A-Mode (Amplitude)

In amplitude, or A-mode, the echoes from tissue interfaces are represented on the monitor as a spike, and the spike height represents the amplitude of the echo.²⁷ The distance of the interface from the transducer is calculated from the time taken for the signal to be sent and received, the “round-trip time.” A-mode US imaging is rarely used and is considered obsolete.

B-Mode (Brightness or Two-Dimension Mode)

Brightness, or B-mode, is the most commonly used US mode. In this mode, the spike is converted to a dot and the brightness of the dot represents the amplitude of the returning signal.^24–26 The position of the dot on the display represents the depth from which the signal is returning and depends on the round-trip time of the US signal. Multiple scan lines across a plane are combined to produce a single two-dimensional (2D) image. A series of frames are then displayed in rapid succession to give the impression of constant motion, the quality of which depends on the number of images displayed per second, that is, the frame rate.

M-Mode (Motion)

Motion, or M-mode, US is directed along a single scan line (sample line), and reflected signals along this scan line are converted to a brightness scale and displayed against a time axis. Because M-mode is produced from US signals along a single scan line, the 2D anatomy of the underlying body tissues should be studied first using the 2D mode (see later). M-mode is of particular interest when time resolution is necessary, such as when examining a target with rapid movement (e.g., the mitral valve during echocardiography).²⁸

Doppler Ultrasound

Doppler US (based on the Doppler principle) detects a shift in frequency between the emitted US waves and their echoes.29–³² It is used to detect and measure blood flow, and the major reflector for this purpose is red blood cells. Several modes are available:

FIGURE 42-2 Doppler ultrasound. A, Color Doppler. B, Power Doppler. C, Pulsed Doppler.

Presets

Most US machines have a number of presets, which are factory set, to allow optimal US imaging of a specific area of the body or type of examination. Some of the categories of presets that are available include small part, vascular, breast, nerve, musculoskeletal, abdominal, and so on. For example, if a small part preset is chosen, the US machine assumes that the operator is scanning for small, relatively superficial structures and automatically adjusts the depth, power, focus, gain, and time-gain compensation (TGC) (see later) to allow optimal imaging of superficial structures. Some US machines also allow the operator to customize presets according to clinical requirements. Power output is the amount of energy transmitted from the US transducer. In most machines, the power cannot be adjusted by the operator but is automatically set when a particular preset is chosen.

Frequency

This control is used to select the desired frequency, within certain limits, of a broadband transducer, that is, a transducer that serves a range of frequencies. In some US systems (M-Turbo, SonoSite Inc, Bothell, Wash.), this is available as an image optimization control. In the “Res” (resolution) setting the highest frequency of the broadband transducer is selected; in the “Pen” (penetration) setting the lowest frequency is selected; and in the “Gen” (general) setting an intermediate frequency is selected.

Gain

The gain control adjusts the amplification of the returning acoustic signals and is used to optimize the US image (Fig. 42-3). Reduced gain produces a dark image (see Fig. 42-3, A) and detail is masked. In contrast, too much gain produces a white image and detail is saturated (see Fig. 42-3, B). In some US machines there are separate controls for overall gain and gain for the near and far fields. “Auto-gain,” by which the US machine automatically adjusts the gain, is also available in some machines.

FIGURE 42-3 Transverse sonogram of the forearm demonstrating reduced gain (A), excess gain (B), and optimal gain (C).

Time-Gain Compensation

US energy is progressively attenuated as it travels through tissue. Therefore signals returning from reflectors at a depth are weaker in strength. By selectively amplifying the echoes from greater depths, using the TGC method or depth-gain compensation (DGC), equal reflectors at unequal depths are displayed as structures of equal brightness on the monitor. TGC is preset to a large degree, and the operator can make fine adjustments if necessary. The TGC control is presented as a series of sliders arranged in a vertical fashion on the control panel. Each of the sliders adjusts the amplification of the returning US signals at a specific image depth.

Depth

Adjustment in the displayed depth may be necessary depending on the location of the target, the patient’s body size, or other anatomic factors. A depth greater than necessary should not be chosen because this reduces the frame rate and resolution of the image.

Focus (Focal Zone)

The focus of the US signal occurs at a point at which the beam is at its narrowest width. It is also the region where lateral resolution is the best. The focus point should therefore be positioned at the depth at which the pertinent anatomic structures are located. In some US machines, the operator can select multiple focal zones, but this markedly reduces the frame rate and thus should not be routinely used.

Freeze and Unfreeze

The “freeze” function allows the operator to lock a static image on the monitor. A number of frames (usually 20 or more) are also simultaneously stored in a memory bank. A trackball or an “arrow” key is then used to scroll back and forth through these frames. The selected still image can then be used for annotation, documentation, storage, review, or teaching. Pressing the freeze button once again will unfreeze the image.

Ultrasound Transducers

The transducer functions both as a transmitter and a receiver of the ultrasound signal.^33–35 Three types of transducers are currently used (Fig. 42-4): (1) in a linear-array transducer, the piezoelectric crystals are arranged in a linear fashion and sequentially fired to produce parallel beams of ultrasound in sequence, creating a field of view that is rectangular and as wide as the footprint of the transducer (see Fig. 42-4, A); (2) a curved linear-array transducer has a curved surface, creating a field of view that is wider than the footprint of the probe (see Fig. 42-4, B), but at the cost of reduced lateral resolution in the far field as the scan lines diverge; (3) a phased-array transducer has a small footprint, but the ultrasound beam is steered electronically to produce a sufficiently wide far field of view. The ultrasound beam diverges from virtually the same point in the transducer (see Fig. 42-4, C ). Phased-array transducers are routinely used for transthoracic echocardiography.³⁴ The footprints of these transducers are small enough to fit between the ribs and still produce a wide far field of view to image the heart. US transducers serve either a single frequency or a range of frequencies (broadband). For example, a transducer with the notation HFL38/13-6 indicates that it is a high-frequency broadband (13-6 MHz) linear transducer with a 38-mm footprint. Note that the nomenclature used for transducers varies among manufacturers of US devices.

FIGURE 42-4 Schematic diagram illustrating the different types of ultrasound transducers. Note how the ultrasound beam is emitted from each of these transducers.

Ultrasound Transducer Selection

Resolution is the ability to distinguish two objects that are close together. Axial resolution is the ability to distinguish two objects that are along the axis of the US beam, and lateral resolution is the ability to distinguish two objects that are side by side. High-frequency US (13-6 MHz) has a higher axial and lateral resolution compared with low-frequency US but cannot penetrate as deeply into body tissue. Therefore high-frequency US is used to image superficial structures such as the brachial plexus in the interscalene groove or supraclavicular fossa. A lower-frequency US transducer (10-5 MHz) is suited for slightly deeper structures, such as the brachial plexus in the infraclavicular fossa, whereas a low-frequency US transducer (5-2 MHz) is used to image deep structures, such as the lumbar plexus or the sciatic nerve. Broadband transducers allow a single transducer to be used for scanning over a wide range of depths. Because regional blocks are performed at relatively shallow depths in neonates, infants, and young children, high-frequency linear transducers are perfectly adequate for most procedures in these age groups. High-frequency linear transducers with a small footprint (13-6 MHz, hockey stick, 25 to 26 mm) are particularly suited for young children.

Contact Artifact

The contact artifact is the most common artifact produced when loss of acoustic coupling occurs between the skin and the transducer. This could occur because the transducer is not touching the skin, but more frequently it is due to air bubbles that are trapped between the skin and the transducer. Therefore it is prudent to apply liberal amounts of gel to exclude air from the skin and transducer interface.

Reverberation Artifact

Reverberation artifacts, also known as repetitive echoes, occur when repeated reflection of US occurs between two highly reflective surfaces.⁵⁸ Some of the US signals returning to the transducer are reflected back, then strike the original interface, and are reflected back toward the transducer a second time. As a result, the first reverberation artifact is twice as far from the skin surface as the original interface. A second or third reverberation artifact also may be seen. Because of attenuation, the intensity of the artifacts decreases with increasing distance from the transducer. Reverberation artifacts are frequently seen during US-guided axillary brachial plexus block, particularly when the needle is inserted in the long axis.

