Ultrasound of the Brachial Plexus

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Chapter 6 Ultrasound of the Brachial Plexus

Robert S. Weller, MD

Key Points

• In the supraclavicular window, ultrasound can image the cervical nerve roots (between the anterior and middle scalene muscles) and below that the brachial plexus trunks and divisions, which appear as a cluster of grapes.

• The infraclavicular window is not used for ultrasound imaging nearly as often as the supraclavicular because of the depth and moderate difficulty visualizing all three cords.

• The terminal branches of the brachial plexus can be seen by placing the transducer in the axilla, where the median, ulnar, radial, and musculocutaneous nerves can be identified in proximity to the axillary vessels.

• Imaging of the brachial plexus is most commonly used for guidance of peripheral nerve blocks, but abnormal findings identified with ultrasound have been reported with trauma, tumors, and acquired demyelinating neuropathies.

• Ultrasound is capable of visualizing detailed anatomy of the structures surrounding the brachial plexus; for those performing interventional procedures (e.g., brachial plexus blocks), such knowledge significantly enhances safety and success.

The use of ultrasound for imaging the brachial plexus and its terminal branches has significantly increased since the first description of its use in anesthesiology for plexus blockade in 1989.¹ With continuous improvement in imaging technology and evidence of the benefits of ultrasound for interventional approaches to, and diagnostic evaluation of, the brachial plexus, it is certain that the technology will increase its penetration into the routine clinical practice of anesthesiologists, neurologists, and musculoskeletal specialists in the coming years.

Equipment, Terminology, and Limitations of Ultrasound

The use of ultrasound for imaging of the brachial plexus has advantages over other imaging modalities, such as computed tomography (CT) and magnetic resonance imaging (MRI). Ultrasound equipment is relatively inexpensive and portable, and can be brought to the bedside for examination in patients who cannot readily be transported. The imaging of vascular and soft tissues around the plexus is excellent, and ultrasound allows for dynamic imaging with the upper extremity in various positions (as is done during imaging of the ulnar nerve at the elbow described in Chapter 5.) This dynamic imaging is ideal for interventional procedures with ultrasound guidance of needle placement, such as local anesthetic injection. Disadvantages of ultrasound imaging include the loss of resolution when imaging deeper structures requiring lower-frequency transducers, and the difficulty of imaging around bony structures, such as the intervertebral foramina, where bone will not allow penetration of ultrasound energy. An additional disadvantage is the requirements for knowledge, training, and perhaps certification in this new imaging modality for nonradiologists. The American and European Societies of Regional Anesthesia have made recommendations for ultrasound training curricula for residents and postgraduate practicing physicians.²

The brachial plexus throughout its course remains relatively close to the surface of the body, and can be imaged using high-frequency ultrasound transducers. Transducers with frequency ranges from 10 to 18 MHz produce images with high axial and lateral resolution and are ideal for brachial plexus imaging, but acoustic attenuation is directly proportional to scanning frequency, and image resolution at depths beyond 5 cm is limited with high-frequency transducers. Linear transducers are most commonly used and produce a truer anatomic image than curvilinear probes, which create a somewhat distorted image.

The terminology used for imaging and ultrasound-guided nerve block describes the probe-to-target orientation and the probe-to-needle orientation. Nerves and vascular structures are most easily located in a cross-sectional view, also known as short-axis. Because nerves display some anisotropy (see Chapter 1), a probe inclination of 90 degrees to the course of the target nerve produces the best image. After locating a nerve using the short-axis transducer position, rotation of the probe on its axis at 90 degrees produces a longitudinal or long-axis view (Fig. 6.1). Once a target nerve is identified in short-axis or long-axis view, a needle may be inserted using ultrasound guidance. If the needle is inserted perpendicular to the ultrasound plane such that only a bright spot is seen as the needle crosses the beam, this is known as out-of-plane. If the needle is inserted parallel to the ultrasound plane such that the tip and needle shaft are seen throughout the insertion, this is known as in-plane (Fig. 6.2). Both out-of-plane and in-plane techniques are used by anesthesiologists, each having advantages and disadvantages; neither has been proved to be superior.

Fig. 6.1 Probe-to-target orientation. A, Shows short-axis view of embedded tubular structures. B, Shows long-axis view of same structures after probe rotation.

