TUCSON, ARIZ. — A novel test for diagnosing carpal and cubital tunnel syndromes was at least as accurate as conventional tests, according to the findings of a prospective, case-control investigation.
During the so-called scratch-collapse test, patients perform a simple resistive motor task, such as pressing their extended arms against a clinician, who then lightly scratches the site of peripheral nerve compression. The patient then immediately attempts to repeat the motor task. If the test is positive, there is a brief loss of proximal postural stability, or “collapse,” in the arm, Dr. Christine Cheng explained at the annual meeting of the American Association for Hand Surgery.
“My first reaction was sort of like everyone else's—'This is crazy,'” she said in an interview. “But it does seem to bear out.”
The test was developed by San Diego orthopedic surgeon Dr. John Beck, based on observations of postural stimulation and muscle control in patients with Parkinson's disease.
The exact mechanism is not fully understood. But it is hypothesized that the test is detecting a short circuit or delay in the proximal muscles, said Dr. Cheng of Washington University, St. Louis.
She presented data from a prospective study in which 169 patients and 109 controls were evaluated for carpal and cubital tunnel syndromes using Tinel's sign, Phalen's test, elbow flexion, and the scratch-collapse test.
Electrodiagnostic studies were used to confirm the diagnosis of carpal tunnel in 119 patients and 175 hands, and cubital tunnel in 70 patients and 81 hands.
In the control group, testing was rarely positive, she said. In the 175 hands with carpal tunnel syndrome, 148 had a positive scratch-collapse test, 141 had a positive Tinel's sign, and 131 had a positive Phalen's test.
In the 81 hands with cubital tunnel syndrome, 64 had a positive scratch-collapse test, 64 had a positive Tinel's sign, and 56 had positive elbow flexion.
Sensitivity of the scratch-collapse test in subjects with carpal tunnel syndrome was 75%, compared with 37% for Tinel's sign and 47% for Phalen's test. Specificity was 62%, 75%, and 66%, respectively. Accuracy was 72%, 47%, and 54%.
Sensitivity of the scratch-collapse test in subjects with cubital tunnel syndrome was 83%, compared with 65% for Tinel's sign and 54% for elbow flexion. Specificity was 82%, 86%, and 81%, respectively. Accuracy was 82%, 77%, and 69%.
Dr. Cheng and her colleagues have been using the test for about 2 years, in conjunction with other testing, to establish a diagnosis of carpal tunnel in patients. Part of the problem in establishing this diagnosis is that it remains primarily clinical, she explained.
The sensitivity and specificity of clinical tests vary widely, electrodiagnostic studies still have significant false-positive and false-negative values, and predictive values depend on the prevalence of disease.
“For something as common and presumably simple as carpal tunnel syndrome, it's not all that easy to diagnose because there really isn't a gold standard that you can use,” she said.
