Peripheral Nerve Blocks: A Color Atlas, 3rd Edition

6.Anatomy of the Brachial Plexus

Laura Clark

Knowledge of anatomy is essential to maximize the likelihood of success when performing peripheral nerve blocks. However, it is important to recognize that variations are frequent, and that “normal anatomy” is only found in 50% to 70% of cases.

Brachial Plexus Anatomy (FIG. 6-1)

The brachial plexus is formed by the anterior rami of C5 to T1 (the posterior roots give innervation for skin and muscle of the paravertebral area). Brachial plexus innervates the scapular belt and the upper limb. It has a triangular shape with the base applied against the cervical spine and the vertex in the axilla. The anterior rami supply the superior (C5-6), middle (C7), and inferior (C8-T1) trunks. At the level of the superior border of the first rib, each trunk divides into an anterior and posterior division. The three posterior divisions join to form the posterior cord. At the level of coracoid process, the anterosuperior and middle divisions form the lateral cord and the anteroinferior division becomes the medial cord. The axillary and radial nerves originate from the posterior cord. The musculocutaneous nerve quickly separates from the main brachial plexus as it passes underneath the clavicle and becomes a single nerve as the plexus transitions from divisions to cords. In the vertex of the axilla, the lateral cord and part of the medial cord join to form the median nerve; the medial cutaneous nerve of the arm and the forearm and finally the ulnar nerve originate from the remainder of the medial cord. See Figure 6-1 for the classical two dimensional picture of the brachial plexus highly recommended to review the anatomy in a fresh tissue cadaver whenever possible. In Figures 6-2 and 6-3 minimal alteration of the anatomy in these fresh tissue dissections demonstrate the brachial plexus and the surrounding anatomy in situ.

Embryology allows us to better understand the apparent complexity of the brachial plexus. During organogenesis, between the 4th and the 8th week of development, the brachial plexus has a cone shape in the bud limb. The cutaneous innervation is distributed at the periphery of the trunk wall. As the bud limb grows, dermatoma stretches out. The cone then divides longitudinally into two independent plans, anterior and posterior, with the appearance of three cords. The posterior cord (posterior plane) provides innervation to the extensor and supinator muscles while the anterior plane, more complex and variable, provides the medial and lateral cords that innervate the flexor and pronator muscles. Generally there is no anastomosis between the anterior and posterior cords. The presence of the subclavian artery explains why there are two anterior cords. Thus, in some species without the subclavian artery (birds) only one anterior cord is found. Embryogenesis also explains the frequency of variations and anastomosis between the nerves originating from the anterior cords.

Figure 6-1. Brachial plexus anatomy.

Usually the anterior rami of C4 and T2 also contribute to the brachial plexus. This contribution allows one to differentiate a “prefixed” plexus with a C4 predominant contribution versus a “postfixed” plexus with a T2 predominant contribution. In humans, the brachial plexus is predominantly “prefixed.” This differentiation may explain the superior extension toward C4 that frequently occurs after an interscalene block (prefixed).

Nerve Courses

Collateral Branches

Twelve branches originate from the brachial plexus. Some are more relevant than others when performing brachial plexus blocks. They include:

· The long thoracic nerve that directly originates from the C5 to C7 roots. It supplies the serratus anterior muscle. The nerve runs posterior to the brachial plexus. Therefore, when performing an interscalene block, the stimulation of the serratus anterior muscle suggests that the needle should be redirected anteriorly.

· The dorsal scapular nerve supplies the levator scapulae and rhomboid minor and the superior part of the rhomboid major muscles. It is one of the first nerves to leave the plexus. The stimulation of this nerve produces an elevation of the scapulae (interscalene approach). The injection of local anesthetic solution following its stimulation produces a block of the deep cervical plexus, which may be of interest in the case of a carotid surgery.

· The suprascapular nerve supplies both the supraspinatus and the infraspinatus muscles and sends branches to the shoulder joints. It originates at the level of the interscalene groove. Moreover, like the long thoracic nerve, the suprascapular nerve runs posterior to the brachial plexus. Its stimulation, which produces a posterior contraction of the rotator cuff (shoulder ascension), indicates that the needle needs to be redirected anteriorly. Diffusion of local anesthetic toward this nerve after a supraclavicular approach is inconsistent.

