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The Diagnostic Anatomy of the Brachial Plexus

Mentioning the phrase “brachial plexus anatomy” is likely to clear a medical school classroom faster than a fire drill. Sure, rout memorization of brachial plexus anatomy out of context can be very frustrating, but this does not have to be the case. For example, when one is familiar with the function of its major, terminal branches (median, ulnar, radial nerves), as well as with that of its spinal nerve input (the C5-T1 nerve roots), learning what comes in between (i.e., the brachial plexus) becomes straightforward. In fact, there is less anatomical detail to absorb than for a single upper extremity nerve.

In this chapter, I will review brachial plexus anatomy with a focus on the student’s needs—by systematically discussing the basics, not the minutia. In this chapter, only the most standard anatomy will be presented. Learning variations, despite being both common and important, is best approached after the basics have been mastered.

The Proximal Brachial Plexus

Nerves and Trunks

The brachial plexus is made up of axons from five spinal nerves: C5, C6, C7, C8, and T1. Theses nerves emerge from their respective intervertebral foramina and merge to form the brachial plexus. From each spinal cord segment, multiple ventral (motor) and dorsal (sensory) rootlets exit the spinal cord, coalescing into motor and sensory roots before entering the intervertebral foramen. In the proximal intervertebral foramen, the sensory root enters its spinal ganglion. At the midportion of the intervertebral foramen, the motor and sensory roots merge to form a single spinal nerve. This merger is short-lived, however, because the spinal nerve almost immediately splits into ventral and dorsal rami after exiting the foramen. The dorsal rami of the spinal nerves run posteriorly to innervate paraspinal muscles and skin; the ventral rami form the brachial plexus.

As the spinal nerves pass through the intervertebral foramen, their enveloping dura gradually turns into epineurium. The primary site of adherence, or stabilization, of these spinal nerves is located just outside the foramen, where the inferior aspect of the nerve is attached to a depression in the transverse process. This is the only bony attachment of the brachial plexus, and serves to protect the weak intradural rootlets from traction injury that may cause traumatic avulsion. This attachment is well developed for C5, C6, and C7, but is weak or absent for C8 and T1, making these latter two nerves more susceptible to avulsion injury.

The two upper roots, C5 and C6, merge to become the upper trunk. The lower two roots, C8 and T1, travel a bit cranially over the first rib to form the lower trunk. This leaves the C7 root to create the middle trunk. These three trunks (upper, middle, and lower), once formed, travel distal toward the clavicle (Fig. 4-1). The middle and lower trunks, as well as the C8 and T1 spinal nerves, usually do not have any branches of clinical significance. Excluding variations, the only branches from the proximal brachial plexus originate from the C5, C6, and C7 spinal nerves, as well as the upper trunk.

From the spine to the clavicle, the spinal nerves and trunks run sandwiched between the anterior and middle scalene muscles (the scalenes). The only components of the supraclavicular brachial plexus not completely covered by these two muscles are C5, C6, and a proximal segment of the upper trunk. The point where C5 and C6 merge to create the upper trunk, is called Erb’s point. C8 and T1 start below the scalenes, but initially rise cranially to be flanked by them.

Figure 4-1 Proximal brachial plexus: spinal nerves to trunks without branches. The two upper roots, C5 and C6, merge to become the upper trunk, while the lower two roots, C8 and T1, travel a bit cranially over the first rib to form the lower trunk. That leaves only the C7 root to create the middle trunk.

Three nerves receive fibers from the C5 nerve root: the phrenic nerve to the diaphragm, long thoracic nerve to the serratus anterior, and the dorsal scapular nerve to the rhomboid muscles. C6 and C7 also provide fibers to the long thoracic nerve (Fig. 4-2). As mentioned, no important branches usually arise from the C8 and T1 spinal nerves.

