Wrist blocks

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CHAPTER 22 Wrist blocks

Dominic Harmon, Jack Barrett

Indications

Surgical

Hand or finger surgery not requiring a tourniquet; supplementation of incomplete brachial plexus block.

Contraindications

Relative

Local neural injury and distorted anatomy (due to previous surgery or trauma).

Clinical anatomy

The hand is innervated by the three nerves that pass through the wrist (Fig. 22.1).

Figure 22.1 Axial MR image showing anatomy of relevant structures at the wrist. 1; Median nerve; 2; palmaris longus tendon; 3; ulnar artery; 4; ulnar nerve; 5; radial artery; 6; radial nerve; 7; radius.

The median nerve approaches the wrist between the palmaris longus (if present) and the flexor carpi radialis. It can also lie beneath the palmaris longus (Fig. 22.2). The median nerve provides sensation to the lateral half of the palm, flexor aspect of the thumb, index finger, middle finger, and radial side of the ring finger.

Figure 22.2 Cadaver structures illustrating anatomy pertinent to nerve blocks at the wrist. 1; Flexor carpi ulnaris; 2; ulnar nerve and artery; 3; flexor digitorum superficialis; 4; palmaris longus; 5; median nerve; 6; flexor carpi radialis; 7; superficial branches of the radial nerve; 8; radial artery.

The ulnar nerve, in the middle of the forearm between the flexor digitorum profundus and the flexor carpi ulnaris, gives off a dorsal and a ventral cutaneous branch. At the wrist, the ulnar nerve lies between the ulnar artery and the lateral border of the flexor carpi ulnaris (Fig. 22.2), which inserts on the pisiform bone. Near the pisiform bone, it passes superficial to the flexor retinaculum and ends by dividing into superficial and deep branches. The ulnar nerve provides sensation for the ulnar half of the back and front of the hand, little finger, and ulnar side of the ring finger.

The radial nerve at the wrist lies between the flexor carpi radialis and the radial artery (Fig. 22.3). The radial nerve provides sensation for the radial half of the dorsum of the hand, back of the thumb, and part of the dorsum of the index finger.

Figure 22.3 Cadaver structures illustrating anatomy pertinent to radial nerve block at the wrist. 1; Cephalic vein; 2; extensor pollicis longus; 3; flexor carpi radialis; 4; superficial branches of the radial nerve; 5; radial artery.

Surface anatomy

Bony landmarks include the ulnar styloid and the radial styloid carpal bones. Other landmarks include the wrist crease, flexor carpi radialis, palmaris longus, and flexor carpi ulnaris tendons (Fig. 22.4). These tendons can be accentuated by having the patient flex the wrist while making a fist. The radial artery can be palpated lateral to the tendon of the flexor carpi radialis, and the ulnar artery lateral to the flexor carpi ulnaris tendon.

Figure 22.4 Landmarks for the nerve blocks at the wrist. Landmarks include radial and ulnar styloids; flexor carpi radialis and ulnaris; palmaris longus; radial and ulnar arteries; and the wrist skin crease.

Sonoanatomy

The examination begins with the patient supine, the arm abducted, the forearm and wrist in supination (Fig. 22.5). A systematic survey should be performed from superficial to deep and medial to lateral. A high frequency ultrasound transducer is used with a transverse orientation.

Figure 22.5 Arm position for ultrasound-guided blocks at the wrist.

The median nerve passes distally in the volar aspect of the forearm between the flexor digitorum superficialis and the flexor digitorum profundus muscles (Fig. 22.6A). Approximately 5 cm proximal to the flexor retinaculum, the median nerve courses around the radial or lateral edge of the flexor digitorum superficialis, where its position becomes more superficial. Just proximal to the carpus, the nerve lies between the tendons of the flexor digitorum superficialis and the flexor carpi radialis, partially deep to the tendon of palmaris longus (if it is present; Fig. 22.6B). The nerve then passes deep to the flexor retinaculum into the carpal tunnel of the wrist.

Figure 22.6 (A) Ultrasound image of the median nerve (MN) in the forearm. (B) Ultrasound image of the median nerve proximal to the carpal tunnel.

