The essential difference between allergic contact dermatitis and photoallergic contact dermatitis is that the latter requires ultraviolet A (UVA) radiation in addition to a light-sensitizing topical agent in order to provoke an allergic attack. Since UVA penetrates glass, a susceptible individual may be exposed while driving, while sitting indoors next to a window, and even while wearing a sunscreen that has inadequate UVA protectants. Some well-known photoallergens include certain colognes, aftershaves, and perfumes (that have, for example, musk ambrette, sandalwood oil, and 6-methyl coumarin); Persian lime rind; diphenhydramine (for example, Benadryl spray); epoxy resins; and halogenated salicylanalides (antibacterial and antifungal agents). Nowadays, ironically, sunscreens containing PABA, PABA esters, or oxybenzone are the leading causes of photocontact dermatitis. And since sunscreen ingredients are also commonly found in moisturizers and makeup, these products must be kept in mind when looking for potential sources of photo-contact allergies.
In a small percentage of people with photoallergies contact dermatitis, the disease may progress to the point where they continue to develop photoallergic reactions to sunlight in the absence of further contact with the topical agent that originally triggered it. When this occurs, dermatologists refer to the condition as persistent light reaction.
Although protected areas of skin are occasionally involved, not surprisingly the sun-exposed areas of the body are the favored sites for photocontact allergy. Nonexposed areas, such as the upper eyelids, the skin behind the ears, and the neck folds, are typically spared. Adults are much more commonly affected than children. Possible allergic reactions include redness and severe blistering, although eczema is the most common manifestation.
While the typical history and clinical examination may suggest the diagnosis, photopatch testing establishes it. In this test duplicate amounts of the suspected topical photoallergens are applied to the skin. After twenty-four hours one set of allergens is irradiated with UVA, and at forty-eight and seventy-two hours the irradiated and non-irradiated sides are examined. Reproducing the allergic skin rash at the irradiated site confirms the diagnosis.
Therapy for photocontact dermatitis is identical to that used in ordinary allergic contact dermatitis. The mainstays include oral antihistamines and topical and oral corticosteroids. Except in cases of persistent light reaction, avoidance of the photoal-lergen(s) will prevent recurrences. Sun protection measures are crucial for controlling persistent light reactions.
