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Photosensitivity may be caused by exogenous or endogenous agents. It occurs when a compound, classically one with unsaturated double bonds in a six-carbon ring, absorbs radiation energy in its action spectrum, usually ultraviolet A (UVA) wavelengths. Exogenous photosensitizers can be agents administered systemically or applied topically. Well-characterized examples of photosensitivity induced by endogenous photosensitizers are the cutaneous porphyrias, which are associated with enzymatic defects in heme biosynthetic pathways that result in elevated levels of porphyrins, known phototoxic agents (see Chapter 132).
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Photosensitivity induced by exogenous agents can be divided into phototoxicity and photoallergy. Phototoxicity is the result of direct tissue injury caused by the phototoxic agent and radiation. It can occur in all individuals exposed to adequate doses of the agent and the activating wavelengths of radiation (Table 92-1). In contrast, photoallergy is a type IV delayed hypersensitivity response to a molecule that has been modified by absorption of photons. It has a sensitization phase, occurs only in sensitized individuals, and requires only a minimal concentration of the photoallergen (see Table 92-1).
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Over 350 medications in the United States have been reported to cause photosensitivity.1 Only a small number of them, however, induce reactions frequently or have been well studied (Tables 92-2, 92-3, 92-4, and 92-5). In evaluations performed at photodermatology centers in New York City, Melbourne, ...