Mirror Image Artifact

Mirror image artifact is a type of reverberation artifact that occurs at highly reflective interfaces.⁵⁹ The first image is displayed in the correct position, and a false image is produced on the other side of the reflector because of its mirrorlike effect.

Propagation Speed Artifact

Propagation speed artifacts occur when the medium through which the US beam passes does not propagate at 1540 m/sec, resulting in echoes that appear at incorrect depths on the monitor. An example of propagation speed artifact is the “bayonet artifact,”⁶⁰ which has been reported during an US-guided axillary brachial plexus block. The shaft of the needle appeared bent when it accidentally traversed the axillary artery. This happens because of the difference in the velocity of sound between whole blood (1580 m/sec) and soft tissue (1540 m/sec).

Acoustic Shadowing

Acoustic shadow is an echo-free area behind surfaces that are highly reflective or attenuating, such as bone (see Fig. 42-11) or metallic implants. The implication for regional anesthesia is that tissues in the area of the shadow cannot be imaged.

Scout Scan

The aim of the scout scan, or the preintervention scan, as the name implies, is to examine the area of interest before the intervention. This has also been referred to as a “mapping scan.” During the scout scan, steps 8 and 9 described in Table 42-2 are performed, the sonoanatomy of the area is visualized, and the image is optimized. Once an optimal view with the target structure is obtained and the best possible site for needle insertion is determined, it is advisable to mark the position⁴⁴ of the transducer on the patient’s skin so the transducer can be returned to the same position after sterile preparations have been completed. It is common to diagnose anatomic variations during the scout scan. The operator can then decide whether to continue with the block in the same location or to choose an alternative approach or technique that may be safer. This assessment of anatomic variation is one of the major benefits of using US for regional anesthesia.

Supraclavicular Brachial Plexus Block

Supraclavicular brachial plexus block has been described in children, although it is rarely used because of fear of pleural puncture and pneumothorax.⁶² US-guided supraclavicular brachial plexus block has recently been successfully performed in a series of 17 children aged younger than 6 years for orthopedic upper limb surgeries.⁶³ Even though the brachial plexus and cervical pleura are clearly delineated using US in the supraclavicular fossa, this technique should be performed only by experienced operators because of the close proximity between the cervical pleura and the brachial plexus.

The child’s head rests on a head ring and is slightly turned to the contralateral side, and a small roll is placed between the scapula. For a right-sided block, a right-handed operator stands or sits at the head end of the child and the US machine is placed directly in front on the ipsilateral side (see Fig. 42-15). The position of the operator and US machine is reversed for a left-sided block. In the supraclavicular fossa, the subclavian artery lies on top of the first rib. The trunks and divisions of the brachial plexus are superficial and lateral to the subclavian artery and sandwiched between the scalenus anterior and scalenus medius muscle (Fig. 42-16). A linear-array transducer (13-10 MHz) is used to perform the block; the hockey stick probe or a linear-array transducer with a small footprint (25 mm) is particularly suited for this block.

FIGURE 42-16 Sonographic appearance of the brachial plexus (white arrows) in the supraclavicular fossa. SA, Scalenus anterior; SCA, subclavian artery; SM, scalenus medius.

During the scout scan the transducer is positioned parallel to and against the clavicle and the subclavian artery is identified. The trunks and divisions of the brachial plexus often look like a bunch of grapes on the superficial and lateral aspect of the subclavian artery (see Fig. 42-16). The cervical pleura and lung are deep to the first rib. The operator should bear in mind that the acoustic shadow of the first rib can obscure the view of the pleura and lung (see also Fig. 41-21 and Chapter 41 for the landmark-guided techniques). The block needle is inserted in the long axis (in-plane) of the transducer in a lateral to medial direction, keeping the pleura in view. A test injection with saline is performed to ensure optimal needle position, after which the calculated dose of local anesthetic is injected in aliquots.

Median Nerve

The median nerve is closely related to the brachial artery throughout its course in the arm. In the upper part, it is lateral to the artery; in the middle of the arm it crosses the artery from the lateral to medial side and continues on the medial side all the way up to the elbow. In the antecubital fossa, the median nerve lies medial to the brachial artery, behind the bicipital aponeurosis, and in front of the brachialis muscle (Fig. 42-23). In the forearm, the median nerve is deep to the flexor digitorum superficialis and on the surface of the flexor digitorum profundus (Fig. 42-24) and accompanied by the median artery that is a branch of the anterior interosseous artery. Pulsations of the latter can occasionally be observed on the US image. A few centimeters proximal to the wrist, the median nerve becomes superficial and lies between the tendon of the flexor carpi radialis (laterally) and the flexor digitorum superficialis (medially) and may also be overlapped by the tendon of the palmaris longus. Because the nerve is superficial at this level and it can be difficult to differentiate the nerve from the tendons, we prefer to perform median nerve block at the midforearm, where it is clearly delineated (see also Fig. 41-22 and Chapter 41 for landmark-guided techniques).

FIGURE 42-23 Transverse sonogram of the cubital fossa. BA, Brachial artery; BCR, brachioradialis.

FIGURE 42-24 Transverse sonogram of the forearm. FDP, Flexor digitorum profundus; FDS, flexor digitorum superficialis; RA, radial artery; UA, ulnar artery.

Ulnar Nerve

In the arm, the ulnar nerve runs on the medial side of the brachial artery up to about the midhumeral level or the insertion of the coracobrachialis muscle, where it pierces the medial intermuscular septum and enters the posterior compartment of the arm. At the elbow, it passes behind the medial epicondyle to enter the ulnar nerve sulcus. Although the nerve is palpable and superficial at the sulcus, it is often difficult to visualize using US because of bony and contact artifacts. The ulnar nerve then enters the proximal forearm and runs between the flexor digitorum profundus (posterior) and the flexor digitorum superficialis (laterally) muscles, and in the distal forearm it is accompanied by the ulnar artery (lateral) (see Fig. 42-24). Close to the wrist, the ulnar nerve is lateral to the flexor carpi ulnaris muscle. The ulnar artery may be used as a reference to locate the ulnar nerve. Once the artery is located, the nerve is traced backward and the local anesthetic injection is performed away from the artery (see Chapter 41 for landmark-guided techniques).⁷⁹

Radial Nerve

In the arm the radial nerve is posterior to the brachial artery. It then leaves the artery to enter the radial (spiral) groove on the back of the arm, where it is accompanied by the profunda brachii artery. In the distal arm the radial nerve pierces the lateral intermuscular septum and enters the anterior compartment. In the antecubital fossa it is lateral to the biceps tendon and lies in an intermuscular gap between the brachialis (medially), the brachioradialis, and the extensor carpi radialis longus (laterally) (Fig. 42-25). At the level of the lateral epicondyle the radial nerve gives off the posterior interosseous nerve (deep branch of the radial nerve), which leaves the fossa by piercing the supinator muscle. In the forearm, the radial nerve (superficial branch of the radial nerve) continues as a pure cutaneous nerve and supplies the radial half of the dorsum of the hand and the proximal parts of the dorsal surfaces of the thumb and index finger. The radial nerve is best blocked before its division in the antecubital fossa (see also Figs. 41-2, 41-23, and Chapter 41 for landmark-guided techniques).⁷⁸

FIGURE 42-25 Transverse sonogram of the arm, just above the elbow, showing the radial nerve. BCR, Brachioradialis; ECRL, extensor carpi radialis longus.

Lumbar Plexus Block

The lumbar plexus results from the union of the ventral rami of the first four lumbar spinal nerves within the substance of the psoas major muscle⁸⁰ The plexus may also receive contributions from the 12th thoracic nerve or the 5th lumbar nerve. In adults, the plexus is located between the fleshy anterior part of the psoas major muscle, which arises from the anterolateral part of the vertebral bodies and the intervertebral disc, and the accessory posterior part of the muscle that originates from the anterior surface of the transverse process.⁸¹ Local anesthetic injected into this intramuscular plane, also referred to as the psoas compartment, produces ipsilateral lumbar plexus block.

