Photosensitivity

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Cutaneous photosensitivity diseases may be idio-pathic, produced by endogenous photosensitizers (e.g. porphyrins) or associated with exogenous photosensitizers like drugs. The association of light and a drug can be responsible for acute inflammation of the skin. The photosensitivity reactions are divided into two types: phototoxicity and photoallergy (Gould et al., 1995).

Phototoxicity

Phototoxic disorders are not rare and always predictable. It can occur in any person who receives sufficient quantities of a phototoxic drug, together with the proper light exposure. The reaction results directly from photochemistry involving the skin. The association of light with a photosensitizing chemical in the skin creates an unstable singlet or triplet state within the electrons. This leads to the generation of reactive oxygen, which is responsible for cell damage.

Clinical manifestations usually present as an exaggerated sunburn occurring in sun-exposed areas only (Figure 34.3, between pp. 426 and 427). This is followed by hyperpigmentation. Photo-onycholysis and pseudoporphyria (blisters on sun-exposed parts of the limbs) are less common clinical forms.

Phototoxicity is histologically characterized by epidermal cell degeneration with necrotic keratino-cytes, oedema, sparse dermal lymphocytic infiltrate and vasodilatation. Phototoxicity is easily documented in vitro or in vivo. A photopatch test will be positive in all individuals and will therefore not be discriminant for a causality assessment. The minimal dose of UV (UVA more often than UVB) inducing an erythema will be decreased in all subjects during treatment.

Photoallergy

A photoallergic reaction is considered as a result of cell-mediated hypersensitivity. Ultraviolet radiation is required to convert a drug into an immunopathologically active compound (photoantigen) that induces the immune response.

Photoallergic eruption is more chronic than phototoxicity and is mainly eczematous and pruri-tic. A lichen planus-like reaction has also been reported. It is usually more marked in exposed sites, but may often progress outside these areas. In the chronic phase, erythema, scaling and lichenification predominate. Photoallergic reactions are usually transient and resolve after a variable length of time when the offending agent has been removed. Rarely, an extreme sensitivity to sun may persist for months or years ("persistent light reactors''). Photopatch testing is valuable when photoallergy is suspected. A multitude of drugs induce photoaller-gic reactions, including antibiotics (sulphonamides, pyrimethamine, fluoroquinolones), fragrances, NSAIDs, phenothiazine, thiazide diuretics, etc.

In phototoxic reactions, the treatment requires removal of the offending agent and/or avoidance of sun exposure. For a drug with a short elimination half-life, administration in the evening may be enough to decrease the risk below the clinical threshold. In photoallergy, drug withdrawal is recommended, because of the risk of worse reactions even with low UV doses. Topical cortico-steroid, systemic antipruritic agents may be useful.

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How To Deal With Rosacea and Eczema

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