When scanning in the axial plane, at the level of the carpal tunnel, it is often difficult to differentiate hyperechoic tendons from the normal median nerve. However, when the transducer is moved proximally in the axial plane, several findings on sonography allow identification of the median nerve. First, the median nerve courses around the radial aspect of the flexor digitorum superficialis to ultimately lie between the flexor digitorum superficialis and the flexor digitorum profundus. Second, the hyperechoic tendons of the flexor digitorum superficialis and the flexor digitorum profundus are contiguous with hypoechoic muscle at the musculotendinous junctions, whereas the median nerve remains relatively hyperechoic. The hyperechoic fat surrounding the median nerve accentuates the contrast between the hyperechoic nerve and hypoechoic muscle proximally. Once the median nerve is identified proximally, it can then be followed distally in the transverse-axial plane to the carpal tunnel. The echogenic surface of the pisiform at the ulnar aspect of the wrist defines the proximal carpal tunnel.

The ulnar nerve at the wrist lies within the Guyon canal, an oblique fibro-osseous tunnel, formed by the flexor retinaculum and palmar carpal ligaments, that lies within the proximal part of the hypothenar eminence. The canal contains the ulnar nerve, the ulnar artery with its venae comitantes, and loose fibrofatty tissue. On transverse sonograms, the ulnar nerve appears as a rounded structure with a location medial to the artery (Fig. 22.7).

Figure 22.7 Ultrasound image of the ulnar nerve at the wrist. UN: ulnar nerve; UA: ulnar artery.

Keeping the patient’s wrist halfway between pronation and supination, place the ultrasound transducer over the lateral aspect of the radial styloid to examine the first compartment of the extensor tendons – abductor pollicis longus (ventral) and extensor pollicis brevis (dorsal). Look at the radial artery and the sensory branch of the radial nerve, the first encroaching deep, the second superficial to the first compartment (Fig. 22.8). Scanning from proximal to distal, note the radial nerve and its branches snapping from ventral to dorsal over these tendons.

Figure 22.8 Ultrasound image of the superficial radial nerve at the wrist. RA: radial artery; APL: abductor pollicis longus tendon; arrow: radial nerve; EPB: extensor pollicis brevis tendon.

Technique

Landmark-based approach

As for all regional anesthetic procedures, after checking that emergency equipment is complete and in working order, intravenous access, ECG, pulse oximetry, and blood pressure monitoring are established. Asepsis is observed.

The patient is placed in the supine position, with the arm abducted and extended at the elbow and wrist joints, and placed on an arm board or on the operator’s knee, with the wrist slightly dorsiflexed. The operator sits facing the patient’s hand. A paresthesia technique is the technique described here. A 15-mm 25-G needle is used.

Median nerve block

Needle insertion for median nerve block is 2 cm cephalad from the wrist crease between the tendons of the flexor carpi radialis and palmaris longus, if present (Fig. 22.9). The needle and syringe are held like a pencil between the thumb and index fingers, with a cephalad needle orientation.

Figure 22.9 Median nerve block technique at the wrist. Needle insertion is 2 cm cephalad from the wrist crease between the tendons of flexor carpi radialis and palmaris longus (if present). The needle and syringe are held like a pencil between the thumb and index fingers; with a cephalad needle orientation.

The patient is instructed to indicate when they feel a paresthesia in the palm and fingers. On obtaining a paresthesia, the needle is withdrawn slightly. Absence of paresthesia is checked prior to injecting 3 mL of local anesthetic. A subcutaneous injection is also made on withdrawal, to block the palmar cutaneous branch of the median nerve. The median nerve may also be blocked by injection deep in the flexor retinaculum in the wrist.

Ulnar nerve block

Needle insertion for ulnar nerve block is 2 cm cephalad from the wrist crease lateral to the flexor carpi ulnaris or medial to the artery. For ulnar nerve block, ventral and medial approaches can be used. The needle and syringe are held like a pencil between the thumb and index fingers; needle orientation is cephalad for the ventral approach (Fig. 22.10).

