Upper Extremity Block Anatomy

The late David Little’s appropriate observations do not always lead anesthesiologists to choose a regional anesthetic for upper extremity surgery. However, those selecting regional anesthesia recognize that there are multiple sites at which the brachial plexus block can be induced. If anesthesiologists are to deliver comprehensive anesthesia care, they should be familiar with brachial plexus blocks. Familiarity with these techniques demands an understanding of brachial plexus anatomy. One problem with understanding this anatomy is that the traditional wiring diagram for the brachial plexus is unnecessarily complex and intimidating.

Figure 3-1 illustrates that the plexus is formed by the ventral rami of the fifth to eighth cervical nerves and the greater part of the ramus of the first thoracic nerve. In addition, small contributions may be made by the fourth cervical and the second thoracic nerves. The intimidating part of this anatomy is what happens from the time these ventral rami emerge from between the middle and anterior scalene muscles until they end in the four terminal branches to the upper extremity: the musculocutaneous, median, ulnar, and radial nerves. Most of what happens to the roots on their way to becoming peripheral nerves is not clinically essential information for an anesthesiologist. There are some broad concepts that may help clinicians understand the brachial plexus anatomy; throughout, my goal in this chapter is to simplify this anatomy.

Figure 3-1. Brachial plexus anatomy.

After the roots pass between the scalene muscles, they reorganize into trunks—superior, middle, and inferior. The trunks continue toward the first rib. At the lateral edge of the first rib, these trunks undergo a primary anatomic division, into ventral and dorsal divisions. This is also the point at which understanding of brachial plexus anatomy gives way to frustration and often unnecessary complexity. This anatomic division is significant because nerves destined to supply the originally ventral part of the upper extremity separate from those that supply the dorsal part. As these divisions enter the axilla, the divisions give way to cords. The posterior divisions of all three trunks unite to form the posterior cord; the anterior divisions of the superior and middle trunks form the lateral cord; and the ununited, anterior division of the inferior trunk forms the medial cord. These cords are named according to their relationship to the second part of the axillary artery.

At the lateral border of the pectoralis minor muscle (which inserts onto the coracoid process), the three cords reorganize to give rise to the peripheral nerves of the upper extremity. Simplified, the branches of the lateral and medial cords are all “ventral” nerves to the upper extremity. The posterior cord, in contrast, provides all “dorsal” innervation to the upper extremity. Thus, the radial nerve supplies all the dorsal musculature in the upper extremity below the shoulder. The musculocutaneous nerve supplies muscular innervation in the arm, while providing cutaneous innervation to the forearm. In contrast, the median and ulnar nerves are nerves of passage in the arm, but in the forearm and hand they provide the ventral musculature with motor innervation. These nerves can be further categorized: the median nerve innervates more heavily in the forearm, whereas the ulnar nerve innervates more heavily in the hand.