Chapter 22. Functional Evaluation of Motor Responses for Upper Limb Blocks

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).

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.

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).

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.

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.

The dorsal scapular nerve (5), which originates from the C5 root, exits 1 cm farther posterior between the middle and posterior scalene muscles (see Figure 22–6). This nerve supplies the rhomboid muscles. Electrical stimulation results in medial and superior movements of the scapula. If the elbow is flexed and the hand of the patient rests on the abdomen, the elbow may be fixed on the bed and the medial movements of the scapula may be confused with abduction of the arm owing to deltoid contractions. This is a very common cause of confusion and failed block.

The suprascapular nerve (6) originates from the superior trunk and exits between the middle and posterior scalene muscles a few cm more lateral than the dorsal scapular nerve (see Figure 22–6). This nerve supplies mainly the muscles of the rotator cuff, which cause internal rotation of the humerus when the nerve is electrically stimulated. Like the dorsal scapular nerve, if the elbow is flexed and the hand of the patient is on the abdomen, rotation of the humerus can create an impression that the arm is abducting. This can mimic deltoid contractions. Electrical stimulation of this nerve, therefore, also frequently leads to confusion and failed block.

Farther posterior is the nerve to the levator scapulae muscle (7), which originates from the cervical plexus. Electrical stimulation of this nerve causes a motor response in the levator scapulae muscle and subsequent elevation of the scapula, which, like the dorsal scapular and suprascapular nerves, can cause confusion and failed block.

The accessory nerve (8), which is a cranial nerve, is farther posterior and superficial to the fascia, which forms the floor of the posterior triangle of the neck (see Figure 22–6). It runs parallel with the trapezius muscle, which it supplies. Electrical stimulation of this nerve causes a motor response on the trapezius muscle, which is elevation of the scapula. Like the other three nerves mentioned previously, electrical stimulation of the accessory nerve can result in failed block.

Sensory distribution of blocks done at the trunk level covers the lateral part of the upper limb if the superior and middle trunks are blocked. The skin around the shoulder joint and clavicle, which is innervated by nerves from the superficial cervical plexus is usually also anesthetized with this block because of the “overflow” of local anesthetic agent to the superficial cervical plexus.

Infraclavicular block is a cord-level block of the brachial plexus. It is essential that the appropriate cord or cords be blocked for successful infraclavicular block.3 Anatomists12 and anesthesiologists13 alike have criticized this simple and practical method because of the anatomic variations in this region. There are 29 described anatomic variations of the brachial plexus, and it is not practical for anesthesiologists to remember all these variations.14

Basically, the pinkie, or fifth digit, moves toward the cord that is being stimulated. Thus, if the lateral cord is stimulated, the forearm pronates and the pinkie moves laterally (Figure 22–7). If the posterior cord is stimulated, the hand extends and the pinkie moves posteriorly. Finally, medial cord stimulation results in ulnar deviation at the wrist and flexion of the hand. The pinkie moves medially. Therefore, a handy acronym to remember is “at the cords, the pinkie towards.”

The Lateral Cord of the Brachial Plexus

The anterior divisions of the superior and middle trunks of the brachial plexus unite to form the lateral cord.15 The musculocutaneous nerve, the lateral head of the median nerve, and the lateral pectoral nerves arise from this cord.16 The musculocutaneous nerve innervates the biceps brachii, coracobrachialis, all the flexors of the upper arm, and the medial portion of the brachialis muscle. The lateral head of the median nerve innervates the pronator teres, flexor carpi radialis, flexor pollicis longus, flexor digitorum profundus (lateral half to digits 2, 3), and the first and second lumbricals and abductor pollicis brevis muscles via the palmar digital and recurrent branches of the median nerve.16–19 Stimulation of the lateral cord thus causes flexion of the elbow and pronation of the forearm due to contractions of the biceps and pronator teres muscles.16 In addition, the first and second lumbricals and thenar muscles also contract (see Figure 22–2). Overall, the fifth digit moves laterally with pronation of the forearm if the hand is held in the anatomic position when the lateral cord is stimulated (Figure 22–8). The sensory innervation of the lateral cord is outlined in Table 22–4.

The Medial Cord of the Brachial Plexus

The anterior division of the inferior trunk forms the medial cord.15 The medial pectoral nerves, the medial brachial nerve, the antebrachial cutaneous nerve, the ulnar nerve, and the medial head of the median nerve all arise from this cord.16 The primary motor nerve of the medial cord is the ulnar nerve, which innervates flexor carpi ulnaris, palmaris brevis, flexor digitorum profundus (medial half to the fourth and fifth digits), and the palmar and dorsal interosseous muscles, the hypothenar muscles, and the third and fourth lumbrical muscles from the deep palmar branch.17,18 Stimulation of the medial cord causes contraction of these deep flexors of the hand, as well as ulnar deviation of the wrist16 (see Figure 22–4). Overall, it appears as if the fifth digit is moving medially if the hand is held in the anatomic position when the medial cord is stimulated (Figure 22–9). The sensory innervation of the medial cord is outlined in Table 22–5.

The Posterior Cord of the Brachial Plexus

The posterior divisions of all three trunks of the brachial plexus form the posterior cord.15 The subscapular and thoracodorsal nerves, axillary nerve, and radial nerve arise from this cord.16 The subscapular nerve innervates the teres major muscle through the lower subscapular nerve and the subscapularis by the upper subscapular nerve. The thoracodorsal (middle subscapular) nerve innervates the latissimus dorsi. The axillary nerve innervates the deltoid muscle, as well as the teres minor muscle. The dominant posterior cord motor nerve is the radial nerve. It supplies all three heads of the triceps brachii muscle, the lateral portion of the brachialis muscle, and the entire extensor–supinator group of muscles.19 Stimulation of the posterior cord causes muscle twitches of the extensor muscles of the forearm, hand, and fingers and contraction of the muscles of the rotator cuff and the deltoid muscle16 (see Figure 22–3). Overall, the fifth digit moves posteriorly from the anatomic position with neurostimulation of the posterior cord (Figure 22–9). The sensory innervation of the posterior cord is outlined in Table 22–6.