Validity of Clinical Small Fiber Sensory Testing to Detect Small Nerve Fiber Degeneration.

Study Design Prospective, cross-sectional diagnostic accuracy study. Background Small nerve fiber degeneration (SFD) commonly occurs in patients with peripheral neuropathies, resulting in deterioration in nerve function. Currently, the gold standard to identify SFD is through skin biopsies. Simple clinical tests aim to identify SFD, but their validity remains unknown. Objectives To examine the validity of clinical tests to assess SFD using carpal tunnel syndrome (CTS) as a model system. Methods 107 participants (22 healthy, 85 with CTS) underwent pinprick testing over the index finger, and had cold and warm detection thresholds taken (CDT, WDT) using quantitative sensory testing (QST). In a sub-group of patients with CTS (n=51), cold and warm sensation were also tested using coins at room and body temperature respectively. The validity of these clinical tests was established against intra-epidermal nerve fiber density (IENFD) measured in skin biopsies from the index finger. Results Optimal validity occurred with clusters of tests: normal warm OR cold sensation is highly sensitive to rule out SFD (sensitivity 0.98, 95% CI 0.85,0.99), but has a low specificity (0.2, 95% CI 0.03,0.52). A reduction in pinprick in contrast is highly specific (0.88, 95% CI 0.72,0.95), thus useful to rule in SFD. For QST, highest specificity (0.77) occurred for WDT and highest sensitivity (0.84, 95% CI, 0.72,0.91) for CDT OR WDT. Conclusion Pin prick testing followed by warm and cold tests if pin prick is normal, is a valid and cost-effective method to detect SFD. For QST, WDT is useful for ruling in SFD. To rule out SFD both CDT and WDT must be negative. Level of Evidence Diagnosis, Level 2. J Orthop Sports Phys Ther, Epub 22 Jun 2018. doi:10.2519/jospt.2018.8230.