photoallergy

Photoallergy is an immune-mediated skin reaction that occurs when a chemical applied to the skin or ingested is altered by ultraviolet or visible light to form a photoactivated compound that elicits an immune response in a person who has become sensitized. The reaction is typically a type IV hypersensitivity and usually requires prior sensitization, meaning it does not occur on first exposure. Clinically, photoallergic dermatitis presents as an itchy, eczematous eruption on sun-exposed areas, often developing 24 to 72 hours after sun exposure in the presence of the photosensitizer. Lesions may include redness, swelling, vesicles, and oozing with crusting, and may be accompanied by a burning sensation. The distribution commonly involves the face, neck, and backs of hands.

Common causes include certain drugs and topical agents that become photoactivated. Systemic medications such as tetracyclines, fluoroquinolones, thiazide diuretics, and some nonsteroidal anti-inflammatory drugs, as well as certain sulfonamides, have been implicated. Topical agents such as neomycin, fragrance components, balsam of Peru, and some sunscreens with specific UV-absorbing compounds can also act as sensitizers. The diagnosis is supported by history and can be confirmed with photopatch testing, which combines patch testing with controlled UVA exposure to identify true photoallergens.

Treatment centers on removing the offending substance and protecting the skin from light exposure. Moderate to severe cases may benefit from topical corticosteroids; antihistamines can ease itch, and systemic steroids are reserved for extensive or unresponsive disease. Prevention involves avoiding known photosensitizers, choosing non-sensitizing products, and using sun protection. Photoallergy is relatively rare compared with phototoxic reactions and generally resolves after the offending agent is discontinued.