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 ultraviolet radiation. Photodermatitis is classified into four groups: (1) primary photodermatoses that are immunologically mediated but are idiopathic in etiology; (2) drug- or chemical-induced photodermatoses; (3) dermatoses that are worsened or aggravated by ultraviolet 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 ultraviolet exposure include systemic lupus erythematosus 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, gastrointestinal 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 these eruptions may become generalized with time to involve even photoprotected areas (eFigure 6–93). The lower lip may be affected.
Photodistribution. V of the chest and arms. (Reproduced, with permission, from Bondi EE, Jegasothy BV, Lazarus GS [editors]. Dermatology: Diagnosis & Treatment. Originally published by Appleton & Lange. Copyright © 1991 by The McGraw-Hill Companies, Inc.)
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 of a topical or systemic photosensitizer is not available and if the eruption is persistent, then a workup including biopsy and light testing may be required. Photodermatitis must be differentiated from contact dermatitis that may develop from one of the many substances in suntan lotions and oils, as these may often have a similar distribution. Sensitivity ...