An US scan for the lumbar plexus block can be performed in the transverse or sagittal axis at the L3-4 vertebral level. The child is positioned in the lateral position with the side to be blocked uppermost with the hip and knees flexed. A linear-array transducer (10-5 MHz) is adequate for imaging in young children, whereas in the older child (older than 6 to 8 years) a curved-array transducer (8-5 or 5-2 MHz) is required.

For a transverse scan,⁸² the US transducer is positioned approximately 2 to 3 cm lateral to the lumbar spine at the L3-4 vertebral level, with its orientation marker directed laterally. We also prefer to align the transducer slightly medially so as to produce a paramedian oblique transverse scan (PMOTS) of the lumbar paravertebral region. Also during a PMOTS of the lumbar paravertebral region, the US beam can be insonated either at the level of the transverse process (PMOTS-TP, Fig. 42-26) or through the gap between the two adjacent transverse process (i.e., the intertransverse space [ITS]) producing a PMOTS-ITS scan (Fig. 42-27). In a typical PMOTS-ITS sonogram of the lumbar paravertebral region, the erector spinae muscle, the vertebral body, the psoas major muscle, quadratus lumborum muscle, and the anterolateral surface of the vertebral body are clearly visualized (see Fig. 42-27). The inferior vena cava (on the right side) and the aorta (on the left side) are also identified anterior to the vertebral body. The lower pole of the kidney is closely related to the anterior surfaces of the quadratus lumborum and psoas muscle and is seen as an oval structure that moves with respiration in the retroperitoneal space. The lumbar plexus is not sonographically visualized in all patients, but when it is visualized, it appears as an oval hyperechoic structure in the posterior part of the psoas muscle (see Fig. 42-27) and close to the intervertebral foramen. In contrast, because the acoustic shadow of the transverse process obscures the posterior part of the psoas muscle and the area close to the intervertebral foramen during a PMOTS-TP, the lumbar plexus is rarely visualized in this US scan window (see Fig. 42-26). Therefore, if a transverse scan is performed during an ultrasound-guided lumbar plexus block, it is our recommendation to use the PMOTS-ITS.

FIGURE 42-26 Paramedian oblique transverse scan of the right lumbar paravertebral region at the level of the transverse process (PMOTS-TP) in a 11-month old infant. Note how the acoustic shadow of the transverse process (TP) obscures the posterior part of the psoas muscle (PM) and how parts of the spinal canal and neuraxial structures (dura, intrathecal space, and cauda equina) are seen through the interlaminar space. CSF, Cerebrospinal fluid; ESM, erector spinae muscle; IVC, inferior vena cava; QLM, quadratus lumborum muscle; SP, spinous process; VB, vertebral body.

FIGURE 42-27 Paramedian oblique transverse scan of the right paravertebral region through the gap between two adjacent transverse processes (PMOTS-ITS) in a 14-month old child. Note the intervertebral foramen (IVF), articular process (AP), and the lumbar plexus in the posterior part of the psoas muscle (PM). ESM, Erector spinae muscle; IVC, inferior vena cava; PM, psoas muscle.

For a sagittal scan of the lumbar paravertebral region, the US transducer is positioned approximately 2 to 3 cm lateral and parallel to the lumbar spine,⁸³ with its orientation marker directed cranially. In a typical sagittal sonogram of the lumbar paravertebral region, the L2, L3, and L4 transverse processes with their acoustic shadow produce what we refer to as the “trident sign” (Fig. 42-28) because of its similarity to the trident (Limia tridens or tridentis) that is often associated with Poseidon (the god of the sea in Greek mythology) and the trishula of the Hindu god Shiva. The hypoechoic psoas muscle is seen between the transverse processes, and the lumbar plexus is identified as hyperechoic longitudinal striations within the posterior aspect of the muscle. A laterally positioned transducer will produce a suboptimal scan without the trident, and the lower pole of the kidney, which can reach the L4-5 level in young children, will come into view.

FIGURE 42-28 Longitudinal sonogram of the lumbar paravertebral region showing the lumbar plexus. ESM, Erector spinae muscle; PM, psoas muscle; TP, transverse process (“trident sign”).

Once an optimal image of lumber paravertebral region is obtained, the insulated block needle is inserted in the plane of the US beam. The needle is inserted from the medial side of transducer when a PMOTS-ITS is used or from the caudal end of the transducer when a sagittal scan is used. The needle is slowly advanced under ultrasound guidance to the posterior part of the psoas muscle, and the correct position of the needle tip close to the lumbar plexus is confirmed by observing ipsilateral quadriceps muscle contraction. After negative aspiration, an appropriate dose of local anesthetic is injected in aliquots over 2 to 3 minutes and the patient is closely monitored.

Femoral Nerve Block

The femoral nerve is the largest branch of the lumbar plexus and is the major nerve of the anterior (extensor) compartment of the thigh. It is formed by the dorsal division of the anterior primary rami of spinal nerves L2, L3, and L4. It exits the pelvis and enters the femoral triangle in the thigh by passing under the inguinal ligament just lateral to the femoral artery. In the thigh it lies in the groove between the iliacus and the psoas major muscle, outside the femoral sheath, and lateral to the artery. A high-frequency (13-10 MHz) linear-array transducer is used to scan the femoral nerve.36,⁸⁴ On a transverse sonogram the femoral nerve is seen as an oval or triangular hyperechoic structure lateral to the femoral artery (Fig. 42-29). The femoral nerve exhibits marked anisotropy, and it may be necessary to tilt or rotate the transducer during the scan before it can be visualized. In young children one must avoid exerting too much pressure during the scan because it is easy to collapse the femoral vein.

FIGURE 42-29 Transverse sonogram of the inguinal region showing the femoral nerve and its relations. FA, Femoral artery; FV, femoral vein.

A femoral nerve block is used to provide postoperative analgesia after femoral fractures. It is performed with the child in the supine position. The ipsilateral lower limb is slightly abducted and externally rotated, and the knee is also slightly flexed. A right-handed operator stands on the right side of the child, and the US machine is positioned directly in front on the contralateral side. The sides of the operator and the US machine are reversed for a left-handed operator. The scout scan is performed with the transducer positioned parallel and just below the inguinal ligament. This ensures that the femoral nerve is scanned before its division. Once an optimal view of the femoral nerve is obtained, the block needle is inserted in the long axis (in-plane) of the US transducer from the lateral to medial side and directed to the lateral aspect of the femoral nerve. A test injection with saline (1 to 2 mL) is performed before the local anesthetic is injected to confirm that the needle is deep to the fascia iliaca and to observe the distribution of the injectate in relation to the femoral nerve (see also Fig. 41-32 and Chapter 41 for landmark-guided techniques).

Subsartorial Saphenous Nerve Block

The saphenous nerve is a branch of the anterior division of the femoral nerve and supplies the skin on the medial aspect of the leg and foot up to the ball of the big toe. In the thigh, the saphenous nerve is located in the subsartorial canal and local anesthetic injected into this intramuscular space produces a saphenous nerve block.85–⁸⁷ The subsartorial canal is also referred to as the adductor canal or Hunter’s canal and is situated on the medial side of the middle one third of the thigh and extends from the apex of the femoral triangle, above, to the tendinous opening in the adductor magnus muscle, below. The canal is triangular in cross section, and its anterior wall is formed by the vastus medialis muscle, the posterior wall or floor is formed by the adductor longus, and the medial wall or roof is formed by a strong fibrous membrane that is overlapped by the sartorius muscle (Fig. 42-30). The subsartorial canal contains the following structures: femoral artery and vein, saphenous nerve, nerve to vastus medialis, and the two divisions of the obturator nerve. The femoral vein lies posterior to the artery in the upper part and lateral to the artery in the lower part of the canal. The saphenous nerve crosses the femoral artery anteriorly from the lateral to the medial side in the canal.

