Contact Dermatitis, Occupational Dermatology, and Photodermatology
Contact dermatitis
Distinguish irritant (direct toxicity, no prior sensitisation, burning, sharply demarcated) from allergic (type IV delayed hypersensitivity, requires sensitisation, itching, spreading, eczematous). Common allergens: nickel, cobalt, chromate, fragrances, preservatives (methylisothiazolinone), rubber accelerators, paraphenylenediamine in hair dye, and topical medicaments. Patch testing is the gold standard; use standard series plus targeted series, read at 48 and 96 hours, and interpret relevance.
Occupational dermatology
Occupational contact dermatitis is the most common occupational skin disease. Recognise patterns by occupation: hairdressers (PPD, nickel, wet work), construction (chromate in cement), healthcare (gloves, disinfectants), and food handlers. Prevention: gloves, barrier creams, substitution, and workplace assessment.
Photodermatology
Learn the action spectrum and features of polymorphic light eruption, solar urticaria, chronic actinic dermatitis, photoallergy (usually topical agents), and phototoxicity (drugs such as tetracyclines, thiazides, amiodarone). Porphyrias (porphyria cutanea tarda) cause fragility and blisters in sun-exposed skin. Photoprotection and phototesting are management anchors.
Traps
- Calling every contact dermatitis allergic - irritant is more common.
- Missing photoallergy when dermatitis is confined to light-exposed sites.
- Patch test reading too early and missing delayed positives.
Rule of thumb
Distribution first: hands and face suggests airborne or hand-transferred allergen; eyelids and genitalia are thin-skin sites that react to low-potency allergens.