Ulnar tunnel syndrome could be broadly defined as a compressive neuropathy of the ulnar nerve at the level of the wrist. The ulnar tunnel, or Guyon’s canal, has a complex and variable anatomy. Various factors may precipitate the onset of ulnar tunnel syndrome. Patient presentation depends on the anatomic zone of ulnar nerve compression: zone I compression, motor and sensory signs and symptoms; zone II compression, isolated motor deficits; and zone III compression; purely sensory deficits. Conservative treatment such as activity modification may be helpful, but often, surgical exploration of the ulnar tunnel with subsequent ulnar nerve decompression is indicated.
A zone I compression elicits motor and sensory signs and symptoms, a zone II compression results in isolated motor deficits, and a zone III compression causes purely sensory deficits.
In select cases, conservative treatment such as activity modification may be helpful, but often, surgical exploration of the ulnar tunnel with subsequent ulnar nerve decompression is indicated.
The anatomy of the ulnar tunnel is complex, but numerous anatomic studies have described the tunnel in significant detail. Because organic lesions are often implicated in the cause, surgical exploration and decompression of the ulnar tunnel represent a common treatment modality.
Ulnar tunnel syndrome (UTS) is broadly defined as a compressive neuropathy of the ulnar nerve at the level of the wrist. The term “ulnar tunnel syndrome” was coined by DuPont and colleagues in 1965 to describe the condition of 4 patients with acquired ulnar neuritis. The ulnar tunnel proper, also known as “Guyon’s canal,” is one potential but not exclusive site of ulnar nerve compression at the wrist. The eponym comes from Guyon’s description in 1861 of a space at the base of the hypothenar eminence at which the ulnar nerve bifurcates and that is vulnerable to compression from surrounding structures. Numerous factors may precipitate the onset of UTS, including space-occupying lesions, vascular lesions, and repetitive trauma. Patient presentation depends on the anatomic zone of ulnar nerve compression and therefore may be purely motor, purely sensory, or a combination of both. In select cases, conservative treatment such as activity modification may be helpful, but often, surgical exploration of the ulnar tunnel with subsequent ulnar nerve decompression is indicated.
As with all nerve-related disease, a thorough understanding of anatomy and potential sites of compression is critical. The ulnar nerve emerges from the medial cord (C8–T1) of the brachial plexus and passes through the axilla into the anterior compartment of the arm, before piercing the intermuscular septum and traveling in the posterior compartment medially. It then courses superficially and passes posterior to the medial epicondyle, into the anatomic cubital tunnel. The nerve then continues deep along the flexor digitorum profundus in the forearm. Before its entrance to the ulnar tunnel, approximately 3.4 cm proximal to the ulnar styloid, the ulnar nerve gives off the dorsal cutaneous branch, which innervates the ulnar and dorsal side of the hand. The main nerve resurfaces at the level of the wrist where it passes through the ulnar tunnel, which is a fibro-osseous structure. The anatomy of the tunnel is complex, and variations in the nomenclature and structures surrounding the tunnel have previously been a source of confusion and misinterpretation. For example, the terms pisohamate tunnel, pisohamate hiatus, and pisohamate arcade have been used variably and interchangeably to describe the ulnar tunnel in part or whole. The entrance of the tunnel is triangular and begins at the proximal edge of the palmar carpal ligament. It extends distally to the fibrous arch of the hypothenar muscles at the level of the hook of the hamate and is approximately 40 to 45 mm in length. The boundaries of the tunnel vary along its length. Generally, the roof of the canal consists of the palmar carpal ligament, palmaris brevis, and hypothenar connective tissue. The floor of the canal consists of the transverse carpal ligament, pisohamate ligament, pisometacarpal ligament, and the tendons of the flexor digitorum profundus and opponens digiti minimi. The medial wall of the canal is formed by the pisiform, the abductor digiti minimi, and the tendon of the flexor carpi ulnaris. The lateral wall is formed by the hook of the hamate, the transverse carpal ligament, and the flexor tendons.
Within the boundaries of the canal lie the ulnar nerve, ulnar artery, accompanying veins, and connective fatty tissue. The ulnar nerve lies slightly deep and ulnar to the ulnar artery. During its course in Guyon’s canal, the ulnar nerve bifurcates into a superficial and a deep branch approximately 6 mm distal to the distal pole of the pisiform. The superficial branch innervates the palmaris brevis and provides the sensory fibers over the hypothenar eminence and small and ring fingers. The motor branch of the nerve exits deep in the canal and courses around the ulnar edge of the hamulus and then runs radially between the abductor digiti minimi and flexor digit minimi and dorsal to the flexor tendons of the small finger ( Fig. 1 ).