Key Facts

Contact dermatitis is common and underdiagnosed. Distinguishing ICD from ACD requires patch testing. Identifying and avoiding the causative agent is the single most effective intervention.

Epidemiology

• Lifetime prevalence: ~15–20% of general population
• Accounts for ~5–7% of dermatology referrals
• Occupational dermatitis: most common occupational skin disease
• Healthcare workers, hairdressers, food handlers most affected

Aetiology

ICD triggers: water, detergents, solvents, friction, alkalis, acids, wet work ACD common allergens: nickel (jewellery, belt buckles), chromate (cement), PPD (hair dye), fragrances, preservatives (MI/MCI), rubber chemicals, colophony (adhesives, plasters), epoxy resin

Pathophysiology

ICD: direct cytotoxic damage to keratinocytes → innate immune response → inflammation. No sensitisation required. Damage is dose-dependent. ACD: hapten penetrates skin → processed by Langerhans cells → presented to T-cells in draining lymph nodes → sensitisation (1–2 weeks). Re-exposure → T-cell mediated type IV hypersensitivity → eczematous reaction at 24–72 hours.

Irritant Contact Dermatitis

• Well-demarcated erythema at contact site
• Dryness, fissuring, glazed appearance
• Burning/stinging more than itch
• Hands most commonly affected (wet work)
• Onset: immediate or cumulative with repeated exposure

Allergic Contact Dermatitis

• Eczematous: erythema, vesicles, weeping, crusting
• Pruritus is prominent
• Distribution reflects contactant (e.g. wrist = watch/nickel; periorbital = nail varnish via transfer)
• Onset: 24–72 hours after exposure (delayed type IV)
• May spread beyond contact site (id reaction)

Red Flags

• Occupational pattern (worse at work, improves on holiday)
• Distribution suggesting specific allergen
• Failure to respond to treatment (unidentified allergen/irritant)

Patch Testing (for ACD)

• Gold standard: European baseline series (28+ allergens) + extended series based on occupation/exposure
• Method: allergens applied to back in Finn chambers for 48 hours; read at 48 hours and 96 hours
• Grading: +, ++, +++ (erythema through to vesicles/bullae); IR = irritant reaction
• Stop topical steroids 1 week before, oral steroids 4 weeks before

Additional

• Skin biopsy: rarely needed; shows spongiotic dermatitis (indistinguishable from atopic eczema)
• IgE (total/specific): to exclude atopic component
• RAST/skin prick testing: for type I latex allergy (NOT for type IV ACD)

Allergen/Irritant Avoidance

• Identify and avoid causative agent — most important step
• Substitution: alternative materials, products
• Protective equipment: gloves (but avoid latex if latex-allergic; use nitrile)
• Occupational health: workplace assessment, RIDDOR reporting

Pharmacological

• Emollients: foundation of care (as per eczema)
• Topical corticosteroids: potent (betamethasone valerate 0.1%) for body; mild (hydrocortisone 1%) for face/flexures
• Topical calcineurin inhibitors: for face/sensitive areas
• Severe acute ACD: short course oral prednisolone 0.5mg/kg for 5–7 days (e.g. severe poison ivy/plant dermatitis)

Referral Criteria

• Dermatology: for patch testing if ACD suspected
• Occupational health: suspected occupational dermatitis
• Urgent: severe widespread ACD not responding to topical treatment

• ACD: excellent if allergen identified and avoided — complete resolution expected
• ICD: improves with irritant avoidance; chronic exposure leads to persistent disease
• Occupational dermatitis: ~25% change occupation; ~50% have persistent symptoms at 5 years
• Poor prognostic factors: continued exposure, hand involvement, atopic background

ICD vs ACD Comparison

Common Allergens and Sources