Along its course above the clavicle, the brachial plexus is bounded by the anterior and middle scalene muscles. The two muscles constitute the scalene triangle with its apex at the transverse processes of cervical vertebrae and its base formed by the first rib.
The phrenic nerve runs anterior to the brachial plexus, separated by the anterior scalene muscle. The distance between the phrenic nerve and the brachial plexus is a few millimeters at the apex of the scalene triangle and reaches a few centimeters towards the base of the triangle.
The vertebral artery lies anterior to the roots of the brachial plexus as they leave the cervical vertebrae.
Nerves supplying the ventral part of the upper extremity originate from the anterior divisions of the trunks of the brachial plexus forming the medial and lateral cords.
Nerves supplying the dorsal part of the upper extremity originate from the posterior divisions of the trunks of the brachial plexus forming the posterior cord.
The cords of the brachial plexus get their names according to their relationship to the second part of the axillary artery in the standard anatomical position (arm adducted, extended parallel to the trunk).
Man uses his arms and hands constantly … as a result he exposes his arms and hands to injury constantly. … Man also eats constantly. … Man’s stomach is never really empty. … The combination of man’s prehensibility and his unflagging appetite keeps a steady flow of patients with injured upper extremities and full stomachs streaming into hospital emergency rooms. This is why the brachial plexus is so frequently the anesthesiologist’s favorite group of nerves. Classical Anesthesia Files, David Little, 1963.
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.
Fig. 4.1 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 this chapter, my goal is to simplify this 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; the un-united 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.
Some writers have focused anesthesiologists’ attention on the fascial investment of the brachial plexus. As the brachial plexus nerve roots leave the transverse processes, they do so between the prevertebral fascia, which divides to invest both the anterior and the middle scalene muscles. Many suggest that this prevertebral fascia surrounding the brachial plexus is tubular throughout its course, thus allowing needle placement within the “sheath” to produce brachial plexus block easily. There is no question that the brachial plexus is invested with prevertebral fascia; however, the fascial covering is discontinuous, with septa subdividing portions of the sheath into compartments that clinically may prevent adequate spread of local anesthetics. Ultrasonographic observation of injections near the brachial plexus confirms our earlier clinical impressions of fascial discontinuity. My clinical impression is that the discontinuity of the “sheath” increases as one moves from transverse process to axilla.
Most upper extremity surgery is performed with the patient resting supine on an operating table with the arm extended on an arm board. Thus anesthesiologists must understand and clearly visualize the innervation of the upper extremity while the patient is in this position. Figs. 4.2 through 4.7 illustrate these features with the arm in the supinated and pronated positions for the cutaneous nerves, and dermatomal and osteotomal patterns, respectively.