The Phrenic Nerve

The C5 spinal nerve provides input to the phrenic nerve. The phrenic nerve is made up of motor fibers from C3, C4, and C5; hence, the phrase “C3, 4, and 5 keeps a man alive.” After being formed, the phrenic nerve runs distally on the superficial aspect of the anterior scalene muscle, laterally to medially (it is the only nerve that runs laterally to medially in the posterior triangle). Along with the anterior scalene, the phrenic nerve passes between the subclavian artery located posteriorly and the subclavian vein anteriorly, to enter the thorax. Each phrenic nerve innervates one hemidiaphragm. Depending on the amount of C5 input to the phrenic nerve, a very proximal lesion to the C5 nerve root may cause an ipsilaterally elevated and paralyzed diaphragm. This may be diagnosed by comparing the percussion of both lungs during inspiration. A radiograph or ultrasound will also confirm this type of paralysis.

Figure 4-2 Proximal brachial plexus: spinal nerves to trunks with branches. Three branches receive a contribution from the C5 nerve root: the phrenic, long thoracic, and dorsal scapular. C6 and C7 also contribute to the long thoracic nerve. One important branch originates from the upper trunk: the suprascapular nerve.

Injury to the long thoracic nerve causes winging of the scapula. At rest, there may be prominence of the scapula’s inferomedial edge, with medial displacement and downward rotation. Winging is classically worsened when the patient pushes forward against resistance with the elbow fully extended and the shoulder girdle protracted forward (anterior) (Fig. 4-3). This latter finding is the hallmark of a long thoracic palsy.

To test this muscle, instruct the patient to reach for a point on the wall in front of him or her, and then apply resistance at the hand or wrist while stabilizing the thorax with the other hand. A common mistake is to not have the patient displace the shoulder girdle forward enough because without doing so, scapular winging from trapezius or rhomboid weakness may be misdiagnosed as a long thoracic palsy. Of note, weakness of any of these three muscles can cause winging when the arm is pushed against resistance across the chest with the arm bent.

The C7 spinal nerve variably provides a branch to the long thoracic nerve. Although weakness itself of the serratus anterior often causes a dull ache in the shoulder region, if the pain is acute and severe, then one should consider acute brachial neuritis as the etiology.

The Dorsal Scapular Nerve

Another branch from the C5 spinal root is the dorsal scapular nerve. This nerve innervates both the major and minor rhomboid muscles (the rhomboids). The rhomboids connect the medial edge of the scapula to the spinal column. When contracted, the rhomboids pull the scapula toward the midline (scapular adduction) and superiorly (scapular downward rotation)–the opposite direction to that of the serratus anterior muscle. The dorsal scapular nerve passes dorsally perforating the middle scalene. It then travels along the undersurface of the levator scapulae down to the rhomboids. With chronic denervation of the rhomboids, interscapular wasting is evident. With rhomboid weakness, there may be mild scapular winging at rest, especially at the inferomedial edge. The scapula may also be laterally displaced and upwardly rotated. To test the rhomboids have the patient place the palm on his or her lower back facing outward. Instruct the patient to push the palm away from the lower back as you apply resistance to the hand as well as to the arm (apply resistance in an anterolateral direction around the thorax) (Fig. 4-4). Observe and palpate the rhomboids during this maneuver. Concurrent damage to the proximal brachial plexus is often present; therefore, the forearm may need to be supported. An alternate method to examine the rhomboids is to have the patient bring the shoulders and scapulae together posteriorly. In this position, the contracted rhomboids can be palpated between the medial aspects of the scapulae.

There is only one branch of significance from the brachial plexus trunks: the suprascapular nerve. This nerve originates from the distal, superior aspect of the upper trunk, just above the clavicle. In summary, therefore, all the proximal brachial plexus branches come from the C5, C6, and C7 spinal nerves, or the upper trunk, with C5 input being present in all of them (Fig. 4-2).

