20 Ulnar Neuropathy at the Wrist

Ulnar neuropathy at the wrist (UNW) is a rare condition that sometimes is confused with ulnar neuropathy at the elbow (UNE) or, more often, with early motor neuron disease. Knowledge of the detailed anatomy of the ulnar nerve at the wrist is necessary to understand the several unique clinical and electrophysiologic patterns that can occur with UNW (Figure 20–1).

FIGURE 20–1 Detailed anatomy of the ulnar nerve at the wrist.

Entrapment of the ulnar nerve at the wrist can take on several patterns: (1) pure motor affecting only the distal deep palmar motor branch, distal to the hypothenar muscles, (2) pure motor affecting the proximal deep palmar branch including the hypothenar motor branches, (3) motor and sensory (proximal canal lesion), and rarely (4) sensory affecting the superificial branch involving the sensory fibers to the volar fourth and fifth fingers. The superficial branch does supply one muscle, the palmaris brevis. This muscle, however, is difficult to assess clinically or by EMG, although there are reports of the “palmaris brevis sign” wherein this muscle is spared or prominent in lesions of the deep palmar motor branch (see Figure 20–2).

(Adapted with permission from Olney, R.K., Hanson, M., 1988. AAEE case report no. 15: ulnar neuropathy at or distal to the wrist. Muscle Nerve 11, 828.)

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 thick 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, 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 20–1 and Box 20–1). 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 which 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.

Table 20–1 Clinical and Electrophysiologic Differentiating Factors in Variants of Ulnar Neuropathy at the Wrist

Box 20–1

Clinical and Electrophysiologic Abnormalities not Consistent with Ulnar Neuropathy at the Wrist

Clinical

• Weakness of thumb abduction (abductor pollicis brevis – median innervated)

• Weakness of the finger flexors of digits 4 and 5 (flexor digitorum profundus – ulnar innervated in the forearm)

• Weakness of index finger extension (extensor indicis proprius – radial innervated)

• Sensory symptoms/signs in the dorsal medial hand/dorsal fifth and fourth fingers (territory of the dorsal ulnar cutaneous sensory branch)

• Sensory symptoms/signs in the medial forearm (territory of the medial antebrachial cutaneous sensory nerve)

Nerve conduction studies

• Abnormal median motor study (unless there is a coexistent carpal tunnel syndrome)

• Abnormal dorsal ulnar cutaneous sensory study

• Focal slowing or conduction block of the ulnar nerve at the elbow

Needle electromyography

• Abnormalities in the proximal ulnar-innervated muscles (flexor carpi ulnaris and flexor digitorum profundus to digits 4 and 5)

• Abnormalities in non-ulnar C8-innervated muscles (abductor pollicis brevis, flexor pollicis brevis, extensor indicis proprius)

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,” Figure 20–2).

FIGURE 20–2 Palmaris brevis sign.

When the palmaris brevis contracts, it results in puckering of the skin along the proximal medial border of the hand. As the palmar brevis is the only muscle supplied by the superficial branch of the ulnar nerve at the wrist, it will be spared in lesions of the deep branch. Thus, prominent contraction of the palmaris brevis may be seen when trying to abduct digit 5 in the more common lesions of the deep palmar motor branch of the ulnar nerve at the wrist. Note the prominent contraction of the palmaris brevis and wrinkling of the skin on the right hand (arrows) compared to the normal left hand.

(Adapted with permission from Iyer, V.G., 1998. Palmaris brevis sign in ulnar neuropathy. Muscle Nerve 21, 675–677.)

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 (Figure 20–3). 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 (Figure 20–4). In such patients, the hypothenar area may be calloused at the compression site.

FIGURE 20–3 Ganglion cyst in Guyon’s canal.

One of the more common causes of ulnar neuropathy at the wrist is compression of the ulnar nerve by a ganglion cyst. Computed tomographic scan of the wrist. Left: Axial scan, volar side up. Note the cyst medial to the hook of the hamate. Right: Coronal scan, lateral hand up. Note the cyst between the pisiform and the hamate.

(From Preston, D.C., Shapiro, B.E., Schecht, H.M., 2001. Ganglion cyst at Guyon’s canal: electrophysiology and pathology. J Clin Neuromusc Dis 3, 89–91.)