Eczema
Atopic Dermatitis (Eczema): Causes, Types, Diagnosis and Evidence-Based Treatment
Atopic dermatitis is a chronic, relapsing inflammatory skin condition caused by a combination of a leaky skin barrier and an over-reactive immune system. It is not an infection, not caused by "dirty blood," and not something a child simply grows out of on schedule.
• It affects up to one in five children and roughly 2–10% of adults worldwide. Hong Kong estimates vary considerably depending on how the diagnosis is defined, and local prevalence has risen steadily over two decades.
• Around 60% of cases begin in the first year of life, but adult-onset atopic dermatitis is real and increasingly recognised.
• Severity is not the whole picture. Sleep loss, concentration, mood and school or work performance are frequently affected more than the visible rash.
• Treatment is now genuinely effective. The modern approach is daily barrier repair, prompt treatment of flares, proactive maintenance to prevent the next flare, and targeted systemic therapy for those who need it — including several biologics and oral JAK inhibitors approved in the last few years.
• Undertreatment caused by fear of topical steroids is one of the most common reasons eczema stays out of control. Used correctly, they are safe.
• Most children improve substantially by adolescence, but the underlying tendency to dry, sensitive skin usually persists. Control, not cure, is the realistic goal — and good control is achievable for most people.
What is atopic dermatitis?
Atopic dermatitis — commonly called eczema — is a chronic inflammatory condition of the skin characterised by intense itch, dryness, and recurring inflamed patches that come and go over months to years.
Two things go wrong at the same time, and they feed each other.
A defective barrier. Healthy skin holds water in and keeps irritants, allergens and bacteria out. In atopic dermatitis this barrier is impaired. The best-understood cause is a loss-of-function mutation in the filaggrin gene, a structural protein essential to the outer layer of the skin. Filaggrin loss-of-function mutation is one recognised driver of epidermal barrier dysfunction in eczema, alongside T-cell-mediated mechanisms. Not everyone with atopic dermatitis has a filaggrin mutation, and the specific mutations found in Chinese populations differ from those described in Europeans — but the end result, a barrier that leaks water outward and lets trouble inward, is the same.
An over-reactive type 2 immune response. Once the barrier is breached, the immune system responds with a characteristic pattern of inflammation driven by signalling molecules including interleukin-4, interleukin-13 and interleukin-31. IL-4 and IL-13 are central drivers of epidermal barrier dysfunction, IgE production and chronic inflammation, which is why targeting this axis transformed the management of moderate-to-severe disease. Interleukin-31 is the principal itch signal.
This matters clinically because inflammation damages the barrier further, and a damaged barrier drives more inflammation. Scratching accelerates both. Breaking that cycle — rather than simply suppressing a rash when it appears — is the whole basis of modern treatment.
What atopic dermatitis is not: it is not contagious, not caused by poor hygiene, not usually caused by food, not a "toxin" problem requiring detoxification, and not a condition that must simply be endured.
How common is atopic dermatitis?
| Population | Reported prevalence |
| Children worldwide | up to 20% |
| Adults worldwide | app. 2% – 10% |
| Chinese children aged 1–7 (diagnosed, 12 cities) | 12.94% |
| Shanghai, children aged 3–6 | 8.3% |
| Hong Kong children - ISAAC Phase 3 | below 5% |
| Hong Kong children - HK allergy prevention guidelines | 15% |
A note on the Hong Kong figures, because they disagree. The two local estimates above differ by a factor of three. This is not an error; it reflects genuine methodological differences - questionnaire-reported "flexural dermatitis" in a single 12-month window captures a very different group from clinician-diagnosed atopic dermatitis over a lifetime. Any Hong Kong prevalence figure quoted without that caveat should be treated with caution. What is not in dispute is the direction of travel: across three ISAAC surveys in Hong Kong between 1995 and 2015, the prevalence of lifetime chronic rash rose by 3.9% and current chronic rash by 4.1%, with the authors concluding there was an increase in eczema prevalence across the surveys. This was the first study to show a continual increase in wheeze, allergic rhinitis and eczema prevalence using ISAAC methodology over 21 years in Hong Kong, a well-developed area.
Hong Kong also appears to sit at the higher end of regional allergic sensitisation. In an ISAAC Phase II comparison of Hong Kong, Beijing and Guangzhou schoolchildren, rates of current wheeze, rhinoconjunctivitis and flexural dermatitis were significantly more common in Hong Kong, and the atopy rate was higher in Hong Kong at 41.2% than in Beijing at 23.9% or Guangzhou at 30.8%.
The most important local finding for parents is this: increased prevalence of parental atopy had the strongest association with increased prevalence of these outcomes, and was the most important associated risk factor identified. Family history matters more than air quality, in this data.
Reference:
- Davis DMR, Drucker AM, Alikhan A, et al. AAD guidelines of care for the management of atopic dermatitis in adults with topical therapies. J Am Acad Dermatol. 2023.
- Sidbury R, Alikhan A, Bercovitch L, et al. AAD guidelines of care for the management of atopic dermatitis in adults with phototherapy and systemic therapies. J Am Acad Dermatol. 2023.
- Guidelines Development Panel of the Hong Kong College of Paediatricians. Management of Atopic Dermatitis in Children: 2020 Review. HK J Paediatr. 2021;26:42-57. — the key local reference; cite this prominently.
- Wollenberg A, Kinberger M, Arents B, et al. EuroGuiDerm guideline on atopic eczema. J Eur Acad Dermatol Venereol. (most recent update).
- National Institute for Health and Care Excellence. Atopic eczema in under 12s: diagnosis and management. CG57.
- Leung R, Wong G, Lau J, et al. Prevalence of asthma and allergy in Hong Kong schoolchildren: an ISAAC study. Eur Respir J 1997.
- Childhood Wheeze, Allergic Rhinitis, and Eczema in Hong Kong: ISAAC Study from 1995 to 2015. — source of the 21-year local trend data.
- Severity scoring of atopic dermatitis: the SCORAD index. Consensus Report of the European Task Force on Atopic Dermatitis. Dermatology 1993;186:23-31.
- Efficacy and Safety of Lebrikizumab in Combination With Topical Corticosteroids in Adolescents and Adults With Moderate-to-Severe Atopic Dermatitis: A Randomized Clinical Trial (ADhere). JAMA Dermatol.
- ARCADIA 1 and ARCADIA 2 — two identically designed pivotal phase III trials enrolling more than 1,700 patients, evaluating nemolizumab administered subcutaneously every four weeks versus placebo with background topical corticosteroids.
- Sanofi. Press release: decision not to submit amlitelimab in atopic dermatitis for global regulatory reviews. Paris, 24 July 2026.
- Simpson EL, Bieber T, Guttman-Yassky E, et al. Two Phase 3 Trials of Dupilumab versus Placebo in Atopic Dermatitis (SOLO 1 and SOLO 2). N Engl J Med. 2016.
- Chalmers JR, et al. BEEP trial: daily emollient during the first year of life for preventing atopic dermatitis. Lancet. 2020. — the negative prevention trial cited in the article.
- Hong Kong Drug Office, Department of Health — verify local registration status of every systemic agent named.