FIGURE 42-30 Transverse sonogram of the thigh showing the adductor canal. AL, Adductor longus; FA, femoral artery; FV, femoral vein; VM, vastus medialis.

A high-frequency (13-6 MHz) linear-array probe is used to scan the saphenous nerve in the subsartorial canal with the child the supine position. The ipsilateral lower limb is slightly abducted and externally rotated, and the knee is also slightly flexed. For a right-sided block, a right-handed operator stands on the right side of the patient and the US machine is positioned directly in front on the contralateral side. The transducer is positioned in the transverse axis over the middle one third of the thigh. The triangular subsartorial canal can be identified between the epimysium of the vastus medialis (closely related to the femur), the adductor longus, and the sartorius muscles. The pulsatile femoral artery lies anterior to the vein in the canal, and the saphenous nerve is seen as a round or oval hyperechoic structure anterior to the artery (12-o’clock position). Because the saphenous nerve is a small nerve, it may not always be visible on US imaging in children. However, owing to the close relation of the saphenous nerve to the femoral artery in the subsartorial canal, a perivascular (arterial) injection in the canal will produce saphenous nerve block. The block needle is inserted in the long-axis (in-plane) of the US transducer from the medial to lateral side and is directed to the anterior aspect of the femoral artery. A test injection of saline (1 to 2 mL) is performed to confirm that the tip of the needle is in the canal, after which the local anesthetic is injected.

Sciatic Nerve Block

The sciatic nerve is the largest mixed nerve in the body and arises from the lumbosacral plexus (L4, L5, S1-3). It innervates the posterior aspect of the thigh and the entire lower limb below the knee except for a small patch of skin on the medial aspect of the leg and ankle, which is innervated by the saphenous nerve. Sciatic nerve block is frequently used to provide analgesia for foot (clubfoot) and leg surgery in children. Several different approaches to the sciatic nerve have been described in the literature. Most approaches described to date rely on surface anatomic landmarks (anterior, transgluteal, infragluteal, lateral, posterior subgluteal, proximal thigh, or at the popliteal fossa). Recently, US-guided sciatic nerve block also has been described.^88,89 A proximal approach to the sciatic nerve is selected when surgery involves the hip (rare in children) or block of the posterior cutaneous nerve of the thigh is warranted. Because sciatic nerve block is most frequently used for foot (clubfoot) surgery in children, a distal approach to the sciatic nerve at the popliteal fossa is usually preferred (see also Fig. 41-31 and Chapter 41 for landmark-guided techniques).

Sciatic Nerve Block at the Subgluteal Space

The sciatic nerve exits the pelvis through the greater sciatic foramen, between the piriformis and the superior gemelli muscles, and enters the subgluteal space below the piriformis muscle. It then descends over the dorsum of the ischium, lying on the dorsal surface of the gemellus superior muscle, tendon of obturator internus, gemellus inferior muscle, and quadratus femoris muscle (in a cranial to caudal relation) before it enters the hollow between the greater trochanter and the ischial tuberosity and then goes on to the posterior compartment of the thigh. The anterior surface of the gluteus maximus covers the upper part of the sciatic nerve; and immediately distal to its lower border (infragluteal position), the sciatic nerve is fairly superficial. In between the greater trochanter and the ischial tuberosity, the sciatic nerve lies in the subgluteal space, which is a well-defined anatomic space between the anterior surface of the gluteus maximus and the posterior surface of the quadratus femoris muscle (Fig. 42-31).^88,89 Other structures that are present in the subgluteal space include the posterior cutaneous nerve of the thigh, inferior gluteal vessels and nerve, nerve to the short and long head of the biceps femoris, the comitans artery and vein of the sciatic nerve, and the ascending branch of the medial circumflex artery (see Fig. 42-31). Local anesthetic injected into the subgluteal space blocks not only the sciatic nerve but also the posterior cutaneous nerve of the thigh. The latter is useful when anesthesia over the posterior aspect of the thigh is needed.

FIGURE 42-31 Transverse section through the gluteal region at the level of the quadratus femoris muscle showing the subgluteal space and its contents.

US-guided sciatic nerve block at the subgluteal space is performed with the child in the lateral position. The side to be anesthetized is placed uppermost, and the hip and knees are flexed (Fig. 42-32). The operator sits or stands behind the child, and the US machine is positioned directly in front. In young children, a linear-array transducer (10-5 MHz) is adequate for imaging the sciatic nerve. In children older than 6 to 8 years, the sciatic nerve can also be imaged using a linear-array transducer (10-5 MHz). However, the increased depth of scan required (as a result of increased muscle bulk) limits the field of vision because as the depth of scan increases the field of vision becomes narrower when a linear transducer is used. Therefore a curved-array transducer (8-5 or 5-2 MHz) with a wide field of vision is preferable in older children.

FIGURE 42-32 Transverse sonogram between the greater trochanter and the ischial tuberosity showing the hypoechoic subgluteal space between the hyperechoic perimysium of the gluteus maximus and the quadratus femoris muscle. The sciatic nerve is seen as a hyperechoic structure in the medial aspect of the subgluteal space. GM, Gluteus maximus; QF, quadratus femoris.

The greater trochanter and the ischial tuberosity are identified, and a line is drawn between these two landmarks. The US transducer is placed parallel to this line, with its orientation marker directed toward the greater trochanter to obtain a transverse scan of the sciatic nerve and the subgluteal space. It may be necessary to slide the transducer slightly cephalad or caudad before an optimal image of the sciatic nerve in the subgluteal space can be obtained. On a sonogram, the subgluteal space is seen as a hypoechoic area between the hyperechoic epimysium of the gluteus maximus and quadratus femoris muscle (see Fig. 42-32) extending from the greater trochanter laterally to the ischial tuberosity medially.^88,89 The subgluteal space is not so well delineated in young children and is better visualized in older children. The sciatic nerve is seen as an oval or triangular hyperechoic structure within the subgluteal space (see Fig. 42-32). The medial limit of the space is obscured by the attachments of the semimembranosus, semitendinosus, and biceps femoris muscles to the ischial tuberosity. Pulsationfs of the inferior gluteal artery often can be detected medial to the sciatic nerve on the sonogram.

The block needle is inserted using in-plane technique from the ischial tuberosity side and advanced slowly toward the sciatic nerve. Once the block needle is deemed to be in the subgluteal space, the position is confirmed by injecting 1 to 3 mL of saline through the needle and observing a distention of the subgluteal space (i.e., separation of the epimysium of the gluteus maximus and quadratus femoris muscle) on the US image (Fig. 42-33). However, if the test injection of saline is seen to spread posterior to the epimysium of the gluteus maximus muscle, it indicates that the tip of the needle is not in the subgluteal space. The needle should be reoriented and advanced a little farther until the typical distention of the subgluteal space to the saline test injection is seen. Occasionally, a subtle pop is felt when the needle tip traverses the epimysium of the gluteus maximus muscle and enters the subgluteal space. Local anesthetic is then injected in aliquots over 2 to 3 minutes while observing for the distention of the subgluteal space and the spread of local anesthetic in relation to the sciatic nerve. It is also easy to pass a catheter into the subgluteal space when a continuous sciatic nerve block is planned. Because the catheter is inserted into an anatomic space, the catheter is also more likely to stay in situ (see also Figs. 41-28, 41-29, and 41-31, and Chapter 41 for landmark-based techniques).

FIGURE 42-33 Transverse sonogram of the sciatic nerve at the subgluteal space after local anesthetic injection. Note the distention of the subgluteal space caused by the local anesthetic and the distribution of local anesthetic around the sciatic nerve. GM, Gluteus maximus; LA, local anesthetic; QF, quadratus femoris.

