10.1 Atopic Dermatitis: Pathogenesis, Phenotypes & European Management

10.1 Atopic Dermatitis: Pathogenesis, Phenotypes & European Management

Overview and Epidemiology

Atopic Dermatitis (AD), also known as atopic eczema, is the most common chronic inflammatory pruritic skin condition globally. In Europe, it affects approximately 15% to 20% of children and 2% to 8% of adults, posing an immense burden on patient quality of life, psychological well-being, and healthcare resources.

The Atopic March

AD frequently represents the initial clinical manifestation of the atopic march—the sequential progression of atopic diathesis from cutaneous barrier impairment in early infancy to systemic allergic comorbidities:

1. Atopic Dermatitis (onset typically within the first 6 months of life).
2. IgE-Mediated Food Allergy (e.g., egg, peanut, cow's milk).
3. Allergic Rhinitis / Rhinoconjunctivitis (emerging in preschool and early school years).
4. Allergic Asthma (manifesting in childhood or adolescence).

The prevailing pathophysiological consensus emphasizes epicutaneous sensitisation: environmental food and aeroallergens penetrate through a genetically disrupted stratum corneum, encountering dendritic cells in an inflammatory microenvironment that promotes systemic IgE class switching and atopic predisposition.

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Molecular Genetics and Epidermal Barrier Dysfunction

The Epidermal Differentiation Complex and Filaggrin (FLG)

The primary structural defect in AD resides within the stratum corneum. The epidermal differentiation complex (EDC) on chromosome 1q21 encodes crucial structural and functional proteins responsible for terminal keratinocyte differentiation, including profilaggrin, loricrin, and involucrin.

• Filaggrin (FLG) Loss-of-Function Mutations: Mutations such as R501X, 2282del4, R2447X, and S3247X represent the strongest and most widely replicated genetic risk factors for AD in European populations.
• Physiological Processing: Profilaggrin is stored within keratohyalin granules in the stratum granulosum. During cornification, it is dephosphorylated and cleaved into 10–12 monomeric filaggrin peptides by endoproteinases (including matriptase and SASPase). Filaggrin bundles keratin intermediate filaments into dense macrofibrils, flattening keratinocytes into protective corneocytes.
• Natural Moisturizing Factor (NMF): In the upper stratum corneum, filaggrin is enzymatically degraded into free hydrophilic amino acids (principally glutamine, histidine, and arginine) and their derivatives, notably pyrrolidone carboxylic acid (PCA) and trans-urocanic acid (UCA). These constitute the hygroscopic Natural Moisturizing Factor (NMF), which maintains stratum corneum hydration, binds ambient water, and sustains an acidic physiological cutaneous surface pH (4.5–5.5).

Consequences of Filaggrin Deficiency

1. Elevated Transepidermal Water Loss (TEWL): Defective lipid lamellar organization (depleted ceramides, particularly long-chain omega-hydroxy ceramides) and diminished NMF lead to chronic dehydration, skin xerosis, and macroscopic microfissuring.
2. Alkalinization of Skin pH: Reduced urocanic acid and lactic acid elevate the cutaneous pH. Alkaline pH activates neutral serine proteases (kallikreins KLK5 and KLK7), which prematurely degrade corneodesmosomes (desmoglein-1, corneodesmosin), accelerating desquamation and barrier breakdown.
3. Protease-Activated Receptor 2 (PAR2) Activation: Unchecked KLK5 activity cleaves PAR2 on keratinocytes and primary nerve endings, triggering non-histaminergic pruritus and stimulating the release of pro-inflammatory epithelial alarmins.
4. Facilitated Microbial and Allergen Entry: Unhindered transit of exogenous protein antigens, haptens, and bacterial toxins across the disrupted physical matrix into the viable nucleated epidermis.