· The subclavius nerve supplies the subclavius muscle and anastomoses with the phrenic nerve, a nerve that also originates from C5. This explains in part why, even with limited extension to the cervical plexus or limited volume of local anesthetic, a blockade of the phrenic nerve with an interscalene block results in a reversible diaphragmatic paralysis.

 

Figure 6-2. A very limited dissection demonstrating how the brachial plexus lies in the neck as transverse from its origin of individual roots from the cervical spine to somewhat join into divisions and group into cords. The divisions travel in a similar plane at this level and are organized in a parallel fashion to each other. The plexus is framed by the sternocleidomastoid at the superior edge of the triangle until it gives way to the anterior border of the anterior scalene. At times the anterior scalene is not prominent as in this specimen. The inferior border is the clavicle. The triangle is completed by the posterior border in the middle scalene muscle.

Terminal Branches (Anterior Plan) (Fig. 6-4)

Median Nerve

The median nerve is formed in the axilla by the union of the lateral and medial cords. It runs first laterally to the axillary artery and progressively crosses over the brachial artery to become medial to the brachial artery at the inferior third of the arm. It does not give any branch until the inferior part of the arm, although in one-third of cases it provides an anastomosis to the musculocutaneous nerve. Around the elbow several branches originate from the median nerve (articular to the elbow, muscular to pronator teres muscle). About 2 or 3 cm below the elbow, the median nerve provides the interosseous nerve, which innervates the anterior aspect of the flexor digitorum profundus and anterior aspect of the carpal joint, with frequent anastomosis with the posterior interosseous nerve. In the forearm the median nerve runs between the flexor digitorum superficialis and flexor digitorum profundus muscles. At the distal third of the forearm the median nerve runs between the tendons of the flexor carpi radialis and palmaris longus muscle. It is of interest to note that the palmaris longus muscle is absent in 15% of the population. Several centimeters above the wrist crease the median nerve provides a sensory palmar branch. Finally, the median nerve enters the hand through the carpal tunnel under the flexor retinaculum and divides into small muscular branches innervating a part of the thenar muscles and branches contributing to the palmar digital nerve.

Figure 6-3. The brachial plexus and its surrounding structures. In this dissection the sternocleidomastoid and the internal jugular vein have been reflected away so that the surrounding structures could be better visualized. The close proximity to the phrenic can be appreciated as well as the relatively close internal carotid artery and vagus nerve. Of note in this specimen is an example of how small in diameter the phrenic nerve can be and how close. At the upper end of the origin of the brachial plexus they are almost side by side. This is often demonstrated during performance of an interscalene block with just a very small change in needle direction causing a phrenic nerve stimulation of the diaphragm or the desired twitch of stimulation of the brachial plexus.

Ulnar Nerve

The ulnar nerve is formed in the upper part of the axilla from the medial cord. Initially the ulnar nerve runs medial to the axillary artery and progressively posterior, reaching the elbow between the medial humeral epicondyle and the olecranon. It leaves its first collateral branch at the elbow for the joint. Below the elbow, branches originate to the flexor carpi ulnaris and part of flexor digitorum profundus muscles. Along the forearm the ulnar nerve runs posterior to the flexor carpi ulnaris and posterior and medial to the ulnar artery. Several centimeters before the wrist crease the ulnar nerve leaves two branches: a palmar branch and a dorsal branch for the palmar and dorsal aspect of the hand. The ulnar nerve enters the hand running under the flexor retinaculum and divides into superficial branches to the palmar digital nerves and deep motor branches that innervate the hypothenar, interosseous, and parts of the lumbrical and thenar muscles.

Musculocutaneous Nerve

The musculocutaneous nerve is a branch of the lateral cord. It leaves the vascular/nervous space high in the axilla, diffusing in the direction of the coracobrachialis muscle, which it perforates. In the arm, it runs in the groove between the biceps and brachialis muscles. It supplies these two muscles and the coracobrachialis muscle. Several variations of the course and distribution of the musculocutaneous nerve may be observed. In 20% of cases, instead of piercing the coracobrachialis muscle it lies against the median nerve for a variable distance along the arm before supplying the three muscles. It pierces the deep fascia just before the elbow and divides in two branches that run on each side of the cephalic vein and become the lateral cutaneous nerve of the forearm. It supplies the skin of the lateral border of the forearm up to the wrist. In some cases it can extend until the thenar region.

