A model for in vivo analysis of sudomotor sympathetic C-fiber activation and human sweat gland output

Quantitative assessment of small fiber peripheral neuropathy often involves an evaluation of the interaction between the C-fiber sudomotor nerve and local sweat rate (SR). Typically, some sort of quantitative sudomotor axon reflex test (QSART) is performed to aid in diagnosing small fiber dysfunction. The currently used QSART demonstrates only moderate test-retest reliability and therefore limits its usefulness in tracking small fiber dysfunction. A new experimental model to examine small C-fiber function in the skin using intradermal electrical stimulation and simultaneous monitoring of SR is proposed. Using intradermal electrical stimulation (1.5 and 2.5 mA) and varying stimulus frequency from 0.2 - 64 Hz a quantitative relationship between the area under the SR - time curve and log₁₀ stimulus frequency is modeled using a four-parameter logistic equation, providing the following parameters: Baseline, Plateau, EC₅₀, and Hill slope. The model has good to excellent repeatability within the same day (ICC = 0.98), on different days at the same skin site (ICC = 0.80), and when comparing two different skin sites (ICC = 0.78) with a small bias estimate and the line of identity always lying within the 95% limits of agreement. Atropine sulfate (0.1 mg•ml^-1) blocked 90 ± 5% of the electrically induced sweating. Overall, the model provides control over sudomotor nerve activity and a quantitative assessment of SR. Finally, the ability to reproduce quantitative stimulus-response curve on different days allows for a robust assessment of the relationship between the activation of a sympathetic C-fiber and local SR.

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