Fig. 28.1
Upper panel illustrates a young woman who reported short-lived eruption secondary to sun exposure. She was asked to reproduce the eruption before the next visit. Histological examination confirmed the diagnosis of acute photosensitive lupus erythematosus. Lower panel illustrates an elderly African American man with chronic actinic dermatitis secondary to persistent light reaction due to hydrochlorothiazide. The patient had involvement of the face and posterior neck as well. His UVA minimum erythema dose was 4 J
Introduction—Pathways to Photosensitivity
The way by which light plays a role in photo-eruptions varies among disorders. Some eruptions such as solar urticaria , polymorphous light eruption (PMLE) , photo-contact dermatitis , and photo-drug eruption, require a specific dose of ultraviolet exposure in order to induce the eruption. Without exposure to the necessary dose of ultraviolet light, patients may not manifest their disorder.
Other disorders, such as lupus erythematosus (LE) and dermatomyositis (DM) usually occur without a specific occasion of light exposure but are often exacerbated by ultraviolet light exposure. Occasionally, however, the first manifestation of lupus (especially subacute lupus) or dermatomyositis may follow an episode of excessive sun exposure.
Finally some disorders, specifically some forms of porphyria, which result from excessive levels of porphyrin in the skin, require chronically sun-exposed or sun-damaged skin in order to manifest skin lesions. Although the levels of porphyrin are similar throughout the skin, the manifestations of the disorder are limited to chronically sun-exposed sites. Ultraviolet light is required for interaction with porphyrin in the skin in order to result in a cascade of events that cause skin fragility and easy trauma-induced blistering.
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