SummaryMany -topically applied or systemically administered -substances may trigger photoallergic skin reactions following exposure to ultraviolet (UV) radiation. While most cases represent photoallergic contact dermatitis due to topically applied agents, systemically triggered photoallergy is considerably less common. The photopatch testa UV-exposed variant of the conventional patch test -is the mainstay in the diagnosis of photoallergic or phototoxic reactions.
IntroductionSun exposure may cause a variety of skin diseases. A distinction is made between primary and secondary photodermatoses, with the former further subdivided into two types. In idiopathic photodermatoses, such as polymorphic light eruption, there is no known photosensitizer or none is present. The second type of primary photodermatoses includes cutaneous disorders resulting from a combination of UV exposure and a photosensitizer. Exposure to photosensitizers may cause phototoxic or photoallergic reactions [1].Photosensitizers are substances that lead to photobiologic reactions after UV exposure, showing either toxic or allergic reaction patterns. Cutaneous manifestations in photoallergic reactions are predominantly eczematous lesions. Photopatch testing is the diagnostic mainstay. In simplified terms, this test corresponds to a duplicate regular patch test divided into a UV-exposed and a non-UV-exposed half. Other tests, especially photoprick, photoscratch, or UV-exposed intracutaneous tests, are far less common in everyday clinical practice [2].The type of UV exposure is crucial in the majority of photodermatoses. While UVB exposure (wavelength: 280-320 nm) causes solar dermatitis 12-24 hours after exposure, it is UVA exposure (wavelength: > 320 nm) that is almost exclusively responsible for phototoxic and photoallergic reactions [1].
PathophysiologyPhotoallergic and phototoxic reactions inevitably require a topical or systemic photosensitizer. There are about 400 known substances with phototoxic or