CHAPTER 14 Brachial plexus anatomy

Before selection of technique, it is important to have a thorough understanding of brachial plexus anatomy. The plexus is composed of roots, trunks, divisions, cords, and branches (Fig. 14.1). The brachial plexus stems from the ventral rami of the C5 to T1 nerve roots in the majority of individuals. Approximately 15% of patients may have contributions to the brachial plexus from the C4 or T2 nerve roots, creating a ‘prefixed’ or ‘postfixed’ plexus (see Fig. 15.9). The cervical roots emerge from the intervertebral foramina and lie on a sulcus on the vertebral transverse processes between the anterior and posterior tubercles to which the scalene muscles are attached. Immediately lateral to the transverse processes of the cervical vertebrae, the nerve roots are sheathed in the prevertebral fascia. The nerve roots then travel between the scalene muscles and form three trunks (upper, middle, and lower). The interscalene groove is a palpable surface anatomy depression between the anterior and middle scalene muscles; it allows clinicians easy and reliable access to the roots and trunks of the plexus. The roots are posterior to the vertebral artery – an important anatomic relation for the interscalene block.

Figure 14.1 Brachial plexus anatomy.

The nerve roots of the plexus, as they exit the intervertebral foramina, typically comprise a single large fascicle, surrounded by a scant amount of epineurium. These then divide and recombine along the length of the plexus until, eventually, most fascicles have been segregated into largely motor or sensory axons, bound for a particular area or structure. The number of fascicles increases and their cross-sectional area decreases, from proximal to distal in the plexus. At the same time, the amount of connective tissue of the epineurium progressively increases from proximal to distal.

The dorsal scapular nerve (to rhomboids) and the long thoracic nerve (to serratus anterior) arise from the C5 and C5, 6, and 7 roots, respectively. The upper (C5, 6) and lower (C8, T1) pairs of roots merge to form the upper and lower trunks of the brachial plexus, while the middle root (C7) continues as the middle trunk. The lower trunk is smaller than the others, and is frequently derived entirely from the eighth cervical nerve. The nerve to subclavius arises from the roots of C5 and C6, where they join to form the upper trunk, which also gives off the suprascapular nerve (to supra- and infraspinatus muscles and the shoulder joint). The trunks are formed in the lower part of the posterior triangle of the neck, between the sternocleidomastoid and trapezius muscles and above the middle third of the clavicle. The trunks travel to the margin of the first rib and divide into anterior and posterior divisions.