Surgical Treatment of Ulnar Nerve Entrapment at the Wrist

Ulnar nerve lesions at the wrist are well described ( ▶ Fig. 64.1). The site of compression is commonly known as Guyon canal, the space of Guyon, the distal ulnar tunnel, and the carpal ulnar neurovascular space—to name a few. Because of its relative rarity, this condition is often misdiagnosed. Despite the frequent finding of advanced muscle atrophy at presentation, operative outcomes can be excellent when the condition is diagnosed in a timely manner and decompression is completed.

Fig. 64.1 Artist’s rendering highlights the course of the ulnar nerve at the level of the wrist and its predilection to traumatic disorders.

64.1.1 Relevant Anatomy

In the distal forearm, the ulnar nerve lies deep and radial to the flexor carpi ulnaris. Radial to the nerve are the ulnar vessels. The dorsal cutaneous branch of the ulnar nerve is given off 6 to 8 cm above the wrist. The ulnar nerve, artery, and vein pass into the hand through Guyon canal, a 4-cm-long tunnel at the level of the wrist, where neurovascular structures are susceptible to compression ( ▶ Fig. 64.2 a). This fibro-osseous tunnel is triangular shaped. The roof of the tunnel consists of the distal extension of the antebrachial fascia and the palmaris brevis. The floor is the ulnar portion of the transverse carpal ligament, the muscles of the hypothenar eminence, and their fibers of origin.

Fig. 64.2 (a) The ulnar nerve (UN) and its branches and the ulnar artery (UA) at the wrist are seen passing between the pisiform (P) and hamate (H). (b) The deep branch can be seen passing beneath a tendinous fibrous arch (FA). ADM, abductor digiti minimi; FDM, flexor digiti minimi; PHL, pisohamate ligament. (Reproduced with permission from Bozkurt MC, Tağil SM, Özçakar L, Ersoy M, Tekdemir I. Anatomical variations as potential risk factors for ulnar tunnel syndrome: a cadaveric study. Clin Anat. 2005;18:274–280.)

The distal ulnar tunnel has been subdivided into three zones. Zone 1 refers to the ulnar nerve proximal to its division into its terminal branches. The division typically occurs in the middle of the canal, and the nerve either bifurcates or trifurcates. Zone 2 includes the deep branch and zone 3 the superficial branch. The deep branch is mainly a motor branch: it innervates intrinsic muscles of the hand but also has some sensory fibers to neighboring carpal joints. The superficial branch, which is mainly sensory, also innervates the palmaris brevis. The deep branch supplies the other ulnar-innervated hand intrinsic muscles. As the motor branch exits the distal (pisohamate) hiatus, it innervates the abductor digiti minimi and flexor digiti minimi. It passes beneath a fibrous arch before taking an acute radial turn around the hook of the hamate. The fibrous (tendinous) nature of the arch may make certain patients more susceptible to compression ( ▶ Fig. 64.2 b). After curving around the hamate, the deep branch typically innervates the opponens digiti minimi, ulnar two lumbricals, interossei, adductor pollicis, and deep head of the flexor pollicis brevis. The superficial branch divides into the digital nerves supplying the skin of the little finger and the ulnar half of the ring finger.

Variant ulnar nerve motor and sensory innervation in the wrist and hand may result in misleading clinical symptoms and signs. Occasionally the branch of the ulnar nerve that supplies the abductor digiti minimi arises proximal to the pisohamate hiatus and travels superficial to the fibrous arch. In these patients, compression of the ulnar nerve at the hiatus may spare the abductor digiti minimi. ¹ The Riche-Cannieu anomaly, marked by an anastomosis between the ulnar nerve and the recurrent branch of the median nerve, may result in partial ulnar and median innervation of the median and ulnar lumbricals, respectively. Ulnar nerve contributions to the thenar musculature have also been described. ² Concomitant ulnar and median neuropathies may further obscure physical examination findings. ³

Incomplete ossification of the carpal bones in young children results in a shallower Guyon canal. Consequently, the ulnar neurovascular bundle may demonstrate a modified course through the wrist in children, traveling directly volar to the hamate instead of medial to its hook. ⁴ Operative exposure of the ulnar nerve at the wrist in pediatric patients requires an appreciation of these anatomical variations to avoid iatrogenic neurovascular injury

64.1.2 Pathoanatomy

The most common causes of ulnar tunnel syndrome include idiopathic, post-traumatic, degenerative, ulnar artery thromboses, variant muscle bellies or fibrous bands, metabolic abnormalities, endocrine disorders, and soft tissue masses. Direct, repetitive trauma induced by bicycle handlebar pressure or vibratory occupational tools, such as jackhammers, are frequently cited causes of ulnar neuropathy at the wrist. Several authors have also proposed an association between carpal tunnel syndrome and compression of the ulnar nerve at Guyon canal, suggesting that the transverse carpal ligament bordering both canals allows for the transmission of increased pressure from one canal to the other. ¹ Soft tissue masses (especially ganglia) may be overrepresented by isolated case reports in the literature. Several articles have tried to correlate pathological entities with specific zones of compression. For example, ganglia are commonly found in patients with zone 2 compression.

The pathophysiology of ulnar nerve entrapment at the wrist is similar to that which underlies other peripheral nerve injury. Increased pressure results in compression of neural microvasculature. Chronic compression may cause mechanical stress and inflammatory changes, which present as demyelination and eventual axonal loss. ⁵

64.2 Patient Selection

64.2.1 Clinical Presentation

Patients with ulnar neuropathy localized to the wrist typically have a combination of wrist pain, hand weakness, and/or sensory abnormalities in the ulnar-sided digits. Symptoms may be exacerbated with wrist flexion. Depending on the zone of compression, clinical symptoms and signs may vary from combined motor and sensory loss to an isolated motor or sensory presentation.

Examination may reveal variable degrees of weakness and atrophy in the hand intrinsic muscles ( ▶ Fig. 64.3). Ulnar innervated extrinsic muscles are normal because the branches to the flexor carpi ulnaris and flexor digitorum profundi are given off more proximally (near the elbow). Clawing may be profound, in fact, more than that seen with patients with ulnar nerve compression at the elbow. A host of eponymous signs have been introduced to describe patterns of ulnar nerve weakness, including Froment’s and Wartenberg’s signs. Two-point discrimination may be diminished on the palmar side of the ulnar 1 ½ digits, but sensation is preserved on the dorsal aspect of the hand. Percussion over the course of the ulnar nerve may produce radiating paresthesias. Wrist flexion may be provocative. The radial and ulnar arteries should be assessed for thrombosis by performing an Allen test or aneurysmal dilation by auscultating for a bruit or palpating a thrill.