Fig. 6.2 Probe-to-needle orientation, with short-axis view of target. A, Demonstrates out-of-plane view with bright spot representing needle, and reverberation artifact (“comet-tail”) deep to the needle. B, Demonstrates in-plane view of needle with shaft most superficial, and reverberation artifact deep to the needle.

Ultrasound Appearance of Tissues

The ultrasonographic appearance of normal nerves and other tissues has been well described.³ The epineurium and connective tissue within nerves appear hyperechoic, and nerve fascicles are relatively hypoechoic. In short-axis, nerves appear honeycomb like (Fig. 6.3), and in long-axis a hyperechoic epineurium is seen at the edges, with discontinuous internal hypoechoic tubular structures representing the fascicles (Fig. 6.4). This ultrasonographic appearance has been closely correlated to the histologic appearance.⁴ Transducer manipulation, particularly tilting, may also influence the hyper- or hypoechogenicity of nerve tissue, and this property is known as anisotropy. Pathologic nerves generally are enlarged, with a homogeneous hypoechoic appearance; the internal hyperechoic connective tissue pattern is lost and this is suggested to be because of peri- or intraneural edema.³ Tendon has an appearance most like nerve tissue, but with finer internal striations and greater anisotropy. Tendon may be distinguished from nerve by moving the ultrasound transducer proximal and distal along the target; nerve varies little, but tendon flattens and blends into muscle.

Fig. 6.3 Ultrasound appearance of the median nerve in the forearm with hyperechoic epineurium and internal fascicles. The bright line from the cortex of the radius (arrow) is seen deep and lateral to the nerve.

Fig. 6.4 A, Ultrasound long-axis view of the ulnar nerve in the axilla, with axillary veins seen deep and superficial to the nerve. Internal tubular organization is evident. B, Ultrasound long-axis view of the musculocutaneous nerve in the axilla showing two distinct large fascicles.

Normal muscle has a generally hypoechoic ultrasonographic appearance with internal hyperechoic connective tissue striations, and vascular structures are anechoic (blood-containing); arteries show pulsation and are difficult to compress, and veins are easily compressed and may show flow or size changes with respiration. Bone has a hyperechoic cortical surface and transmits little ultrasound energy such that shadowing is apparent deep to the bone surface (Fig. 6.5). Air is also a poor ultrasound conductor, and ultrasound imaging through air-containing structures is unsatisfactory.

Fig. 6.5 Ultrasound appearance of nonneural tissues, with vein, muscle, tendon, and bone identified.

Supraclavicular Anatomy

The ultrasonographic appearance of the brachial plexus is best understood by first reviewing the gross anatomic three-dimensional (3D) appearance of the elements of the plexus and adjacent structures. With this knowledge, the expected appearance in sequential two-dimensional anatomic “cuts” of the brachial plexus can be predicted, and correlated with the ultrasound images at the same location. A review paper has recently shown the correlations of anatomic cross section, histology, and ultrasound appearance of the brachial plexus.⁵ Many of the gross anatomic cross-sectional images included here as figures were made after access of the brachial artery and basilic vein at the elbows of unembalmed cadavers. Fluid infused under pressure was used to distend and identify vessels during ultrasound imaging. Later, injection of liquid red and blue latex into the arterial and venous systems was performed prior to freezing and sectioning for cross-sectional specimens.

One must first address the misnomer of the cervical “roots” as the first portion of the brachial plexus. The true spinal roots arise from the spinal cord and traverse the subarachnoid space within the cervical spinal dura mater. The dorsal and ventral roots combine to form the segmental spinal nerve within the neural foramina of the cervical spine, still surrounded by dura and cerebrospinal fluid (CSF). This mixed spinal nerve divides into ventral and dorsal rami, and the ventral rami of the spinal nerves that form the brachial plexus have long been called the cervical “roots.” I will maintain this historical convention in this chapter, acknowledging that it is not entirely accurate.

The ventral branches of cervical roots C5 to T1 form the brachial plexus, with occasional contribution from C4 (if pre-fixed) or T2 (if post-fixed), providing most of the innervation of the upper extremity. The five subsections of the plexus are the five (or six) roots, the three trunks, the three anterior and three posterior divisions of each trunk, the three cords, and the terminal branches.

