Indications: hand and finger surgery
Transducer position: transverse at wrist crease or distal third of the forearm (Figure 32G–1)
Goal: local anesthetic injection next to the median and ulnar nerves and the sensory branch of the radial nerve
Local anesthetic: 10–15 mL (total volume)
Ultrasound-guided wrist block. Transducer and needle positions for (A) median nerve block; (B) ulnar nerve block; (C) radial nerve block.
The wrist block is an effective method to provide anesthesia of the hand and fingers without the arm immobility that occurs with more proximal brachial plexus blocks. Traditional wrist block technique involves advancing needles using surface landmarks toward the three nerves that supply the hand: the median, ulnar, and radial nerves. Since the nerves are located relatively close to the surface, this is a technically easy block to perform, but knowledge of the anatomy of the soft tissues of the wrist is essential for successful blockade with minimum patient discomfort. In addition to providing anesthesia and analgesia, wrist blocks using botulinum toxin to treat hyperhidrosis have been described.1
Three individual nerves are involved in a wrist block: the median, ulnar, and radial nerves.
The median nerve crosses the elbow medial to the brachial artery and courses toward the wrist deep to the flexor digitorum superficialis in the center of the forearm. As the muscles taper toward tendons near the wrist, the nerve assumes an increasingly superficial position until it is located beneath the flexor retinaculum in the carpal tunnel with the tendons of the flexor digitorum profundus, flexor digitorum superficialis, and flexor pollicis longus. A linear transducer placed transversely at the level of the wrist crease will reveal a cluster of oval hyperechoic structures, one of which is the median nerve. At this location, it is easy to confuse the tendons for the nerve and vice versa; for this reason, it is recommended to slide the transducer 5–10 cm proximally the volar side of the forearm, to confirm the location of the nerve. The tendons will have disappeared on the image, leaving just muscle and the solitary median nerve (Figures 32G-2 and 32G-3), which then can be carefully traced back to the wrist, if desired. In many instances, however, it is much simpler to perform a median nerve block at the midforearm, where the nerve is easier to recognize.
(A) Cross-sectional anatomy of the distal forearm. (B) Sonoanatomy of the median nerve (MN) at the forearm. RA, radial artery; FCRM, flexor carpi radialis muscle; FPLM, flexor pollicis longus muscle; FDSM, flexor digitorum superficialis muscle.