Phototherapy and photochemotherapy of skin diseases.

One important aspect of photomedicine is the use of nonionizing electromagnetic radiation with and without exogenous photosensitizers to treat diseases. Phototoxicity (cell injury by photons) is a likely mechanism for phototherapy and photochemotherapy of several skin diseases. The mechanism of action for phototherapy of hyperbilirubinemia and of uremic pruritus appears to be photochemical alteration of extracellular metabolites. Psoriasis is an example of a disease benefitted by several forms of phototherapy and photochemotherapy with varying relative effectiveness and safety. Two successful forms of treatment are oral psoralen photochemotherapy and UVB plus topical adjunctive agents. New information about UVB therapy of psoriasis includes data about the therapeutic action spectrum and about the relative roles of various topical agents such as coal tar, mineral oil, "lubricants" and steroids. Although there are many surface similarities, phototherapy and psoralen photochemotherapy have fundamental differences which may alter longterm risks in quantitative and qualitative ways.