Photoallergic contact dermatitis.

Photoallergic contact dermatitis (PCD) is a delayed-type hypersensitivity cutaneous reaction in response to a photoantigen applied to the skin in subjects previously sensitized to the same substance. For the development of PCD, irradiation with ultraviolet (UV) radiations, usually UVA, is required to create a complete antigen, and the culprit substance needs to be within the skin at the time of UVA exposure. The incidence of PCD in the general population is unknown and is considered uncommon. Epidemiological data have been obtained from studies performed using photo-patch tests in patients with suspected photodermatoses in tertiary care outpatient units. Prevalence of PCD over time has been also dependent on prescription and/or usage patterns of potential photosensitizers, and was particularly high in the past, causing mini-epidemics in some countries because of the widespread use of halogenated salicylanilides and other photosensitizing compounds. Many topical substances with photosensitizing properties are currently available, with the most important ones being sunscreen agents and nonsteroidal antiinflammatory drugs. The diagnosis of PCD, in patients with a history of photosensitivity and especially with an eczematous form of photodermatosis, should be confirmed by photo-patch testing.