I've had all kinds of tests on my arms and hands to find out what's causing the numbness and tingling in my thumb and index finger. It wasn't until they did nerve conduction tests, that the physician finally diagnosed me with carpal tunnel syndrome. Why don't they just start with that test in the first place?
Pressure on a nerve can cause entrapment leading to symptoms such as pain, numbness, and/or weakness. 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.
Making the diagnosis for this problem can be difficult. The physician relies on your history, clinical presentation, and results of specific tests. Most of the tests depend on your 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. For example, nerve conduction tests are done by passing electrical energy through the nerve and measuring the time it takes to generate a response. 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 results of a new 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. During the test, the patient is sitting facing the examiner with the elbows 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 carpal tunnel syndrome, the skin is scratched over the palm-side of the wrist. Then the resistance test is repeated. A positive response for nerve entrapment is a sudden (but temporary) weakness of the forearm.
This test has other advantages. It is easy to use and repeatable. In other words, 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 scratch collapse test appears to be sensitive and specific enough to be reliable. The studied 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 www.jhandsurg.org. Tests like this may eventually replace the more painful and expensive nerve conduction tests.
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.
*Disclaimer:* The information contained herein is compiled from a variety of sources. It may not be complete or timely. It does not cover all diseases, physical conditions, ailments or treatments. The information should NOT be used in place of visit with your healthcare provider, nor should you disregard the advice of your health care provider because of any information you read in this topic.