Supraclavicular Brachial Plexus

The brachial plexus is a complex structure located in the lower half of the lateral neck, extending from the cervical spine to the axilla ( ▶ Fig. 1.1). It provides motor, sensory, and autonomic innervation to the upper limb, except for the skin of the upper half of the medial and posterior part of the arm, which is supplied by the intercostobrachial nerve. The brachial plexus can be divided into: (1) a supraclavicular portion, constituted by roots C5 to T1, the upper, middle, and inferior trunks, and its divisions; and (2) an infraclavicular portion, formed by the cords and its terminal branches. Commonly when we refer to the brachial plexus, we say that it is formed by the union of the C5–C8, and T1 roots. ¹ However, this is not entirely accurate. In actuality, the brachial plexus is an anastomosis of the ventral rami of spinal nerves C5–C8 and T1, its posterior rami being directed to the spinal muscles ( ▶ Fig. 1.2). There are two anatomic variants: the prefixed brachial plexus receiving fibers from C4, with little to no contribution from T1; and the postfixed plexus receiving fibers from T2, with little to no contribution from C5. ² Each spinal nerve consists of a ventral root, which has motor and autonomic functions, and a dorsal root, which is sensory. The dorsal root enters the spinal ganglion. Distal to the spinal ganglion, both roots coalesce to emerge through the intervertebral foramen as the spinal nerve. Almost immediately, this nerve divides into two rami: ventral and dorsal. The dorsal rami supply the paraspinal muscles and the skin of the back, while the ventral rami form the brachial plexus. ³ The ventral and dorsal roots and the intra-axial portion of the spinal nerve are only covered by an arachnoid sheath. As it exits the intervertebral foramen, this sheath continues as the epineurium. At this level, there are adhesions between the nerve sheath and the transverse process. These adhesions anchor the nerve structures, protecting the roots from traction injury. They are more important at the C5–C7 levels, and weaker at the C8 and T1 levels; for this reason, the C8 and T1 roots are more susceptible to root avulsion. ^{3,​ 4,​ 5}

Before continuing with a description of how the brachial plexus is formed, it is important to mention the relationship it has with the sympathetic nervous system. Immediately distal to its origin, the ventral rami of the spinal nerves that form the brachial plexus receive gray rami communications from the middle and inferior cervical sympathetic ganglia and the first thoracic sympathetic ganglion. Sympathetic fibers destined for the face, via the trigeminal nerve, pass through spinal nerves T1 and T2. It is for this reason that proximal lesions of T1 and/or T2 can cause Horner’s syndrome, which consists of anhidrosis, miosis, ptosis, and enophthalmos. ^{3,​ 4}

The most proximal structures in the brachial plexus are the trunks. The upper trunk is formed by the anastomosis of C5 and C6, C7 continues as the middle trunk, and C8 and T1 form the lower trunk. Between the spine and the clavicle, the three trunks are located between the anterior and middle scalene muscles ( ▶ Fig. 1.3). The three trunks emerge from the interscalene space and traverse the inferior region of the posterior triangle of the neck. ⁶

1.1.1 Collateral Branches of the Supraclavicular Brachial Plexus

The most proximal collateral branches of the brachial plexus arise from spinal nerves C5–C7 (the phrenic, long thoracic, and dorsal scapular nerves) and upper trunk (suprascapular nerve). They are intended to innervate the muscles of the proximal upper limb. The function of this is to stabilize and mobilize the shoulder, with the exception of the phrenic nerve, which actually is not considered a collateral branch of the plexus, but has a contribution from C5. More detailed descriptions of the nerves and their consistency follow:

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  Phrenic nerve: The phrenic nerve receives contributions from C3–C5. It is purely motor and supplies the ipsilateral hemidiaphragm. It runs along the surface of the anterior scalene, its direction being from medial to lateral (making it the only nerve having this direction in the posterior triangle). A proximal lesion of C5 (root avulsion) can cause ipsilateral diaphragmatic paralysis. ⁷

  
  
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  Long thoracic nerve: This nerve receives contributions from C5–C7. It passes between the anterior and middle scalene, behind the brachial plexus. It innervates the serratus anterior muscle, the function of which is to stabilize the scapula and allow for scapular rotation and anterior displacement. Injury to this nerve results in a winged scapula.

  
  
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  Dorsal scapular nerve: The dorsal scapular nerve is a branch of C5, directed dorsally to pierce the middle scalene muscle, after which it continues to run below the levator scapulae to ultimately reach and innervate the rhomboid muscles and the levator scapulae. Its function is to approximate the scapula to the midline. Interscapular injuries cause atrophy, which may manifest as a slightly winged scapula at rest. When this nerve is affected in the context of a brachial plexus injury, it indicates a proximal lesion affecting C5.