Sciatic Nerve Block at the Popliteal Fossa

The sciatic nerve block at the popliteal fossa is also referred to as the popliteal fossa block and is the preferred approach for sciatic nerve block in children undergoing foot surgery. The popliteal fossa is a diamond-shaped space lying behind the knee joint, the lower part of the femur, and the upper part of the tibia. It is bound superolaterally by the tendon of biceps femoris, superomedially by the tendon of semitendinosus and the semimembranosus, inferolaterally by the lateral head of gastrocnemius, and inferomedially by the medial head of gastrocnemius. The sciatic nerve enters the posterior aspect of the thigh at the lower border of the gluteus maximus and runs vertically downward to the apex of the superior triangle of the popliteal fossa, where it terminates by dividing into the tibial and the common peroneal nerve, usually 3 to 7 cm above the popliteal crease.^90,91 The division of the sciatic nerve into its terminal branches may, however, take place anywhere above this level. This accounts for the occasional sparing of either division of the sciatic nerve after distal sciatic nerve block techniques using nerve stimulation.

Although the classic teaching is to perform popliteal sciatic nerve block with the patient in the prone position, it is very commonly performed in children with the child in the supine position with the leg elevated by an assistant (Fig. 42-34). During US-guided popliteal sciatic nerve block the operator sits on the ipsilateral side facing the head of the patient and the US machine is positioned directly in front. In young children, a linear-array transducer (10-5 MHz) is adequate for imaging the sciatic nerve in the popliteal fossa. In the adolescent child or children with muscular thighs, a curved-array transducer (8-5 MHz) may be preferable. The transducer is positioned just above the apex of the upper triangle of the popliteal fossa in the transverse axis (see Fig. 42-34). It may be necessary to first scan for the sciatic nerve in the middle of the thigh and then trace it distally to the popliteal fossa, where the sciatic nerve is seen as a round, hyperechoic structure. Division of the sciatic nerve into the tibial and common peroneal nerve can also be visualized in children; location of this division varies widely among children. In the popliteal fossa and proximal to the popliteal crease, the tibial and common peroneal nerve are seen as hyperechoic structures superficial and lateral to the popliteal artery.

FIGURE 42-34 Popliteal sciatic nerve block. Transverse sonogram of the sciatic nerve at the apex of the popliteal fossa. AM, Adductor magnus; BF, biceps femoris.

The block needle is inserted using in-plane technique from the lateral aspect of the thigh with its point of entry being anterior to the tendon of the biceps femoris (if it is palpable). This places the needle in the same orientation as for the lateral approach to the sciatic nerve at the popliteal fossa. The exact point of needle entry will depend on where the sciatic nerve is best visualized. The needle is gradually advanced under US guidance, and the tip is positioned just posterior to the sciatic nerve. This is confirmed by injecting 1 to 3 mL of saline through the needle, after which half of the calculated dose of local anesthetic is injected. The same process is repeated by repositioning the tip of the needle anterior to the sciatic nerve. This ensures that the local anesthetic spreads optimally around the sciatic nerve (see also Fig. 41-30 and Chapter 41 for landmark-guided techniques).

Rectus Sheath Block

Rectus sheath block is the technique of injecting local anesthetic into the potential space between the rectus muscle and the posterior rectus sheath. This produces anesthesia of the ventral rami of the intercostal nerves as they traverse the rectus muscle to supply the skin of the anterior abdominal wall on either side of the midline. Because the seventh and eight intercostal nerves also provide motor innervation to the rectus abdominis muscle, a rectus sheath block at this level should also produce relaxation of the muscle. Bilateral rectus sheath blocks are frequently used in children to provide perioperative analgesia during umbilical and paraumbilical hernia.⁹² We have also found it to be useful for analgesia after laparoscopic procedures in children in whom the port insertion sites are close to the midline.

The rectus abdominis muscle arises as two tendinous heads from the lateral part of the pubic crest and the anterior pubic ligament. The fibers run vertically upward and are inserted to the front of the chest wall through the xiphoid process and the 5th, 6th, and 7th costal cartilages. The muscle is enclosed in a fibrous aponeurotic sheath, the rectus sheath, which is formed by the aponeurosis of the external oblique, internal oblique, and transversus abdominis muscles. The anterior rectus sheath is complete and covers the muscle from end to end. However, the posterior rectus sheath is incomplete, being deficient above the costal cartilage and below the arcuate line (or the fold of Douglas), which is located midway between the umbilicus and the pubic symphysis. The rectus sheath on both sides is held together in the midline by a raphe, the linea alba, which is formed by the fusion of the fibers of the three aponeuroses that form the rectus sheath. The ventral rami of the 7th to 12th intercostal nerves pass anteriorly and downward from the intercostal spaces and pierce the posterolateral aspect of the rectus sheath before passing anteriorly through the muscle to supply the skin of the anterior abdominal wall. Three transverse fibrous bands (tendinous insertions), one at the level of the umbilicus, one at the level of the xiphoid process, and one midway between the two, divide the rectus muscle into three smaller parts. These fibrous bands are adherent to the anterior rectus sheath and traverse only the anterior half of the rectus muscle. Therefore a potential space exists between the rectus muscle and the posterior rectus sheath that communicates from the xiphisternum to the pubic crest. Local anesthetic injected into this space can spread up and down the sheath and is the basis of a rectus sheath tumor.

A linear-array transducer (13-10 MHz) with a small footprint (25 mm) is used for imaging in children (E-Fig. 42-2). The operator stands or sits on one side of the anesthetized child, and the US machine is positioned directly opposite on the contralateral side. The transducer is positioned in the longitudinal axis midway between the umbilicus and the xiphisternum. The rectus muscle is seen as a hypoechoic structure between the hyperechoic anterior and posterior rectus sheath (see E-Fig. 42-2). Deep to the posterior rectus sheath the hyperechoic peritoneum can be recognized by its typical peritoneal sliding movement and the “comet-tail” artifacts produced. The fibrous bands (tendinous insertions) in the rectus sheath can also be recognized in the longitudinal sonogram as hyperechoic areas on the anterior surface of the hypoechoic muscle that do not traverse the whole muscle belly.^93,94 The linea alba can be recognized on a transverse sonogram, and the posterior deficiency of the rectus sheath is also readily recognized below the level of the arcuate line.

E-FIGURE 42-2 Longitudinal sonogram of the rectus muscle showing the anterior and posterior rectus sheath.

The block needle is inserted using in-plane technique in a caudal to cranial direction (E-Fig. 42-3). When the tip of the needle is visualized in the potential space between the rectus muscle and the posterior rectus sheath, a test injection of saline (1 to 3 mL) is performed to confirm the longitudinal spread of the injectate between the muscle and the rectus sheath and the widening of the space (see E-Fig. 42-3, C and D). Injection into the muscle will offer resistance to injection and is readily recognized in the sonogram. The needle should be repositioned, and the typical spread of the test injection under the rectus muscle should be confirmed before a calculated dose of the local anesthetic is injected. For umbilical hernia surgery, bilateral rectus sheath blocks are performed just above the umbilicus on either side (see also Fig. 41-16 and Chapter 41 for landmark-guided techniques).

E-FIGURE 42-3 Rectus sheath block. The in-plane needle insertion technique. LA, Local anesthetic; LS, longitudinal sonogram; PRS, posterior rectus sheath; RM, rectus muscle; TS, transverse sonogram.

Ilioinguinal and Iliohypogastric Nerve Blocks

Ilioinguinal and iliohypogastric nerve blocks are used to provide analgesia after inguinal hernia repair, orchidopexy, or hydrocele surgery. The analgesia is comparable to caudal epidural analgesia, and it offers the advantage of not affecting micturition after surgery,⁹⁵ making it ideal for analgesia in children undergoing an outpatient procedure. Currently, anatomic landmarks and tactile sensations are relied on to perform the block,⁹⁵ which unfortunately can result in failure in up to 20% to 30% of cases. Moreover, it can also lead to complications such as femoral nerve palsy,^96,97 pelvic hematoma,⁹⁸ and bowel perforation.⁹⁹ A US-guided ilioinguinal iliohypogastric nerve block has been described in children.^21,100 Compared with the traditional landmark-based method, US-guided ilioinguinal iliohypogastric nerve block is accomplished more accurately, using smaller volumes of local anesthetic, and has a greater success rate.

