# Atopic Dermatitis: Pathogenesis and Modern Management

## Overview

Atopic dermatitis (AD) is the most common chronic inflammatory skin disease, affecting up to 20 percent of children and 10 percent of adults. It is characterized by intense pruritus, eczematous lesions with age-dependent morphology and distribution, and a relapsing-remitting course. The past decade has seen a revolution in understanding its pathogenesis and a dramatic expansion of targeted therapies.

## Epidemiology

AD prevalence is increasing worldwide, especially in industrialized nations. Sixty percent of cases present in the first year of life, and 90 percent by age 5. Up to half of childhood AD resolves by adolescence, but adult-onset AD is increasingly recognized. There is a strong association with other atopic conditions — allergic rhinitis and asthma — known as the "atopic march." A family history of atopy is the strongest risk factor.

## Pathogenesis

### Barrier Dysfunction

Filaggrin (FLG) loss-of-function mutations are found in approximately 30 to 50 percent of patients with moderate-to-severe AD, though FLG mutations are neither necessary nor sufficient for AD development. When present, they lead to impaired natural moisturizing factor, elevated skin pH, and enhanced transepidermal water loss. AD skin also shows reduced ceramide levels (especially ceramides 1 and 3) in stratum corneum lipids, tight junction abnormalities (claudin-1 downregulation), and enhanced protease activity from kallikrein-related peptidases that degrades barrier components.

### Immune Dysregulation

The hallmark of AD immunology is a Th2-skewed immune response. IL-4 and IL-13 drive IgE class switching, suppress filaggrin and antimicrobial peptide expression, and promote barrier dysfunction. IL-31 is a major pruritogen that signals via IL-31RA/OSMR on sensory neurons. IL-5 recruits eosinophils. TSLP (thymic stromal lymphopoietin), released by damaged keratinocytes, activates dendritic cells to polarize Th2 responses. Acute AD lesions are Th2/Th22 dominant, while chronic lesions acquire Th1 and Th22 contributions with epidermal hyperplasia. The Th17/Th22 contribution is more prominent in Asian and pediatric AD subtypes. Deficiency of antimicrobial peptides (cathelicidin LL-37, human beta-defensins) predisposes to infection.

### The Hygiene Hypothesis

Reduced early microbial exposure may promote Th2-skewed immunity, supported by lower AD rates in children raised on farms, in early daycare, or with multiple siblings. This remains a simplification; the "biodiversity hypothesis" and "old friends hypothesis" are more nuanced modern formulations.

### Neuroimmune Itch Pathway

IL-31 binds IL-31RA on dorsal root ganglion neurons. IL-4 and IL-13 sensitize sensory neurons via JAK1 signaling. TSLP directly activates itch-specific TRPA1+ neurons. The resulting itch-scratch cycle perpetuates barrier damage and inflammation in a self-reinforcing loop.

## Clinical Features

### Age-Dependent Distribution

In infants, AD favors the face, scalp, and extensor surfaces. In children, it shifts to flexural involvement — the antecubital and popliteal fossae, wrists, and ankles. In adults, the hands, eyelids, and flexures predominate, sometimes with a head/neck pattern, and nummular or papular morphology may appear. Elderly-onset AD is often generalized with marked xerosis.

### Morphology

Acute AD presents with erythematous papules, vesicles, weeping, and crusting. Subacute disease shows erythematous, excoriated, scaly plaques. Chronic disease produces lichenified, thickened plaques with accentuated skin markings and sometimes prurigo nodularis-like lesions.

### Associated Features

Several clinical signs are associated with AD: Dennie-Morgan infraorbital folds, Hertoghe sign (thinning of the lateral eyebrows), keratosis pilaris, ichthyosis vulgaris (sharing FLG mutations), hyperlinear palms, and white dermographism (paradoxical blanching to stroking).

## Severity Assessment

Several validated tools are used to assess AD severity. SCORAD combines extent, intensity, and subjective symptoms. EASI (Eczema Area and Severity Index) measures extent and intensity only and is widely used in clinical trials. IGA (Investigator Global Assessment) provides a global severity scale from 0 to 4. Quality-of-life measures include the DLQI and CDLQI. The Peak Pruritus NRS captures patient-reported itch intensity.

