Atopic dermatitis and the atopic march — short-answer questions

This stem concerns a classic infantile atopic dermatitis presentation in a child with a strong atopic family history, illustrating diagnostic criteria, barrier-and-immune pathophysiology, stepwise management, and the atopic march. ^[1]

Question 1 (10 marks)

a) Give the most likely diagnosis, apply the UK Working Party diagnostic criteria to this child, and outline the barrier-and-Th2 pathophysiology. (6 marks) ^[1]

The most likely diagnosis is atopic dermatitis of infancy. The UK Working Party criteria require an itchy skin condition plus at least three of five supporting features, and this child meets all of them: he has an intensely itchy rash (rubbing, scratching and sleep disturbance); the distribution involves the typical infantile sites of cheeks, scalp and extensor surfaces; the onset was before two years of age; he has a strong family history of atopy (maternal asthma, paternal childhood eczema); and he has generally dry skin. The sparing of the nappy area is characteristic and supports the diagnosis. ^[1]

The pathophysiology rests on two reinforcing pillars. The first is a defective epidermal barrier, classically from loss-of-function variants in the filaggrin gene, which reduce natural moisturising factor and ceramides, raise transepidermal water loss, and produce a dry cracked skin surface that admits antigens and supports Staphylococcus aureus colonisation. The second is a Th2-skewed immune response: antigens breaching the barrier are taken up by Langerhans cells, presented to naïve T cells that polarise toward Th2, and the resulting interleukin-4, interleukin-13 and interleukin-31 drive immunoglobulin class-switching to IgE, recruit eosinophils, and — through interleukin-31 — directly stimulate itch. The itch-scratch cycle that follows further damages the barrier and sustains chronic relapsing disease. ^[3]

b) Describe four characteristic clinical features and state how you would grade severity. (4 marks) ^[1]

The four characteristic features are the age-typical infantile distribution on cheeks, scalp and extensors with sparing of the nappy area, the intensely itchy and excoriated erythematous crusted plaques, the generally dry skin (xerosis), and the nocturnal itch disrupting sleep. The strong bilateral parental atopic history is a further supporting feature. ^[1]

Severity is graded with a validated instrument such as SCORAD (which combines body-surface-area extent, six clinical signs and subjective itch-and-sleep scores), EASI (a physician-assessed area-and-severity index), or POEM (a parent-completed seven-item score), with the Children's Dermatology Life Quality Index capturing quality-of-life impact. Because this child's itch disrupts sleep, the disease is at least moderate, and a SCORAD or POEM should be recorded at baseline and at each review to guide step-up and step-down of therapy. ^[1]