The suprascapular nerve (C5, C6) descends posteriorly and distally along the superior portion of the upper trunk, then between the inferior belly of the omohyoid and the trapezius muscle toward the suprascapular notch. The suprascapular nerve’s convergence with the omohyoid muscle at the scapula confirms its identity. The suprascapular artery and vein traverse the brachial plexus just superior and deep to the clavicle, joining the suprascapular nerve as it approaches the suprascapular notch. The suprascapular nerve passes through the notch and under the superior scapular ligament; the artery and vein pass over this ligament (Fig. 4-5). The nerve and vessels join again as they pass around the lateral edge of the scapular spine through the spinoglenoid fossa. Here, all components of the neurovascular bundle pass under the inferior scapular ligament. The suprascapular nerve innervates the supraspinatus and infraspinatus muscles. The supraspinatus attaches to the superior aspect of the humeral head and mediates the initial 20 to 30 degrees of arm abduction. The infraspinatus attaches to the posterior aspect of the humeral head and is the primary external rotator of the arm. Test the supraspinatus muscle by having the patient abduct a straight arm from his or her side against resistance (Fig. 4-6). To test the infraspinatus muscle, instruct the patient to flex the forearm to 90 degrees. Then stabilize the elbow against the side of the patient, and ask him or her to rotate the arm externally against resistance, like a tennis swing (Fig. 4-7). Contraction of these two muscles can be observed and palpated during testing. With chronic denervation, atrophy above (supraspinatus) or below (infraspinatus) the scapular spine is readily appreciated.

Figure 4-6 Supraspinatus (C5, C6) assessment: Test the supraspinatus muscle by having the patient abduct a straight arm from his or her side against resistance.

The distal portion of the brachial plexus is composed of cords, which are named according to their relationship to the axillary artery deep to the pectoralis minor muscle (lateral, medial, posterior)(Fig. 4-8). The cords are intimately associated with the axillary artery and vein in this region. As the brachial plexus cords pass further distal to the pectoralis minor, their anatomical relationship to this artery changes-they are no longer lateral, medial, and posterior.

The musculocutaneous nerve (C5, C6) is the distal continuation of the lateral cord containing fibers from the upper trunk (Fig. 4-9). In the axilla, the musculocutaneous nerve travels distal, and somewhat lateral, to pierce the coracobrachialis muscle, which it innervates. The coracobrachialis muscle assists the anterior deltoid in flexing the arm (forward in front of the body). It also stabilizes the humerus during forearm flexion. The coracobrachialis cannot be isolated or palpated, and therefore, is not examined clinically. After passing through, and then deep to this muscle, the musculocutaneous nerve continues on the superficial surface of the brachialis muscle deep to the lateral border of the biceps brachii. These two muscles are also innervated via multiple branches off the musculocutaneous nerve. Distally, the musculocutaneous nerve enters the antecubital fossa where it pierces the superficial fascia just lateral to the biceps tendon, entering the subcutaneous space as the lateral antebrachial cutaneous nerve. The territory of this sensory nerve includes, as the name implies, the lateral half of the forearm (Fig. 4-10). The lateral antebrachial cutaneous nerve has an anterior and posterior division. The biceps brachii, with the assistance of the brachialis and brachioradialis, flexes the forearm. The biceps brachii is also a strong supinator of the forearm when the elbow is flexed. To test the biceps brachii and brachialis, have the patient flex a fully supinated arm against resistance (Fig. 4-11). By testing the arm in full supination, the contribution from the brachioradialis is minimized.

Figure 4-10 Musculocutaneous nerve sensory territory. The territory of the lateral antebrachial cutaneous nerve includes, as the name implies, the lateral one half of the forearm.

The axillary nerve (C5, C6) arises from the posterior cord deep to the axillary artery. A branch off the axillary artery, the posterior humeral circumflex artery, joins the axillary nerve, passing inferiorly and medially to it. This neurovascular bundle passes briefly upon the subscapularis muscle toward the surgical neck of the humerus. It then passes through the quadrangular, or quadrilateral, space, which is bordered superiorly by the teres minor, inferiorly by the teres major, laterally by the neck of the humerus, and medially by the long head of the triceps (Fig. 4-12). The axillary nerve is relatively fixed at the quadrangular space, and analogous to the suprascapular nerve in the suprascapular notch, is especially vulnerable to stretch injury when blunt or traction forces are applied to the brachial plexus. Immediately after passing through the quadrangular space, the axillary nerve divides into anterior and posterior divisions. The anterior division curves anteriorly and somewhat superiorly under the deltoid muscle, which it innervates. The posterior division gives an immediate branch to the teres minor after passing through the quadrangular space (Fig. 4-13). The posterior division then pierces the brachial fascia distal and posterior to the deltoid’s insertion into the humerus to become cutaneous. This cutaneous portion of the axillary nerve carries sensation from the upper lateral arm (Fig. 4-14); it is called the upper lateral brachial cutaneous nerve. The axillary nerve also carries sensory fibers from the shoulder joint.

