Systemic photoallergic contact dermatitis

Photodermatitis is a cutaneous reaction caused by the interaction between UV radiation and visible light with chemical compounds that may reach the skin by systemic or topical means. Based on the pathogenetic mechanism such reactions are classified into phototoxic and photoallergic. The former have no immunological bases and are the consequence of the combined action of sunlight and phototoxic substances. The latter have a lower incidence since caused by type IV hypersensitivity along with a series of pathogenetic factors. In particular, allergic photocontact dermatitis is provoked by the application of sensitizing substances with sunlight, leading to the formation of new allergens that are the targets of the immunological response. A peculiar subtype of photoallergy is the one that may be secondary to the systemic administration or the transdermal absorption of specific drugs. This form, named systemic photoallergic contact dermatitis, may present clinical signs similar to those due to other dermatitis such as drug-induced phototoxicity. Therefore diagnosis of systemic photoallergic contact dermatitis is still a matter of discussion, even more so because the necessary diagnostic procedures are invasive, potentially harmful and impractical. The aim of this review is to analyze the few reported cases of systemic photoallergy in the literature, focusing on the peculiar features that can lead the physician to the diagnosis of this rare dermatitis. © Monte Meru Editrice.