Phototoxicity and photoallergy

Photosensitivity may be phototoxic or photoallergic. Phototoxicity is much more common. There are 2 types of photoxicity: photodynamic, which requires oxygen, and nonphotodynamic, which does not. Reactions induced by porphyri n molecules, coal tar derivatives, and many drugs are photodynamic. The rea ction induced by psoralens, for the most part, is nonphotodynamic. Acute ph ototoxic reactions are characterized by erythema and edema followed by hype rpigmentation. Long-term ultraviolet phototoxicity results in chronic sun d amage and skin cancer formation. Also, certain chemicals such as psoralen m olecules and coal tar are photocarcinogenic. Phototoxic reactions to certai n drugs produce unusual clinical patterns, that is lichenoid eruptions, dys chromia, photo-onycholysis, and pseudoporphyria. Photoallergy is an uncommo n acquired altered reactivity dependent on an immediate antibody or a delay ed cell mediated reaction. Solar urticaria is an example of the former, whe reas photoallergy to exogenous chemicals is an example of the latter. Photo allergy to systemic drugs does occur but is difficult to characterize. The action spectrum for photoreactions to exogenous agents usually at least inc ludes the ultraviolet A rays for both phototoxicity and photoallergy. Copyr ight (C) 1999 by W.B. Saunders Company.