Volume 89, Issue 5 , Pages 887-889, May 2008
Ulnar Nerve and Cubital Tunnel Ultrasound in Ulnar Neuropathy at the Elbow
Article Outline
Abstract
Yoon JS, Hong S-J, Kim B-J, Kim SJ, Kim JM, Walker FO, Cartwright MS. Ulnar nerve and cubital tunnel ultrasound in ulnar neuropathy at the elbow.
Objective
To determine the accuracy of the ultrasonographic measurement of ulnar nerve to cubital tunnel area for diagnosis of ulnar neuropathy at the elbow.
Design
Patients with confirmed ulnar neuropathy at the elbow and normative, healthy volunteers were evaluated with high-resolution ultrasound. The cross-sectional areas (CSAs) of the ulnar nerve and cubital tunnel were measured with the elbow extended and flexed, and results from the 2 groups were compared.
Setting
Electromyography laboratory and radiology department of a tertiary care center.
Participants
Twenty-seven patients with ulnar neuropathy at the elbow and 20 controls.
Interventions
Not applicable.
Main Outcome Measure
The ratio of ulnar nerve to cubital tunnel CSA with the elbow flexed.
Results
The ulnar nerve, with the elbow flexed, was larger in those with ulnar neuropathy at the elbow, and this group also had larger cubital tunnels than did controls. In those with ulnar neuropathy at the elbow, the ratio of the ulnar nerve to cubital tunnel was .31, and in the controls it was .32, which was not significantly different (P=.89).
Conclusions
The ratio of ulnar nerve to cubital tunnel did not differentiate those with ulnar neuropathy at the elbow from controls.
Key Words: Elbow, Electrodiagnosis, Rehabilitation, Ulnar nerve, Ulnar neuropathies, Ultrasonography
ULNAR NEUROPATHY AT THE elbow is the second most common type of nerve entrapment in the upper extremity.1 This condition is traditionally evaluated with a combination of history, clinical examination, and electrodiagnostic studies. Recently, imaging of the ulnar nerve at the elbow has been used to help confirm the diagnosis of ulnar nerve entrapment.2 Ultrasound is the most commonly used imaging modality because it is inexpensive, provides high-resolution, is readily available, and allows for dynamic imaging during elbow flexion, and most studies suggest that the key ultrasonographic finding is enlargement of the ulnar nerve at the site of entrapment.3, 4, 5
In a previous study without a control group, we examined several ultrasonographic measurements in people with cubital tunnel syndrome, including the cross-sectional area (CSA) of the ulnar nerve and cubital tunnel, both with the elbow extended and flexed.6 These measurements were then correlated with ulnar nerve conduction velocities across the elbow, and we found the ratio of the ulnar nerve to cubital tunnel CSA at the tunnel inlet, with the elbow flexed, had the highest correlation with nerve conduction study findings.6 Therefore, we postulated that this ratio may be more important than the nerve CSA alone for diagnosis of ulnar neuropathy at the elbow. To address this question, we designed this current study using the same ultrasonographic technique to determine which measurement best differentiates those with ulnar neuropathy at the elbow from normative, healthy controls.
