle trunk, and the rami of C8 and T1 unite to form the inferior trunk.[1,3–5] A short distance above the clavicle, each of these trunks split into anterior and posterior divisions.[4]

The brachial plexus is of great clinical importance to the surgeon. It may be damaged in open, closed or obstetrical injuries, be pressed upon by a cervical rib or be involved in a tumor. It is encountered in operations upon the root of the neck, and hence it is in danger.[3] Anatomical variations of brachial plexus are quite common. Knowledge of the variations of the brachial plexus in the anatomy is important to anatomists, radiologists, anesthesiologists, and surgeons. Also knowledge of the variation of the brachial plexus may be useful for surgeons for improved guidance during supraclavicular block procedures, and for surgical approaches for brachial plexus. Here, we are reporting a case, in which the superior trunk of the brachial plexus was absent unilaterally on the right side.

CASE REPORT

During a study on the formalin-fixed, adult cadaver-neck specimens in the Department of Anatomy of the Medical College, anatomical variations in the formation of the brachial plexus were recorded. In one of the specimens the ventral rami of C5 and C6 were not joined to form the upper trunk [Figure 1]. The variation was unilateral and was found only on the right side of the specimen. No such variation was found on the left side. No other anatomical variation was found on the right side.

Figure 1.

Dissected lower neck and axilla showing absence of the upper trunk

DISCUSSION

The brachial plexus is formed by the ventral rami of C5, 6, 7, 8, and T1. In some individuals, trunk divisions or cord formations may be absent in one or the other part of the plexus; however, the makeup of the terminal branches was unchanged.[5]

Fazan, et al., reported a variation on the superior trunk found in two cases, both on the right side. In both cases, the C5 and C6 roots were split in the anterior and posterior divisions. Both anterior divisions joined to give origin to an ‘anterior superior trunk’ and both posterior divisions joined to give origin to a ‘posterior superior trunk’. These trunks joined to give origin to the superior trunk.[6]

Prakash et al., reported a case in which the middle trunk of brachial plexus was found missing. The C5, C6, C7 roots of brachial plexus united to form the upper trunk; and the C8 and T1 roots formed its lower trunk. After giving two branches, that is, the suprascapular nerve and the nerve to the subclavius, the upper trunk divided into two divisions. The anterior division of the upper trunk continued as a lateral cord, while the posterior division combined with the posterior division of the lower trunk to form a posterior cord of brachial plexus. The anterior division of the lower trunk continued as a medial cord. All the other parts of the brachial plexus were normal.[7]

Although variations of brachial plexus are common, the absence of the upper trunk of the brachial plexus is sparesly reported. This may be the result of a lack of connection between C5 and C6 during the early stages of inra uterine life. The unilateral absence of the upper trunk of the brachial plexus is rarer. This case is unique, as it occurred unilaterall