## Complications

S. aureus colonizes over 90 percent of AD skin. Eczema herpeticum — HSV superinfection — is a dermatologic emergency presenting with punched-out erosions and fever. Eczema coxsackium (coxsackievirus superinfection) and eczema vaccinatum (historical, with smallpox vaccination) are additional viral complications. Sleep disturbance and psychosocial impact are substantial.

## Management

### Foundational Therapy

Emollient therapy is the cornerstone, applied immediately after bathing; ceramide-containing emollients are preferred. Bathing should use lukewarm water with limited soap in a "soak and seal" technique. Trigger avoidance (relevant irritants and allergens), environmental controls (avoiding overheating, wearing soft cotton fabrics), and patient education are essential.

### Topical Anti-Inflammatory Therapies

**Topical corticosteroids (TCS)** remain the mainstay, with potency matched to body site and age. Proactive (maintenance) use two times per week to previously affected areas reduces flares and is the most evidence-based strategy for flare prevention. Corticosteroid phobia is a major barrier to adherence. **Topical calcineurin inhibitors** — tacrolimus 0.03%/0.1% and pimecrolimus 1% — are preferred for the face, eyelids, and intertriginous areas, carrying no risk of atrophy; the black box warning for theoretical malignancy risk is not supported by long-term data. **Crisaborole** (PDE4 inhibitor) offers modest efficacy for mild-to-moderate AD, with application site burning as its main side effect. **Ruxolitinib cream** (JAK1/2 inhibitor) provides rapid itch relief without atrophy risk but should be limited to less than 20 percent BSA application. **Tapinarof** (aryl hydrocarbon receptor agonist) is a non-steroidal topical approved for AD, with folliculitis as its most common side effect.

### Systemic Therapies

#### Biologics

| Biologic | Target | Key Efficacy | Notable Adverse Effects |
|---|---|---|---|
| Dupilumab | IL-4Ralpha (blocks IL-4 and IL-13) | EASI-75 ~40-50% at 16 wk | Conjunctivitis (10-25%), injection site reactions, facial erythema |
| Tralokinumab | IL-13 | Similar to dupilumab | Possibly lower conjunctivitis rates |
| Lebrikizumab | IL-13 (high affinity) | Similar to dupilumab | Monthly maintenance dosing advantage |
| Nemolizumab | IL-31RA | Dramatic pruritus reduction | Targets itch pathway directly |

**Dupilumab** (anti-IL-4Ralpha) blocks both IL-4 and IL-13 signaling and was the first biologic approved for moderate-to-severe AD (in adults and children aged 6 months and older). It achieves EASI-75 response in approximately 40 to 50 percent of patients at 16 weeks. Adverse effects include conjunctivitis (10 to 25 percent), injection site reactions, and facial erythema, but no laboratory monitoring is required. **Tralokinumab** (anti-IL-13) selectively neutralizes IL-13 with similar efficacy and possibly lower conjunctivitis rates, approved for adults. **Lebrikizumab** (anti-IL-13) is a high-affinity IL-13 inhibitor with the advantage of monthly maintenance dosing after induction. **Nemolizumab** (anti-IL-31RA) targets the itch pathway directly and produces dramatic pruritus reduction.

#### JAK Inhibitors (Oral)

| JAK Inhibitor | Selectivity | Dose | Key Considerations |
|---|---|---|---|
| Abrocitinib | JAK1 | 100 or 200 mg daily | Rapid itch relief; monitor CBC, lipids, LFTs; herpes zoster risk |
| Upadacitinib | JAK1 | 15 or 30 mg daily | Highest efficacy in head-to-head trials; acne as side effect |
| Baricitinib | JAK1/2 | 2 or 4 mg daily | EU/Japan approved (not FDA); lower efficacy than JAK1-selective agents |

**Abrocitinib** (JAK1-selective) at 100 or 200 mg daily provides rapid itch relief within days, with monitoring required for CBC, lipids, and hepatic function, plus herpes zoster risk. **Upadacitinib** (JAK1-selective) at 15 or 30 mg daily has the highest efficacy among oral JAK inhibitors in head-to-head trials, with acne as a notable side effect and the same monitoring concerns. **Baricitinib** (JAK1/2) is approved in the EU and Japan but not FDA-approved for AD, with lower efficacy than selective JAK1 inhibitors.