Figure 6-4. Terminal branches (anterior plan).

Medial Cutaneous Nerve of the Arm and the Forearm

The medial cutaneous nerve of the arm and the forearm are both formed in the medial cord. The medial cutaneous nerve of the arm quickly leaves the plexus in the axilla and dives in the direction of the armpit where it joins the intercostobrachial nerve (Hyrtl anastomosis). It supplies the skin of the medial border of the arm. The cutaneous nerve of the forearm arises from the medial cord and runs with the ulnar nerve until the elbow. In the upper part of the arm it leaves some branches (one or two) for the anterior and internal part of the arm. It pierces the deep fascia just before the elbow and divides in two branches on each side of the basilic vein. It supplies the skin of the medial aspect of the forearm until the wrist.

Terminal Branches (Posterior Plan)

Axillary Nerve

The axillary nerve originates from the posterior cord. It runs deeply against the capsule of the shoulder joint, and it curves posteriorly along the surgical neck of the humerus through a quadrangular space formed laterally and medially by the humerus and the triceps brachii muscle and superior and inferior by the teres minor and major muscles. Then it divides into anterior and posterior branches. It supplies the skin of the inferior half of the shoulder and the lateral aspect of the arm. It also supplies the teres minor and deltoid muscles and gives branches to the shoulder joint.

Radial Nerve

The radial nerve is the extension of the posterior cord in the axilla. It runs initially posterior to the axillary artery and progressively in a spiral course around the humerus in a specific sulcus to run laterally and then anteriorly at the elbow. It supplies the triceps brachii muscle through a branch leaving at various levels in the axilla. Frequently it leaves a spindly branch to the brachialis muscle. At the third distal part of the arm it leaves branches for the brachioradialis and extensor carpi radialis longus muscles. It supplies skin of the posterior aspect of the arm and the forearm and the lateral border of the elbow. Around the elbow, it gives articular branches to the joint. The cutaneous branch for the forearm leaves the trunk at the inferior part of the radial sulcus. At various levels around the elbow, the radial nerve divides into sensory (superficial) and motor (deep) terminal branches. The superficial branch runs along the lateral border of the forearm and pierces the fascia at the lower third. At this level it is in close relation to the cephalic vein. It supplies the lateral part of skin of the dorsal aspect of the hand. The deep branch (posterior interosseous muscle) winds posteroinferiorly around the neck of the radius and runs between the superficial and deep planes of the extensor muscles. It supplies all extensor and supinator muscles and the dorsal face of the carpal joint.

Anastomosis

Anastomosis is a simple fiber exchange between nearby nerves. Anastomoses are frequent with many variations, and some are more important than others. In approximately one-third of cases an anastomosis exists between the musculocutaneous and the median nerve. In most cases it is a lateromedial anastomosis that follows the course of the musculocutaneous nerve, but sometimes it is a mediolateral anastomosis that follows the course of the median nerve. Consequently, if during an axillary or humeral block it is difficult to locate the musculocutaneous nerve, the clinician should determine if the musculocutaneous nerve is not already blocked by an injection of local anesthetic following the identification of the median nerve.

At the upper part of the forearm, the Martin and Grüber anastomosis between median and ulnar nerves occurs frequently (20%). In general, it is median fibers that are found in the deep branch of the ulnar nerve, which could explain why when one uses a nerve stimulator it is sometimes difficult to differentiate between these two nerves. In cases of doubt, one can search for a response of the flexor carpi ulnaris muscle (always supplied by the ulnar nerve) or a response of the flexor carpi radialis muscle (always supplied by the median nerve).

Upper Limb Innervation (Fig. 6-5)

Upper limb innervation is complex, and it is important to recognize the difference between skin, muscle, and bone innervation. Such knowledge is essential to predict the extension of the motor and sensory block and determine the most appropriate approach for a given surgery (especially if a distal block is chosen). It also explains the motor response associated with the stimulation of a given nerve.

