Ultrasonography complements electrodiagnostic (EDX) studies in the evaluation of disorders of the peripheral nervous system1 including that of distal ulnar nerve neuropathy (Guyon canal syndrome, ulnar tunnel syndrome) by providing insight into the underlying cause; it is also useful in the evaluation of ulnar nerve neuropathies, which are nonlocalizable by EDX studies,2 as in instances of severe distal median nerve neuropathies.3 A recent review by Karvelas and Walker4 has emphasized the role of ultrasonography in all cases of distal ulnar neuropathy because of the ease of use, current quality of imaging, and low cost.
Although there are several reports illustrating the value of ultrasonography (US) in isolated cases of distal ulnar nerve neuropathy, a large series with US evaluation is still rare. Our Neurodiagnostic Center performs EDX evaluation in 2,000 to 2,500 patients annually, referred by hand, orthopaedic, and neurosurgery clinics; during the 8-year period, a total of 5,204 cases of ulnar neuropathies were diagnosed, of which 33 were localized to the Guyon canal; all these patients had undergone US imaging of the ulnar nerve at the time of EDX studies. This study evaluates the role of ultrasonography in the diagnosis of patients with distal ulnar nerve neuropathy, specifically from the point of view of localization and detection of the underlying cause.
MATERIALS AND METHODS
This is a retrospective analysis of patients with distal ulnar nerve neuropathy (confirmed by EDX studies) who underwent ultrasonic imaging as part of their evaluation.
Inclusion criteria (EDX) for ulnar nerve neuropathy at the Guyon canal were as follows:
- Zone 1:
- a. Prolonged distal motor latency to the abductor digiti minimi (ADM) and/or the first dorsal interosseous (FDI) with normal motor conduction proximal to the wrist with or without denervation/reinnervation changes confined to the FDI and ADM.
- b. Prolonged latency/decreased amplitude/absence of sensory nerve action potential (SNAP) over the digital branch and normal SNAP over the dorsal cutaneous branch.
- a. Prolonged distal motor latency (ADM/FDI or both) with normal motor conduction proximal to the wrist with or without denervation/reinnervation changes confined to the FDI/ADM.
- b. Normal SNAP over the digital branch.
- a. Prolonged latency, decreased amplitude, or absence of SNAP in the digital branch.
- b. Normal SNAP over the dorsal cutaneous branch.
- c. Normal motor distal latency to the ADM and FDI.
- d. Absence of denervation/reinnervation changes in the ADM and FDI.
Ulnar nerve conduction studies were performed with recording electrodes over the ADM and FDI. If the compound muscle action potential over the FDI showed a decreased amplitude or was absent, the nerve was stimulated at the palm to confirm or rule out conduction block at the Guyon canal.
Patients with additional presence of peripheral neuropathy, ulnar nerve neuropathy at the elbow, or carpal tunnel syndrome were excluded.
All ultrasonic imaging was performed by a single physician with 10 years of experience in neuromuscular US using a linear array transducer 8 to 18 MHz (GE LOGIQ system). The cross-sectional area of the ulnar nerve was measured at the distal wrist crease, at the level of the pisiform bone, and at the hook of the hamate; the tracing technique was used to measure the cross-sectional area inner to the hyperechoic epineurium, as described in the literature.5,6 The upper normal cross-sectional area cutoff value for the ulnar nerve at the Guyon canal was 8 mm2. Imaging also included documentation of abnormal echo intensity, blood flow, and presence of cysts and other masses.
Thirty-three patients satisfying the EDX criteria for distal ulnar nerve neuropathy were included in this study. The age range was 20 to 76 years. The male to female ratio was 19:14. There were 16 patients with right and 17 with left hand involvement.
In 22 of 33 patients, the clinical history indicated a specific etiology in the form of acute/recurrent trauma. The acute injuries (11) included fractures and crush injuries, iatrogenic trauma, gunshot injuries, and dog bite. Prolonged or recurrent pressure on the volar aspect of the palm appeared to be the underlying cause in 11 patients (Table 1). Some examples are pressure on the palm caused by tools such as pruning scissors, by bicycle handlebar/cane/walker, or by activities such as prolonged scrubbing of floors or sanding wood.
TABLE 1. -
Traumatic Ulnar Neuropathy at the Guyon Canal
||Prolonged/Recurrent Trauma to the Hypothenar Area
|Crush injury/blunt trauma (5)
||Use of tools causing pressure in the palm (3)
|Dog bite (1)
||Bicycling, use of cane/walker (3)
||Scrubbing, sanding, habitual leaning with pressure on the palm (5)
Ultrasonography showed no abnormality in five cases; four of these patients had a history of recurrent or prolonged pressure at the hypothenar area, and one case was considered idiopathic. The most common abnormal finding was the increase in cross-sectional area (16 cases). Two cases showed neuroma, and cysts were detected in 10 patients (Table 2).
TABLE 2. -
Findings on Ultrasonography in 33 Cases
||No. of Cases
||Site of Involvement Based on EDX Studies
||Z1 (1), Z2 (3), Z3 (1)
||Z1 (1), Z2 (1)
||Z1 (5), Z2 (5)
|Increase in CSA
||Z1 (11), Z2 (5)
CSA, cross-sectional area.
