Electrodiagnostic Underpinnings of the Scratch Collapse Test
Nirbhay S. Jain, MD1; Meaghan L Barr, MD2; Amanda Miller, BS1; Chris Gajewski, MD3; Prosper Benhaim, MD4
1University of California, Los Angeles, Los Angeles, CA; 2University of California Los Angeles, Los Angeles, CA; 3UCLA, Los Angeles, CA; 4Division of Plastic Surgery, University of California, Los Angeles, Los Angeles, CA
Introduction: Compressive neuropathies remain a common disorder. Diagnosis by physical exam is difficult. The scratch collapse test (SCT) was introduced to address this, though it is controversial. To address these controversies, we explored the electrodiagnostic underpinnings evaluated by the SCT by correlating positivity on examination to nerve conduction studies.
Methods: We reviewed patients who underwent nerve conduction studies and SCT for carpal tunnel syndrome (CTS) and/or cubital tunnel syndrome (CuTS). Demographic data, including age and BMI, as well as amplitudes, latencies, and velocities on nerve conduction studies were collected. Values between patients who had positive and negative SCT were compared to determine statistical significance.
Results: 360 patients with CTS and 319 patients with CuTS were identified. For patients with CTS, velocities and amplitudes were significantly lower in patients with a positive SCT when compared to a negative SCT. Similarly, CuTS patients had a statistical decrease in velocity but equivalent amplitudes. In both populations, the SCT was more predictive of actual physiologic change in patients with lower BMI than with higher BMI. Similarly, patients of older age had less consistent results correlating the SCT and electrodiagnostic studies. A history of diabetes did not have a major effect on the relationship between SCT and electrodiagnostic studies. Two providers performed the SCT on patients; results were statistically similar between the two. Interestingly, even if values did not reach significance, the trend was consistent - patients with a positive SCT consistently had lower velocities and amplitudes and higher latencies than those with a negative SCT, suggesting some sort of physiologic damage to the nerve.
Discussion: Compression neuropathies remain among the most common conditions for which patients seek out hand and upper extremity surgeons, especially when it comes to carpal tunnel and cubital tunnel syndromes. The SCT, a powerful but controversial maneuver, was created to address this. In our study, a positive SCT was associated with lower amplitudes as well as longer latencies and slower velocities. These findings suggest that patients have both a decrease in axonal density, as evidenced by the lower amplitudes, as well as a decrease in myelination, as evidenced by the prolonged latencies and velocities. BMI impacted the power of the SCT by inhibiting the ability to accurately irritate the nerve to trigger the collapse resulting in increased false negative tests. Together, this suggests that the SCT does measure nerve damage and is justified for use in clinical context.
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