  
  
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  Suprascapular nerve: This nerve is the only branch that arises from the trunks of the brachial plexus, with contributions from C5 and C6. It originates in the superior portion of the upper trunk, immediately proximal to the clavicle ( ▶ Fig. 1.3). It then redirects back toward the suprascapular notch. In the notch, it joins the suprascapular artery and vein, both of which are located above the upper scapular ligament, while the nerve lies below. It innervates the supraspinatus and infraspinatus muscles—the former stabilizes the humeral head and contributes to the first 30 degrees of shoulder abduction, while the latter is an external rotator. ⁸

1.2 Infraclavicular Brachial Plexus

Within the posterior triangle of the neck, each trunk is divided into an anterior and a posterior division. ⁹ Each division passes under the midclavicle, thereby entering the axilla. The combination of these divisions will form the cords (i.e., the infraclavicular brachial plexus). The cords are named according to their relationship with the axillary artery, so that we have the lateral, medial, and posterior cords. The anterior divisions of the upper and middle trunks form the lateral cord, carrying fibers from C5–C7. The anterior division of the lower trunk continues as the medial cord, carrying fibers from C8 and T1. The posterior divisions of the three trunks are joined to form the posterior cord, carrying fibers from C5–C8 and T1. In the projection of the lateral border of the pectoralis minor muscle, the three cords divide to give rise to the five terminal branches of the brachial plexus. The lateral cord gives rise to the lateral contribution to the median nerve (mainly sensitive) and the musculocutaneous nerve. The medial cord gives rise to the medial contribution of the median nerve (mainly motor) and the ulnar nerve ( ▶ Fig. 1.4). The axillary and radial nerves arise from the posterior cord.

One way to potentially simplify learning the anatomy of the brachial plexus is to relate each structure to a certain function. Consequently, the posterior divisions form the posterior cord, which in turn gives birth to the radial and axillary nerves, both of which are responsible for upper limb extension. Conversely, the anterior divisions form the lateral and medial cords that are responsible for upper limb flexion through their terminal branches: the musculocutaneous, median, and ulnar nerves.

1.2.1 Collateral Branches of the Infraclavicular Brachial Plexus

In addition to the above-mentioned terminal branches, the infraclavicular brachial plexus also gives out collateral branches. ¹⁰ They are the following:

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  Medial cutaneous nerve of the arm: This branch arises from the medial cord, and has its axonal origin in C8 and T1. After its origin, it descends on the medial side of the axillary artery in an anterior direction. In the arm, it initially is located medial and then anterior to the ulnar nerve, descending in front of the basilic vein. It pierces the aponeurosis next to the basilic vein, and is distributed to the skin of the lower third of the medial surface of the arm.

  
  
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  Medial cutaneous nerve of the forearm: This nerve is also a branch of the medial cord that supplies the skin of the anterior and posterior surfaces of the medial aspect of the forearm. Within the axilla, it anastomoses with the intercostobrachial nerve, which provides sensory innervation to the distal axillary region and proximal inner arm.

  
  
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  Medial pectoral nerve: This is a collateral branch of the medial cord. It passes through the pectoralis minor and pectoralis major, supplying both muscle groups.

  
  
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  Upper and lower subscapular nerve: These are branches of the posterior cord. The former innervates the subscapularis muscle, while the latter supplies the teres major and the distal portion of the subscapularis.

  
  
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  Thoracodorsal nerve: Also a branch of the posterior cord, this nerve innervates the latissimus dorsi muscle. ¹¹

  
  
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  Lateral pectoral nerve: A collateral branch of the lateral cord, the lateral pectoral nerve pierces the clavipectoral fascia and innervates the pectoralis major muscle.

1.3 Terminal Branches of the Brachial Plexus

1.3.1 Radial Nerve

The radial nerve has its axonal origin in the roots C5–C8, and its macroscopic origin in the posterior cord, posterior to the third portion of the axillary artery and anterior to the subscapularis, teres major, and latissimus dorsi muscles. In the arm, it is directed obliquely downward, laterally and posteriorly, and passes through the humeral-tricipital slit located below the teres major and the latissimus dorsi. ⁴ This is a musculoskeletal conduit between the radial sulcus of the humerus dorsally and the long and lateral heads of the triceps ventrally, accompanied by the deep humeral artery. Near its origin, the nerve emits a variable number of branches to the triceps directed to innervate its long, medial, and lateral portions separately, as well as a sensory branch, the posterior cutaneous nerve of the arm, which, as its name suggests, provides sensation to the proximal posterior region of the arm. It then continues its course as a satellite of the deep artery of the arm, located between the long and medial heads of the triceps ( ▶ Fig. 1.5). From there, it comes into contact with the humerus within the radial sulcus or spiral groove, where it gives off additional muscular branches for the lateral and medial heads of the triceps, as well as for the anconeus muscle. It is important to note the anatomy of the collateral sensory branches: the inferior lateral cutaneous nerve of the arm and the posterior cutaneous nerve of the forearm. They provide sensory innervation to the posterior and medial region of the distal arm, elbow, and proximal forearm. At the level of the elbow, the radial nerve can be found between the biceps medially and the brachioradialis laterally, along with the radial recurrent artery; and 2 to 3 cm from this point, it provides a motor branch for the brachioradialis. Once it is beyond the elbow crease, it divides into its two terminals branches, anterior or superficial (sensory) and posterior or deep (motor) ( ▶ Fig. 1.6).

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