Peripheral Nerve Blocks: A Color Atlas, 3rd Edition

43. Pediatric Ultrasound

Giovanni Cucchiaro

Introduction

The use of ultrasound for pediatric nerve blocks is especially useful because the blocks are often performed under general anesthesia. In small children (less than 30 kg) a high frequency transducer (7 MHz to 15 MHz) may be used for deep blocks such as sciatic and infraclavicular blocks as well as superficial blocks. The following probes are recommended.

1.   25 mm linear array (6 MHz to 13 MHz): This probe is used for all blocks in children less than 15 Kg with the exception of infraclavicular blocks where a probe with a smaller footprint is useful. This probe is also useful for interscalene and supraclavicular blocks in larger children who weigh more than 30 kg.

2.   11 mm curved array (4 MHz to 8 MHz): This probe is useful for infraclavicular blocks in children of all sizes because of its small footprint. The small footprint allows the user to place the probe near to the clavicle and still have room to manipulate the needle which is directed toward the plexus in a saggital plane superior to the probe. This probe may also be useful for sciatic blocks in the popliteal fossa and for femoral nerve blocks.

3.   38 mm linear array (6 MHz to 13 MHz): This probe is useful for sciatic blocks in larger children.

1. Upper Extremities

Interscalene Nerve Block

A linear transducer is placed in a transverse position over the interscalene area (Fig. 43-1). The brachial plexus is visualized in cross section and a series of anechoic circles is seen between the anterior and middle scalene muscle and beneath the sternocleidomastoid muscle (Fig. 43-1). The block needle is inserted posterior to the probe using an in plane technique (Fig. 43-1). This technique is especially useful when the practitioner wishes to target the lower roots of the brachial plexus. This approach allows for direct visualization of the needle in real time. When inserting a catheter for postoperative analgesia after shoulder surgery, the upper roots of the brachial plexus need to be blocked. In this setting, it may be preferable to insert the needle inferior to the probe using an out of plane technique (Fig. 43-2). The needle is directed cephalad to the plexus and the catheter is placed adjacent to roots C5 and C6. Diffusion of the local anesthetic around the roots is seen after injection (Fig. 43-3).

Figure 43-1. The arrow shows the direction of the block needle, which enters the neck parallel to the longitudinal axis of the probe. The placement of a catheter using this approach to the brachial plexus will most likely result in positioning the catheter next to the lower roots of the brachial plexu (C8-T1). CA, carotid artery; IJV, internal jugular vein; SCM, sternocleidomastoid muscle; BP, brachial plexus; SA, anterior scalene; SM, middle scalene.

Infraclavicular Nerve Block

The landmark point for the infraclavicular nerve block is the coracoid process. With the arm abducted, to better expose the axillary vessels, the probe is placed 1 to 1.5 cm below the coracoid process in the saggital plane. The needle is introduced superior to the probe and introduced nearly perpendicular to the skin (Fig. 43-4). The needle is advanced through the pectoralis major and minor muscles toward the lateral cord. Local anesthetic is injected around the lateral cord (Fig. 43-5). The needle is then redirected underneath the axillary artery to reach the posterior cord (Fig. 43-6). The same technique can be used to selectively block the medial cord. In older children it is useful to block each cord. In younger children it is sufficient to inject the local anesthetic around one cord and allow it to diffuse to the other cords.

Figure 43-2. The arrow shows the direction of the block needle, which enters the neck perpendicular to the longitudinal axis of the probe. The placement of a catheter using this approach to the brachial plexus will most likely result in positioning the catheter next to the higher roots of the brachial plexus (C5-C6). CA, carotid artery; IJV, internal jugular vein; SCM, sternocleidomastoid muscle; BP, brachial plexus; SA, anterior scalene; SM, middle scalene.

Figure 43-3. The local anesthetic has been injected and the brachial plexus structures are now isolated from the anterior and middle scalene muscle and are easily identifiable. IJV, internal jugular vein; SCM, sternocleidomastoid muscle; BP, brachial plexus; LA, local anesthetic.

Median Nerve Block

The median nerve can be identified above the elbow, where it lies underneath the biceps and is adjacent to the brachial artery (Fig. 43-7).

Figure 43-4. The probe is held 1 cm below the coracoid process and the needle is advanced at a 5° to 10° angle, in relationship to the probe. The brachial plexus cords surround the axillary artery. AA, axillary artery; AV, axillary vein; PMAJOR, pectoralis major muscle; PMINOR, pectoralis minor muscle; LC-PC-MC, lateral, posterior, medial cord of the brachial plexus.

 

Figure 43-5. The tip of the needle is in the pectoralis minor muscle and the local anesthetic is collecting above the lateral cord. LA, local anesthetic; AA, axillary artery; LC, lateral cord of the brachial plexus.

Figure 43-6. The tip of the needle is below the pectoralis minor and the local anesthetic is collecting around the axillary artery, pushing the lateral cord more laterally. LA, local anesthetic; AA, axillary artery; LC, lateral cord; PC, posterior cord; PMINOR, pectoralis minor.

Radial Nerve Block

The radial nerve can be visualized superior to the elbow where it lies anterior to the humerus (Fig. 43-8).