The ilioinguinal and iliohypogastric nerves are branches of the primary ventral ramus of L1, which arises from the lumbar plexus. It also receives contributions from the T12 spinal nerve. Superomedial to the anterior superior iliac spine (ASIS), the two nerves pierce the transversus abdominis muscle to lie between it and the internal oblique muscle. The iliohypogastric nerve is situated superior to the ilioinguinal nerve and continues inferomedially for a short distance, after which their ventral rami transverse the internal oblique muscle to lie between the internal and external oblique muscles before giving off branches that pierce the external oblique muscle to provide cutaneous sensation. The iliohypogastric nerve supplies sensory innervation to the skin over the inguinal region. The ilioinguinal nerve continues anteroinferiorly with the spermatic cord or the round ligament of the uterus in the inguinal canal and becomes superficial by emerging through the superficial inguinal ring to innervate the skin of the upper medial aspect of the thigh and either the skin of the upper part of the scrotum and the root of the penis or the skin covering the labia majora and mons pubis.

A linear-array transducer (13-10 MHz) is used for the imaging and the ilioinguinal and iliohypogastric nerves, which are best visualized close to the ASIS. The operator stands on the ipsilateral side to be blocked, and the US machine is positioned directly opposite on the contralateral side. The transducer is positioned close to the ASIS and parallel to a line joining the ASIS and the umbilicus (Fig. 42-35). The ilioinguinal and iliohypogastric nerves are identified as two small, rounded structures lying side by side between the internal oblique and transversus abdominis muscle (see Fig. 42-35). The external oblique is frequently identified only as a hyperechoic aponeurotic layer at the point of needle insertion. Deep to the transversus abdominis muscle the peritoneum and the bowel are also visualized (see Fig. 42-35).

FIGURE 42-35 Transverse sonogram of the inguinal region showing the ilioinguinal and iliohypogastric nerves and its relation to the abdominal musculature. ASIS, Anterior superior iliac spine; IO, internal oblique; TA, transverse abdominis.

The block needle is inserted in the long axis (in-plane) of the US beam in a medial to lateral direction (Fig. 42-36). We prefer this orientation because it facilitates visualization of the needle (in-plane) and, in the event that the needle is inadvertently inserted too deep, further passage is obstructed by the iliac bone, thus reducing the potential for a major complication such as bowel perforation. When the tip of the needle is close to the two nerves, a test injection is performed with 0.5 to 1 mL of normal saline. Correct position of the needle tip is confirmed by observing the widening of the tissue plane between the internal oblique and the transversus abdominis muscle. A calculated dose of a long-acting local anesthetic (0.2 mL/kg) is then injected through the needle, and the spread of the local anesthetic to both nerves is visualized (see also Fig. 41-14 and Chapter 41 for landmark-guided techniques).

FIGURE 42-36 Ilioinguinal iliohypogastric nerve block. The in-plane needle insertion technique. ASIS, Anterior superior iliac spine; IO, internal oblique; TA, transverse abdominis.

Transversus Abdominis Plane (TAP) Block

The sensory supply of the abdominal wall is provided by the anterior rami of the T7 to L1 thoracolumbar nerves. T7 to T9 innervate the skin above the umbilicus, T10 innervates the skin around the umbilicus, and T11, T12 (cutaneous branches of the subcostal nerve), and L1 (iliohypogastric and ilioinguinal nerves) innervate the skin below the umbilicus. These nerves pass inferoanteriorly in a plane between the internal oblique and transversus abdominis muscles, also known as the transversus abdominis plane (TAP). The lateral cutaneous branch is given off at the midaxillary line and innervates the abdominal wall up to the lateral edge of the rectus abdominis muscle. The segmental nerve then courses anteriorly and medially toward the midline in the TAP to penetrate the lateral margin of the rectus sheath and emerge anteriorly though the rectus muscle as the anterior cutaneous branch. The lateral and anterior cutaneous branches of the thoracolumbar nerves supply the skin form the midline to the anterior axillary line. The thoracolumbar nerves, as they course through the TAP, also supply muscular branches to the abdominal musculature.

Local anesthetic injected into the TAP plane produce sensorimotor blockade (segmental) of the abdominal wall, and two techniques, the posterior TAP block and subcostal TAP block, have been described.^101,102 In the posterior TAP block the local anesthetic is injected in the TAP plane between the iliac crest and the costal margin, and during a subcostal TAP block the local anesthetic is injected into the TAP plane just below the costal margin at the midclavicular line. Anesthesia and analgesia produced by a TAP block is unilateral; thus it is useful for surgical procedures (e.g., orchidopexy, herniotomy, appendicectomy) in which the incision does not cross the midline. Bilateral TAP blocks are indicated for abdominal surgical procedures in which the surgical incision is in the midline or across the midline. It is also the authors’ practice to perform bilateral TAP blocks for laparoscopic procedures. Because TAP blocks do not produce visceral analgesia, they should be used as part of a multimodal analgesic regimen for perioperative analgesia. Currently, published data on the use of TAP blocks for perioperative analgesia in children are limited^101,103 but may be useful as an alternative when central neuraxial blocks are contraindicated (e.g., in children with underlying coagulopathy or spinal dysraphism).

For a posterior TAP block, a high-frequency linear array transducer (15-7 MHz) is placed midway between the iliac crest and the costal margin along the midaxillary line (Fig. 42-37, A). For infants and young children, a linear transducer with a small footprint (25 mm) is preferred. The operator stands or sits on one side of the anesthetized child, and the US machine is positioned directly opposite on the contralateral side. The various layers of the abdominal wall are identified on the sonogram (see Fig. 42-37, B). From superficial to deep, they include a layer of subcutaneous tissue and fat and the three abdominal muscles with their fascial layers (i.e., the external oblique, internal oblique, and the transversus abdominis muscles, respectively). Deep to the transversus abdominis muscle the hypoechoic peritoneum is also visualized (see Fig. 42-37, B). The block needle (50 mm) is inserted 1 to 2 cm medial to the medial edge of the ultrasound transducer (Fig. 42-37, C ), depending on the depth at which the TAP is located (can be determined using the electronic caliper in the US system), and in the plane of the US beam. Because the abdominal wall in infants and young children is relatively thin, and to avoid inadvertent deep needle insertion and visceral puncture it is the authors’ practice to initially insert the block needle directed toward the US transducer rather than in an anteroposterior direction. Once the needle tip penetrates the skin and enters the abdominal muscle, it is redirected and slowly advanced under direct vision and penetration of the tip through the external oblique and internal oblique muscles, and finally the TAP is identified between the internal oblique and transversus abdominis muscles. A test injection (1 mL) of saline is performed to confirm correct needle placement in the TAP, which is indicated by distention of the TAP by the hypoechoic fluid. A calculated dose of a long-acting local anesthetic (0.2 mL/kg) is then injected through the needle, while distention of the TAP is visualized in realtime (see Fig. 42-37, D).

FIGURE 42-37 Posterior transverse abdominis plane (TAP) block. A, Note how the US transducer is positioned between the iliac crest and the costal margin along the mid-axillary line. B, The three layers of the abdominal musculature. C, In-pane needle insertion during a posterior TAP block. D, Distention of the TAP by the injected local anesthetic (LA). EOM, External oblique muscle; IOM, internal oblique muscle; TAM, transversus abdomen muscle.

For a subcostal TAP block, a high-frequency linear array transducer (15-6 MHz) is placed immediately below the costal margin along the midclavicular line and the block needle (50 to 80 mm) is inserted in the plane of the US beam and from a medial to lateral direction until the tip is identified to be in the TAP. As described earlier, saline 1 to 2 mL is injected to confirm correct needle placement in the TAP. A calculated dose of local anesthetic (0.2 mL/kg) is then injected, and as the local anesthetic hydro-dissects the TAP plane, the block needle is advanced posteriorly in the TAP to improve spread of the local anesthetic. Bilateral subcostal TAP blocks are indicated for midline upper abdominal incision. Currently, no published data exist on the use of subcostal TAP blocks in children and its role in children is still not defined.

