Differential time courses of skin blood flow and hyperalgesia in the human sunburn reaction following ultraviolet irradiation of the skin

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Abstract
The response of skin to ultraviolet (UV) irradiation is an inflammation with pronounced vasodilation and hyperalgesia. Volunteers underwent UV irradiation of patches of forearm skin 3 cm in diameter. The intensity of the UV irradiation (290–320 nm) ranged between 133 mJ/cm2and 400 mJ/cm2. Changes in skin blood flow were measured with laser Doppler technique at 3, 6, 9, 12, 24, 30, 36, 48, 60, 72, 96 and 216 h post UV irradiation. The alteration in pain threshold was measured for heat with a Peltier thermode and for pressure with a metal stylus. The effect of repeated topical pre‐treatment 4 days prior to UV irradiation with 1.5 g of 0.04% capsaicin containing ointment was also investigated. UV irradiation resulted in a dose‐dependent increase of skin blood flow for more than 96 h post‐irradiation. There were two peaks of blood flow at 12 h and 36 h. Topical application of capsaicin prior to UV irradiation attenuated blood flow from 30 h to 45 h post UV irradiation. Enhanced blood flow was also present 5 mm outside the irradiated area (flare reaction). The control level of heat pain threshold was 44.5±0.7 C in normal skin. Heat pain thresholds were lowered by up to 7.761.2C in UV‐irradiated areas in a dose‐dependent fashion. The control level of pressure pain threshold was 15.260.3N. Pressure pain thresholds were lowered by up to 6 N in irradiated areas. Maximal hyperalgesia coincided with the second peak of skin blood flow between 30 h and 60 h post UV irradiation. The effects of topical application of capsaicin suggests an involvement of neuropeptide mechanisms in the late phase of the human sunburn reaction.