FIGURE 33-1. (A) Radial nerve block above the elbow. The needle is inserted in-plane from lateral to medial direction. (B) Median nerve block at the level of the midforearm. (C) Ulnar nerve block at the level of the midforearm.
Ultrasound imaging of individual nerves in the distal upper limb allows for reliable nerve blockade. The two main indications for these blocks are a stand-alone technique for hand and/or wrist surgery and as a means of rescuing or supplementing a patchy or failed proximal brachial plexus block. The main advantages of the ultrasound-guided technique over the surface-based or nerve stimulator–based techniques are the avoidance of unnecessary proximal motor and sensory blockade, that is, greater specificity. Additional advantages are avoidance of the risk of vascular puncture and a reduction in the overall volume of local anesthetic used. There are a variety of locations where a practitioner could approach each of these nerves, most of which are similar in efficacy. In this chapter, we present the approach for each nerve that we favor in our practice.
The radial nerve is best visualized above the lateral aspect of the elbow, lying in the fascia between the brachioradialis and the brachialis muscles (Figure 33-2). The transducer is placed transversely on the anterolateral aspect of the distal arm, 3–4 cm above the elbow crease (Figure 33-1A). The nerve appears as a hyperechoic, triangular, or oval structure with the characteristic stippled appearance of a distal peripheral nerve. The nerve divides just above the elbow crease into superficial (sensory) and deep (motor) branches. These smaller divisions of the radial nerve are more challenging to identify in the forearm; therefore, a single injection above the elbow is favored because it ensures blockade of both. The transducer can be slid up and down the axis of the arm to better appreciate the nerve within the musculature surrounding it. As the transducer is moved proximally, the nerve will be seen to travel posteriorly and closer to the humerus, to lie deep to the triceps muscles in the spiral groove (Figure 33-3).
FIGURE 33-2. (A) Radial nerve anatomy at the distal third of the humerus. (B) Sonoanatomy of the radial nerve at the distal humerus. Radial nerve (RN) is shown between the biceps and triceps muscles at a depth of approximately 2 cm.
FIGURE 33-3. Sonoanatomy of the radial nerve in the spiral groove of the humerus. RN, radial nerve.
The median nerve is easily imaged in the midforearm, between the flexor digitorum superficialis and flexor digitorum profundus, where the nerve typically appears as a round or oval hyperechoic structure (Figure 33-4A and B). The transducer is placed on the volar aspect of the arm in the transverse orientation and tilted back and forth until the nerve is identified (Figure 33-1B). The nerve is located in the midline of the forearm, 1–2 cm medial and deep to the pulsating radial artery. The course of the median nerve can be traced with the transducer up and down the forearm, but as it approaches the elbow or the wrist, its differentiation from adjacent tendons and connective tissue becomes more challenging.
FIGURE 33-4. (A) Anatomy of the medianus nerve (MN) of the midforearm. (B) Sonoanatomy of the MN at the midforearm. FDSM, Flexor Digitorum Superficialis Muscle; FCRM, Flexor Carpi Radialis Muscle & PLM, Palmaris Longus Muscle; FPLM, Flexor Palmaris Longus Muscle.
The ulnar nerve can be easily imaged in the midforearm, immediately medial to the ulnar artery, which acts as a useful landmark. Similar to the radial and median nerves, the ulnar nerve appears as a hyperechoic stippled structure, with a triangular to oval shape (Figure 33-5A and B). The ulnar artery and nerve separate, when the transducer is slid more proximally on the forearm, with the artery taking a more lateral and deeper course. The ulnar nerve can be traced easily proximally toward the ulnar notch, when desired, and the level of the blockade can be decided based on the desired distribution of the anesthesia as well as the ease of imaging and accessing the nerve. Sliding the transducer distally shows the nerve and artery becoming progressively shallower together as they approach the wrist where the ulnar nerve lies medial to the artery.
FIGURE 33-5. (A) Anatomy of the ulnar nerve at the midforearm. The ulnar nerve (UN) is closely related to the ulnar artery (UA). (B) Sonoanatomy of the ulnar nerve at the midforearm. UN is shown closely related to the UA, sandwiched between the flexor carpi ulnaris (FCUM) and flexor digitorum profundus muscles (FDPM). FDSM = Flexor Digitorum Superficialis Muscle.
Distribution of Blockade
As is the case with the landmark-based distal blocks, anesthetizing the radial, median, and/or ulnar nerves provides sensory anesthesia and analgesia to the respective territories of the hand and wrist. Note that the lateral cutaneous nerve of the forearm (a branch of the musculocutaneous nerve) supplies the lateral aspect of the forearm, and it may need to be blocked separately by a subcutaneous wheal distal to the elbow if lateral wrist surgery is planned. For a more comprehensive review of the innervation of the hand, see Chapter 1, Essential Regional Anesthesia Anatomy.