Skin Innervation

Shoulder

The superficial cervical plexus innervates most the shoulder, while the axillary nerve innervates the lower half; however, overlapping with intercostal nerves for the anterior aspect of the shoulder and with dorsal ramus of C3 to T3 roots for the posterior aspect of the shoulder is possible. This explains why, even with a successful interscalene block, cutaneous anesthesia could be insufficient to provide adequate anesthesia for the insertion of a posterior trocar or deltopectoral groove incision.

Arm and Forearm

A block of the radial nerve performed at the elbow does not produce a sensory block of the posterior aspect of the forearm because this territory is supplied by the posterior cutaneous nerve, a branch of the radial nerve that originates in the middle of the arm. In general, at the level of the elbow and thereafter, none of the three nerves (median, ulnar, and radial) have branches to supply the shin of the forearm.

Figure 6-5. Upper limb innervation.

Hand

Innervation of the distal part of the palm is supplied by palmar branches of the ulnar and median nerves. These branches, as the dorsal branch of the ulnar nerve, originate several centimeters above the wrist crease. This explains why median and ulnar blocks at the wrist need to be performed at the distal third of the forearm (5 to 6 cm above the wrist crease). In addition to the palmar aspect of the second, third, and fourth fingers, the palmar digital branches of the median and ulnar nerves supply the two-thirds dorsal part of these fingers. Consequently, a block of the common palmar branch provides anesthesia to 75% of the finger with a single injection. In contrast, the palmar aspect of the second, third, and fourth fingers depends on the palmar digital branch of the median nerve. Consequently, a block of the common palmar branch provides anesthesia to 75% of the palm with a single injection.

The dorsal aspect of the thumb is completely supplied by the radial superficialis nerve. At the level of the hand there is also a considerable overlap in zones of distribution of the different nerves. This must be taken into account when the intensity of each block is being tested. The median nerve contains the most important contingent of sensitive fibers for the hand.

Muscle Innervation

All muscles of the shoulder and arm except the trapezius are supplied by the brachial plexus. This explains why it is difficult to obtain a complete muscle relaxation of the shoulder even with an efficient brachial plexus block. This could be of importance for some types of surgery, like shoulder arthroplasty, even if anesthesia is sufficient with an interscalene block.

The muscles of the arm are divided into two groups, an anterior group (biceps, brachial, and coracobrachialis muscles) all supplied by the musculocutaneous nerve and a posterior group (triceps muscle) supplied by the radial nerve.

The muscles of the forearm are divided into four groups: an anterior and medial group in charge of flexion and pronation movement supplied by the median and the ulnar nerve; a lateral group for supination supplied by the deep radial nerve; and a posterior group for extension that is also supplied by the radial nerve.

The muscles of the hand are mainly supplied by the ulnar nerve except for a part of the thenar muscles and the first and second lumbrical muscles, which are supplied by the median nerve.

Bone and Articular Innervation

Bone and articular innervation are not well documented. The figures derived from the book published by Déjerine in 1926 are in agreement with the concept that the innervation of the joint is generally supplied by the same nerves that supply the muscles mobilizing the joint (Hilton's law).

The shoulder joint is supplied by the axillary, suprascapular, subscapular, and pectoral lateral nerves. All these nerves originate from the C5-6 roots and are effectively blocked by an interscalene approach.

The elbow is supplied by the radial, musculocutaneous, median, and ulnar nerves.

The wrist is supplied by the anterior interosseous nerve (a branch of the median nerve) with frequent ulnar anastomosis and by the posterior interosseous nerve (a branch of the radial nerve).

Although most representations of upper extremity innervation are based on superficial distribution, it is important to realize that the muscular and bone innervation is not strictly superimposed. The only location in which a single nerve innervates all structures is the medial aspect of the hand and the fifth finger (ulnar nerve). For example, in the case of an exploration of a first interdigit wound, it is necessary to block the median and radial nerves to provide adequate anesthesia of the anterior and posterior aspect of the skin, and to block the ulnar nerve to provide anesthesia of the interosseous muscles.

Topographic Anatomy (Fig. 6-6)

Supraclavicular Region (Posterior Cervical Triangle)

In front of the brachial plexus, the omohyoid muscle delineates two triangles: one superior, the omotrapezius triangle with the trapezius muscle; and one inferior, the omoclavicular triangle with the clavicle. Roughly, we can consider that we find the trunk of the brachial plexus in the superior triangle and the cords in the inferior triangle.

