Ultrasound-guided median nerve block at the wrist is useful in the management of the pain subserved by the median nerve. This technique serves as an excellent adjunct to brachial plexus block and for general anesthesia when performing surgery at the wrist or below. Ultrasound-guided median nerve block at the wrist with local anesthetic may be used to palliate acute pain emergencies, including postoperative pain, pain secondary to traumatic injuries of the distal radius, and portions of the wrist and carpal bones innervated by the distal median nerve, as well as cancer pain, while waiting for pharmacologic, surgical, and antiblastic methods to become effective.

Ultrasound-guided median nerve block can also be used as a diagnostic tool when performing differential neural blockade on an anatomic basis in the evaluation of distal upper extremity pain as well as in a prognostic manner to determine the degree of neurologic impairment the patient will suffer when destruction of the median nerve is being considered or when there is a possibility that the nerve may be sacrificed during surgeries in the anatomic region of the median nerve at the level of the wrist. This technique may also be useful in those patients suffering symptoms from compromise of the median nerve due to carpal tunnel syndrome. Ultrasound-guided median nerve block at the wrist may also be used to palliate the pain and dysesthesias associated with stretch injuries to the median nerve.

The median nerve at the wrist is susceptible to trauma during surgery for carpal tunnel syndrome or may be damaged by wrist fractures or compressed by mass or tumor (Fig. 66.1). Entrapment neuropathy of the median nerve at the wrist is known as carpal tunnel syndrome and is the most common entrapment neuropathy encountered in clinical practice (Figs. 66.2 and 66.3) Carpal tunnel syndrome presents as pain and dysesthesias with associated numbness and weakness in the hand and wrist that radiate to the thumb, index finger, middle finger, and radial half of the ring finger. These symptoms may also radiate proximal to the level of nerve entrapment into the distal forearm.

Physical findings associated with entrapment or trauma of the median nerve at the wrist include a positive Tinel sign over the median nerve at the site of injury (Fig. 66.4). Decreased sensation in the distribution of the median nerve of the thumb, index finger, middle finger, and radial half of the ring finger is often present as weakness of thumb opposition. A positive Phalen test is highly suggestive of the diagnosis of carpal tunnel syndrome. Phalen test is performed by having the patient place the wrists in complete unforced flexion for at least 30 seconds (Fig. 66.5). The test is considered positive if this maneuver elicits dysesthesia, pain, or numbness in the distribution of the median nerve.

Arising from fibers from the ventral roots of C5 and C6 of the lateral cord and C8 and T1 of the medial cord of the brachial plexus, the median nerve lies anterior and superior to the axillary artery in the 12:00 o’clock to 3:00 o’clock quadrant as it passes through the axilla. As the median nerve exits the axilla, it passes inferiorly adjacent to the brachial artery. At the antecubital fossa, the median nerve lies just medial to the brachial artery. Continuing its downward path, the median nerve gives off a number of motor branches to the flexor muscles of the upper arm. These branches are susceptible to nerve entrapment by aberrant ligaments, muscle hypertrophy, and direct trauma. As the median nerve approaches the wrist, it overlies the radius where it is susceptible to trauma from radial fractures and lacerations. The nerve lies deep to and between the tendons of the palmaris longus muscle and the flexor carpi radialis muscle at the wrist. It is susceptible to entrapment as it passes through the carpal tunnel (Fig. 66.6). The terminal branches of the median nerve provide sensory innervation to a portion of the palmar surface of the hand as well as the palmar surface of the thumb, index, and middle fingers and the radial portion of the ring finger. The median nerve also provides sensory innervation to the distal dorsal surface of the index and middle fingers and the radial portion of the ring finger (Fig. 66.7).