Scratch Collapse Test for the Evaluation of Carpal and Cubital Tunnel Syndrome

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Painful cutaneous stimulus has been noted to cause a period of inhibition in tonic voluntary muscle activity, and a brief period of electromyographic silence in humans termed the cutaneous silent period. The authors introduced a new diagnostic test for carpal tunnel syndrome (CTS) and cubital tunnel syndrome (CuTS) in which the examiner scratches the patient's skin lightly over the area of nerve compression, while the patient performs resisted shoulder external rotation (ER) bilaterally. One hundred sixty-nine patients were assessed for CTS or CuTS. Diagnosis was made by a single surgeon and supported by electrodiagnostic studies. A control group was recruited based on their absence of signs of CTS or CuTS. Both groups had the following tests performed: Tinel's over wrist and elbow; wrist flexion with direct compression over the median nerve; elbow flexion with direct compression over the ulnar nerve at the elbow; and the Scratch Collapse Test (SCT). The new test was performed as follows: patient facing examiner, arms adducted, elbows flexed, hands outstretched, and wrist neutral. The examiner gently pushed against both the forearms, asking him/her to sustain steady resistance. With fingertips, examiner scratched/swiped the skin overlying the potentially compressed nerve. A positive test was recorded if the patient demonstrated a loss of resistance in the affected side after “scratching” the respective tunnel. Loss of resistance was brief with the patient regaining strength essentially immediately. The patient has not likely had it administered before, making it potentially more difficult to feign. It can be repeated in succession, without observable fatigue, so several trials can be performed for verification. It provides a more objective method than most of the clinical tests because it does not rely on patient's report. Data analysis showed that the SCT was reproducible, with excellent inter-rater reliability, and had higher sensitivity than the other tests performed, for both CTS and CuTS. In this study, the positive predictive value for all tests was high for CTS, with the SCT having the highest value (99%) followed by wrist flexion/nerve compression (98%) and Tinel's (96%). Negative predictive values were highest for the SCT (73%) followed by wrist flexion/nerve compression (65%) and Tinel's (59%). For CuTS, the SCT had the highest positive predictive value (99%), followed by Tinel's (98%), and elbow flexion/nerve compression (96%). Negative value, however, was highest for Tinel's (98%), followed by the SCT (86%) and elbow flexion/nerve compression (78%). Data analysis showed that the SCT was reproducible, with excellent inter-rater reliability, and had higher sensitivity than the other tests performed, for both CTS and CuTS. The accuracy rate was 82% for diagnosing CTS and 89% for CuTS.