Photoallergic Dermatitis

Photoallergy is increased reactivity of skin to ultraviolet and visible light and is brought about by a chemical agent on an immunologic basis . The eruption of photoallergic dermatitis may be due to topical application of, or oral ingestion of, a photosensitizing agent, resulting in a photocontact dermatitis or photodrug eruption, respectively. Agents that may elicit photocontact dermatitis include soaps and cleansers (containing halogenated salicylanilide), perfumes (such as musk ambrette), topical sulfonamides, sunscreens, benzocaine, and diphenhydramine. Common causes of photodrug eruption include thiazide diuretics, oral hypoglycemics, and phenothiazines. The eruption is pruritic and composed of erythematous papules and confluent plaques on sun-exposed skin, usually the face, dorsal aspect of the arms, and the "V" of the neck.

Histopathology.

The features are similar to that of an acute allergic contact dermatitis, showing variable spongiosis, in some cases with vesiculation, and a superficial perivascular Iymphohistiocytic infiltrate with exocytosis. A deeper perivascular infiltrate and eosinophils are more common in photoallergic dermatitis induced by systemic ingestion of medications. With chronic exposure to the antigen, there is progression to chronic dermatitis, with diminished spongiosis, less intense inflammation, and more acanthosis. Phototoxic reactions are "sunburn" reactions with epidermal apoptosis and necrosis with intraepidermal to subepidermal blisters-the changes paralleling the degree of damage. Neutrophils may be prominent.