Phototoxic and photoallergic reactions

Abstract

An exogenous substance may cause photosensitivity by phototoxic or photoallergic mechanisms, or by inducing a dermatosis which is exacerbated by exposure to ultraviolet (UV) radiation (Table 1). Phototoxicity is commoner than photoallergy, and is distinguished from it by the lack of an immunological basis. The characteristics of these two reaction patterns are shown in Table 2.However, it must be recognized that all attempts to classify substances causing these reactions are partly arbitrary; in particular, many agents are capable of producing photosensitivity by multiple and unique mechanisms, with corresponding differences in clinical presentation. Phototoxicity may be due to systemically administered agents (usually drugs), or contact with substances (most commonly plants). Photoallergy is almost always due to topically applied substances (including sunscreens). Mechanisms of phototoxicity and photoallergy are discussed in greater detail in Chap. 6. With regard to photoallergy, its predisposing factors and prevalence, individual photoallergens and the investigation of suspected photoallergy are described in the chapter on photopatch testing (Chap. 27).