New Test for Nerve Entrapment

Pressure on a nerve can cause entrapment leading to symptoms such as pain, numbness, and/or weakness. Two of the most common nerve entrapment syndromes are carpal tunnel syndrome and cubital tunnel syndrome. A new diagnostic test for these conditions is now available.

Carpal tunnel syndrome is a common problem affecting the hand and wrist. Symptoms begin when the median nerve gets squeezed inside the carpal tunnel of the wrist. The carpal tunnel is an opening through the wrist to the hand that is formed by the bones of the wrist on one side and the transverse carpal ligament on the other. Any condition that decreases the size of the carpal tunnel or enlarges the tissues inside the tunnel can produce the symptoms of carpal tunnel syndrome.

Cubital tunnel syndrome is a condition that affects the ulnar nerve where it crosses the inside edge of the elbow. The symptoms are very similar to the pain that comes from hitting your funny bone. When you hit your funny bone, you are actually hitting the ulnar nerve on the inside of the elbow. There, the nerve runs through a passage called the cubital tunnel. When this area becomes irritated from injury or pressure, it can lead to cubital tunnel syndrome.

Making the diagnosis for either of these nerve entrapments can be difficult. The physician relies on the patient's history, clinical presentation, and results of specific tests. Most of the tests depend on the patient's subjective response to provocative tests. The examiner taps over the nerve or places the arm, wrist, and/or hand in a position that makes the symptoms better or worse. These tests are provocative because they can irritate an already compromised nerve and confirm that there is a problem.

Electrodiagnostic tests can also help make the diagnosis. Electrical energy passed through the nerve is measured. A lag time in nerve messages sent to the skin (sensory) or to the muscles (motor) is a sign that the nerve isn't functioning properly. But nerve conduction tests of this sort aren't always accurate enough.

With all of these tests, patient responses can vary widely. Studies of test sensitivity and specificity have not found one test that is both sensitive and specific. Sensitivity means the test shows a true positive when there's a problem. Specificity refers to the ability of the test to also show a true negative (the person doesn't have the problem). If a test isn't sensitive enough, patients who have the problem will be missed. If the test isn't specific enough, patients who don't have the problem won't be eliminated.

To overcome low sensitivity and/or low specificity, the examiner must use more than one test. After all the tests are done, then the results are compared. The physician looks for a pattern to confirm or rule out nerve entrapment. What physicians really need is one test that is both specific and sensitive to replace the whole battery of other tests.

The authors of this study suggest that the scratch collapse test may be that test. The test is done by first resisting the patient's forearms as he or she tries to move the forearms away from the body. At the start of the test, the patient is sitting facing the examiner. The elbows are at the sides and bent 90 degrees. The palms of the hands are facing each other.

After the resistance, the examiner scratches the patient's skin over the area of the affected nerve. For cubital tunnel syndrome, this would be along the inside of the forearm at the level of the elbow. For carpal tunnel syndrome, the skin is scratched over the palm-side of the wrist. The resistance test is repeated. A positive response for nerve entrapment is a sudden (but temporary) weakness of the forearm.

To test the scratch collapse test, the researchers compared two groups of people. One group had a nerve entrapment (either carpal tunnel syndrome or cubital tunnel syndrome). The other (control) group had no symptoms of either condition. Although electrodiagnostic tests were positive for everyone in group one, these tests were not done on the control group. Provocative tests (e.g., Tinel's test, flexion/compression test) were done on everyone in both groups.

The scratch collapse test was also done on everyone in both groups. Sensitivity and specificity was compared for all three clinical tests (Tinel's, flexion/compression, scratch collapse). The scratch collapse test had higher sensitivity than the other two tests for both types of nerve entrapment. Overall accuracy for diagnosing nerve entrapment was 82 per cent for carpal tunnel syndrome and 89 per cent for cubital tunnel syndrome.

Other benefits of the test include its ease of use and repeatability. The test can be done more than once without a rest. Patients don't seem to get fatigued, so the test can be repeated to verify results. Mild-to-severe entrapment can be assessed by how severe the weakness is. The test can be used to sort out patients who might be seeking secondary gain (a money settlement) for their injury.

The authors conclude that the scratch collapse test is sensitive and specific enough to be reliable. They showed that it was also reproducible. This means the test was reliable no matter who performed it (so long as the examiner was trained to do the test). And since the results don't depend on the patient reporting on results, it is considered a more objective clinical test. A video of this test is available at


Christine J. Cheng, MD, MPH, et al. Scratch Collapse Test for Evaluation of Carpal and Cubital Tunnel Syndrome. In The Journal of Hand Surgery. November 2008. Vol. 33A. No. 9. Pp. 1518-1524.


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