Figure 51-1. A: Ultrasound probe position to obtain a transverse view of the brachial plexus in the interscalene area. B: Ultrasound image of the brachial plexus in the interscalene area. SCM = sternocleidomastoid muscle, ASM = anterior scalene muscle, MSM = middle scalene muscle, IJ = internal jugular vein, CA = carotid artery, VA = vertebral artery. Arrows mark brachial plexus roots in the interscalene groove.
The Interscalene Region
In the interscalene region, the cervical roots forming the plexus are located between the anterior and middle scalene muscles. They are best visualized when scanned in the lateral aspect of the neck in an axial oblique plane (Figure 51–1 A). In this manner, the sternocleidomastoid muscle can be identified superficially. Deep to it are the anterior and middle scalene muscles where one or more roots are visualized in the interscalene groove.13 They appear mostly hypoechoic, with few internal punctuate echoes (Figure 51–1B). Deeper to this plane, the vertebral artery and vein are seen next to the vertebral transverse process. The carotid artery and internal jugular vein can be identified medially.
The Supraclavicular Region
In the supravlavicular region, the brachial plexus is best scanned with a linear probe in a coronal oblique plane (Figure 51–2A).14 The subclavian artery is the most prominent landmark identified immediately superior to the first rib (Figure 51–2B). The trunks, or divisions, of the plexus in this region are tightly arranged within what seems to be a single sheath, immediately lateral and cephalad to the subclavian artery. The anterior and middle scalene muscles can be identified as they insert on the first rib. The pleura can be seen immediately deep to the first rib.
Figure 51-2. A: Ultrasound probe position for imaging the brachial plexus in the supraclavicular area. B: Ultrasound image of the brachial plexus in the supraclavicular area. ASM = anterior scalene muscle, MSM = middle scalene muscle, SA = subclavian artery, RIB 1 = first rib. The arrow signals the brachial plexus located in the most distal part of the interscalene space, just cephalad and lateral to the subclavian artery.
Figure 51-3. A: Ultrasound probe position for imaging the brachial plexus in the infraclavicular area.(Drawing on the patient with a “C” indicates the clavicle.) B: Ultrasound image of the brachial plexus in the infraclavicular area. PEC M = pectoralis major muscle, PEC m = pectoralis minor muscle, A = axillary artery, V = axillary vein, L = lateral cord, M = medial cord, P = posterior cord.
The Infraclavicular Region
In the infraclavicular region next to the coracoid process, the cords of the plexus lie deep to the pectoralis major and pectoralis minor muscles. They can be best imaged with a linear probe in the range of 4 to 7 MHz, in a parasagittal plane, immediately medial to the coracoid process (Figure 51–3A).15, 16 In this manner, a transverse view of the cords adjacent to the axillary vessels can be obtained (Figure 51–3B). The cords appear hyperechoic, with the lateral cord commonly cephalad and the posterior cord posterior to the artery. The medial cord in this region can often be seen between the artery and vein, but is not always visible.
The Axillary Region
In the axilla and the upper arm, the neurovascular bundle is located in the internal bicipital sulcus, which separates the flexor muscle compartment of the arm (biceps and coracobrachialis muscles) from the extensor compartment (triceps). At this level, terminal braches of the brachial plexus, such as the musculocutaneous, median, ulnar, and radial nerves, are located superficially, usually within 1 to 2 cm of the skin. A linear 10- to 15-MHz probe is therefore recommended. To obtain a transverse view of the neurovascular bundle with the arm abducted at 90 degrees and the forearm flexed, the probe is positioned perpendicular to the long axis of the arm, as close to the axilla as possible (Figure 51–4A). The round pulsatile axillary artery is easily identified in the bicipital sulcus and is distinguished from the axillary veins that are readily compressed. Nerves in the axilla are round to oval-shaped and hypoechoic with internal hyperechoic areas, presumably the epineurium. In the axillary region, the median and ulnar nerves are usually lateral and medial to the artery, respectively (Figure 51–4B). The radial nerve is often posterior or posteromedial to the artery, but nerve location is highly variable.17 The musculocutaneous nerve often branches off more proximally and can be seen as a hyperechoic structure. It can be found between the biceps and coracobrachialis muscles for a short distance before entering the body of the coracobrachialis muscle (see Figure 51–4B). When performing an axillary block, it is best to inject local anesthetic around each nerve individually to achieve consistent success. Local anesthetic spread within the sheath compartment may be presumably restricted by the septa when observed under ultrasound.18
LUMBOSACRAL PLEXUS
The lumbar plexus (LI to L5) and the sacral plexus (Si to S4) provide innervation to the lower extremity. Unlike the brachial plexus, the lumbosacral plexus and its proximal branches are quite deep. Sonographic imaging can be more challenging except for the distal peripheral branches.
Paravertebral Anatomy & Lumbar Plexus Blocks
Ultrasound imaging of the lumbar plexus in the paravertebral region in adults is technically difficult because of its deep location. Using curved 4- to 5-MHz transducers, Kirchmair and associates identified the lumbar plexus within the psoas muscle and correlated ultrasound images with anatomic specimens.19 Scanning is performed with the patient either prone with a pillow under the abdomen to reduce lumbar lordosis or in the sitting position. The transducer is placed longitudinally, in a parasagittal plane, approximately 3 cm from the midline to determine the location of the lumbar transverse processes. Once accomplished, the transducer is turned 90 degrees to the transverse axial plane and positioned between two transverse processes so that bony interference to ultrasound beam penetration is minimized (Figure 51–5A). In the axial image, two muscles are identified deep to the subcutaneous plane: the erector spinae muscle immediately lateral to the spinous process and the smaller quadratus lumborum more laterally. The psoas muscle lies deep (anterior) to these two muscles and is adjacent to the vertebral bodies and intervertebral discs (Figure 51–5B). Previous anatomic studies demonstrate that the lumbar plexus most often lies between the posterior third and anterior two thirds of the psoas muscle; the average skin-to-plexus distance is 5-6 cm.20 For this reason it has been recommended that local anesthetic be administered in the posterior one third of the muscle. Ultrasound also identifies the inferior pole of the kidney (as low as the L3 to L4 level) and can potentially avoid renal hematoma due to inadvertent needle trauma.21
Figure 51–4. A: Ultrasound probe position for imaging the brachial plexus in the axillary area. B: Ultrasound image of the brachial plexus in the axillary area. A = axillary artery, M = median nerve, U = ulnar nerve, R = radial nerve, MC = musculocutaneous nerve. CORACOB = coracobrachialis muscle, arrowheads signal the needle shaft.
Figure 51–5. A: Ultrasound probe position for imaging the paravertebral anatomy relevant to performing lumbar plexus block. B: Ultrasound image of the paravertebral anatomy at the L2 to L3 level. AP = articular process, VB = vertebral body, ES = erector spinae muscle, PM = psoas muscle, K = kidney. The actual lumbar plexus roots are not seen in this image. They traverse the posterior third of the psoas muscle and are difficult to identify due to the depth and similar echogenicity of surrounding muscle tissue.
Femoral Nerve
The three main terminal branches of the lumbar plexus are the femoral, obturator, and lateral femoral cutaneous nerves. The femoral nerve derived from L2 to L4 is the largest branch and can be easily imaged in the inguinal region using a linear 8-12 MHz transducer (Figure 51-6A).22
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