Ulnar Neuropathy at the Wrist

Anatomy

At the wrist, the ulnar nerve enters Guyon’s canal at the level of the distal wrist crease. The canal is formed proximally by the pisiform bone and distally by the hook of the hamate. The floor is formed by a combination of the transverse carpal ligament and the adjacent hamate and triquetrum bones. The roof is loosely formed. In contrast, there is a thick band at the outlet that runs from the hook of the hamate to the pisiform bone, creating the pisohamate hiatus. In the canal, the nerve divides into superficial and deep branches. Before exiting through the pisohamate hiatus, motor fibers are given off the deep branch (also known as the deep palmar motor branch) to three of the four hypothenar muscles (abductor digiti minimi [ADM], flexor digiti minimi, and opponens digiti minimi). After the hiatus, the superficial branch supplies sensation to the volar fifth and medial fourth digits and also supplies motor innervation to the one remaining hypothenar muscle, the palmaris brevis. The deep palmar motor branch goes on to innervate the third and fourth lumbricals, the four dorsal and three palmar interossei, the adductor pollicis, and the deep head of the flexor pollicis brevis.

Clinical

Several subtypes of UNW occur, depending on the exact location of the lesion and which fibers are affected ( Table 23.1 and Box 23.1 ). The following lesions have been described:

• Distal deep palmar motor lesion : affects all muscles supplied by the deep palmar motor branch except the hypothenar muscles; the superficial branch containing the sensory fibers and motor innervation to the palmaris brevis is not affected.

• Proximal deep palmar motor lesion : affects all ulnar-innervated hand muscles, including the hypothenar muscles, with the exception of the palmaris brevis; the superficial branch containing the sensory fibers and motor innervation to the palmaris brevis is not affected.

• Proximal canal lesion : affects all branches of the ulnar nerve, including the proximal and distal deep palmar motor and the superficial branches that contain the sensory fibers and motor innervation to the palmaris brevis.

• Superficial branch lesion : affects only the superficial branch, which is primarily sensory. Note that while the palmaris brevis muscle is affected, this is not clinically apparent.

The first two patterns are the most common, accounting for more than 75% of all cases of UNW. In both, the superficial branch is not affected; thus, there are no sensory symptoms or sensory loss. Patients present with painless weakness and atrophy of ulnar intrinsic hand muscles. Because the ulnar-innervated adductor pollicis and deep head of the flexor pollicis brevis are in the thenar eminence, both the hypothenar and thenar eminences may be wasted, if the lesion is in the proximal deep palmar motor branch. Similar to UNE, the Benediction hand posture, Froment’s sign, and Wartenberg’s sign may be present in advanced cases. In addition, another somewhat obscure sign, known as the “palmaris brevis sign,” may be seen in severe lesions of the deep palmar motor branch. Remember that the palmar brevis is the only muscle supplied by the superficial branch and is therefore spared in lesions of the deep branch. When the palmaris brevis contracts, it results in puckering of the skin along the proximal medial border of the hand. Because the other intrinsic hand muscles are wasted, prominent contraction (and possibly hypertrophy) of the palmaris brevis may be seen when the fifth digit is forcibly contracted in the more common lesions of the deep palmar motor branch of the ulnar nerve at the wrist (the “palmaris brevis sign,” Fig. 23.2 ).

In more proximal lesions, the superficial branch will also be affected, leading to sensory disturbance of the volar fifth and medial fourth digits. The dorsal medial aspect of the hand and fingers will be spared because they are innervated by the dorsal ulnar cutaneous sensory branch, which arises several centimeters proximal to the wrist. This is an important clinical point to remember when trying to discern if the ulnar nerve lesion is at the wrist or more proximal. In addition, the proximal volar medial hand should be spared because the palmar cutaneous branch also arises just proximal to the wrist.

Etiology

Entrapment of the ulnar nerve at the wrist is far less common than at the more usual sites at the elbow. It has been described in association with trauma and wrist fracture. However, more common is a ganglion cyst within Guyon’s canal that compresses the ulnar nerve ( Fig. 23.3 ). Rarely, an anomalous muscle or other mass lesions have been reported, including ulnar artery aneurysms, lipomas, and other tumors. In addition, certain occupations or activities that involve repetitive movement or pressure against the ulnar wrist predispose to lesions at this location. This is especially true for bikers or laborers who use the same hand tools repetitively, which results in pressure on the hypothenar eminence ( Fig. 23.4 ). In such patients, the hypothenar area may be callused at the compression site.

Differential Diagnosis

In lesions where the superficial branch containing the sensory fibers is not affected, UNW is most often confused with early motor neuron disease. Motor neuron disease is well known to present with painless atrophy and weakness of a distal limb, a pattern essentially identical to distal UNW lesions. The key differentiating finding on physical examination in UNW is the intact strength and bulk of the abductor pollicis brevis muscle, supplied by the median nerve. In motor neuron disease, one would expect all C8–T1-innervated muscles supplied by different peripheral nerves to be affected. In UNW, there is a marked difference between ulnar C8–T1-innervated muscles (which are weak and wasted) and median C8–T1-innervated muscles (which are spared). However, this difference in ulnar versus median innervated muscles can also be seen in some atypical motor neuron disorders, such as multifocal motor neuropathy with conduction block, a rare autoimmune mediated motor neuropathy that preferentially affects distal muscles in a non-myotomal pattern of weakness (see Chapter 29 ).

In proximal lesions at the wrist where the superficial branch (and hence sensory fibers) is affected, the differential diagnosis is similar to that of UNE. Indeed, in UNW with sensory involvement, the most important diagnosis to exclude is UNE. Unequivocal sensory loss over the medial dorsal aspect of the hand and fingers and/or weakness of the distal flexors of the ring and little fingers are consistent with a lesion at the elbow, not at the wrist. However, in mild or early cases of UNE, these signs may not be present. In addition to UNE, one must keep in mind the possibilities of C8–T1 radiculopathy, lower trunk or medial cord brachial plexopathy, and rare cases of ulnar nerve entrapment in the arm or forearm, which can present with similar symptoms and signs.