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 ).
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:
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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.
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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.
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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.
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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.
| Deep Palmar Motor Branch Distal | Deep Palmar Motor Branch Proximal | Proximal Canal | Superficial Branch † | |
|---|---|---|---|---|
| Weakness – interossei and 3rd/4th lumbricals | X | X | X | |
| Weakness – hypothenar muscles (ADM, ODM, FDM) | X | X | ||
| Sensory loss – volar medial hand and little finger, medial half ring finger | X | X | ||
| Reduced CMAP at FDI | X | X | X | |
| Reduced CMAP at ADM | X | X | ||
| Prolonged FDI latency | X | X | X | |
| Prolonged ADM latency | X | X | ||
| Reduced SNAP to digit 5 | X | X | ||
| Prolonged latency comparing INT to 2nd LUM | X | X | X | |
| Conduction block at the wrist | X | X | X | |
| CV slowing at the wrist | X | X | X | |
| EMG abnormalities in FDI | X | X | X | |
| EMG abnormalities in ADM | X | X |
† The superficial branch is often thought of as a “sensory branch.” However, it does supply one muscle, the palmaris brevis.
Clinical
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Weakness of thumb abduction (abductor pollicis brevis – median innervated)
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Weakness of the finger flexors of digits 4 and 5 (flexor digitorum profundus – ulnar innervated in the forearm)
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Weakness of index finger extension (extensor indicis proprius – radial innervated)
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Sensory symptoms/signs in the dorsal medial hand/dorsal fifth and fourth fingers (territory of the dorsal ulnar cutaneous sensory branch)
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Sensory symptoms/signs in the medial forearm (territory of the medial antebrachial cutaneous sensory nerve)
Nerve conduction studies
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Abnormal median motor study (unless there is a coexistent carpal tunnel syndrome)
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Abnormal dorsal ulnar cutaneous sensory study
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Focal slowing or conduction block of the ulnar nerve at the elbow
Needle electromyography
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Abnormalities in the proximal ulnar-innervated muscles (flexor carpi ulnaris and flexor digitorum profundus to digits 4 and 5)
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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 ).
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.
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 to be equally 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 26 ).
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.
Electrophysiologic Evaluation
Nerve Conduction Studies
The findings on nerve conduction studies in UNW depend on (1) whether the superficial sensory branch is involved and (2) if the deep motor branch is involved, whether it is affected proximal or distal to the hypothenar muscles. If the lesion is distal, affecting only the deep palmar motor branch after the take-off to the hypothenar muscles, then the routine ulnar sensory study, recording the fifth digit, and the routine ulnar motor conduction study, recording the ADM, will be normal. In suspected UNW, additional nerve conduction studies must always be performed in order to detect abnormalities that may not be present on routine ulnar motor and sensory studies ( Box 20–2 ).
Routine studies:
- 1
Ulnar motor study recording abductor digiti minimi, stimulating wrist, below groove, and above groove in the flexed elbow position
- 2
Ulnar motor study recording first dorsal interosseous, stimulating wrist, below groove, and above groove in the flexed elbow position
- 3
Ulnar motor study recording first dorsal interosseous, stimulating the wrist (3 cm proximal to the distal wrist crease) and palm (4 cm distal to the distal wrist crease)
- 4
Median motor study recording abductor pollicis brevis, stimulating wrist and antecubital fossa
- 5
Median and ulnar F responses
- 6
Ulnar sensory response, recording digit 5, stimulating wrist (bilateral studies)
- 7
Median sensory response, recording digit 2 or 3, stimulating wrist
- 8
Dorsal ulnar cutaneous sensory response (bilateral studies)
Additional studies to consider:
- 9
Ulnar motor study recording the contralateral first dorsal interosseous, stimulating the wrist (in order to compare distal latencies and amplitudes side to side)
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Lumbrical–interossei distal latency comparison study
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Ulnar motor study recording the first dorsal interosseous, inching across the wrist in 1 cm increments
The following patterns are consistent with ulnar neuropathy at the wrist:
DML to FDI: >4.5 ms (provided CMAP amplitude is not markedly reduced)
DML comparing FDI to ADM: >2.0 ms difference
DML comparing symptomatic FDI to contralateral FDI: >1.3 ms difference
DML comparing ulnar INT to second lumbrical: >0.4 ms difference
The following patterns denote ulnar neuropathy at the wrist with certainty:
DML to FDI in the demyelinating range: >130% upper limit of normal (i.e., any DML to the FDI >6.0 ms)
Focal slowing across the wrist during inching studies: ≥0.5 ms over a 1 cm increment, recording FDI
Conduction block, comparing palm and wrist stimulations, recording FDI
Conduction velocity slowing across the wrist recording FDI
Special considerations:
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If the superficial sensory branch is affected, the SNAP amplitude will be low or absent, with a normal dorsal ulnar cutaneous SNAP. (Caution must be taken in interpreting this pattern, which also can occur in patients with UNE.)
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Occasional false-positive results occur when using the DML to FDI or ADM; comparing DML to FDI versus ADM; and the lumbrical–interossei study, especially in cases of moderate or severe UNE with axonal loss. Wrist versus palmar stimulation studies, or inching studies across the wrist should be done to demonstrate UNW with certainty.
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If the dorsal ulnar cutaneous sensory study is performed and is absent, it is prudent to stimulate the superficial radial sensory nerve along the lateral radius with the recording electrodes in place for the dorsal ulnar cutaneous sensory study to ensure that an anomalous innervation is not present (recall there is a very rare anomalous innervation wherein the superficial radial sensory nerve supplies the entire dorsum of the hand, including the usual territory of the dorsal ulnar cutaneous sensory nerve).
SNAP, sensory nerve action potential; CMAP, compound muscle action potential; DML, distal motor latency; UNE, ulnar neuropathy at the elbow; ADM, abductor digiti minimi; FDI, first dorsal interosseous; INT, interossei.
In addition to routine ulnar motor studies recording ADM and sensory studies recording digit 5, the following studies often are helpful.
Ulnar Motor Studies Recording the First Dorsal Interosseous
In all cases of suspected UNW, it is imperative to perform ulnar motor studies recording the first dorsal interosseous (FDI). In lesions of the distal deep palmar motor branch, the latency to the FDI may be prolonged with a decreased compound muscle action potential (CMAP) amplitude. Comparison with the contralateral asymptomatic side often is helpful as well. In cases where the lesion is more proximal, affecting the hypothenar branches, the distal motor latency (DML) to the ADM also may be prolonged, with a decreased CMAP amplitude. However, one of the patterns highly suggestive of UNW is preferential involvement of the distal deep palmar motor branch, whereby the ulnar motor study recording the FDI is affected out of proportion to the ulnar motor study recording the ADM . Comparison of their relative distal motor latencies often can be helpful:
| DML to FDI: | ≤4.5 ms |
| DML comparing FDI to ADM: | ≤2.0 ms difference |
| DML comparing symptomatic FDI to contralateral FDI: | ≤1.3 ms difference |
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