Flare Size but Not Intensity Reflects Histamine-Induced Itch.

Introduction: Flare reactions arise due to the release of vasodilators from sensory nerves caused by antidromic transmission of action potentials after the induction of itch.

Objective: We investigated the link between flare and itch using 3 models of itch.

Methods: Skin provocations with histamine, capsaicin, and cowhage were performed in 31 subjects. Itch was quantified using the visual analog scale. Flare was assessed using laser speckle contrast imaging (LSCI) and digital photography.

Results: The duration, intensity, and area under the curve of histamine-induced itch correlated with the area of increased blood flow measured with LSCI (r = 0.545, p = 0.002; r = 0.575, p = 0.001; and r = 0.649, p < 0.001, respectively). Itch and skin blood flow in response to capsaicin or cowhage did not correlate.

Conclusion: In histamine-induced skin inflammation, itch and increased blood flow are linked. Thus, the area of histamine-induced flare may be used as a surrogate marker for histamine-induced itch.