Phototoxicity & Photoallergy
These photosensitivity (phototoxicity and photoallergy) disorders are related to the topical and/or systemic administration of drugs and as well as other chemicals. Both reactions (phototoxicity and photoallergy) require the absorption of energy by a drug or chemical resulting in the production of an excited-state photosensitizer that can transfer its absorbed energy to a bystander molecule or to molecular oxygen, thereby generating tissue-destructive chemical species.
Phototoxicity is a nonimmunologic reaction caused by drugs and chemicals. The clinical manifestations of phototoxicity include erythema (redness of skin) resembling a sunburn reaction that quickly “peels,” within several days. In addition there may also be edema, vesicles, and bullae.
Photoallergy is much less common in compare to phototoxicity and is distinct in that this is an immunopathologic (phototoxicity is a nonimmunologic reaction) process. The excited-state photo sensitizer may create highly unstable free radicals that bind covalently to macromolecules to form a functional antigen capable of evoking a delayed hypersensitivity response. The clinical manifestations of photoallergy typically differ from those of phototoxicity in that there is severe itching type dermatitis in photoallergy. This dermatitis predominate and evolves into lichenified, thickened, “leathery” changes in sun-exposed areas. A small subset (perhaps 5–10%) of patients with photoallergy may develop a persistent exquisite hypersensitivity to light even when the offending drug or chemical is identified and eliminated, a condition known as persistent light reaction.
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