The roots exit the cervical spine sandwiched between the anterior scalene muscle, originating from the anterior tubercles of the transverse processes of C3-6 and inserting on the first rib between the subclavian artery and vein, and the middle scalene muscle, originating from the posterior tubercles of the transverse processes of C2-7 and inserting on the first rib posterior to the plexus (Fig. 6.6). Anterior to the anterior scalene muscle and deep to the sternocleidomastoid muscle is the carotid sheath containing the common carotid artery and internal jugular vein. The formation of the upper trunk from C5 and C6, the middle trunk as the continuation of C7, and the lower trunk from C8 and T1 are commonly described at the lateral edge of the scalene muscles. The three trunks cross the first rib closely approximated to the posterior surface of the subclavian artery, and this is where the plexus is most compact and readily and completely blocked by injection of local anesthetic; this is known as a supraclavicular block (see following section). Each trunk separates into anterior and posterior divisions, and these divisions may occur proximal (Fig. 6.7), on, or distal to the first rib.

Fig. 6.6 Anatomic dissection of the right neck demonstrating the supraclavicular brachial plexus emerging between the anterior and middle scalene muscles.

Fig. 6.7 Dissection of the right neck showing three trunks, the subclavian artery, the phrenic and suprascapular nerves, and the separation of the upper trunk into anterior and posterior divisions.

The cross-sectional anatomic appearance of the neck can be predicted from the previously described relationships, and correlated with the ultrasound appearance. If an anatomic section is made, or a high-frequency ultrasound probe positioned short-axis to the course of the brachial plexus roots at approximately the level of the cricoid cartilage (C6 level) (Fig. 6.8), the trachea is seen anteriorly, adjacent to the (usually) homogeneous hyperechoic thyroid gland. Posterolateral to the thyroid is the carotid sheath, and the common carotid artery, internal jugular vein, and vagus nerve may be identified. Posterolateral to the carotid sheath, the scalene muscles and brachial plexus are located deep to the sternocleidomastoid. The cross-sectional appearance of the scalene muscles is characteristic and helps identify the cervical roots passing between them (Fig. 6.9). The best ultrasound image is usually obtained with a modest caudad inclination of the transducer such that it is perpendicular to the plexus. The roots appear as round or oval hypoechoic structures with few internal echoes. A significantly higher fascicle–to–connective tissue ratio above the clavicle compared with below has been demonstrated (Fig. 6.10), and because the epineurium and connective tissue separating the hypoechoic fascicles is hyperechoic, this may explain the relative hypoechoic appearance of brachial plexus elements above the clavicle compared with a hyperechoic appearance distal to the clavicle (Fig. 6.11).⁶ The transverse processes of the cervical spine can be identified by their hyperechoic cortical edge and shadowing deep to their surface, with the root seen between the anterior and posterior tubercles (Fig. 6.12). The characteristic absence of the anterior tubercle of C7 has been used to identify the level of the roots imaged.⁷ The vertebral artery and vein are often seen adjacent to the C7 transverse process and can also be useful in identifying root level (Fig. 6.13). Once the root level is determined, the probe may be rotated 90 degrees to achieve a long-axis view and visualize the root exit zones (Fig. 6.14). The phrenic nerve is sometimes seen adjacent to the C5 root, diverging from it anteriorly and traveling just superficial to the anterior scalene fascia as the transducer is moved caudad (Fig. 6.15).⁸

Fig. 6.8 Ultrasound probe position for imaging the interscalene brachial plexus in short-axis view. Note the caudad tilt of the probe to be perpendicular to the course of the roots.

Fig. 6.9 Anatomic cross section at the C7 level, with nearly a horizontal cut.

Fig. 6.10 Relative increase in connective tissue to nerve tissue below the clavicle compared with above. Absolute values (mm²) of neural and nonneural (connective) tissue inside the epineurium (median ± SD).

(Modified and reprinted with permission from Moayeri N, Bigeleisen PE, Groen GJ: Quantitative architecture of the brachial plexus and surrounding compartments, and their possible significance for plexus blocks, Anesthesiology 108:299–304, 2008.)