Of the 33 cases, 15 underwent surgery; all patients with cysts detected by ultrasonic imaging did have cysts confirmed and ablated during surgery. There was one case where an aberrant muscle was found, not detected by the author during preoperative US imaging. In two additional cases, the surgeon found fibrous bands constricting the nerve, also not detected preoperatively by US imaging.
A 48-year-old woman presented with gradual onset of weakness of the left hand of 6 months duration along with numbness of the ulnar two digits and ulnar palm. She had no medical problems such as diabetes mellitus or history of acute or recurrent local trauma.
Examination showed wasting and marked weakness of the FDI muscle; the ADM was also weak, but the flexor digitorum profundus was normal. Wartenberg and Froment signs were positive (Figs. 1A and 1B). Pain and light touch sensations were absent over the small finger and ulnar side of the palm.
The EDX study showed normal motor conduction in the ulnar nerve across the elbow and forearm but slowing across the wrist with recording electrode placed over ADM; no compound muscle action potential could be recorded over the FDI on stimulation at the wrist and at the palm distal to the hook of the hamate. The ulnar nerve SNAP was absent over digital branches, whereas it was normal over the dorsal cutaneous branch. Needle EMG showed denervation changes predominantly in the FDI.
Ultrasonic imaging revealed a large cyst compressing the ulnar nerve at the Guyon canal (Fig. 2A). This was confirmed by intraoperative findings (Fig. 2B).
EDX studies are most useful in providing insight into the pathophysiology of nerve dysfunction, i.e., the extent of conduction block, demyelination, and axon loss. They are also valuable in finding the location of nerve involvement, but with some limitations; they may not be accurate when there is no compound muscle action potential and SNAP, when only sensory axons are involved, or when anatomic aberrations lead to confusing findings. Selective fascicular involvement and the existence of tandem lesions may also lead to faulty localization. However, another limitation of the EDX study is that it provides no clue to the underlying cause of focal neuropathy; this often needs additional studies such as MRI,7 which is expensive and may not always provide an answer.8 Hence, combining ultrasonic imaging with EDX is cheaper and more convenient than other imaging modalities such as MRI.
An important drawback of the EDX study is its failure to reliably make the distinction between axonotmesis and neurotmesis in cases of nerve injury, which is crucial for planning surgical management; ultrasonic imaging can be used to differentiate between the two.9,10 In this series, although there were several cases of ulnar nerve injury, we did not find any example of neurotmesis; there were two cases of neuroma in continuity.
Ulnar nerve lesions at the elbow may selectively involve motor fascicles destined to the FDI, leading to faulty localization to the Guyon canal (misled by the topography of denervation); US imaging can confirm the location to the elbow or wrist and potentially prevent surgery at the wrong location. Another area of confusion is when there are tandem lesions at multiple sites. In patients with significant slowing of ulnar nerve motor conduction across the elbow and to a lesser extent the forearm and the wrist, the usual explanation is ulnar neuropathy at the elbow with both focal and anterograde demyelination causing diffuse slowing of conduction (often related to longstanding entrapment at the elbow); however, in such patients, additional pathology in the forearm or wrist cannot be totally ruled out. This is a situation where US imaging may be particularly useful.
In this series, the EDX studies confirmed the location of ulnar nerve neuropathy to the Guyon canal and determined the underlying pathology (demyelination vs. conduction block vs. axon loss) but gave no clue to the etiology. US imaging was abnormal in 28 cases, although findings suggestive of specific etiology were found only in 12; there were 2 cases of neuroma and 10 cases of cysts, subsequently verified at surgery. Ultrasonic imaging was normal in five patients, indicating that a normal study does not necessarily rule out ulnar nerve neuropathy at the Guyon canal.
There are several reports of cysts compressing the ulnar nerve at the Guyon canal.11–13 However, the presence of cysts in 30% of our cases of Guyon canal syndrome is somewhat high when compared with past reports; in one of the larger series reported in 2003 (without US imaging) from the same geographic location, only one ganglion cyst was found among 31 cases of ulnar tunnel syndrome.14 It is conceivable that the routine use of US imaging in our patients with distal ulnar nerve neuropathy might have led to the discovery of cysts in more patients. This raises the question whether cysts in the Guyon canal are underdiagnosed (without US imaging) and may not be getting to the surgeon. To our surprise, in this series, we did not encounter some of the rarer causes of Guyon canal syndrome reported in the literature.15–23
There are limitations to this study. Although one of the larger series, the total number of cases is still too small for generalizations; the study of a larger number of patients will be useful in further verifying the higher percentage of cysts noted in this study. More detailed imaging using higher frequency probes24 may have enabled better visualization of the smaller ulnar nerve and revealed causes such as fibrous bands missed by the author in this series. Despite these limitations, this study illustrates the significant advantage of combining US imaging and EDX in the evaluation of distal ulnar nerve neuropathy.
In conclusion, US imaging provides a cheap, readily available sensitive measure for detecting the underlying cause of distal ulnar nerve neuropathy preoperatively. The advantage of making a definitive etiological diagnosis is that it often leads to better preoperative planning and prompt surgical intervention, potentially facilitating a better outcome.
Dr. Luke Robinson, hand surgeon in Louisville, Kentucky, provided the intraoperative photograph.
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