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Immunopathogenesis and Neuro-Immune Crosstalk

                                  [EPIDERMAL BARRIER BREACH]
                                 (FLG mutation, elevated TEWL)
                                              │
                                              ▼
                              [EPITHELIAL ALARMIN RELEASE]
                                 (TSLP, IL-25, IL-33)
                                              │
                                              ▼
                        ┌───────────────────────────────────────────┐
                        │   Dendritic Cells & ILC2 Activation       │
                        └─────────────────────┬─────────────────────┘
                                              │
                                              ▼
                        ┌───────────────────────────────────────────┐
                        │    Th2 / Th22 Differentiation & Influx    │
                        │   (IL-4, IL-13, IL-31, IL-22)             │
                        └───────┬───────────────────────────┬───────┘
                                │                           │
               ┌────────────────┴───────────────┐           │
               ▼                                ▼           ▼
    [IL-4 / IL-13 Signaling]           [IL-31 Neuro-Itch] [IL-22 Epidermal Remodeling]
    • STAT6 Phosphorylation             • Binds IL-31RA/    • Acanthosis & Hyperplasia
    • Downregulates FLG & Involucrin      OSMRβ on C-fibers • Barrier Defect Perpetuation
    • Suppresses AMPs (beta-defensins) • Severe Pruritus   
    • Promotes B-cell IgE Switching    • Scratch Damage

Epithelial Alarmins

Mechanical scratching and barrier disruption trigger basal and spinous keratinocytes to release preformed alarmins:

• Thymic Stromal Lymphopoietin (TSLP): Binds TSLP receptors on myeloid dendritic cells, inducing OX40L expression, which instructs naive CD4+ T cells to differentiate into Th2 effector cells.
• Interleukin-33 (IL-33) and Interleukin-25 (IL-25): Rapidly activate resident group 2 innate lymphoid cells (ILC2s), stimulating copious production of IL-5 and IL-13 independently of adaptive immunity.

The Th2 and Th22 Cytokine Cascade

• Interleukin-4 (IL-4) & Interleukin-13 (IL-13): The master cytokines of atopic inflammation. IL-4 acts through the heterodimeric Type I (IL-4Rα/γc) and Type II (IL-4Rα/IL-13Rα1) receptors; IL-13 signals exclusively through the Type II receptor. Activation of JAK1/JAK3 and JAK1/Tyk2 leads to STAT6 phosphorylation, orchestrating:
  • Downregulation of structural barrier genes (FLG, LOR, IVL).
  • Profound suppression of antimicrobial peptide synthesis (cathelicidin LL-37, human beta-defensins 2 and 3), creating an permissive environment for bacterial colonisation.
  • B-cell heavy-chain class switching to immunoglobulin E (IgE).
  • Eosinophil recruitment via eotaxins (CCL11, CCL26).
• Interleukin-31 (IL-31): Known as the master pruritus cytokine. IL-31 is produced by activated Th2 cells and binds to a heterodimeric receptor complex consisting of IL-31 Receptor A (IL-31RA) and Oncostatin M Receptor beta (OSMRβ) expressed on cutaneous unmyelinated peptidergic C-fiber sensory neurons and keratinocytes. IL-31 directly depolarizes sensory neurons, transmitting intense itch signals to the dorsal root ganglion and the central nervous system, driving the destructive itch-scratch cycle.
• Interleukin-22 (IL-22): Secreted by Th22 cells, IL-22 induces epidermal acanthosis, parakeratosis, and downregulates terminal differentiation markers, exacerbating barrier leakage.
• Biphasic Evolution (Acute vs. Chronic): While acute AD lesions are overwhelmingly driven by Th2/Th22 cytokines, chronic lichenified lesions demonstrate an additional influx of Th1 cells producing Interferon-gamma (IFN-γ) and IL-12, accompanied by tissue remodeling, collagen deposition, and dermal thickening.

