The flare response in skin largely depends on an intact primary sensory fiber, the C-fiber. We measured the flare response to the intradermal injection of substance P, histamine, and capsaicin in control subjects and in diabetic patients with and without clinically obvious polyneuropathy. The neuropathic diabetic patients had a reduced flare response to substance P, histamine, and capsaicin, compared with control and nonneuropathic diabetic subjects. The smaller flareresponse in the neuropathic diabetics after capsaicin administration suggested a dysfunction of the peripheral component of the C-fiber. Alternatively, dysfunction of the mast cell or vascular reactivity may contribute to the diminished flare. Because C-fibers participate in nociception in addition to the flare response, the findings of this study, by a method that permits a quantifiable measurement of the function of peripheral sensory neurons in diabetic subjects, has potential usefulness in evaluating sensory neuropathy in diabetic patients.

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