Figure 4-13 Axillary nerve anatomy, posterior view with deltoid removed. Immediately after passing through the quadrangular space, the axillary nerve divides into an anterior and posterior division. The anterior division curves anteriorly and somewhat superiorly under the deltoid muscle, which it innervates. The posterior division gives an immediate branch to the teres minor after passing through the quadrangular space, and then becomes subcutaneous by piercing the brachial fascia.

The teres minor assists the infraspinatus in externally rotating the arm. It also weakly assists the teres major in adducting a straight arm. It is not possible to test the teres minor in complete isolation, but one may observe and palpate its contraction if the patient is thin. The deltoid is the prime abductor of the arm, especially between 30 and 90 degrees. The initial 30 degrees of abduction is primarily controlled by the supraspinatus, whereas abduction above 90 degrees has an important trapezial component, which rotates the shoulder girdle upward. Test the deltoid by having the patient abduct the arm against resistance (Fig. 4-15). The deltoid has three separate heads: the anterior, lateral, and posterior. Abducting the arm to the side and slightly in front of the body tests the anterior and lateral heads of the deltoid. To assess the posterior head, have the patient place a straightened arm almost 90 degrees abducted, and then ask the patient to move the arm posteriorly and superiorly against resistance (Fig. 4-16). The absence of posterior deltoid contraction can help confirm an axillary palsy, especially in those patients with a powerful supraspinatus muscle that alone can abduct the arm to 90 degrees. Arm flexion (in front of the body) is mediated by the anterior deltoid. Although the initial 60 degrees of arm flexion is mostly anterior deltoid, the serratus anterior assists the deltoid during flexion above 60 degrees.

The axillary nerve is usually injured in isolation by shoulder trauma, including shoulder dislocations or humeral fractures. For patients with suspected axillary neuropathy following shoulder trauma, one should always exclude partial damage to the posterior division of the brachial plexus as an alternate diagnosis. By examining the latissimus dorsi (the thoracodorsal nerve), as well as all the muscles innervated by the radial nerve, posterior cord involvement can be excluded. The axillary nerve may be compressed at the quadrangular space, which is known as the quadrangular space syndrome. The etiology of this entrapment is uncertain.

Figure 4-14 Axillary nerve sensory territory. The posterior division of the axillary nerve ends by piercing the brachial fascia distal and posterior to the deltoid’s insertion into the humerus. This cutaneous extension of the axillary nerve carries sensation from the upper lateral arm, and is called the upper lateral brachial cutaneous nerve.

Figure 4-16 Posterior deltoid (C5, C6) assessment: To assess the posterior deltoid, have the patient place a straightened arm almost 90 degrees abducted, and then ask him or her to move the arm posteriorly and superiorly against resistance. Contraction of the posterior head can be observed and palpated.

Aside from the major terminal branches described in the previous section (musculocutaneous and axillary nerves), each cord has other smaller branches (Fig. 4-17). The lateral cord gives one minor branch, the lateral pectoral nerve to the pectoralis major. The medial cord gives three minor branches, a medial pectoral nerve to both the pectoralis minor and major, and two cutaneous branches: the medial brachial and antebrachial cutaneous nerves. The posterior cord also has three small branches: It has two subscapular nerves, the upper subscapular to the subscapularis and the lower subscapular to the subscapularis and teres major, as well as a third branch called the thoracodorsal nerve, which innervates the latissimus dorsi. Damage to any of these nerves helps localize a lesion to the cord level.

The Lateral Cord

The lateral pectoral nerve (C5, C6) branches from the proximal lateral cord, travels over the brachial plexus and axillary artery, pierces the clavipectoral fascia (which separates the pectoralis major from all other deep structures, including the pectoralis minor), and then fans out to innervate the pectoralis major from below. The lateral pectoral nerve concentrates its innervation upon the clavicular head of the pectoralis major. When this branch passes over the plexus, there is a reliable connection, or temporary merger, with the medial pectoral nerve, which as the name implies, comes from the medial cord. The pectoralis major is a strong adductor and internal rotator of the arm. To test the clavicular head of the pectoralis major, and therefore the lateral pectoral nerve, have the patient abduct the arm 90 degrees with the forearm flexed and palm facing forward (Fig. 4-18). Then, against resistance at the medial elbow, have the patient swing his or her arm toward mid-line. The lateral pectoral nerve is the only minor branch from the lateral cord.

