Photodermatoses — expanded notes
Action spectrum and mechanisms
UVA penetrates more deeply and contributes to photoageing and many photosensitivity reactions. UVB is strongly erythemogenic and produces direct DNA photoproducts. Visible light may worsen pigmentary disease. Phototoxicity is direct non-immune cellular injury; photoallergy is delayed T-cell-mediated hypersensitivity to a photoactivated chemical.
Polymorphic light eruption
PLE typically develops hours to days after sun exposure, especially in spring and early summer. It causes itchy papules, papulovesicles or plaques on exposed sites. Morphology may be polymorphic between patients but relatively consistent within an individual. It often improves with gradual seasonal exposure (hardening). Management includes photoprotection, topical corticosteroids for flares and supervised phototherapy in recurrent cases.
Drug-induced photosensitivity
Review prescribed and over-the-counter medicines. Phototoxic associations include tetracyclines, fluoroquinolones, thiazides, amiodarone, voriconazole and some NSAIDs. Phototoxic reactions resemble exaggerated sunburn in exposed areas. Photoallergic reactions are eczematous and may extend beyond exposed skin. Do not stop essential medication without prescriber review.
Porphyrias and other important conditions
Porphyria cutanea tarda causes skin fragility, erosions, milia and bullae on dorsal hands, often with hypertrichosis and hyperpigmentation; investigate porphyrins and associated risk factors such as iron overload, hepatitis C and alcohol. Erythropoietic protoporphyria causes painful burning and oedema after light exposure, often without blistering. Chronic actinic dermatitis causes severe chronic eczematous photodistributed disease. Solar urticaria causes rapid wheals within minutes.
Investigation and treatment
Use phototesting/photopatch testing in specialist settings when diagnosis is uncertain. Investigations may include porphyrin studies or autoimmune serology guided by clinical features. Recommend broad-spectrum SPF 50+, UVA protection, clothing, shade and review of photosensitising agents. Treat the underlying condition and inflammation.