• Steven C. Borene, MD

I.	INTRODUCTION
II.	BRACHIAL PLEXUS ROOT STIMULATION Fifth, Sixth, & Seventh Cervical Roots (C5, C6, & C7) Eighth Cervical & First Thoracic Root (C8 & T1)
III.	BRACHIAL PLEXUS TRUNK STIMULATION
IV.	THE CORDS OF THE BRACHIAL PLEXUS The Lateral Cord of the Brachial Plexus The Medial Cord of the Brachial Plexus The Posterior Cord of the Brachial Plexus

INTRODUCTION

A nerve stimulator delivers a charge to a motor nerve fiber, which causes a flow of ions through the nerve membrane and initiates an action potential in the nerve fiber.¹ The larger the delivered charge, the more motor fibers in the motor nerve fascicle will fire and the stronger the resulting motor response until a maximal motor response is reached. Several factors influence the total charge delivered to a nerve fiber. The duration and intensity of the current are the main variables. The impedance of the tissue between the stimulating surface (needle or catheter) and the nerve further influence the current intensity. This is reduced as the distance from the stimulating surface to the nerve is reduced. In addition, the type of tissue between the stimulating surface and the nerve, the type of electrode used, and the polarity of the electrode affect the total impedance of the system and thus affect the motor response to stimulation. Muscle mass is also important, such that a stimulus in a frail 80-year-old woman differs substantially from a motor response from a similar electrical response delivered to a young muscular patient.¹

Beyond exposure to regional anesthesia during training, an important aspect in developing confidence and proficiency in the placement of peripheral nerve blocks is through observing a well-defined motor response to neurostimulation. At the level of the roots, the brachial plexus originates from five separate nerve bundles, mainly divided into posterior sensory fibers and anterior motor fibers² (Figure 22-1). More distally, the five roots of the brachial plexus converge to form three trunks: the superior, middle, and inferior trunks. The brachial plexus then divides into the three cords, and finally the seven terminal branches are formed. The three cords are named according to their relation to the axillary artery: posterior, lateral, and medial (see Figure 22-1).

Figure 22-1. Schematic representation of the nerves of the brachial plexus and upper extremity.

Success with brachial plexus blockade requires the identification and block of the appropriate roots, trunks, cords, and peripheral nerves for the proposed surgery.³ Accurately identifying the correct nerve bundles and avoiding injection of incorrect nerves may lead to higher success rates if the single stimulation technique of the infraclavicular block, for example, is followed,^4,5 or it may lead to shorter latency periods if the multiple stimulation technique⁶ is chosen. In the case of blocks at the trunk level, it is also important to recognize false motor responses and avoid injecting local anesthetic when these are elicited.⁷

Whichever approach to blocks of the brachial plexus is chosen, elicitation of specific muscle twitches when each nerve bundle is stimulated is often confusing for trainees and experienced anesthesiologists alike. This chapter describes a simple and consistent way to teach and remember the motor responses elicited by electrical stimulation of the different nerve bundles.

Figure 22-2. Schematic representation of the nerves derived from the lateral cord of the brachial plexus.

BRACHIAL PLEXUS ROOT STIMULATION

Fifth, Sixth, & Seventh Cervical Roots (C5, C6, & C7)

When studying Figure 22-1, it can be seen that the fifth cervical root (C5) and the superior parts of the sixth and seventh cervical roots (C6 and C7) form most of the superior and middle trunk and all of the lateral cord of the brachial plexus (Figure 22-2). Fibers from the C5, C6, and C7 roots then terminate in the dorsal scapular nerve, suprascapular nerve, lateral pectoral nerve, musculocutaneous nerve, and part of the median nerve (Table 22-1). Electrical stimulation of only the C5 root, as with the cervical paravertebral block,^8,9 for example, therefore results in a motor response in any or all of the muscles supplied by the above nerves (rhomboid muscles, rotator cuff muscles, major pectoral muscle, biceps muscle, pronators of the forearm, superficial flexors of the forearm, and first and second lumbricals of the hand and abductor of the thumb).

Table 22–1.

Motor Response Due to Stimulation of Nerves Originating from the C5 Root

It is, however, almost never possible to achieve pure C5 stimulation. With cervical paravertebral block, the needle is aimed toward the C6 root.⁸ Electrical stimulation of the C6 and C7 roots results in motor responses in the muscles supplied by nerves branching off the posterior cord (Figure 22-3), that is, the deltoid muscle (axillary nerve), extensors of the arm (radial nerve) (Table 22-2) and also any combination of the above muscles supplied by the nerves branching from the lateral cord (see Table 22-1).