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Cutaneous Microbiome and Staphylococcus aureus Dysbiosis

In healthy human skin, Staphylococcus epidermidis and Cutibacterium acnes dominate the cutaneous microbiome. In atopic dermatitis, a severe state of microbial dysbiosis develops:

• Colonization Rates: Over 90% of lesional skin and up to 70% of non-lesional skin in AD patients is colonized with Staphylococcus aureus, compared to <5% in healthy individuals.
• Virulence Mechanisms:
  • Alpha-toxin (hemolysin A): Creates membrane pores in human keratinocytes, triggering cell necrosis, IL-1α release, and further barrier degradation.
  • Delta-toxin: Induces rapid mast cell degranulation in an IgE-independent manner, directly fueling erythema and pruritus.
  • Staphylococcal Superantigens: Enterotoxins A, B (SEA, SEB) and Toxic Shock Syndrome Toxin-1 (TSST-1) bypass conventional MHC-restricted antigen processing. They cross-link the invariant region of MHC Class II molecules with the variable beta (Vβ) domain of the T-cell receptor (TCR), activating up to 20% of circulating T cells simultaneously. This induces massive cytokine release and generates corticosteroid insensitivity via induction of the glucocorticoid receptor-beta (GR-β) isoform.

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Clinical Phenotypes Across the Lifespan

The clinical presentation and anatomical distribution of atopic dermatitis evolve characteristically with patient age:

Minor Clinical Stigmata of Atopy

• Dennie-Morgan Fold: Accentuated double infraorbital fold or crease below the lower eyelid caused by chronic eyelid edema and rubbing.
• Hertoghe Sign: Thinning or absence of the lateral third of the eyebrows due to constant rubbing and scratching.
• Pityriasis Alba: Ill-defined, mildly scaly, hypopigmented macules located predominantly on the sun-exposed cheeks of children.
• Keratosis Pilaris & Ichthyosis Vulgaris: Hyperkeratotic follicular papules on lateral upper arms and thighs, and fine polygonal fish-like scaling on the shins, closely linked to underlying heterozygous FLG mutations.
• Hyperlinear Palms & Soles: Exaggerated cutaneous markings on palms and soles associated with filaggrin deficiency.
• White Dermographism: Stroking involved or uninvolved atopic skin with a blunt instrument causes a blanched white line rather than a normal red wheal, resulting from paradoxical local capillary vasoconstriction.

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Diagnostic Criteria and Severity Scoring

Hanifin and Rajka Diagnostic Criteria

The historical gold standard for clinical trials and formal classification mandates satisfying at least 3 of 4 Major Criteria and at least 3 of 23 Minor Criteria:

                             [HANIFIN & RAJKA CRITERIA]
                                        │
          ┌─────────────────────────────┴─────────────────────────────┐
          ▼                                                           ▼
   [MAJOR CRITERIA]                                            [MINOR CRITERIA]
   (Must satisfy >= 3 of 4)                                    (Must satisfy >= 3 of 23)
   1. Pruritus                                                 • Xerosis / Dry skin
   2. Typical morphology & distribution:                       • Ichthyosis / Keratosis pilaris
      - Flexural lichenification (adults)                      • Hyperlinear palms
      - Facial/extensor involvement (infants)                  • Elevated serum IgE
   3. Chronic or chronically relapsing course                  • Early age of onset (<2 yrs)
   4. Personal or family history of atopy                      • Cutaneous infections (S. aureus, HSV)
      (AD, allergic rhinitis, asthma)                          • Pityriasis alba
                                                               • Dennie-Morgan infraorbital fold
                                                               • Cheilitis / Nipple eczema
                                                               • White dermographism
                                                               • Food intolerances / Cataracts

UK Working Party Diagnostic Criteria

Validated as a streamlined, highly sensitive and specific clinical tool without requiring invasive laboratory tests. The patient must have an itchy skin condition (or parental report of scratching) in the last 12 months, PLUS 3 or more of the following:

1. History of involvement of the skin creases (folds of elbows, behind knees, fronts of ankles, around neck, or around eyes).
2. Personal history of asthma or allergic rhinitis (or history of atopic disease in a first-degree relative if the child is under 4 years of age).
3. History of a general dry skin in the past year.
4. Visible flexural eczema (or dermatitis of cheeks/forehead and outer limbs in children under 4 years).
5. Onset of rash under 2 years of age (not applied if the child is under 4 years of age).