Figure 4-17 Distal brachial plexus and branches. Aside from the major terminal branches, each cord has other smaller branches: The lateral cord has one, and both the medial and posterior cords have three.

There are three minor branches from the medial trunk: the more proximal medial pectoral nerve, and the more distal medial brachial and antebrachial cutaneous nerves. The medial pectoral nerve (C6-T1) innervates the pectoralis minor, which it passes through, and then pierces the clavipectoral fascia to innervate the sternal head of the pectoralis major. As mentioned, this nerve almost always communicates with the lateral pectoral nerve. To test the sternal head of the pectoralis major, the patient should begin with the forearm flexed 90 degrees and the arm abducted approximately 30 degrees. Instruct the patient to adduct the arm against resistance placed at the medial elbow (Fig. 4-19). The pectoralis minor cannot be adequately isolated from the pectoralis major, and is therefore not assessed clinically.

Just prior to formation of the ulnar nerve, the medial cord gives off two branches: the medial brachial cutaneous nerve and the medial antebrachial cutaneous nerve. Both of these nerves were discussed in the ulnar nerve chapter because they are more readily understood in that context. In summary, sensory loss on the medial one half of the arm (medial brachial cutaneous) and forearm (medial antebrachial cutaneous) should be used to confirm involvement of the medial cord.

The first and last minor branches from the posterior cord are aptly named the upper and lower subscapular nerves (C5, C6). The upper subscapular nerve is not very long and enters the subscapularis muscle to innervate it. The subscapularis muscle, along with the teres major (and latissimus dorsi and pectoralis major), internally rotates the arm. The subscapularis muscle cannot be completely isolated, but one can test internal arm rotation as a composite test (Fig. 4-20). The lower subscapular nerve innervates the lower half of the subscapularis muscle, as well as the teres major. The teres major, along with the latissimus dorsi and pectoralis major, are the main arm adductors. To test the teres major, begin with a straightened arm abducted horizontally with the palm down. Instruct the patient to adduct the extended arm against resistance while you inspect the teres major (Fig. 4-21). The third branch from the posterior cord is the thoracodorsal nerve, which arises between the upper and lower subscapular nerves. The thoracodorsal nerve innervates the latissimus dorsi muscle. To assess the latissimus dorsi, the patient should adduct the arm when the forearm is flexed 90 degrees with the palm facing forward (Fig. 4-22). In summary, all of the branches from the posterior cord act to adduct and internally rotate the arm, a point worth remembering.

Previous chapters reviewed the median, ulnar, and radial nerves. This section discussed the remaining branches of the brachial plexus, including the musculocutaneous and axillary nerves, as well as several more minor branches. The hard part is over; connecting the proximal and distal ends of the plexus is easy.

The brachial plexus divisions connect the trunks to the cords (Fig. 4-23). There are no distinct branches from the divisions. Each of the three trunks has both an anterior and posterior division. All three posterior divisions merge to create the posterior cord. The anterior divisions from the upper and middle trunks create the lateral cord; only the anterior division of the lower trunk forms the medial cord. The posterior cord receives the largest number of divisions (three); this can be remembered by the fact that the posterior cord subsequently yields the largest terminal branch of the plexus, the radial nerve. The medial cord receives the anterior division of the lower trunk, which is made up of the C8 and T1 spinal nerves. Knowing that the ulnar nerve is predominantly C8 and T1, it makes sense that the medial cord, which continues distally as the ulnar nerve, originates from the anterior division of the lower trunk (Fig. 4-24).

Figure 4-23 Brachial plexus divisions. The posterior cord is made up of all three posterior divisions, the lateral cord from the anterior divisions of the upper and middle trunks, and the medial cord from the lower trunk’s anterior division.