Eighth Cervical & First Thoracic Root (C8 & T1)

The roots of C8 and T1 continue to form the inferior trunk (see Figure 22-1). Fibers coming from C8 continue to join the posterior and medial cords, whereas fibers of the T1 root continue to form the medial cord (Figure 22-4). Electrical stimulation of the inferior trunk causes contractions of all, or combinations of, the muscles supplied by nerves branching off the posterior cord (see Table 22-2), but mainly of the muscles supplied by the nerves coming from the medial cord (Table 22-3). These are the minor pectoral muscle (medial pectoral nerve) and muscles supplied by the ulnar nerve (flexor carpi ulnaris, flexor digitorum profundus, adductor pollicis, flexor pollicis, the interosseous muscles of the hand, the third and fourth lumbricals and flexors, and flexor and opponens digiti minimi (Table 22-3).

Sensory distribution of blocks performed at the level of root cover the total upper limb with the exception of the skin around the shoulder joint and clavicle, which is innervated by nerves from the superficial cervical plexus (Figure 22-5), and the skin on the medial upper arm, just distal to the axilla, which is innervated by the intercostobrachial nerve.

Figure 22-3. Schematic representation of the nerves derived from the posterior cord of the brachial plexus.

Clinical Pearls

Electrical stimulation of the superior three roots of the brachial plexus (C5, C6, and C7), as seen with cervical paravertebral block for shoulder surgery, results mainly in motor responses in the posterior and anterior shoulder girdle muscles and flexor and extensor muscles of the proximal arm.

Electrical stimulation of the inferior two roots of the brachial plexus (C8 and T1), as seen with cervical paravertebral block for wrist surgery, results mainly in motor responses in the deep flexor muscles of the hand.

The upper and lower limits of the current intensity for successful blocks at the level of the roots have not yet been established. For example, Wehling and colleagues¹⁰ reported a case in which successful cervical paravertebral block was obtained without any untoward sequelae after stimulating through a catheter at a nerve stimulator current output of 0.05 mA and 300 μsec. On the other hand, in the original description of the continuous cervical paravertebral block currents of up to 3 mA were advocated.^8,9

Blocks at the level of the roots of the brachial plexus are ideal for continuous blocks for major surgery to all three large joints of the upper limb.

BRACHIAL PLEXUS TRUNK STIMULATION

Nerve conduction studies have demonstrated that electrical stimulation of the proximal aspects of all three brachial plexus trunks, as classically done during interscalene block, results in a motor response mainly of the muscles supplied by the radial nerve¹¹ (see Table 22-2). Moving the stimulating probe to the distal part of the superior trunk of the brachial plexus results in motor responses in the muscles supplied by the musculocutaneous nerve (flexion at the elbow due to biceps contraction) and axillary nerve (abduction of the arm due to deltoid contraction)¹¹ (see Table 22-1). If the inferior part of the inferior trunk is electrically stimulated, a flexor response can be observed in the hand resulting from a motor response in the muscles supplied by the median and ulnar nerves¹¹ (see Tables 22-1 and 22-3).

When attempting an interscalene block, it is equally important to know which motor responses are not acceptable. At the level at which the interscalene block is done, there are five other nerves that do not form part of the brachial plexus. Stimulating these nerves can lead to misleading motor responses, and if these “false” motor responses are incorrectly accepted, the confusion may ultimately lead to failed blocks.

From Figure 22-6, it can be seen that the phrenic nerve (1) is situated on the belly of the anterior scalene muscle. Stimulating this nerve results in a motor response of the diaphragm, which can clearly be seen as abdominal twitches. Phrenic nerve twitch should not be relied upon.

Approximately 1 cm posterior from the phrenic nerve between the anterior (2) and middle scalene muscles (4) are the trunks of the brachial plexus (3). Electrical nerve stimulation of the trunks here results in a triceps muscle motor response if stimulated proximally and in biceps and deltoid responses if stimulated distally on the superior trunk. A flexion hand motor response indicates that the inferior part of the inferior trunk is being stimulated, which means that the needle is deep to the superior and middle trunks and probably close to the dome of the lung.

Table 22–2.

Motor Response Due to Stimulation of Nerves Originating from C6 and C7 Roots