Ultrasound-guided injection for carpal tunnel syndrome is useful in the management of the symptoms associated with carpal tunnel syndrome. Because the median nerve is contained within a relatively noncompliant space as it passes through the carpal canal, the addition of ultrasound guidance allows for more accurate needle placement within the borders of the canal while at the same time avoiding needleinduced trauma to the median nerve. Furthermore, the ability to observe the actual flow of the injectate within this closed space utilizing real-time ultrasound imaging allows the clinician to identify any further compression of the nerve as the injection proceeds.

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 (Fig. 68.1). While the clinical presentation of carpal tunnel syndrome is consistent, this entrapment neuropathy has many causes and is associated with many pathologic conditions (Table 68.1). 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 carpal tunnel syndrome include a positive Tinel sign over the median nerve at the site of injury (Fig. 68.2). 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. 68.3). 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. 68.4). 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. 68.5).

The benefits, risks, and alternative treatments are explained to the patient, and informed consent is obtained. The patient is then placed in the sitting position with the elbow flexed to about 100 degrees and the forearm resting comfortably palm up on a padded bedside table with the fingers slightly flexed, which will relax the flexor tendons (Fig. 68.6). With the patient in the above position, the distal crease of the wrist is identified (Fig. 68.7). A high-frequency linear ultrasound transducer is placed in a transverse position over the distal crease of the wrist, and an ultrasound survey scan is taken (Fig. 68.8). The median nerve will appear as a bundle of hyperechoic nerve fibers surrounded by a slightly more hyperechoic neural sheath lying beneath the flexor retinaculum and above the superficial flexor tendons (Fig. 68.9). The median nerve can be distinguished from the flexor tendons by simply having the patient flex and extend their fingers and
observing the movement for the tendons. The flexor tendons will also exhibit the property of anisotropy with the tipping of the ultrasound transducer back and forth over the tendons. The ulnar artery is then identified on the ulnar side of the wrist (Fig. 68.10). Color Doppler may aid in the identification of the ulnar artery so it can be avoided when performing inplane needle placement (Fig. 68.11). After the ulnar artery is identified, the ultrasound transducer is slowly moved medially until the median nerve is again easily identifiable in the transverse ultrasound image. The skin overlying the area beneath the ultrasound transducer is prepped with antiseptic solution. A sterile syringe containing 2.0 mL of 0.25% preservative-free bupivacaine and 40 mg of methylprednisolone is attached to a 1½-inch, 22-gauge needle using strict aseptic technique. The needle is placed through the skin just above the level of where the ulnar artery was previously identified and is advanced using an in-plane approach beneath the ulnar border of the ultrasound transducer with the needle trajectory adjusted under real-time ultrasound guidance so that the needle tip ultimately rests in proximity to the median nerve as it lies within the carpal tunnel beneath the flexor retinaculum (Fig. 68.12). When the tip of the needle is thought to be in satisfactory position, after careful aspiration, a small amount of local anesthetic and steroid is injected under real-time ultrasound guidance to confirm that the needle tip is in the proper position. After proper needle tip placement is confirmed, the remainder of the contents of the syringe are slowly injected. There should be minimal resistance to injection, and no paresthesias should be elicited. Alternatively, this injection may be performed utilizing an out-of-plane technique by introducing the needle from
the inferior margin of the ultrasound transducer and advancing it under real-time ultrasound guidance (Fig. 68.13).