A sheath formed by the surrounding muscular aponeurosis and fascia surrounds all the brachial plexus and vessels. In the supraclavicular area this space is limited anteriorly by the anterior scalene muscle and posteriorly by the middle scalene. Anteriorly the superficial cervical fascia lies between the trapezius and the sternocleidomastoid muscles, and posteriorly the profound cervical aponeurosis is formed by the fusion of the pretracheal and prevertebral fasciae.

Omotrapezius Triangle (Occipital Triangle)

In the interscalene groove we have to differentiate the superior brachial plexus, composed of the superior (C5-6) and middle (C7) trunk, and the inferior plexus, deeply localized behind the subclavian vessels and lying on the pleural space. In some cases the C5-6 root is not found in the interscalene groove but runs in front of the anterior scalene muscle. Moreover, Hovelacque described a frequent muscular lamina tight between the scalene muscles, which separates the first two roots of the plexus from the others. These considerations explain in part why it is difficult to obtain a complete brachial plexus anesthesia after an interscalene block. They also explain that:

· When the needle is positioned too medially, there is an increased risk of vertebral artery puncture or perimedullar puncture.

· The proximity of the inferior cervical node explains the frequent Horner syndrome.

 

Figure 6-6. Topographic anatomy.

· It is important to recognize the presence of the phrenic nerve anteriorly and the supra-scapular and long thoracic nerves posteriorly to the brachial plexus. Their stimulation provides critical information about how to reorient the needle.

· The external jugular vein is anterior to the brachial plexus and prolongs the line perpendicular to the middle of the clavicle. Its localization is helpful when the interscalene groove is difficult to feel, especially in obese patients.

Even if there are important anatomic variations, knowledge of the general disposition of the trunk fibers provides helpful information in the understanding of nerve stimulation for the interscalene approach. The superior trunk could be separated in two contingents, one superior with fiber for musculocutaneous and axillary nerves and one inferior for radial and median nerves. The middle trunk contains fibers mainly for the radial and to a lesser degree for the musculocutaneous and median nerves. Consequently, the stimulation of the superior trunk (C5-6) produces a contraction of either the biceps brachii or the deltoid muscles while the stimulation of the brachioradialis or triceps brachii muscles can be related to either a stimulation of the posterior part of the superior trunk or a stimulation of the middle trunk. A finger flexion suggests a stimulation of the inferior trunk with a needle close to the pleura.

Omoclavicular Triangle (Supraclavicular Triangle)

It is in this region that the differentiation between trunks and cords occurs corresponding to the cleavage between innervation of flexors (anterior plan) and extensor muscles (posterior plan). The main interest of this region is the closeness of the brachial plexus nerves, which suggest extensive diffusion with a single injection. But this zone corresponding to the supraclavicular fossa is considered quite dangerous by anatomists and surgeons because of the close relations between nerves, vessels, and pleural space. Several collaterals of the subclavian artery (transversa colli and the suprascapular or the cervical superficialis arteries) cross or interlace the brachial plexus. Consequently, independently of a direct vascular injection f local anesthetic, a vascular puncture can lead to the development of a profound hematoma with an increasing risk of secondary neurologic compressions or fibrosis.

Figure 6-7. Frontal dissection of the brachial plexus demonstrating its course from the interscalene area to the beginning of the axilla. The pectoralis major and minor have been removed. The plexus exits the interscalene region, passes under the clavicle, and surrounds the artery. The axillary artery has been retracted by the blue band to show the posterior cord of the plexus or it would not have been seen at all. The medial cord is too deep to be shown in this frontal view. The vein is quite inferior and if it is punctured by the needle, the needle should be removed and reentered 1 to 2 cm superiorly. Note in this section the boundaries of the lung as seen by the edges of the intercostal muscles and why it is possible to enter the lung with both the vertical infraclavicular block and the coracoid infraclavicular block.

It seems better and safer to avoid plexus blocks based on the direct approach of the supraclavicular fossa and instead select a more lateral approach, in search of a stimulation of the lateral cord.