Figure 22.10 Ulnar nerve block technique at the wrist: ventral approach. Needle insertion is 2 cm cephalad from the wrist crease lateral to the flexor carpi ulnaris. The needle and syringe are held like a pencil between the thumb and index fingers; needle orientation is cephalad.

The patient is instructed to indicate when they feel a paresthesia in the palm and fingers. On obtaining a paresthesia, the needle is withdrawn slightly. Absence of paresthesia is checked prior to injecting 3 mL of local anesthetic. The ulnar nerve at the wrist may also be blocked by injection medial and deep to flexor carpi ulnaris (Fig. 22.11). The medial approach is preferable because ulnar artery damage is less likely, and both dorsal and palmar cutaneous branches may be blocked from the same needle insertion point.

Figure 22.11 Ulnar nerve block technique at the wrist: medial approach. Needle insertion is 2 cm cephalad from the wrist crease; medial; and deep to the flexor carpi ulnaris.

The ulnar nerve can also be blocked 6 cm cephalad from the proximal wrist crease by injection of 4 mL of local anesthetic beneath the tendon of the flexor carpi ulnaris. This method will block the dorsal and ventral branches of the ulnar nerve, and is the method of choice if anesthesia is required on the dorsal aspect of the little finger.

Radial nerve block

Needle insertion for radial nerve block is at the level of the wrist crease (Fig. 22.12). For radial nerve block at the wrist, a 30-mm 22-G needle is used. A subcutaneous injection is made from the radial styloid across the tendon of the extensor pollicis brevis to the middle of the dorsal surface of the wrist (Fig. 22.13). The needle is redirected, infiltrating now across the tendon of the extensor pollicis brevis to the ventral surface of the wrist and over the radial artery. A paresthesia is not sought because the radial nerve is now superficial fibers only. Local anesthetic solution is massaged in to improve the subcutaneous spread. Local anesthetic (8–10 mL) is injected.

Figure 22.12 Radial nerve block technique at the wrist: needle insertion. Needle insertion is at the level of the wrist crease. A subcutaneous injection is made from the radial styloid across the tendon of the extensor pollicis longus.

Figure 22.13 Radial nerve block technique at the wrist: subcutaneous injection. The subcutaneous injection is continued to the dorsal surface of the wrist.

Ultrasound-guided approach

Intravenous access, electrocardiogram (ECG), pulse oximetry and blood pressure monitoring are established. Maximized comfort for the operator and patient is an important step in pre-procedure preparation. For ultrasound-guided nerve blocks at the wrist, the patient is placed in the supine position, the arm abducted, the forearm and wrist in supination (Fig. 21.5). The operator sits adjacent to the side to be blocked. The ultrasound screen, transducer, needle, and plane of imaging should all be placed in one view for the operator. For ultrasound-guided nerve blocks at the wrist the ultrasound screen is placed at the elbow level on the side to be blocked (Fig. 22.14). Room lights may be turned down to enhance image viewing. The operating lights can be used to maintain some working lighting in the background.

Figure 22.14 Global view of the block field for ultrasound-guided nerve blocks at the wrist.

The skin is disinfected with antiseptic solution and draped. A sterile sheath (CIVCO Medical Instruments, Kalona, IA, USA) is applied over the ultrasound transducer with sterile ultrasound gel (Aquasonic, Parker Laboratories, Fairfield, NJ, USA). Another layer of sterile gel is placed between the sterile sheath and the skin. The wrist is scanned in the transverse plane using a high frequency transducer. The ultrasound screen should be made to look like the scanning field. That is, the right side of the screen represents the right side of the field. Adjustable ultrasound variables such as scanning mode, depth of field, and gain are optimized. Developing and maintaining a predetermined basic scanning routine is of enormous help in improving operator confidence and success.

The median nerve is identified just proximal to the wrist crease. A 15-mm 25-G needle is inserted parallel to the axis of the beam of the ultrasound transducer (Fig. 22.15). The needle is slowly advanced under ‘real-time’ imaging to bring the needle tip to rest adjacent to the median nerve. The needle is readjusted to allow complete encirclement of the nerve with local anesthetic (5 mL). Local anesthetic appears as a hypoechoic image (Fig. 22.16).