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Photoallergic vs. Phototoxic contact:
Differentiating features of phototoxic and photoallergic contact reactions:
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Feature
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Phototoxicity
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Photoallergy
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Incidence (for a given chemical)
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High (i.e. universal)
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Low
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Amount of agent required for photosensitivity
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Large
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Small
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Onset after exposure to photosensitizer and light
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Minutes to hours
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24 – 72hrs
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Clinical characteristics
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Exaggerated sunburn
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Exczematous pattern (acute, subacute or chronic dermatitis)
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Distribution
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Exposed skin only
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Exposed skin; may spread to unexposed skin
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Pigmentary changes
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Frequent
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Unusual
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Histopathologic features
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Epidermal cell degeneration; dermal edema and vasodilatation; sparse mononuclear infiltrate
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Epidermal spongiosis; exocytosis of mononuclear cells; dermal mononuclear cell infiltrate
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Development of persistent light reaction
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No
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Yes
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Action spectra for most drugs/ chemicals
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UVA
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UVA
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Phototoxic contact dermatitis:
| • | universal, therefore don’t patch test (i.e. everyone will be positive) |
| • | “gin and tonic dermatitis” - limes or limon (but not really lemon) rind contains psoralens |
| • | PIPA with no history of prior inflammation: think about phototoxic contact |
Photoallergic contact dermatitis:
| • | diagnose by photopatch testing |
| • | e.g. musk ambrette (no longer in US), PABA, oxybenzone |
| • | ddx: airborne contact (airborne contact concentrates in skin folds vs. photo: spares eyelid, submental area, and folds) |
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