Polymorphic light eruption
This is the most frequent cause of a so-called ‘sun allergy’.
It is speculated that UVR causes a natural body chem-ical to change into an allergen. Mechanisms are sim-ilar to those in drug photoallergy.
Small itchy red papulovesicles or eczematous plaques arise from 2 h to 5 days, most commonly at 24 h, after exposure to UVR. The eruption is itchy and usually confined to sun-exposed areas (Fig. 16.8), remember-ing that some UVR passes through thin clothing.
The disorder tends to recur each spring after UVR exposure. Tanning protects some patients so that if the initial exposures are limited, few or no symptoms occur later. Such patients can still enjoy sun exposure and outdoor activities. Others are so sensitive, or their skin pigments so poorly, that fresh exposures con-tinue to induce reactions throughout the summer. These patients require photoprotection, and must limit their sun exposure and outdoor activities. The rash disappears during the winter.
Phototoxic reactions, photoallergic reactions, miliaria rubra, chronic actinic dermatitis, ordinary eczemas, allergic reactions to sunscreens and airborne allergic contact dermatitis should be considered.
It may be possible to reproduce the dermatitis by test-ing non-sun-exposed skin with UVB and UVA.
If normal tanning does not confer protection, sunscreens (Formulary 1) should be used. Protective clothing, such as wide-brimmed hats, long-sleeved shirts and long trousers, is helpful. In some patients, a 4-week PUVA course in the late spring can create enough tan to confer protection for the rest of the season. Moderately potent topical steroids (Formulary 1) usually improve the eruption. Hydroxychloroquine (Formulary 2) may be effective when used over the sunny season.