Solar urticaria: mechanism and treatment.

A patient with solar urticaria induced by wavelengths 290-420 nm is reported. Wheals appeared after a few seconds of exposure to the sun; longer exposure caused general malaise and syncope. Intradermal injection of in vitro irradiated plasma caused a local whealing which was not seen with plasma kept dark. The wheals induced by irradiation could be inhibited by local injection of an antihistamine. Local injection of lidocaine and hydrocortisone was ineffective. Depletion of substance P in the skin by topical application of capsaicin did not change the sensitivity to irradiation with 313 nm and a single PUVA treatment did not change the minimal urticarial dose (MUD). Sunscreens were in practice of limited value with the exception of a protective plastic helmet. Repeated daily irradiation with UVA in increasing doses normalized his response to sunlight.