Methods
We performed ultrasonography on 27 consecutive patients with unilateral ulnar neuropathy at the elbow (59% right-sided) and 20 normative, healthy volunteers. Prior to the collection of data, the study protocol was approved by our institutional review board and consent forms signed by all participants. All patients diagnosed with ulnar neuropathy at the elbow had clinical symptoms (pain at the elbow and/or numbness and weakness in the hand), examination findings (weakness and/or sensory loss in an ulnar distribution), and electrodiagnostic findings (based on American Association of Neuromuscular and Electrodiagnostic Medicine criteria)7 consistent with ulnar neuropathy at the elbow. We excluded patients with previous ulnar nerve surgery, polyneuropathy, and acute traumatic etiology. The normative, healthy comparison group was recruited from hospital personnel and had no numbness, pain, or weakness in any extremity, and only ultrasound data from the right arm was used (a previous study showed no significant side-to-side differences in ulnar nerve CSA in normative, healthy controls).8
Ultrasonography was performed with an ATL HDI 5000 machinea with a 12MHz linear array transducer. A musculoskeletal radiologist, blinded to all participant information, performed the ultrasound studies. Each subject was placed in a supine position and imaged with the elbow extended and flexed 135° (in an effort to maximize the pressure within the cubital tunnel). The ulnar nerve and the cubital tunnel CSA were measured at the inlet and outlet of the tunnel, both with the arm extended and flexed. The inlet of the cubital tunnel was defined as the triangular space bordered by the first hyperechoic Osborne's band, the medial epicondyle, and the olecranon. The outlet of the tunnel was defined as the level at which the 2 heads of the flexor carpi ulnaris joined, and the borders were the flexor carpi ulnaris heads and the ulna.
The CSAs of the ulnar nerve and cubital tunnel were measured by tracing along the hyperechoic rim of the nerve and tunnel using the continuous trace function on the ultrasound device. Each measurement was taken twice and the average was recorded. There were a total of 8 measurements at each elbow because both the nerve and tunnel were measured at the inlet and outlet, with the elbow extended and flexed. Statistical analyses were performed with SPSS.b Comparisons were made using 2-tailed t tests, and P values of .05 were considered significant.
Results
The mean age of the 27 patients with ulnar neuropathy at the elbow was 46.1 years and the comparison group was 40.7 years (P>.05), and 41% of the ulnar neuropathy group were women, whereas 40% of the comparison group were women (P>.05). The clinical characteristics of the patients with ulnar neuropathy are presented in table 1. All patients with ulnar neuropathy at the elbow had some degree of nerve conduction velocity slowing (defined as <50m/s) across the elbow, with a mean velocity 32.9±3.6m/s. In addition, they all had at least a 20% drop in compound motor amplitude potential comparing stimulation below the elbow to above the elbow. Because multiple short segment stimulation studies were not performed, the exact localization of the neuropathy was not known. However, in all patients a short segment across the elbow was used for the nerve conduction studies, so all neuropathies were within a segment of the ulnar nerve extending 3cm distal to the medial epicondyle and 7cm proximal.
Table 1. The Clinical Characteristics of the 27 Patients With Ulnar Neuropathy at the Elbow
| Characteristic | Percentage |
|---|---|
| Subjective symptom | |
 Pain | 81.5 |
| Numbness | 74.1 |
| Weakness | 59.3 |
| Examination finding | |
| Sensory loss | 85.2 |
| Weakness | 48.1 |
| Both sensory loss and weakness | 44.4 |
The mean CSA of the ulnar nerve in those with neuropathy was greater than in the control group, but this was only statistically significant when the elbow was flexed (table 2). The mean CSA of the cubital tunnel was significantly larger in those with ulnar neuropathy than in the control group, in both extension and flexion (see table 2). No difference was detected when the mean ratio of ulnar nerve to cubital tunnel CSA was used to compare the 2 groups. In those with ulnar neuropathy at the elbow the ratio was .31, and in the controls it was .32 (P=.89).
Table 2. The Mean CSAs of the Ulnar Nerve and the Cubital Tunnel for Patients and Controls at the Tunnel Inlet and Outlet with the Elbow Extended and Flexed
| Measurement | Location | Elbow Position | Patients | Controls | P |
|---|---|---|---|---|---|
| Ulnar nerve CSA (mm2) | Inlet | Extended | 10 | 7 | .06 |
| Inlet | Flexed | 10 | 6 | <.01 | |
| Outlet | Extended | 9 | 8 | .56 | |
| Outlet | Flexed | 9 | 5 | <.01 | |
| Cubital tunnel CSA (mm2) | Inlet | Extended | 33 | 21 | <.01 |
| Inlet | Flexed | 32 | 18 | <.01 | |
| Outlet | Extended | 20 | 14 | <.01 | |
| Outlet | Flexed | 19 | 12 | <.01 | |
| Ulnar nerve/cubital tunnel CSA | Inlet | Flexed | 0.31 | 0.32 | .89 |
Discussion
The ulnar nerve at the elbow was easily imaged using ultrasound, and no difficulty occurred in locating the landmarks of the cubital tunnel. As with previous studies,3, 4, 5 the ulnar nerve was enlarged in those with entrapment when compared with controls. In addition, the CSA of the cubital tunnel was larger in those with ulnar neuropathy at the elbow than in controls, and this finding has not previously been reported. However, the main measurement of interest, the ratio of ulnar nerve to cubital tunnel area at the tunnel inlet, was the same in those with ulnar neuropathy and in controls.
