Contact dermatitis — formative SAQs

SAQ 1 (10 marks)

A nine-year-old girl with well-controlled atopic eczema is referred because an intensely itchy, well-demarcated band of eczema around her umbilicus has recurred three times over six months, each time settling with topical corticosteroid and returning within days of wearing her favourite jeans. Her mother is frustrated and the girl is missing school. ^[6]

1. Give the most likely diagnosis and the mechanism, and contrast it with an irritant dermatitis in terms of timing, symptom quality and spread. (3) ^[6]
2. Describe the investigation you would arrange, the procedure, the timing of its readings, and the principles of interpretation. (4) ^{[1] [3]}
3. Outline the definitive management and the advice you would give the family. (3) ^{[2] [6]}

Model answer — SAQ 1

(1) Diagnosis and mechanism (3). The most likely diagnosis is allergic contact dermatitis to nickel, released from the metal button of her jeans, on the background of atopic eczema. It is a delayed type IV hypersensitivity reaction: she was sensitised in a prior induction phase in which the nickel hapten was taken up by Langerhans cells and presented to naive T-cells in the draining lymph node, generating memory T-cells, and on re-exposure those memory T-cells elicit the eczematous inflammation within 24 to 72 hours. Unlike irritant dermatitis, it requires prior sensitisation, never occurs on first exposure, is triggered by tiny amounts of allergen, and may spread beyond the contact site; it is intensely itchy rather than stinging, and the periumbilical distribution is the classic nickel clue. ^[6]

(2) Investigation, procedure and interpretation (4). The investigation is epicutaneous patch testing, the gold standard for allergic contact dermatitis, which detects type IV and not type I allergy. The allergens are applied to the 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, with a further reading around day seven to catch late reactions, particularly to metals and antibiotics. A positive reaction is a localised eczematous response graded from doubtful erythema through papular and vesicular reactions. Interpretation demands assessment of relevance — whether the allergen explains her current or past dermatitis — because a positive patch test without relevance does not by itself make the diagnosis. Testing is avoided on inflamed or sun-exposed skin and while the back is under topical or systemic corticosteroid, and a tailored paediatric baseline series is used. ^{[1] [3]}

(3) Management and family advice (3). Management has three pillars: identify and remove the cause, repair the barrier, and reduce inflammation. She should avoid nickel-releasing items — cover or replace the jeans button, avoid costume jewellery and buckles — and use fragrance-free emollients and a potency-matched topical corticosteroid, a moderate preparation for the trunk, until the dermatitis settles. I would give the family a written avoidance card listing nickel sources, advise on reading product labels, and safety-net for the warning signs of secondary infection. The long-term cure of allergic contact dermatitis is sustained avoidance, so reinforcement of the avoidance plan at follow-up is central. ^{[2] [6]}