Chapter 29. Wrist Block

The wrist block is a technique for blocking branches of the ulnar, medial, and radial nerves at the level of the wrist. The wrist block is a basic peripheral nerve block technique that involves anesthesia of the median, ulnar, and radial nerves, as well as the dorsal sensory branch of the ulnar nerve. The wrist block is simple to perform, essentially devoid of systemic complications, and highly effective for a variety of procedures on the hand and fingers. As such, skill in performing a wrist block should be in the armamentarium of every anesthesiologist. Wrist blocks can be used in the outpatient setting and office setting along with the standard operating room setting, resulting in safe, effective, and cost-effective anesthesia that is well accepted by both surgeons and patients.1–8 Wrist blocks are also useful in the emergency setting to provide anesthesia for repair of hand injuries in the emergency room because there is adequate anesthesia of the hand without motor blockade of the extrinsic hand muscles.9

A wrist block is most commonly used for carpal tunnel and hand and finger surgery.10,11 The most common hand surgery in the United States is carpal tunnel release. Paget described carpal tunnel syndrome in 1853.1,12 Although Learmonth reported release of the carpal tunnel at the wrist in 1933, it was not until the 1950s that the surgery became popular through the efforts of Phalen.13–15 Because of the ease of performing a wrist block, wrist blocks are used in a variety of settings including the emergency room, outpatient surgery centers, and office-based anesthesia practices. Hand surgeons also make use of the wrist block to perform minor procedures in their offices. A wrist block can be used in a patient with a full stomach requiring emergeny surgery, thereby obviating the need for general anesthesia and reducing the risk of aspiration.

The contraindications to wrist blocks are few, but include local infection at the sites of needle insertion, preexisting central or peripheral nervous systems disorders, and allergy to local anesthesia. Patients are usually able to tolerate a tourniquet on the arm without anesthesia for 20 min; a wrist tourniquet can be tolerated for about 120 min.

Innervation of the hand is shared by the ulnar, median, and radial nerves (Figure 29–1). The ulnar nerve innervates more intrinsic muscles than the median nerve, which in turn innervates digital branches to the skin of the medial one and a half digits (Figure 29–2). A corresponding area of the palm is innervated by palmar branches that arise from the ulnar nerve in the forearm. The deep branch of the ulnar nerve accompanies the deep palmar arch and supplies innervation to the three hypothenar muscles, the medial two lumbrical muscles, all the interossei, and the adductor pollicis. The ulnar nerve also innervates the palmaris brevis muscle.

The median nerve traverses the carpal tunnel and terminates as digital and recurrent branches. The digital branches innervate the skin of the lateral three and a half digits and, usually, the lateral two lumbrical muscles. A corresponding area of the palm is innervated by palmar branches that arise from the median nerve in the forearm. The recurrent branch of the median nerve supplies the three thenar muscles. In the palm, the digital branches of the ulnar and median nerves lie deep in the superficial palmar arch, but in the fingers, they lie anterior to the digital arteries that arise from the superficial arch. Although the innervation of the ring and middle fingers may vary, the skin on the anterior surface of the thumb is always supplied by the median nerve and that of the little finger by the ulnar nerve. The palmar digital branches of the median and ulnar nerves also innervate the nail beds of their respective digits.

The radial nerve passes along the front of the radial side of the forearm. It arises first from the lateral side of the radial artery and beneath the supinator muscle. About 3 inches above the wrist, it leaves the artery, pierces the deep fascia, and divides into two branches (Figure 29–3). The superficial branch, the smaller of the two branches, supplies the skin of the radial side and base of the thumb, and joins the anterior branch of the musculocutaneous nerve. The deep branch of the radial nerve communicates with the posterior branch of the musculocutaneous nerve. On the dorsum of the hand, the deep branch of the radial nerve forms an arch with the dorsal cutaneous branch of the ulnar nerve.

The superficial branch of the radial nerve runs along the medial aspect of the brachioradialis muscle (see Figure 29–3). It then passes between the tendon of the brachioradialis and radius to pierce the fascia on the dorsal aspect. Just above the styloid process of the radius (circle), it gives off digital branches for the dorsal skin of the thumb, index finger, and lateral half of the middle finger. Several of its branches pass superficially over the anatomic “snuff box.”

The median nerve is located between the tendons of the palmaris longus and the flexor carpi radialis (see Figures 29–2 and 29–4). The palmaris longus tendon is usually the more prominent of the two tendons, and the median nerve passes just lateral to it.

