Contact Dermatitis
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Direct answer
Contact dermatitis is eczema produced by external agents acting on the skin, in two immunologically distinct forms: irritant contact dermatitis, a direct toxic effect of substances such as detergents, alkalis, solvents and wet work that can affect anyone on first exposure, and allergic contact dermatitis, a delayed type IV hypersensitivity reaction in a previously sensitised person to agents such as nickel, chromates, rubber chemicals, fragrances and plants. The morphology ranges from acute vesiculation and weeping to chronic lichenified plaques confined to or centred on the contact area. Patch testing is the gold standard investigation for the allergic type, and complete allergen avoidance is the definitive treatment.
What you must remember
- Irritant contact dermatitis: occurs on first exposure without sensitisation, is dose-dependent, hurts more than it itches, and has sharply demarcated boundaries matching the irritant contact — hands of houseworkers, cement burns in masons, napkin dermatitis in infants.
- Allergic contact dermatitis: requires prior sensitisation (10-14 days), appears 24-72 hours after re-exposure, itches intensely, may spread beyond the contact site, and shows spongiotic eczema histologically with a positive allergic reaction on patch testing.
- Patch testing: standardised allergens in Finn chambers applied on the back, occluded for 48 hours, then read at 48 and 72 (often 96) hours; a positive reaction is eczematous and graded by intensity — this is the investigation of choice for allergic contact dermatitis, unlike prick testing which detects type I allergy.
- Classical allergen-source patterns: nickel — earlobes, wrist watch, jeans button, umbilicus; potassium dichromate — cement (masons) and leather (footwear); rubber accelerators — gloves, elastic and shoes; fragrance and preservatives — cosmetics; paraphenylenediamine — black hair dye and temporary tattoos; epoxy resin — adhesives and industrial occupations.
- Indian scenario: Parthenium hysterophorus (congress grass) is the leading cause of airborne allergic contact dermatitis in India, producing chronic lichenified dermatitis of the face, neck, V of chest and forearms, often with cross-reactions to Compositae plants.
- Shoe dermatitis (chromate or rubber) involves the dorsa of feet and toes but characteristically spares the web spaces — the reverse of tinea pedis, which starts in the fourth web space.
- Treatment: identify and avoid the allergen, protective barriers such as gloves and barrier creams, topical corticosteroids appropriate to site, emollients for the disrupted barrier, and a short course of systemic corticosteroids for severe widespread reactions; occupational cases deserve workplace modification.
Common confusion
The examiner's favourite discrimination is irritant versus allergic contact dermatitis. Irritant dermatitis affects many exposed workers from the first day, is painful, sharply bordered and heals on protection; allergic dermatitis affects only sensitised individuals, is delayed, itchy, may spread beyond contact and is confirmed by patch testing. A second classic trap is shoe dermatitis versus tinea pedis — web-space sparing points to shoes. Finally, airborne contact dermatitis due to parthenium mimics chronic photosensitive dermatitis of the face; the clue is that airborne dermatitis involves the upper eyelids and retro-auricular folds — sites shielded from light but not from dust.
Exam-focused takeaway
Questions typically pair an occupation or object with a likely allergen — mason with cement (chromate), hair-dye user with paraphenylenediamine, nurse with rubber gloves, farmer with parthenium. Patch test methodology (48 hours occlusion, 72-hour reading, Finn chamber) and its interpretation is a recurring one-liner. Expect clinical photographs of well-demarcated eczema matching a garment or watch strap, asking whether it is irritant or allergic, and single-best-answer stems on the first step after diagnosis — allergen elimination, not stronger steroids.
Frequently asked questions
How do irritant and allergic contact dermatitis differ?
Irritant dermatitis is a direct, dose-dependent toxic reaction possible in anyone from first exposure and is painful with sharp margins; allergic dermatitis is a cell-mediated type IV reaction in a sensitised person, delayed by 24-72 hours, intensely itchy and capable of spreading beyond the contact area.
What is patch testing and when is it read?
Standardised concentrations of allergens are applied in Finn chambers to intact skin on the back under occlusion for 48 hours, and reactions are graded at 48 and 72 (often 96) hours; eczematous reactions identify clinically relevant allergens.
Which allergen is most often implicated in jewellery dermatitis?
Nickel, producing itchy eczema under earrings, watches, spectacle frames and jeans buttons; it is among the commonest positive patch test allergens worldwide, including in Indian series.
Why does cement dermatitis occur in masons?
Wet cement is highly alkaline and irritant, and it contains hexavalent chromium which is a potent allergen, so masons develop both irritant and allergic contact dermatitis, including cement burns.
What is airborne contact dermatitis?
Allergic dermatitis to particles such as parthenium pollen settling on exposed skin, mimicking photosensitivity but involving shielded areas like eyelids and retro-auricular skin; it is the commonest type of plant contact dermatitis in India.
How is contact dermatitis from shoes distinguished from tinea pedis?
Shoe dermatitis affects dorsa of toes and feet with web-space sparing, may be asymmetric and patch-test positive, whereas tinea pedis begins in the interdigital spaces, is scaly with an active edge, and shows fungal hyphae on potassium hydroxide mount.