Photodermatology and Pigmentary Disorders
Photobiology
UVB causes sunburn and direct DNA damage; UVA penetrates deeper and drives photoaging and drug photosensitivity. Know Fitzpatrick skin types and minimal erythema dose concepts.
Photodermatoses
Polymorphous light eruption (delayed, pruritic papules), solar urticaria (immediate), chronic actinic dermatitis, and porphyrias (blisters, urine porphyrins). Phototesting and photopatch testing help.
Pigmentary disorders
Vitiligo (depigmented macules, autoimmune association), melasma (sun-exposed, hormonal), postinflammatory hyperpigmentation, and albinism. Treatment includes photoprotection, topical steroids/calcineurin inhibitors, and narrowband UVB.
Traps
- Confusing melasma with postinflammatory hyperpigmentation; history of inflammation favors PIH.
- Missing porphyria because blisters are attributed to sun sensitivity alone.
- Overlooking drug-induced photosensitivity (thiazides, tetracyclines, NSAIDs).
Rule of thumb
For any photodistributed eruption, ask: timing (immediate vs delayed), drug history, and porphyrin screen when blisters occur.
Worked example
A young woman with pruritic papules on sun-exposed forearms each spring: diagnose polymorphous light eruption; treat with photoprotection and topical steroids.