Equipment needed includes the following:
• Ultrasound machine with linear transducer (8–14 MHz), sterile sleeve, and gel
• Standard nerve block tray
• One 20-mL syringe containing local anesthetic
• A 2-in, 22–25 gauge short-bevel insulated stimulating needle
• Peripheral nerve stimulator (optional)
• Sterile gloves
• Because these are superficial blocks of distal peripheral nerves, some practitioners choose to use a small-gauge (i.e., 25-gauge) needle. When a using small-gauge needle, however, meticulous attention should be paid to avoid an intraneural injection, which is more likely with a smaller diameter and sharp tip design.
Landmarks and Patient Positioning
Any patient position that allows for comfortable placement of the ultrasound transducer and needle advancement is appropriate. Typically, the block is performed with the patient in the supine position. For the radial nerve block, the arm is flexed at the elbow and the hand is placed on the patient’s abdomen (Figure 33-1A). This position allows for the most practical application of the transducer. The median and ulnar nerves are blocked with the arm abducted and placed on an armboard, palm facing up. (Figures 33-1 B, C)
The goal is to place the needle tip immediately adjacent to the nerve(s) of choice and to deposit 4–5 mL of local anesthetic in the vicinity of the nerve. It is unnecessary to completely surround the entire nerve in a doughnut pattern, although this can enhance the speed of onset of the block. As with all peripheral blocks, avoidance of resistance to injection is important to decrease the risk of an intrafascicular injection.
With the patient in the proper position, the skin is disinfected and the transducer positioned so as to identify the radial nerve. The needle is inserted in-plane, with the goal of traversing the biceps brachii muscle and placing the tip next to the radial nerve (Figure 33-6A). If nerve stimulation is used, a wrist or finger extension response should be elicited when the needle is in proximity to the nerve. After negative aspiration, 4–5 mL of local anesthetic is injected (Figure 33-6B). If the spread is inadequate, slight adjustments can be made and a further 2–3 mL of local anesthetic administered.
FIGURE 33-6. (A) Needle position to block the radial nerve (RN) at the elbow. BM - Brachialis Muscle, BrM - Brachioradialis muscle. (B) Local anesthetic (area shaded in blue) distribution to block the RN above the elbow. (1) Biceps brachii muscle.
Median and Ulnar Nerves
With the arm abducted and the palm up, the skin of the volar forearm is disinfected and the transducer positioned transversely on the midforearm. The median nerve should be identified between the previously mentioned muscle layers. If it is not immediately visualized, the transducer should be positioned slightly more laterally and the radial artery identified, using color Doppler ultrasound. Sliding back to the midline, the nerve can be seen approximately 1–2 cm medial and 1 cm deep to the radial artery. The needle is inserted in-plane from either side of the transducer (Figure 33-7A). After negative aspiration, 4–5 mL of local anesthetic is injected (Figure 33-7B). If the spread is inadequate, slight adjustments can be made and a further 2–3 mL of local anesthetic administered.
FIGURE 33-7. (A) Needle (1) position for the block of the median nerve (MN) at the forearm. (B) Distribution of local anesthetic for block of the MN at the forearm.
• The median nerve can often “hide” in the background of the musculature. Tilting the transducer proximally or distally will bring the nerve out of the background.
Imaging at the level of the elbow crease readily reveals the nerve positioned medial to the brachial artery. From this location, the nerve can be traced distally.
When in doubt, nerve stimulation (0.5-1.0 mA) can be used to confirm localization of the correct nerve.
In some patients, the median and ulnar nerves often can both be anesthetized with a single skin puncture.
Then the transducer is positioned more medially until the ulnar nerve is identified. The use of color Doppler ultrasound can aid in finding the ulnar artery, which always lies lateral to the nerve at this level. The nerve should then be traced up until the artery “splits off,” to minimize the likelihood of arterial puncture. The needle is inserted in-plane from either side of the transducer (the lateral side is often more ergonomic) (Figure 33-8A). After negative aspiration, 4–5 mL of local anesthetic is injected (Figure 33-8B). If the spread of the local anesthetic is inadequate, slight adjustments can be made and a further 2–3 mL administered.
FIGURE 33-8. (A) Needle (1) position for the block of ulnar nerve (UN) at the forearm. (B) Distribution of local anesthetic (area shaded in blue) for the block of the UN at the forearm.
• The out-of-plane approach can also be used for all three blocks; however, we find that visualizing the needle path makes for greater consistency in placement and a lesser chance of nerve impalement.
The use of a tourniquet, either on the arm or forearm, usually requires sedation and/or additional analgesia.
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