2. Lower Extremities

Femoral Nerve

A linear probe is oriented in the transverse direction at or below the level of the inguinal crease. The femoral nerve can be visualized in short axis, lateral to the femoral artery, beneath the fascia iliaca and superficial to the iliopsoas muscle. The typical triad, femoral nerve-femoral artery-femoral vein can be seen in Fig. 43-9. The needle is introduced inferior to the probe using an out of plane technique. Direct visualization of the needle is difficult with this approach, however the position of the needle relative to the nerve can be inferred by the movement of adjacent tissues such as the fascia iliaca as the needle is advanced. Alternatively, injection of a small amount of local anesthetic may demonstrate the position of the needle relative to the nerve (Fig. 43-10). A femoral block conducted solely on neurostimulation may fail because stimulation across the thin fascia iliaca is possible in small children even at low stimulation thresholds. Figure 43-11 shows the femoral nerve and artery pushed apart by the local anesthetic, indicating the correct placement of the block needle. Ultrasound can also confirm the proper distribution of the local anesthetic around the femoral nerve. Larger needles (18–19 gauge) can make significant holes in the fascia allowing the local anesthetic to extravasate above the plane of the fascia iliaca (Fig. 43-12).

Figure 43-7. Ultrasound image of the radial nerve, approximately 2 cm above the medial epicondyle of the humerus. B, brachialis muscle; BA, brachial artery; BB, biceps brachii muscle; MN, median nerve; M, medial side of the arm; L, lateral side of the arm.

Figure 43-8. Ultrasound image of the radial nerve, approximately 2 cm above the lateral epicondyle of the humerus. B, brachialis muscle; BB, biceps brachii muscle; BR, brachioradialis muscle; ECRL, extensor carpi radialis longus; RN, radial nerve; M, medial side of the arm; L, lateral side of the arm.

Figure 43-9. The femoral vein and femoral artery lie medial to the femoral nerve, underneath the fascia iliaca. The distance between the femoral nerve and the femoral artery varies, depending on the child's age. FN, femoral nerve; FA, femoral artery; FV, femoral vein.

Figure 43-10. Indentation of the fascia iliaca by the block needle as it is advanced toward the femoral nerve. Because of the thinness of the fascia iliaca in children, it is often possible to obtain elevation of the patella, resulting from femoral nerve stimulation, despite the stimulating needle being above the fascia iliaca. The use of ultrasound helps to avoid this mistake. FV, femoral vein; FA, femoral artery; FN, femoral nerve; M, medial side of the patient; L, lateral side of the patient.

Figure 43-11. The needle is below the fascia iliaca and the ultrasound visualizes the local anesthetic surrounding the femoral nerve. FN, femoral nerve; arrows, block needle.

Figure 43-12. The local anesthetic has leaked around the block needle from the area surrounding the femoral nerve into the space above the fascia iliaca. This may explain the high failure rate of so-called 3-in-1 blocks. FA, femoral artery; FN, femoral nerve; arrowheads, collection of local anesthetic above the fascia iliaca; yellow arrow, block needle.

Sciatic Nerve (Lateral Approach)

The use of ultrasound in the lateral approach to the sciatic nerve is relatively simple compared with the posterior approach: the probe is far from the field and thus does not require a sterile cover. Visualization of the needle is relatively simple because the needle trajectory is within the plane of the transducer (Fig. 43-13). The identification of the sciatic nerve in the middle of the thigh can be difficult because it is surrounded by the biceps, semitendinosus and the semimembranosis muscles. Neural structures at this level can be easily confused with artifacts. An easy approach consists in initially localizing the popliteal vessels in the popliteal fossa. The patient is placed supine and the knee is bent to facilitate the placement of the probe in a transverse orientation posterior to the popliteal fossa. The vessels are identified and the tibial nerve and common peroneal nerves are identified (Fig. 43-14). These nerves are traced cephalad until they join to form the sciatic nerve which lies deep and lateral to the femoral artery (Fig. 43-13). The needle is introduced perpendicular to skin in line with the transducer until the needle tip is adjacent to the sciatic nerve. Local anesthetic is then injected around the sciatic nerve. Some practitioners prefer to inject the local anesthetic more distally in the popliteal fossa. In this case, an injection around the tibial nerve and common peroneal nerve are required (Fig. 43-15).

Figure 43-13. Visualization of the sciatic nerve at the mid-thigh level. SN, sciatic nerve; FA, femoral artery; M, medial side of the patient; L, lateral side of the patient.

Figure 43-14. Visualization of the sciatic nerve terminal branches (common peroneal and tibial nerve) at the popliteal fossa level. TN, tibial nerve; CPN, common peroneal nerve; FV, femoral vein; FA, femoral artery; M, medial side of the patient; L, lateral side of the patient.

Figure 43-15. The block needle tip is in proximity of the tibial nerve and the local anesthetic is collecting around the components of the sciatic nerve. LA, local anesthetic; CPN, common peroneal nerve; TN, tibial nerve; FA, femoral artery; M, medial side of the patient; L, lateral side of the patient.