There are several possible explanations why the ulnar nerve to cubital tunnel ratio did not differentiate these 2 groups. One unexpected finding in this study was that the cubital tunnel was larger in those with ulnar neuropathy than in controls. Although we expected the nerve would enlarge in the group with neuropathy, we thought the area of the tunnel would not differ based on the presence or absence of ulnar neuropathy at the elbow. Because the nerve and tunnel enlarged together in those with entrapment, the nerve to tunnel ratio remained the same as those without entrapment. This suggests that the cubital tunnel is a flexible compartment that expands as the ulnar nerve enlarges during entrapment. This finding is supported by cadaveric studies of the cubital tunnel, which show that the tunnel is flexible and changes size with elbow movement.9
Study Limitations
Limitations in the study design also may have contributed to the inability of the nerve to tunnel ratio to differentiate those with ulnar neuropathy from controls. Our previous study enrolled only patients with cubital tunnel syndrome (defined by nerve conduction short segment stimulation studies),6 whereas this study included all sites of ulnar neuropathy at the elbow. Because the ultrasonographic measurements focused on the cubital tunnel, this may have limited the ability to fully evaluate ulnar neuropathy at the elbow outside the cubital tunnel. Previous studies of entrapment suggest that identifying the area of maximal nerve enlargement improves diagnostic accuracy,3, 4, 5 and this was not done in this study. This may also explain why the ulnar nerve in those with neuropathy was not significantly larger than controls at every site and elbow position.
This study design was also limited because the duration of symptoms was not recorded for people with ulnar neuropathy at the elbow, which prevented us from determining if this variable affected ulnar nerve and cubital tunnel enlargement. Finally, this study enrolled a relatively small number of participants. However, it is unlikely that even a large increase in sample size would have made a difference in outcome, particularly for the ratio of nerve to tunnel, because this measurement was so similar in the 2 groups.
Further studies exploring techniques to maximize the accuracy of ultrasound in the diagnosis of ulnar neuropathy at the elbow are warranted. Possible techniques worthy of exploration include comparing the CSA at the site of maximal nerve enlargement to either the contralateral ulnar nerve or an unaffected segment of the same nerve, measuring the amount of mobility of the ulnar nerve at several segments around the elbow, and quantitatively measuring the echogenicity of the enlarged ulnar nerve.
Conclusions
Although several different ultrasonographic measures were evaluated in this study, including the area ratio of ulnar nerve to cubital tunnel, no measurement was clearly better in differentiating the 2 groups than the CSA of the ulnar nerve. Previous studies of ulnar neuropathy at the elbow, as well as studies of other focal neuropathies, suggest that the nerve CSA at the point of maximal enlargement is the most appropriate measurement in evaluating entrapment neuropathy.10, 11 This current study shows that more sophisticated ultrasonographic measurement techniques do not appear to increase diagnostic accuracy for ulnar neuropathy at the elbow.
Suppliers
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No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the authors or upon any organization with which the authors are associated.
PII: S0003-9993(08)00106-8
doi:10.1016/j.apmr.2007.10.024
© 2008 American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation. Published by Elsevier Inc. All rights reserved.
Volume 89, Issue 5 , Pages 887-889, May 2008