The ulnar nerve passes between the ulnar artery and tendon of the flexor carpi ulnaris (see Figures 29–2 and 29–4. The tendon of the flexor carpi ulnaris is superficial to the ulnar nerve.

A standard regional anesthesia tray is prepared with the following equipment.

• Sterile towels and 4 × 4-in. gauze pads
• 10-mL syringes with local anesthetic (LA)
• One 1½-in., 25-gauge needle

The patient is in the supine position with the arm abducted. The wrist should be kept in a slight dorsiflexion.

Block of the Radial Nerve

The radial nerve is essentially a “field block” and requires a more extensive infiltration because of its less predictable anatomic location and division into multiple, smaller cutaneous branches. Five milliliters of LA is injected subcutaneously just above the radial styloid while advancing the needle medially (Figure 29–5). The infiltration is then extended laterally, using an additional 5 mL of LA.

Block of the Dorsal Sensory Branch of the Radial Nerve

The dorsal sensory branch of the radial nerve is blocked by inserting the needle 1 cm proximal to the radial styloid, which is radial to the radial artery (Figure 29–6). This branch of the radial nerve exits from between the brachioradialis and extensor carpi radialis longus 5 to 8 cm proximal to the radial styloid. The needle is advanced to Lister's tubercle, and if there are no paresthesias, 5 mL of LA is injected subcutaneously throughout this area.

Block of the Ulnar Nerve

The ulnar nerve is anesthetized by inserting the needle under the tendon of the flexor carpi ulnaris muscle close to its distal attachment just above the styloid process of the ulna (Figures 29–4, 29–7, and 29–8). The needle is advanced 5 to 10 mm to just past the tendon of the flexor carpi ulnaris. Three to 5 mL of LA solution is injected. A subcutaneous injection of 2 to 3 mL of local anesthesia just above the tendon of the flexor carpi ulnaris is also advisable in blocking the cutaneous branches of the ulnar nerve, which often extend to the hypothenar area.

Block of the Dorsal Sensory Branch of the Ulnar Nerve

The dorsal sensory branch of the ulnar nerve is blocked by inserting the needle at the level of the ulnar styloid because it travels from palmar to dorsal in the area of the ulnar styloid (Figure 29–9). Start the injection at the flexor carpi ulnaris and extend subcutaneously dorsally toward the distal radioulnar joint. Five mL of LA is injected subcutaneously throughout the area.

Block of the Median Nerve

The median nerve is anesthetized by inserting the needle between the tendons of the palmaris longus and flexor carpi radialis (Figures 29–8, 29–10, and 29–11). The needle is inserted until it pierces the deep fascia. Three to 5 mL of LA is injected. Although the piercing of the deep fascia has been described to result in a fascial “click,” it is more reliable to simply insert the needle until it contacts the bone. At that point, the needle is withdrawn 2 to 3 mm and the LA is injected. Figure 29–12 demonstrates the spread of the LA after injection of 5 mL using the described technique.

Nerve Stimulation Technique

Median and ulnar nerves can also be blocked at the wrist using a nerve stimulator. These blocks may be used for finger flexor tendon repairs, when the surgeon wishes to test their function intraoperatively (function of the forearm muscles is not affected). The median nerve is found in the carpal tunnel between the palmaris longus and the flexor carpi radialis tendons, and the ulnar nerve is found between the flexor carpi ulnaris and the ulnar artery. Twitches are similar to elbow blocks except for the forearm pronation, which is missing. Two to 3 mL of LA is sufficient to block either nerve.

The choice of the type and concentration of LA for wrist blockade is based on the desired duration. Lidocaine is the most used anesthetic for wrist block, but bupivacaine or ropivacaine can be used safely, also.16Table 29–1 provides onset times and duration for some commonly used LAs mixtures.

Wrist block is associated with moderate patient discomfort because multiple insertions and subcutaneous injections are required. Appropriate sedation and analgesia, midazolam (2–4 mg) and alfentanil (250–500 mcg), are required to ensure the patient's comfort. A typical onset time for a wrist block is 5 to 15 min, depending primarily on the concentration and volume of LA used. Sensory anesthesia of the skin develops faster than the motor block. Placement of an Esmarch latex-free bandage or a tourniquet at the level of the wrist is well tolerated and does not require additional blockade.

Complications after wrist block are typically limited to residual paresthesias due to an inadvertent intraneural injection. Systemic toxicity is rare because of the distal location of the blockade and relatively small volumes (doses) of local anesthetic NMR required (Table 29–2).