Diagnosis of small fiber neuropathy: A comparative study of five neurophysiological tests

The diagnosis of small fiber neuropathy (SFN) is a challenge for clinical neurophysiology. Conventional nerve conduction studies are inappropriate for this purpose and therefore various neurophysiological tests have been proposed. In this study, we compared the diagnostic value of five of these tests in 87 patients with clinically definite (n=33) or possible (n=54) SFN related to amyloid neuropathy secondary to transthyretin gene mutation or monoclonal gammopathy (n=30), primary Sjögren's syndrome (n=20), Fabry's disease (n=2), or unknown cause (n=35). Neurophysiological tests included quantitative sensory testing with determination of warm and cold detection thresholds (WDT, CDT), recording of laser-evoked potentials (LEP) and sympathetic skin responses (SSRs), and measurement of electrochemical skin conductance (ESC) using Sudoscan(®) device. All tests were performed at the four extremities (hands and feet). All patients with clinically definite SFN and 70% of the patients with possible SFN had at least one abnormal test. The LEP was the most sensitive test (altered in 79% of the patients with at least one abnormal test), followed by ESC (61%), WDT (55%), SSR (41%), and CDT (32%). The combination of LEP, assessing A-delta sensory fibers, WDT, assessing sensory C fibers, and ESC, assessing autonomic C fibers, appears a relevant approach for the diagnosis of SFN. Compared to SSR and CDT, these three tests, LEP, WDT, and ESC, had a significantly better diagnostic sensitivity and their combination further improved diagnostic accuracy.