Axillary Region (FIG. 6-7)

The axillary region is limited anteriorly by the pectoral muscle, posteriorly by the latissimus dorsi muscle, laterally by the biceps brachii and coracobrachialis muscles, medially by the thorax and anterior serratus muscle, and superiorly by the scapula, clavicle, and the first rib. In this area the three cords divide into the terminal branches of the brachial plexus at the level of the scapulohumeral joint and posteriorly to the inferior border of the pectoralis minor muscle. At this level, the plexus is in close proximity to vessels and especially the axillary artery: In the upper part of the axilla behind the clavicle, the artery is anterior to the plexus and progressively becomes central as it runs along the axilla. As with the suprascapular region, the entire plexus is located in a common diffusion space between the clavipectoral fascia in front, the subscapular teres major latissimus dorsi posteriorly, the coracobrachialis laterally, and the anterior serratus medially. It is necessary to distinguish the upper part of the axillary region corresponding to the infraclavicular region and the lower part corresponding to the axillary hollow.

Infraclavicular Region (Fig. 6-8)

It is at this level that the terminal branches of the brachial plexus originate. As for the supraclavicular region, we can roughly define an anterior plan centered by the median nerve with the musculocutaneous nerve laterally and the ulnar and medial antebrachial cutaneous nerves medially. A posterior plane can be made with the radial and axillary nerves. After their differentiation, the musculocutaneous and axillary nerves quickly leave the common diffusion space, which explains why there is an increased risk of block failure if an injection of local anesthetic is made following the stimulation of either the musculocutaneous or axillary nerve. In contrast, a stimulation of the median nerve produces more consistent results.

As for the supraclavicular region, it is very important to note the close proximity of the nerves and vessels (axillary vessels, thoracoacromial artery, and cephalic vein) in this region where hemostasis with a simple compression could be difficult.

Axillary Hollow Region

At this level, the axillary artery constitutes the central axis of the nerves and vessel bundle. Although Partridge and Retzl have shown variation of localization of the nerves around the artery, we can consider that the median nerve is lateral, the ulnar nerve is medial and anterior, and the radial nerve is medial and posterior to the axillary artery. At this level, the diffusion space is divided into secondary sheaths. This concept is very old: In 1855 Marcellin Duval described a common sheath for vessels and nerves partitioned by septa. Lassale and Ang demonstrated the existence of a common space between the median and ulnar nerves and the axillary vessels but without the radial, axillary, and musculocutaneous nerves. The presence of these septa was confirmed by Thompson and Rorie, who compared clinical, radiologic, and anatomic data. Even though some subsequent studies have limited the value of these findings, they explain why it is difficult to obtain more than 80% of a complete block with a single injection.

Although there is also a close relation between vessels and nerves, vascular puncture in this area is easy to control with external compression.

Humeral Region (Brachial Region)

At this level there is no common diffusion space including nerves and vessels, except between the brachial artery and the median nerve (in some rare cases the ulnar nerve is included in this space). Therefore, a high humeral block requires each nerve to be blocked individually. Although the distribution of the nerves around the brachial artery is comparable, a greater distance separates each nerve. To perform a high humeral block, the needle must be introduced at the proximal third of the arm (and not more distal) to avoid crossing the radial nerve on the lateral aspect of the humerus and it becoming inaccessible to the needle. This explains why the term “humeral or brachial canal block” is preferred to the term “midhumeral block” that was initially proposed. When stimulating the radial nerve, a contraction of the triceps brachii must not be accepted as the end point but rather an extension of the thumb. This response corresponds to a stimulation of the muscular branch, which at this level is at some distance from the radial nerve.

Distal Third of the Arm and Elbow Regions

Individual nerve blocks at the elbow are realized at the distal third of the arm. It is possible to distinguish the superficial nerves, which supply the skin of the forearm, and the deep nerves that supply muscles of the forearm and the skin and the muscles of the hand.

Superficial Nerves

The lateral and the medial antebrachial cutaneous nerves are near the elbow. The lateral antebrachial cutaneous nerve (terminal branch of the musculocutaneous nerve) has two branches: one anterior and one posterior on each side of the cephalic vein. The medial antebrachial cutaneous nerve also has two branches, which are satellites of the basilic vein. The posterior antebrachial cutaneous nerve (a branch of the radial nerve) is at the level of the posterior aspect of the elbow. All of these nerves can be blocked by subcutaneous infiltration.