Thoracic Paravertebral Block

Thoracic paravertebral block is the technique of injecting local anesthetic alongside the thoracic vertebra close to the intervertebral foramen.¹⁰⁴ This produces ipsilateral, segmental, somatic, and sympathetic nerve blockade that is effective for relieving postoperative pain of unilateral origin from the chest¹⁰⁵ and abdomen.¹⁰⁶ Traditionally, thoracic paravertebral block is performed using surface anatomic landmarks¹⁰⁶ or a catheter is placed in the paravertebral space under direct vision during thoracic surgery (see Chapter 41).¹⁰⁵ Currently, no published data exist on US-guided thoracic paravertebral block in children, although there are several reports in adults.^107,108 The following section describes how the authors perform real-time, in-plane US-guided single-shot thoracic paravertebral block and paravertebral catheter placement in young infants.

US-guided thoracic paravertebral block is performed with the child in the lateral position and with the side to be blocked uppermost. The operator sits or stands behind the child, and the US machine is positioned directly in front on the opposite side of the operating table. In most children a 15-6–MHz linear array transducer is adequate for imaging the paravertebral anatomy. However, a high-frequency linear array transducer with a small footprint (25 mm) is ideal for US-guided thoracic paravertebral block in neonates and young infants. For a US-guided thoracic paravertebral block for thoracotomy, the US transducer is placed at the midthoracic region and a transverse scan of the thoracic paravertebral region is performed with the orientation marker directed laterally. On a transverse sonogram of the thoracic paravertebral region and with the US beam being insonated between two adjacent ribs, the paraspinal muscles are identified as hypoechoic structures on either side of the midline and posterior to the spinal osseous elements (Fig. 42-38). The spinous process and lamina are identified as hyperechoic structures. Because the posterior spinal elements are cartilaginous in neonates and young infants, the acoustic window for imaging is relatively large and it is possible to visualize the neuraxial elements in the same transverse sonogram (see Fig. 42-38). Anterior to the lamina and transverse process the pleura can be visualized as a hyperechoic structure that moves synchronously with respiration or ventilation (see Fig. 42-38). A hypoechoic triangular space may also be visualized lateral to the paravertebral region and posterior to the pleura, which represents the apex of the thoracic paravertebral space (see Fig. 42-38).

FIGURE 42-38 Transverse sonogram demonstrating the sonoanatomy of the right thoracic paravertebral region in a neonate. Note the hyperechoic pleura anteriorly and the hypoechoic apical part of the thoracic paravertebral space (TPVS) posterior to the pleura. Also note the large acoustic window resulting from incomplete ossification of the posterior spinal elements that allows visualization of the neuraxial structures within the spinal canal. CSF, Cerebrospinal fluid; PSM, paraspinal muscles.

For a US-guided thoracic paravertebral block, the block needle (50 mm) is inserted lateral to the US transducer and advanced toward the apical part of the paravertebral space in the plane of the US beam. The advancing needle is visualized in real time, and entry into the apex of the thoracic paravertebral space is confirmed using a test bolus injection (1 mL) of saline. Correct placement of the needle in the paravertebral space is indicated by anterior displacement of the parietal pleura, distention of the paravertebral space, and increased echogenicity of the parietal pleura (Fig. 42-39). A calculated dose of local anesthetic (0.2 mL/kg) is then injected with the needle in situ before a catheter is inserted through the needle, leaving approximately 2 cm of catheter in situ if a continuous thoracic paravertebral block is planned (see also Figs. 41-17, 41-18, and Chapter 41 for landmark-guided techniques).

FIGURE 42-39 Transverse sonogram of the right thoracic paravertebral region in a neonate after an ultrasound-guided thoracic paravertebral block. Note the direction of needle insertion and distention of the right thoracic paravertebral and intercostal spaces, adjacent to the level of injection, with the local anesthetic (LA). PSM, Paraspinal muscle.

Spinal Sonography and Central Neuraxial Blocks

Since the early 1980s, spinal ultrasonography has been used as a diagnostic screening tool in neonates and infants suspected of spinal dysraphism and for detecting spinal tumors, vascular malformations, and trauma.^109,110 Today it is considered the first-line screening test for spinal dysraphism, with a diagnostic sensitivity comparable to that of MRI. Spinal ultrasonography is possible in neonates and infants because the incomplete ossification of the predominantly cartilaginous posterior spinal elements creates an acoustic window that allows the transmission of the US beam. The overall visibility of neuraxial structures decreases with age.¹⁰⁹ Neuraxial structures are best visualized in neonates and infants younger than 3 months of age; progressive ossification of the posterior spinal elements makes detailed sonographic evaluation of the spine difficult beyond 6 months of age unless the child has a persistent posterior spinal defect.¹¹¹ Some reports have demonstrated that neuraxial structures can be visualized in older children, although their details are limited. The overall visibility of neuraxial structures also decreases as one progresses up the spine, with the best visibility in the sacral level followed by the lumbar and then at the thoracic level.

In diagnostic radiology, spinal US is most frequently performed with the child in the prone position, whereas in an anesthetized child it is generally performed with the child in the lateral position. Because the neuraxial structures are relatively superficial in children, they are best visualized using high-frequency (10-5 MHz) transducers, which also produce better images than sector transducers. Scans are obtained in both the transverse (axial) and longitudinal (sagittal) axes, and they are performed either through the midline or parallel and lateral (paramedian) to the spinous processes. The paramedian scan is preferable in older children because it avoids the ossified spinous processes that can interfere with US transmission. In neonates and young infants, the spinal canal can be scanned with the transducer placed directly over the spinous processes (i.e., a median longitudinal scan). Beyond this age group a paramedian longitudinal scan provides the best overall view of neuraxial structures.^112–114

On a longitudinal sonogram, the neonatal spinal cord is seen as a hypoechoic tubular structure with hyperechoic anterior and posterior walls (Fig. 42-40). A thin strip of variable intense echo, “the central echo complex” (see Fig. 42-40), extends longitudinally through the center of the spinal cord and represents the area between the myelinated ventral white commissure and the central portion of the anterior median fissure. This produces the “triplet echoes” that are characteristic of the spinal cord at all levels (see Fig. 42-40). The diameter of the spinal cord varies, being largest in the cervical and lumbar regions and smallest at the thoracic region. Anterior and posterior to the spinal cord are two well-defined linear and hyperechoic echoes that represent the arachnoid-dural layer (see Fig. 42-40). The ligamentum flavum, which is relevant for epidural access, is also readily visualized in young children and appears less echogenic than the dura (E-Fig. 42-4). The dural layers taper distally to close the thecal sac at the S2 level. The epidural space is the hypoechoic area between the dura and the ligamentum flavum (see E-Fig. 42-4), and arterial pulsations also may be visible on US between these two layers. The cerebrospinal fluid (CSF) surrounds the spinal cord as an anechoic layer between the dura and spinal cord (E-Fig. 42-5). The vertebral bodies are seen as echogenic structures anterior to the spinal cord. The spinal cord tapers distally to form the conus medullaris (Fig. 42-41) at the level of first and second lumbar vertebral bodies. The conus medullaris is continuous with the filum terminale, which extends into the sacral canal as a hyperechoic structure. It is surrounded by the roots of the cauda equina, which appear as multiple parallel echogenic lines surrounding the filum terminale (E-Fig. 42-6). Differentiation of the filum terminale from the roots of the cauda equina can sometimes be difficult. The cauda equina typically lies in the anterior half of the spinal canal when the child is in the prone position, but it moves freely within the CSF with change in position and with crying. Slight anteroposterior movement of the spinal cord, superimposed on the arterial pulsations, is commonly seen during real-time imaging.

FIGURE 42-40 Longitudinal paramedian sonogram of the thoracic spine in a neonate. CSF, Cerebrospinal fluid.