Objective Severity Scoring Systems

• SCORAD (SCORing Atopic Dermatitis): $\text{SCORAD} = \frac{A}{5} + \frac{7B}{2} + C$
  • A (Extent): Based on the rule of nines (0–100).
  • B (Intensity): Sum of 6 clinical signs (erythema, edema/papulation, oozing/crusts, excoriations, lichenification, dryness) graded 0–3 each (maximum 18).
  • C (Subjective Symptoms): Visual analogue scale (0–10) for pruritus plus sleep loss over the past 3 days (maximum 20).
  • Interpretation: Mild (<25), Moderate (25–50), Severe (>50).
• EASI (Eczema Area and Severity Index):
  • Assesses four anatomical regions: Head/Neck (weighted 0.1), Upper Limbs (0.2), Trunk (0.3), Lower Limbs (0.4).
  • Evaluates 4 signs: Erythema (E), Induration/Papulation (I), Excoriation (Ex), Lichenification (L) on a 0–3 scale.
  • Calculated score ranges from 0 to 72.
  • Stratification: Mild (0.1–7.0), Moderate (7.1–21.0), Severe (21.1–50.0), Very Severe (>50.0). EASI-75 (75% reduction from baseline) is the regulatory benchmark endpoint for modern clinical trials.
• Patient-Reported Outcomes:
  • POEM (Patient-Oriented Eczema Measure): Evaluates frequency of symptoms over the past week (0–28).
  • DLQI (Dermatology Life Quality Index): 10-item instrument for adult health-related quality of life (0–30).

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Cutaneous Complications and Red Flags

1. Eczema Herpeticum (Kaposi Varicelliform Eruption)

• Etiology: Disseminated viral superinfection, predominantly caused by Herpes Simplex Virus Type 1 (HSV-1), occasionally HSV-2 or Coxsackievirus A6 (eczema coxsackium).
• Pathophysiology: Profound local suppression of antimicrobial peptides (defensins, cathelicidins) and impaired plasmacytoid dendritic cell IFN-alpha production allow unrestricted viral replication across compromised epidermal surfaces.
• Clinical Features: Sudden, explosive eruption of monomorphic, grouped, dome-shaped umbilicated vesicles that rapidly progress into circular, punched-out erosions with hemorrhagic crusting. Predominantly affects the face, neck, and upper trunk.
• Systemic Manifestations: High spiking fever, painful regional lymphadenopathy, malaise, and leukocytosis.
• Life-Threatening Risks: Keratoconjunctivitis, corneal ulceration, dendritic keratitis (risk of blindness if periorbital lesions are involved; mandates immediate ophthalmological assessment), HSV sepsis, and meningoencephalitis.
• Emergency Management: Do not await virological confirmation (PCR or Tzanck smear) before initiating therapy. Administer immediate intravenous acyclovir (5–10 mg/kg every 8 hours) in toxic or hospitalized patients, or high-dose oral valacyclovir (1000 mg three times daily) in mild outpatients. Concomitant bacterial coverage targeting S. aureus with flucloxacillin or cefuroxime is frequently required.

2. Other Cutaneous Superinfections

• Eczema Molluscatum: Widespread dissemination of Molluscum contagiosum (poxvirus) lesions across eczematous sites.
• Impetiginisation: Extensive golden-yellow ('honey-colored') crusting, pustulation, and moist erosions caused by secondary S. aureus or Streptococcus pyogenes infection.
• Eczema Vaccinatum: Historic and bioterrorism-relevant severe disseminated infection following live smallpox (vaccinia) vaccination in individuals with active or quiescent AD.

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European Stepwise Management (EuroGuiDerm / ETFAD Guidance)

European guidance from the European Task Force on Atopic Dermatitis (ETFAD) and the EuroGuiDerm consortium escalates therapy with disease severity. The ladder below is a teaching summary: the EASI and SCORAD cut-offs are commonly used severity bands, not thresholds set by the guideline itself.