The brachial plexus runs through the interscalene triangle, which is bordered by the anterior scalene, the middle scalene, and the first rib. The apex of the interscalene triangle is located in the posterior triangle of the neck. The anterior and middle scalenes originate from the anterior and posterior tubercles, respectively, of the cervical transverse processes. These muscles run down and attach along the first rib, with the spinal nerves and brachial plexus being sandwiched between them. The region where the brachial plexus and subclavian artery exit together from between the scalenes and over the first rib is a potential site of entrapment (thoracic outlet syndrome). The brachial plexus divisions lie deep to the clavicle, while the cords and their branches are deep to the pectoralis minor and coracoid process. In the axilla, the distal brachial plexus cords lie between the clavipectoral fascia (superficial) and subscapularis muscle (deep).

Figure 4-25 Head and neck sensory coverage, excluding the face. Sensory branches from the cervical plexus, along with the dorsal rami of the upper cervical spinal nerves, provide innervation to this area. Of note, the greater occipital nerve is made up solely of the dorsal rami of C2, and is not considered a branch of the cervical plexus.

In contrast to the deep branches, the superficial branches of the cervical plexus pour over (around and superficial to) the posterior edge of the sternocleidomastoid muscle to provide sensory coverage from the neck down to the shoulder and upper chest, including the area just below the clavicle (Fig. 4-25). Four named sensory branches originate from the cervical plexus, from superior to inferior: the lesser occipital, greater auricular, transverse cervical, and supraclavicular (Fig. 4-26). The only nerve from the cervical plexus that crosses the posterior triangle of the neck is the supraclavicular nerve and its terminal branches. If one follows the great auricular nerve back to the point where it emerges from below the sternocleidomastoid muscle, the spinal accessory nerve can often be located just a few millimeters more cranial to this point.

The greater occipital nerve originates from the dorsal ramus of C2. C1 provides no sensation to the skin. The spinal accessory nerve (CN XI) crosses the most-cranial aspect of the neck’s posterior triangle.

Spinal Accessory Nerve (CN XI)

The spinal accessory nerve is made up of intraspinal branches from C1 to C4, and the medulla. These branches merge intradurally, with the nerve, once formed, exiting the skull base via the jugular foramen. Its relationship to the jugular vein just below the skull base is variable. After the spinal accessory nerve innervates the sternocleidomastoid muscle, which is also innervated by deep motor branches of the cervical plexus, it exits from below the sternocleidomastoid and enters the posterior triangle of the neck, approximately 8 cm superior to the clavicle. The spinal accessory nerve travels toward the top of the shoulder, eventually passing under the trapezius muscle, which it innervates (Fig. 4-26). As mentioned previously, this nerve commonly emerges from under the sternocleidomastoid muscle just cranial to the greater auricular nerve, the latter being an excellent surgical landmark. Across the posterior triangle, the spinal accessory nerve is interwoven with a major chain of lymph nodes. For this reason, the most common cause of isolated spinal accessory nerve palsy is iatrogenic injury following cervical lymph node dissection or biopsy. Alternatively, a painful, idiopathic spinal accessory neuropathy may also occur, which is thought to be a variant of acute brachial plexitis. Vernet syndrome consists of spinal accessory, vagus, and glossopharyngeal damage at the jugular foramen, through which all three of these nerves pass. This syndrome is usually from a local metastatic deposit or schwannoma.

Secondary to the trapezial weakness, a spinal accessory palsy also causes scapular winging. Trapezial winging is mild at rest and usually involves the upper border of the scapula, although this is variable. All types of winging (serratus anterior, trapezial, and rhomboid) are worse when the arm (partially flexed at the elbow) is pushed across the chest or in front of the body against resistance. However, only serratus anterior weakness causes winging when an extended, protracted arm is resisted. The presence of rhomboid weakness helps differentiate rhomboid versus trapezial winging. Table 4-1 compares the three muscular causes of scapular winging.

Related posts:

The Diagnostic Anatomy of the Lumbosacral Plexus The Clinical Evaluation of the Brachial Plexus The Diagnostic Anatomy of the Inguinal Complex of Nerves The Diagnostic Anatomy of the Median Nerve The Diagnostic Anatomy of the Radial Nerve The Diagnostic Anatomy of the Ulnar Nerve

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