Exogenous Photosensitivity Disorders

These disorders result from topical application or ingestion of an agent that causes the skin to be more sensitive to ultraviolet light. Photosensitivity disorders may be phototoxic or photoallergic. The clinical differences are subtle and do not impact on acute management of a patient; therefore, the reader is referred to a reference or dermatology textbook for further information.

Topical photosensitizers usually result in a cutaneous eruption at their sites of application. Ultraviolet exposure is necessary for the eruption to occur. Furocoumarins are the most common group of agents causing topical photoeruptions. Lime juice applied to the skin, fragrances, figs, celery, and parsnips are examples of furocoumarins. Other topically applied agents causing photosensitivity include PABA esters, topical psoralens, musk ambrette, and salicylanilide antibacterials. The typical clinical eruption is a severe sunburn-like reaction, often with vesiculation. Often a linear appearance suggests that an externally applied substance is the culprit.

Numerous ingested substances can result in a photosensitivity eruption as well. T.ab.!.e 2.3.8.-1 lists the most common offending agents. Because these agents are ingested and distributed throughout the body, the eruption involves all sun-exposed areas. The characteristic distribution of a photosensitivity eruption is the face, posterior neck, dorsal hands, and extensor arms. Certain areas including the creases of the eyelids, the upper lip, the V of the anterior neck, and the posterior auricular neck are spared.

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