Ulnar Nerve Block at the Wrist

Indications and Clinical Considerations

Ulnar nerve block at the wrist is most often used for surgical anesthesia and treatment of ulnar tunnel syndrome. Ulnar tunnel syndrome is caused by compression of the ulnar nerve as it passes through the Guyon canal at the wrist ( Fig. 82.1 ). The most common causes of compression of the ulnar nerve at this anatomic location are space-occupying lesions, including ganglion cysts, hematoma, and ulnar artery aneurysms; fractures of the distal ulna and carpals; and repetitive motion injuries that compromise the ulnar nerve as it passes through this closed space ( Fig. 82.2 ). This entrapment neuropathy manifests most often as a pure motor neuropathy without pain, which is caused by compression of the deep palmar branch of the ulnar nerve as it passes through the Guyon canal ( Fig. 82.3 ). This pure motor neuropathy manifests as painless paralysis of the intrinsic muscles of the hand.

Ulnar tunnel syndrome may also occur as a mixed sensory and motor neuropathy. Clinically, this mixed neuropathy manifests as pain, numbness, and paresthesias of the wrist that radiate into the ulnar aspect of the palm and dorsum of the hand and the little finger, as well as the ulnar half of the ring finger. These symptoms also may radiate proximal to the nerve entrapment into the forearm. As with carpal tunnel syndrome, the pain of ulnar tunnel syndrome is frequently worse at night and made worse by vigorous flexion and extension of the wrist. If the condition is not treated, progressive motor deficit and ultimately flexion contracture of the affected fingers can result. The onset of symptoms is usually after repetitive wrist motions or from direct trauma to the wrist, such as wrist fractures or direct trauma to the proximal hypothenar eminence, which may occur when the hand is used to hammer on hubcaps or from handlebar compression during long-distance cycling ( Fig. 82.4 ).

Physical findings include tenderness over the ulnar nerve at the wrist. A positive Tinel sign over the ulnar nerve as it passes beneath the transverse carpal ligament is usually present. If the sensory branches are involved, there is decreased sensation into the ulnar aspect of the hand and the little finger, as well as the ulnar half of the ring finger. Depending on the location of neural compromise, the patient may have weakness of the intrinsic muscles of the hand, as evidenced by the inability to spread the fingers or weakness of the hypothenar eminence.

Ulnar tunnel syndrome is often misdiagnosed as arthritis of the carpometacarpal joints, cervical radiculopathy, or diabetic polyneuropathy. Patients with arthritis of the carpometacarpal joint usually have radiographic evidence and physical findings suggestive of arthritis. Most patients with cervical radiculopathy have reflex, motor, and sensory changes associated with neck pain, whereas patients with ulnar tunnel syndrome have no reflex changes and motor and sensory changes are limited to the distal ulnar nerve. Diabetic polyneuropathy generally manifests as symmetric sensory deficit involving the entire hand rather than limited just to the distribution of the ulnar nerve. Cervical radiculopathy and ulnar nerve entrapment may coexist as the so-called double crush syndrome. Furthermore, because ulnar tunnel syndrome is commonly seen in patients with diabetes, it is not surprising that diabetic polyneuropathy is usually present in diabetic patients with ulnar tunnel syndrome. Pancoast tumor invading the medial cord of the brachial plexus also may mimic an isolated ulnar nerve entrapment and should be ruled out by apical lordotic chest radiography.

Electromyography helps distinguish cervical radiculopathy, diabetic polyneuropathy, and Pancoast tumor from ulnar tunnel syndrome. Plain radiographs are indicated for all patients with ulnar tunnel syndrome to rule out occult bony disease. On the basis of the patient’s clinical presentation, additional testing may be indicated, including complete blood cell count, uric acid, sedimentation rate, and antinuclear antibody testing. Magnetic resonance imaging or ultrasound imaging of the wrist is indicated if joint instability or a space-occupying lesion is suspected ( Fig. 82.5 ). The injection technique described later serves as both a diagnostic and a therapeutic maneuver.

Clinically Relevant Anatomy

The ulnar nerve is made up of fibers from C6-T1 spinal roots. The nerve lies anterior and inferior to the axillary artery in the 3 o’clock to 6 o’clock quadrant. Exiting the axilla, the ulnar nerve descends into the upper arm along with the brachial artery. At the middle of the upper arm, the nerve courses medially to pass between the olecranon process and medial epicondyle of the humerus. The nerve then passes between the heads of the flexor carpi ulnaris muscle, continuing downward, moving radially along with the ulnar artery. At a point approximately 1 inch proximal to the crease of the wrist, the ulnar nerve divides into the dorsal and palmar branches ( Fig. 82.6 ). The dorsal branch provides sensation to the ulnar aspect of the dorsum of the hand, the dorsal aspect of the little finger, and the ulnar half of the ring finger. The palmar branch provides sensory innervation to the ulnar aspect of the palm of the hand, the palmar aspect of the little finger, and the ulnar half of the ring finger. As with the carpal tunnel, the ulnar tunnel is a closed space and is bounded on 1 side by the pisiform and on the other side by the hook of the hamate ( Fig. 82.7 ). The ulnar nerve must pass between the transverse carpal ligament and the volar carpal ligament ( Fig. 82.8 ). In addition to the ulnar nerve, the ulnar tunnel contains the ulnar artery, which may compress the nerve. Unlike the carpal tunnel, the ulnar tunnel does not contain flexor tendon sheaths.