Figure 6-8. Sagittal section of the infraclavicular area. This dissection is a sagittal section of the right side demonstrating the relationship of the brachial plexus below the clavicle. The pectoralis major and minor are virtually the only muscles above the plexus. The lung is not visible and has not been violated. At the level of a vertical infraclavicular block, the lung would be visible. The axillary vein is prominently seen inferior to the plexus, which surrounds the axillary artery. The fascicles of the lateral cord (which contains one-half of the median nerve) is virtually laying on the artery. The posterior cord (which contains 100% of the radial nerve) is actually superior but is deeper than the artery from the surface of the skin. It is large and round in this specimen compared with the lateral cord, which is flattened and surrounds the upper surface of the artery. The medial cord (which contains the other part of the median nerve and the ulnar nerve) is just under the artery and is the deepest cord to the skin.

Deep Nerves

Approximately 3 to 4 cm above the elbow crease, the median nerve lies between the medial border of the biceps brachii tendon and the lateral border of the pronator teres muscle tendon. In this region the nerve is located medially to the brachial artery, which constitutes an easy landmark for a median block. The radial nerve lies between the lateral border of the biceps brachii tendon and the medial border of the brachioradialis biceps tendon. It has a mirror position with the median nerve regarding the biceps brachii tendon, but is more profound. Frequently, contraction of the pronator muscles for the median nerve and contraction of the brachioradialis nerve for the radial nerve are first observed with the nerve stimulator. These responses must not be accepted as an end point, and the needle must be moved more laterally for the median nerve and more medially for the radial nerve. The ulnar nerve runs on the posterior aspect of the elbow, very superficially, in the groove between the olecranon and the medial epicondyle.

Distal Third of the Forearm Region

As with the elbow region, we can distinguish two plans.

Superficial Branches

The lateral antebrachial cutaneous, the medial antebrachial cutaneous, and the posterior antebrachial cutaneous nerves are the three superficial branches of the distal third of the forearm. These three nerves supply the skin of the forearm until the wrist crease with a frequent thenar extension for the lateral antebrachial cutaneous nerve. These nerves can be blocked by subcutaneous infiltration. Moreover, the superficial radial branch, which supplies part of the dorsal face of the hand located on the lateral border of the forearm, can be blocked at the level of the wrist crease.

Deep Branches

Four deep nerves can be identified at the distal third of the forearm.

The median nerve runs between the palmaris longus tendon medially and the flexor carpi radialis tendon laterally. Located 3 to 4 cm above the wrist, it gives a palmar branch for the hand. Consequently, a median nerve block at the wrist must be performed at least 5 cm above the crease. Since at this level the median nerve is mainly composed of sensitive fibers with a small motor contingent, the use of a nerve stimulator produces inconsistent motor results. Finally, at this level the median nerve is surrounded by the flexor tendons, which constitute a real space of diffusion for local anesthetics.

The anterior interosseous nerve supplies the anterior aspect of the carpal joint. It lies in front of the interosseous membrane posterior to the median nerve and is separated from it by the pronator quadratus muscle. It is unlikely that injection of local anesthetic for a median nerve block at the wrist could extend to the anterior interosseous nerve.

The ulnar nerve on the medial border of the forearm lies posterior to the flexor carpi ulnaris tendon and the ulnar artery. Between 4 and 6 cm above the wrist crease it contains two branches, one posterior for the dorsal face of the hand and one palmar branch. Consequently, a block of the ulnar nerve should be performed more than 5 cm above the wrist crease. In contrast to the median nerve, however, the ulnar nerve at this level is mainly motor and very easy to locate with the nerve stimulator. Since its location under the flexor carpi ulnaris is variable, a blind injection of local anesthetics would be associated with a high failure rate.

The posterior interosseous nerve is the terminal part of the deep branch of the radial nerve. It supplies the posterior face of the carpal joint and lies on the posterior face of the interosseous membrane. It can be blocked with a perpendicular injection 3 cm above the dorsal wrist crease.

Suggested Readings

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