ESSENTIALS OF DIAGNOSIS
Painful or pruritic erythema, edema, or vesiculation on sun-exposed surfaces (face, neck, hands, and “V” of the chest).
Inner upper eyelids and area under the chin are spared.
Photodermatitis is a cutaneous reaction to UV radiation. It comprises four groups: (1) primary, idiopathic immunologically mediated photodermatoses; (2) drug- or chemical-induced photodermatoses; (3) dermatoses that are worsened or aggravated by UV exposure; and (4) genetic diseases with mutations predisposing to photodermatitis.
Primary photodermatoses include polymorphic light eruption, chronic actinic dermatitis, and actinic prurigo. Drug- or chemical-induced photodermatitis may be either exogenous or endogenous in origin. Porphyria cutanea tarda and pellagra are examples of endogenous phototoxic dermatoses. Exogenous drug- or chemical-induced photodermatitis manifests either as phototoxicity (a tendency for the individual to sunburn more easily than expected) or as photoallergy (a true immunologic reaction that presents with dermatitis). Drug-induced phototoxicity is triggered by UVA. Contact photosensitivity may occur with plants, perfumes, and sunscreens. The sunscreen oxybenzone (a benzophenone) is a common cause of photoallergic dermatitis. Dermatoses that are worsened or aggravated by UV exposure include SLE and dermatomyositis. Three percent of persons with atopic dermatitis, especially middle-aged women, are photosensitive.
The acute inflammatory phase of phototoxicity, if severe enough, is accompanied by pain, fever, GI symptoms, malaise, and even prostration. Signs include erythema, edema, and possibly vesiculation and oozing on exposed surfaces. Peeling of the epidermis and pigmentary changes often result. The key to diagnosis is localization of the rash to photoexposed areas, though eruptions may become generalized with time to involve photoprotected areas (eFigure 6–104). The lower lip may be affected.
A: Polymorphous light eruption on the arm of a young man. Note the sparing of the skin under his watchband. B: A photoallergic drug reaction characterized by widespread eczema in the photodistribution areas such as the face, upper chest, arms, and back of hands. A punch biopsy showed a spongiotic dermatitis. The exact photoallergen was not found. (Reproduced with permission from Richard P. Usatine, MD, in Usatine RP, Smith MA, Mayeaux EJ Jr, Chumley HS. The Color Atlas and Synopsis of Family Medicine, 3rd ed. McGraw-Hill, 2019.)
Blood and urine tests are generally not helpful unless porphyria cutanea tarda is suggested by the presence of blistering, scarring, milia (white cysts 1–2 mm in diameter) and skin fragility of the dorsal hands, and facial hypertrichosis. Eosinophilia may be present in chronic photoallergic responses.
The differential diagnosis is long. If a clear history of the use ...