#### Conventional Systemics

Cyclosporine offers rapid onset but nephrotoxicity limits its duration as bridging therapy. Methotrexate has slower onset with modest efficacy. Azathioprine requires TPMT checking before initiation and has limited AD-specific data. Mycophenolate mofetil is used off-label as a steroid-sparing agent.

### Phototherapy

Narrowband UVB (311 nm) is first-line phototherapy for moderate AD. UVA1 is useful for acute flares. Limitations include access, time commitment, and long-term UV exposure risk.

## Controversy: Long-Term JAK Inhibitor Safety

The FDA black box warning on JAK inhibitors was extrapolated from tofacitinib cardiovascular data (the ORAL Surveillance trial) in rheumatoid arthritis patients over age 50 with cardiovascular risk factors. Dermatology patients are generally younger with fewer comorbidities. Real-world data is emerging, with a consistent signal for herpes zoster reactivation. MACE, VTE, and malignancy risk remain debated. Current consensus holds that JAK inhibitors are appropriate for AD patients failing or intolerant of other therapies, with individualized risk-benefit discussion.

## Controversy: Topical vs. Systemic Treatment Ladder

The traditional step-up approach (emollients, then TCS, then TCI, then phototherapy, then systemics) is increasingly questioned. "Proactive" early systemic intervention may prevent disease progression, reduce the burden of chronic inflammation, and improve long-term outcomes. The counter-argument is that AD has a high spontaneous remission rate in children, and overtreating with systemics carries risk. Shared decision-making and treat-to-target approaches are gaining traction.

<image>Clinical photograph series showing age-dependent distribution of atopic dermatitis: (A) infant with facial and extensor eczema, (B) child with flexural involvement of antecubital fossae, (C) adult with lichenified hand eczema and eyelid dermatitis. Show characteristic morphology at each stage with erythema, scale, and excoriations visible on representative skin tones.</image>

<image>Immunopathogenesis diagram of atopic dermatitis showing damaged keratinocytes releasing TSLP, IL-25, and IL-33, activating dendritic cells that polarize Th2 responses. Show the IL-4/IL-13 axis driving IgE class-switching in B cells and barrier dysfunction, IL-31 signaling to sensory neurons causing itch, and the corresponding therapeutic targets (dupilumab blocking IL-4Ralpha, nemolizumab blocking IL-31RA, JAK inhibitors blocking downstream signaling).</image>

<image>Treatment algorithm flowchart for atopic dermatitis showing the step-up approach from emollients and trigger avoidance, through topical corticosteroids and calcineurin inhibitors, phototherapy, and systemic therapies (dupilumab, JAK inhibitors, conventional immunosuppressants). Include decision points based on EASI score and quality of life measures.</image>

## Clinical Pearls

Colonization with S. aureus is nearly universal in AD; routine antibiotics are not indicated unless there is clinical superinfection. Eczema herpeticum (punched-out erosions, monomorphic vesicles, fever) requires urgent systemic acyclovir and is a contraindication to topical calcineurin inhibitors until resolved. Dupilumab-associated conjunctivitis may require ophthalmology referral and can be managed with tacrolimus ophthalmic ointment or fluorometholone drops. Food allergy testing in AD should be guided by clinical history; routine panels lead to false positives and unnecessary dietary restriction. The "proactive" use of TCS or TCI (two times per week to previously affected sites) is the most evidence-based strategy to prevent flares. White dermographism and the "headlight sign" (perinasal sparing) are useful clinical clues to AD in the differential diagnosis.

## References
- Langan SM, Irvine AD, Weidinger S. Atopic dermatitis. Lancet. 2020;396(10247):345-360.
- Simpson EL, Bieber T, Guttman-Yassky E, et al. Two phase 3 trials of dupilumab versus placebo in atopic dermatitis. N Engl J Med. 2016;375(24):2335-2348.
- Weidinger S, Beck LA, Bieber T, Kabashima K, Irvine AD. Atopic dermatitis. Nat Rev Dis Primers. 2018;4(1):1.
- Silverberg JI, et al. Upadacitinib plus topical corticosteroids in atopic dermatitis (Measure Up 1 and Measure Up 2). Lancet. 2021;397(10290):2151-2168.
- Paller AS, et al. Efficacy and safety of dupilumab in children aged 6 months to 5 years with atopic dermatitis. Lancet. 2022;400(10356):908-919.