- Indications: surgery on the hand and fingers
- Transducer position: transverse at wrist crease or distal third of the forearm
- Goal: local anesthetic injection next to medians and ulnar nerve and local anesthetic infiltration for the radial nerve
- Local anesthetic: 10–15 mL
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, namely the median, ulnar, and radial nerves. The ultrasound-guided approach has the advantage of direct visualization of the needle and target nerve, which may decrease the incidence of needle-related trauma. In addition, because the needle can be placed with precision immediately adjacent to the nerve, smaller volumes of local anesthetic are required for successful blockade than with a blind technique. 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.
Three individual nerves are involved:
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 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 (Figures 34-2A and B and 34-3A and B). At this location it is easy to confuse the tendons for the nerve, and vice versa; for this reason, it is recommended that the practitioner slides the transducer 5 to 10 cm up the volar side of the forearm, leaving the tendons more distally to confirm the location of the nerve. The tendons will have disappeared on the image, leaving just muscle and the solitary median nerve, which then can be carefully traced back to the wrist, if desired. In many instances, however, it is much simpler to perform a medianus block at the midforearm, where the nerve is easier to recognize.