Mechanisms of phototherapy and photochemotherapy for photodermatoses.

Most photodermatoses represent indications for preventive ultraviolet (UV) phototherapy and/or psoralen plus ultraviolet A (PUVA) photochemotherapy. The aim of treatment is to prevent the outbreak of disease by increasing the patient's tolerance to sunlight. The mechanisms by which ultraviolet B (UVB) and PUVA induce such tolerance are not completely understood. Pigmentation and skin thickening may be important factors in the protective effect, but they cannot sufficiently explain the degree of protection induced. Other mechanisms that may be of critical importance for the therapeutic efficacy encompass a variety of immunomodulatory effects on human skin known to be induced by UVA, UVB, and PUVA. Obviously the mechanisms of prophylactic phototherapy are strongly intertwined with the pathogenesis of the photodermatoses. The possible mechanisms of photoprevention are discussed for polymorphic light eruption (PMLE), actinic prurigo, chronic actinic dermatitis, and solar urticaria.