Chapter 3 – Upper Extremity Block Anatomy
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 who do select regional anesthesia recognize that there are multiple sites at which the brachial plexus block can be induced. I believe that 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. Additionally, small contributions may be made by the fourth cervical and 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. However, there are some broad concepts that may help clinicians understand brachial plexus anatomy; throughout, my goal in this chapter 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 an 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 medial cord is the nonunited, anterior division of the inferior trunk. These cords are named according to their relation 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. Once again, in an effort to simplify, 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 prevertebral fascia that divides to invest both the anterior and the middle scalene muscles. Many suggest that this prevertebral fascia surrounding the brachial plexus is tubular in form throughout its course, thereby 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 apparently discontinuous, with septa subdividing portions of the sheath into compartments that clinically may prevent adequate spread of local anesthetics. 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 room table with the arm extended on an arm board; and it seems logical for anesthesiologists to be able to understand and clearly visualize the innervation of the upper extremity while the patient is in this position. Figures 3-2 , 3–3 , 3–4 , 3–5 , 3–6 , 3–7 illustrate these features with the arm in the supinated and pronated positions for the cutaneous nerves and the dermatomal and osteotomal patterns, respectively.
An additional clinical “pearl” that can help anesthesiologists check a brachial plexus block before initiating the surgical procedure is the “four Ps.” Figure 3-8 shows how the mnemonic “push, pull, pinch, pinch” can help an anesthesiologist remember how to check the four peripheral nerves of interest during a brachial plexus block. By having the patient resist the anesthesiologist’s pulling the forearm away from the upper arm, motor innervation to the biceps muscle can be assessed. If this muscle has been weakened, one can be certain that the local anesthetic has reached the musculocutaneous nerve. Likewise, by asking the patient to attempt to extend the forearm by contracting the triceps muscle, one can assess the radial nerve. Finally, pinching the fingers in the distribution of the ulnar or median nerve—that is, at the base of the fifth or second digit, respectively—can help the anesthesiologist develop a sense of the adequacy of the block of both the ulnar and median nerves. Typically, if these maneuvers are performed shortly after a brachial plexus block, motor weakness is evident before the sensory block. As a historical highlight, this technique for checking the upper extremity was developed during World War II to allow medics a method for quickly analyzing injuries to the brachial plexus.
Although some of the brachial plexus neural anatomy of interest to anesthesiologists has been outlined, there are some anatomic details that should be highlighted ( Fig. 3-9 ). As the cervical roots leave the transverse processes on their way to the brachial plexus, they exit in the gutter of the transverse process immediately posterior to the vertebral artery. The vertebral arteries leave the brachiocephalic and subclavian arteries on the right and left, respectively, and travel cephalad to enter a bony canal in the transverse process at the level of C6 and above. Thus, one must be constantly aware of needle tip location in relation to the vertebral artery. It should be remembered that the vertebral artery lies anterior to the roots of the brachial plexus as they leave the cervical vertebrae.
Another structure of interest in the brachial plexus anatomy is the phrenic nerve. It is formed from branches of the third, fourth, and fifth cervical nerves and passes through the neck on its way to the thorax on the ventral surface of the anterior scalene muscle. It is almost always blocked during interscalene block and less frequently with supraclavicular techniques. Avoidance of phrenic blockade is important in only a small percentage of patients, although its location should be kept in mind for those with significantly decreased pulmonary function—that is, those whose day-to-day activities are limited by their pulmonary impairment.
Another detail of the brachial plexus anatomy that needs amplification is the organization of the brachial plexus nerves (divisions) as they cross the first rib. Textbooks often depict the nerves in a stacked arrangement at this point. However, radiologic, clinical, and anatomic investigations demonstrate that the nerves are not discretely “stacked” here but, rather, assume a posterior and cephalad relation to the subclavian artery ( Fig. 3-10 ). This point is important when one is carrying out a supraclavicular nerve block and is using the rib as an anatomic landmark. The relation of the nerves to the artery means that if one simply walks the needle tip closely along the first rib paresthesias may not be easily elicited because the nerves are more cephalad in relation to the first rib.
Another anatomic detail that requires highlighting is the proximal axillary anatomy at a parasagittal section through the coracoid process. At this transition site, the brachial plexus is changing from the brachial plexus cords to the peripheral nerves as it surrounds the subclavian and axillary arteries ( Fig. 3-11 ). At the site of this parasagittal section the borders of the proximal axilla are formed by the following anatomic structures.
|Anterior: posterior border of the pectoralis minor muscle and brachial head of the biceps|
|Posterior: scapula and subscapularis, latissimus dorsi, and teres major muscles|
|Medial: lateral aspect of the chest wall including the ribs and intercostal and serratus anterior muscles|
|Lateral: medial aspect of upper arm|
These anatomic relations are important during continuous infraclavicular block techniques.