                                [STEPWISE LADDER - TEACHING SUMMARY]

  STEP 4: SEVERE / REFRACTORY AD (EASI >= 21 or SCORAD >= 50)
  ═══════════════════════════════════════════════════════════════════════════════════════
  • Biologics: Dupilumab (anti-IL-4Rα), Tralokinumab (anti-IL-13), Lebrikizumab (anti-IL-13)
  • Oral JAK Inhibitors: Upadacitinib (15/30 mg), Abrocitinib (100/200 mg), Baricitinib (2/4 mg)
  • Conventional Immunosuppressants: Ciclosporin (3–5 mg/kg/day, short-term), MTX, Azathioprine
                                        ▲
                                        │ Escalation
  STEP 3: MODERATE AD (EASI 7.1–21 or SCORAD 25–50)
  ═══════════════════════════════════════════════════════════════════════════════════════
  • Phototherapy: Narrowband UVB (NB-UVB 311 nm) or Medium-Dose UVA1
  • Potent Topical Corticosteroids (Class III) or Tacrolimus 0.1% Ointment
  • Proactive Maintenance Therapy (twice-weekly application)
                                        ▲
                                        │ Escalation
  STEP 2: MILD AD (EASI 0.1–7 or SCORAD < 25)
  ═══════════════════════════════════════════════════════════════════════════════════════
  • Mild-to-Moderate Potency Topical Corticosteroids (Class I–II)
  • Topical Calcineurin Inhibitors (TCI: Tacrolimus 0.03%, Pimecrolimus 1%)
  • Topical PDE4 Inhibitor: Crisaborole 2% Ointment (US; not marketed in EU)
                                        ▲
                                        │ Escalation
  STEP 1: BASELINE FOUNDATIONAL THERAPY (ALL PATIENTS)
  ═══════════════════════════════════════════════════════════════════════════════════════
  • Liberal Emollient Therapy (>250–500 g/week in adults) with high lipid/ceramide content
  • Soap-free syndet cleansing, brief lukewarm bathing/showering
  • Trigger avoidance (harsh detergents, synthetic/wool clothing, overheating, confirmed allergens)
  • Structured patient education programmes (Eczema School)

1. Baseline and Topical Anti-Inflammatory Therapy

• Emollients: Liberal, frequent application (>250–500 g per week in adults). Applied immediately following bathing ('soak and seal' method) to restore stratum corneum hydration and reduce flare frequencies.
• Topical Corticosteroids (TCS): The cornerstone of reactive flare management. Categorized according to the European 4-class system:
  • Class I (Weak): Hydrocortisone acetate 1%.
  • Class II (Moderate): Clobetasone butyrate, triamcinolone acetonide.
  • Class III (Potent): Betamethasone valerate, mometasone furoate, fluticasone propionate.
  • Class IV (Very Potent): Clobetasol propionate.
  • Application Strategies: Reactive daily application during flares, transitioning upon clearance to proactive therapy—applying mid-potency TCS twice weekly (e.g., weekend therapy) to historically vulnerable sites to forestall subclinical inflammatory relapse.
• Topical Calcineurin Inhibitors (TCI):
  • Tacrolimus ointment (0.03% and 0.1%) and Pimecrolimus cream (1%).
  • Mechanism: Bind cytosolic macrophilin-12 (FKBP-12), inhibiting calcineurin phosphatase, preventing dephosphorylation and nuclear translocation of NFAT (Nuclear Factor of Activated T cells), blocking IL-2, IL-4, and TNF-alpha transcription.
  • Advantages: Highly effective for delicate cutaneous regions prone to corticosteroid-induced atrophy (face, eyelids, perioral skin, neck, intertriginous folds, anogenital area). Do not cause cutaneous thinning, telangiectasia, striae, or intraocular hypertension.
  • Adverse Effect: Transient application-site burning and stinging sensation, mitigating after several days of treatment.
• Topical PDE4 & JAK Inhibitors:
  • Crisaborole 2% ointment: Phosphodiesterase-4 inhibitor approved in the US for mild-to-moderate AD from 3 months of age. An EU marketing authorisation was granted in 2020 but later withdrawn, so it is not available in most of Europe.
  • Ruxolitinib 1.5% cream: Topical JAK1/JAK2 inhibitor. It is approved in the US for short-term, non-continuous treatment of mild-to-moderate AD; in the EU it is authorised for non-segmental vitiligo, not AD.

