Test for carpal tunnel syndrome found unreliable

A nerve test widely used to diagnose carpal tunnel syndrome may not be reliable, according to a new study from the University of Michigan (U-M) in Ann Arbor. People without any typical carpal tunnel syndrome symptoms can have abnormal results on nerve conduction tests, casting doubt on the test’s usefulness, the study says.

Part of the problem lies in the fact that the test results are greatly influenced by the person doing the test, according to the study. Published in a recent issue of the journal Muscle & Nerve, the study is the first of its size and kind.1 A team of experts from several areas of the university concluded that doctors should use a continuous scale to evaluate test results rather than the current practice of choosing a cutoff point to distinguish between normal and abnormal results.

"When job placement or surgery hangs in the balance, the reliability of a test like this becomes even more important," says co-author and U-M health scientist Deborah Salerno. "The results of our study were mixed, and, at times, the difference in reliability was striking. For reliable results, you want to have an experienced examiner do the test and use appropriate criteria to see if the results are normal."

The study involved 158 workers, mostly women, whose work included a large amount of time spent at a computer keyboard. Not all subjects complained of symptoms typical of carpal tunnel syndrome. All participants underwent two rounds of nerve conduction tests, spaced three weeks apart, that electrically stimulated two nerves in the wrist. The difference in response time was examined.

Two physicians performed the tests. In the first round, each physician tested both wrists of each subject. In the second round, three weeks later, tests were performed by one of the physicians in the dominant wrist only. The examiners tested two nerves for abnormalities:

1. the median nerve, which runs down the middle of the arm and wrist and controls the palm, thumb, and several fingers;

2. the ulnar nerve, which runs through the outer arm and wrist to control the back of the hand and the small and ring fingers.

The median nerve is more affected by carpal tunnel syndrome, in which overuse of the hand and wrist causes tissue swelling. This swelling, in turn, narrows the gap between the carpal (or wrist), bones, and ligaments through which the nerve runs, which causes pressure on the nerve. Women between ages 30 and 60 are most commonly affected.

Although a variety of tests can detect conditions such as carpal tunnel syndrome, nerve conduction studies have been used as the gold standard. The test uses a small jolt of electricity to stimulate each nerve. Next, the time it takes for the stimulus to reach the muscle is measured. A second stimulation of the same nerve at a different spot, or of the other major nerve, allows the lag time (known as the latency) to be compared. This comparison lets doctors spot the existence and the extent of nerve damage in different locations.

In the study, the results recorded by each examiner for each patient were compared, as were each examiner’s results from both their exams of the workers. In both cases, results showed that certain tests were better than others. Between examiners and within the same examiner, median nerve tests were more reliable than ulnar nerve tests.

A key question was how differences between examiners would affect a diagnosis of carpal tunnel syndrome. That is, would the first examiner rate a test result as "normal" when the second rated it as "abnormal"?

To explore this, the researchers used two cutoff points to define "normal." The first defined a difference of 0.5 milliseconds (Msec) or more between the median and ulnar tests in the same wrist as abnormal. The second used a difference of 0.8 Msec or more. Results showed that between examiners, both cutpoints had good to excellent reliability. Using the two definitions of normal, however, resulted in marked differences in results. Each examiner had higher reliability with the higher cutoff point.

"Presumably, the higher cutpoint reflected more serious nerve problems. This would make the abnormalities easier to notice, and results more reliable," says Salerno.

In light of this, the researchers concluded that a system with more ordered categories (such as normal, equivocal, mild, or definite abnormality) would better describe results, rather than using absolute cutoff points to define "normal."

Interestingly, between the two examiners, 36 (22%) of the workers in the first round had abnormal results. In contrast, over half (53%) of the workers in the study reported pain, burning, tingling, or numbness in the hands or wrists, which are typical symptoms of certain nerve disorders.

"Nerve conduction studies have been considered the gold standard for carpal tunnel syndrome; but abnormal results alone, without symptoms, do not define the disorder," says Salerno. "Both symptoms and physical findings are critical for accurate test interpretations." Accurate and reliable testing — using tests beyond nerve conduction studies — should be performed, considering that a diagnosis of carpal tunnel syndrome can lead to work-area and duty adjustments, prescription of medication, or surgery, the researchers conclude.

Reference

1. Salerno D, Werner R, Albers J, et al. Reliability of nerve conduction studies among active workers. Muscle & Nerve 1999; 22:1,372-1,379.

For more information on the study, contact:

Deborah Salerno, University of Michigan, 412 Maynard, Ann Arbor, MI 48109-1399. Telephone: (734) 662-1572.