FIGURE 42-41 Longitudinal paramedian sonogram of the thoracolumbar spine in a neonate showing the termination of the spinal cord. CSF, Cerebrospinal fluid.

E-FIGURE 42-4 Longitudinal midline sonogram of the lumbar spine in a neonate. CSF, Cerebrospinal fluid; SP L2, spinous process of L2; SP L3, spinous process of L3; SP L4, spinous process of L4.

E-FIGURE 42-5 Transverse sonogram of the lumbar spine in a neonate showing the cauda equina. CSF, Cerebrospinal fluid.

E-FIGURE 42-6 Longitudinal sonogram of the sacrum showing the tapered end of the thecal sac. CSF, Cerebrospinal fluid.

On a transverse (axial) sonogram, the spinal cord is seen as a round or oval hypoechoic structure, with its bright central echo complex (Fig. 42-42). The spinal cord is fixed laterally by the dentate ligament (see Fig. 42-42), which represents the transversely oriented, echogenic arachnoid duplications that are seen in parts of the thoracic spinal canal. Paired (ventral and dorsal) echogenic nerve roots are seen below the L2 level. Farther caudally in the lumbar region, a transverse scan shows the filum terminale surrounded by the nerve roots of the cauda equina (E-Fig. 42-7). The arachnoid–dura mater complex is hyperechoic and forms the anterior and posterior border of the subarachnoid space with the anechoic CSF and the spinal cord in the thoracic region (see Fig. 42-42) and the cauda equina in the lumbar region (see E-Fig. 42-7). The vertebral bodies are the hyperechoic structures anterior to the spinal canal. The vertebral arches are also echogenic and cast an acoustic shadow anteriorly (see E-Fig. 42-7). The paraspinal muscles appear hypoechoic on US.

FIGURE 42-42 Transverse sonogram of the thoracic spine in a neonate. CSF, Cerebrospinal fluid.

E-FIGURE 42-7 Longitudinal sonogram of the sacrum after a test bolus injection of saline through an indwelling caudal catheter during a single-shot caudal epidural injection. Note the displacement of the dura. CSF, Cerebrospinal fluid.

Epidural Catheterization

Continuous epidural analgesia via an indwelling epidural catheter is a well-established method of providing perioperative analgesia in children.¹²¹ The loss-of-resistance technique^5,122 is the most frequently used method of identifying the epidural space. Although popular, this technique relies on the operator’s tactile sensation and can rarely result in dural puncture and serious neurologic complication, such as spinal cord injury. Alternative methods for identifying the epidural space in children have been described¹²³ but have not gained widespread acceptance. Epidural catheters have been successfully placed in young children under US guidance.^122,124 Compared with the traditional loss-of-resistance method, when epidural catheterization is performed using the loss-of-resistance method in conjunction with US guidance, fewer bony contacts occur during the procedure and the time required for catheter placement is reduced. The spread of local anesthetic in the epidural space can also be visualized in real time. In addition, the underlying anatomy and the depth from the skin to the ligamentum flavum, dura, and epidural space can be assessed before needle insertion. However, epidural catheterization under US guidance requires two anesthesiologists who are familiar with epidural anesthesia and spinal sonography in children. The first anesthesiologist performs the scan in the longitudinal paramedian axis and maintains a steady image while the second anesthesiologist performs the needle insertion through the midline. Entry of the needle into the epidural space is confirmed by the loss of resistance to saline and observing the local sonographic changes resulting from the saline injection (i.e., anterior displacement of the posterior dura, widening of the epidural space, and compression of the thecal sac). US-guided epidural catheterization in young children is a very demanding procedure and requires a great deal of skill and dexterity. It should be undertaken only by pediatric anesthesiologists who have acquired adequate training and skill in US-guided regional anesthesia (see Chapter 41 for landmark-guided techniques).

Assessment of Indwelling Epidural Catheters

For optimal epidural anesthesia or analgesia, the epidural catheter tip must be located in the correct dermatomal level. This is often achieved by directly placing the catheter in the desired level via the lumbar or thoracic route. Even with this approach, and in particular when loss of resistance is used to locate the epidural space, it is not possible to determine with any certainty the exact location of the catheter tip. Epidurography can locate the catheter tip,¹²⁵ but this exposes the child to radiation and the risk of anaphylaxis from the contrast medium (see Figure 41-7). An epidural catheter inserted via the caudal route can be advanced to the lumbar or thoracic epidural space. However, this is technically difficult in children older than 1 year of age,¹²⁶ and even in children younger than 6 months of age, the catheter can be misplaced in the sacral, lumbar, and even the cervical region.^127,128 Whether styleted epidural catheters are easier to advance from the caudal to the thoracic epidural space than nonstyleted ones is not known, although better success has been reported with styleted catheters.¹²⁹ Even if a catheter is successfully advanced to the thoracic epidural space, radiographic confirmation is still required to confirm the position of the catheter tip.¹²⁷ Electric nerve stimulation through an indwelling styleted epidural catheter and observation of myotomal contractions have been used to locate the position of the catheter tip.¹³⁰ In experienced hands this technique has a success rate of 89%. However, this test cannot be performed if neuromuscular blocking drugs or local anesthetics have been administered. Epidural electrocardiography also can be used to locate the position of a thoracic epidural catheter¹³¹ and is not affected by the use of neuromuscular blocking drugs or local anesthetics in the epidural space. This method relies on matching the evolving electrocardiogram (ECG) recorded from the tip of a specially adapted epidural catheter, as it is advanced, to the surface ECG recorded at the target vertebral level. However, this method is not specific because even a subcutaneous electrocardiographic electrode at the same position relative to the heart will produce a similar electrocardiographic pattern.

US has been used to directly visualize the position of epidural catheters in children (E-Fig. 42-10).^132,133 US localization is noninvasive, does not involve exposure to radiation or contrast medium, and is not affected by the use of neuromuscular blocking drugs or epidural local anesthetics. It also provides real-time images of the catheter during catheter insertion. However, not all epidural catheters or their tips can be identified using US, and it may be limited by the acoustic window available for scanning in children of different ages. Gently jiggling the catheter and observing tissue movement within the epidural space also has been used to facilitate epidural catheter localization.¹³³ Injecting saline with air bubbles may improve catheter localization using US, but the use of air in the injectate can lead to inadvertent air embolism or a patchy block.¹³⁴ Therefore it is suggested that observing surrogate markers such as the widening of the epidural space, dural displacement, and compression of the thecal sac, subsequent to a saline test injection, may be more accurate in locating the position of an epidural catheter in children.

E-FIGURE 42-10 Longitudinal paramedian sonogram of the lumbar spine demonstrating an indwelling epidural catheter.

Advantages of Ultrasound Imaging

US-guided regional anesthetic blocks appear to offer several advantages. US imaging is noninvasive and simple to use and does not involve exposure to radiation. It allows the target nerves and the surrounding structures to be directly visualized during block placement, which is particularly advantageous in children with difficult or variant anatomy, obesity, or amputated limbs in whom the evoked motor responses cannot be visualized. US is also useful in young children because the majority of regional anesthetic procedures in this age group are performed under general anesthesia when the use of a muscle relaxant makes nerve stimulation impractical. US guidance also helps determine the best possible site and maximum safe depth for needle insertion, allows real-time guidance of the needle and needle tip to the target site, avoids unintended vascular or pleural puncture, and allows visualization of the spread of the injected local anesthetic in real time. Together, these should lead to fewer needle insertions, improved comfort during block placement in awake and sedated children, reduced complications, improved quality of block, and greater success rates. However, limited studies exist comparing US with conventional methods of performing regional nerve blocks in children. Cumulative evidence (from adults and children) suggests that US speeds the execution of peripheral nerve and central neuraxial blocks, reduces the discomfort experienced during block placement, reduces the amount of local anesthetic required, speeds the onset of sensory blockade, improves the quality of sensorimotor blockade, and prolongs the duration of sensory block. Additional studies are required to determine whether US guidance reduces complications related to regional anesthesia in children.