Contact dermatitis — branching viva

Examiner opening (Examiner)

You are the general paediatric registrar in the outpatient clinic. A nine-year-old girl with known atopic eczema is referred because an intensely itchy, well-demarcated band of eczema around her umbilicus keeps coming back whenever she wears her favourite jeans, despite her usual eczema creams. Talk me through your assessment and your working diagnosis. [5]

Exemplar opening (Candidate)

This is a classic distribution clue. A recurrent, intensely itchy, well-demarcated periumbilical dermatitis that tracks her clothing is allergic contact dermatitis to nickel from the metal button of her jeans until proven otherwise, on a background of atopic eczema. The key features are the recurrence at the same anatomical site, the clear relationship to a metal contact, the itch rather than soreness, and the failure of her usual eczema therapy — all of which point away from a simple flare of her atopic eczema. I will take a careful exposure history covering her jewellery, belt buckles and buttons, examine the whole skin including the earlobes for any similar reaction, and arrange patch testing to confirm the allergen. [5]

Branch 1 — irritant versus allergic (Examiner)

What is the difference between irritant and allergic contact dermatitis, and why does it matter here? [5]

Exemplar (Candidate)

Irritant contact dermatitis is a direct, non-immunological injury to the skin barrier by a toxic agent; it is concentration- and duration-dependent, occurs on first contact if the irritant is strong enough, stays confined to the contact site, and tends to sting or burn rather than itch. Allergic contact dermatitis is a delayed type IV hypersensitivity reaction requiring prior sensitisation, so it never occurs on first exposure; it is triggered by tiny amounts of allergen once sensitised, is intensely itchy, and may spread beyond the contact site. It matters because the management differs — both need removal of the cause and barrier repair, but only allergic disease benefits from patch testing, allergen identification and long-term avoidance counselling. This girl's recurrent, intensely itchy, distribution-clued eruption fits allergic disease. [5]

Branch 2 — the patch test (Examiner)

You decide to patch test her. Describe the procedure, the timing of the readings, and how you interpret the result. [1]

Exemplar (Candidate)

Patch testing detects delayed type IV allergy, not type I immediate allergy. The allergens are applied to her upper back in standardised chambers held under occlusive tape, left in place for 48 hours, then removed and read. The decisive reading is at 72 to 96 hours, because delayed reactions take time to declare, and a further reading around day seven catches late reactions, particularly to metals such as nickel and to antibiotics. A positive reaction is a localised eczematous response graded from doubtful erythema through papular, vesicular and confluent reactions. I then assess relevance — whether the allergen explains her current and past dermatitis — because a positive without relevance does not make the diagnosis. I avoid testing on inflamed or sun-exposed skin and while her back is under topical or systemic corticosteroid, and I use a tailored paediatric baseline series with additional allergens as her history dictates. [1]

Branch 3 — pitfalls in interpretation (Examiner)

Her patch test comes back with several positives. What are the pitfalls, and how do you avoid them? [1]

Exemplar (Candidate)

Several positives raise two pitfalls. The first is excited skin, or angry back, syndrome, in which one strongly positive reaction inflames the surrounding skin and generates a crop of false positives; the response is to wait for the skin to settle and repeat the individual allergens in isolation. The second is over-interpreting a positive without relevance — a positive patch test is only clinically meaningful if it explains her dermatitis now or in the past, so each positive must be matched to her history and exposures. I would also guard against reading too early, which produces false negatives, and testing on steroid-suppressed skin. A clear, relevant positive to nickel, with the rest non-relevant, would confirm the diagnosis and drive her avoidance plan. [1]

Branch 4 — the atopic connection (Examiner)

Her atopic eczema is otherwise well controlled. Does her atopy change your thinking about contact allergy? [4]

Exemplar (Candidate)

Yes, and in the opposite direction from the old teaching. The older view that atopy protected against contact allergy has been overturned by systematic reviews showing that children with atopic dermatitis have a higher prevalence of contact allergy, because their compromised barrier allows easier penetration of allergens and because they are exposed to many topical products that can sensitise. So atopy raises rather than lowers my suspicion, and a recurrent, same-site or treatment-resistant eczema in an atopic child is a clear indication to patch test. The lesson is that atopic eczema and contact dermatitis coexist, and the failure of standard therapy is a signal to look for a contactant. [4]

Branch 5 — management (Examiner)

How do you manage her once you have confirmed nickel allergy? [2]

Exemplar (Candidate)

Management rests on three pillars: remove the cause, repair the barrier, and reduce inflammation. She avoids nickel-releasing items — she covers or replaces the jeans button, avoids costume jewellery and buckles, and learns to read product and metal labels — and I give the family a written nickel avoidance card. For the barrier she uses fragrance-free emollients generously and frequently, and for the inflammation I apply a potency-matched topical corticosteroid, a moderate preparation for the trunk, once or twice daily until the dermatitis settles, then step down. The long-term cure is sustained avoidance, so I reinforce the plan at follow-up and safety-net for the warning signs of secondary infection. Most children do very well with this combination. [2]

Branch 6 — a different scenario (Examiner)

A six-month-old infant presents with glazed erythema over the convex buttock surfaces that spares the folds, with a few satellite pustules. What is this and how do you manage it? [5]

Exemplar (Candidate)

This is irritant napkin (diaper) dermatitis with probable candidal superinfection. The distribution over the convex surfaces that spares the folds reflects direct contact with urine and faeces — the proteases and lipases of which damage the stratum corneum — at the points of maximal contact, while the skin folds are protected by skin-on-skin apposition; the satellite pustules indicate candidal extension into the folds. Management combines frequent nappy changes, gentle cleansing with water rather than alkaline soaps, air exposure, and a barrier cream such as zinc oxide or petrolatum, with a topical antifungal such as clotrimazole for the candidal component. A mild topical corticosteroid such as hydrocortisone is used only if inflammation is severe, and sparingly because of the high surface-area-to-mass ratio. The diagnosis is clinical and I safety-net the parents for secondary bacterial infection. [5]

Examiner wrap-up (Examiner)

Thank you. Summarise the three points you most want the examiner to remember. [5]

Exemplar (Candidate)

First, the distribution is the key to the cause — a recurrent periumbilical or earlobe dermatitis is nickel allergic contact dermatitis until proven otherwise. Second, patch testing detects type IV allergy, with allergens under occlusion for 48 hours, a decisive read at 72 to 96 hours and a day-seven read for late reactions, and every positive must be assessed for relevance. Third, manage in three steps — remove the cause, repair the barrier, and reduce inflammation with a potency-matched topical corticosteroid — and remember that atopy raises rather than lowers the risk of contact allergy, so a resistant atopic eczema is an indication to patch test. [5] [4]