2. Phototherapy

• Modality of Choice: Narrowband UVB (NB-UVB 311 nm) administered 3 times weekly. Alternative for acute vesicular exacerbations: medium-dose UVA1 (340–400 nm, 30–50 J/cm²).
• Mechanism: Induces apoptosis of infiltrating dermal T cells, suppresses Langerhans cell antigen-presenting capacity, downregulates Th2/Th22 cytokine expression, and stimulates antimicrobial peptide production.
• Contraindications & Precautions: Concomitant use with topical calcineurin inhibitors or systemic immunosuppressive drugs (ciclosporin) is contraindicated due to theoretical photo-carcinogenesis risks.

3. Targeted Systemic Biologics

4. Oral Janus Kinase (JAK) Inhibitors

Oral JAK inhibitors disrupt downstream signaling of key atopic cytokines (IL-4, IL-13, IL-31, TSLP, IL-22, IFN-γ) by binding the ATP-binding catalytic domain of JAK enzymes.

• Upadacitinib: Highly selective JAK1 inhibitor. Dosing: 15 mg or 30 mg orally once daily. Exceptional speed of onset, significantly reducing pruritus within 24 to 48 hours.
• Abrocitinib: Highly selective JAK1 inhibitor. Dosing: 100 mg or 200 mg orally once daily. Patients aged 65 years or older start at 100 mg; moderate or severe renal impairment requires dose reduction (for example 50 mg daily in moderate impairment).
• Baricitinib: Reversible JAK1/JAK2 inhibitor. Dosing: 2 mg or 4 mg orally once daily.
• Pre-treatment Screening & Safety Protocols:
  • Mandatory Baseline Screening: Latent tuberculosis (IGRA / QuantiFERON and chest radiography), Hepatitis B (HBsAg, anti-HBc), Hepatitis C, pregnancy test, full blood count, renal function, liver enzymes, and fasting lipid panel.
  • Black-Box Warnings & Class Toxicities: Major Adverse Cardiovascular Events (MACE), Venous Thromboembolism (Deep Vein Thrombosis [DVT] and Pulmonary Embolism [PE]), serious bacterial/opportunistic infections, reactivation of Herpes Zoster (varicella-zoster vaccination with recombinant subunit vaccine is recommended prior to initiation), non-melanoma skin cancer, elevated serum lipids, and transient creatine kinase (CPK) elevation.
  • Contraindications: Active severe infections, active malignancy, severe hepatic impairment, pregnancy/breastfeeding, and high baseline thromboembolic risk unless no alternative exists.

5. Conventional Systemic Immunosuppressants

• Ciclosporin (Cyclosporine A): The only traditional systemic agent licensed for AD across most European member states. Dosing: 3 to 5 mg/kg/day administered in two divided daily doses. Induces rapid remission. Licensed course lengths differ between countries; guidelines favour short courses (typically a few months) and specialist review before longer use, because of nephrotoxicity (reduced GFR, tubular interstitial fibrosis) and arterial hypertension. Requires baseline and bi-weekly serum creatinine and blood pressure monitoring.
• Methotrexate (MTX): Off-label in AD. Dosing: 10 to 25 mg orally or subcutaneously once weekly with mandatory folic acid supplementation (5 mg once weekly, 24–48 hours post-MTX). Slower onset of action (8–12 weeks). Monitor liver function (PIIINP or FibroScan), complete blood count, and renal parameters.
• Azathioprine: Off-label in AD. Dosing: 1.5 to 2.5 mg/kg/day. Mandatory baseline determination of Thiopurine S-methyltransferase (TPMT) enzymatic activity or genotype to avoid fatal myelosuppression.
• Systemic Corticosteroids: Strongly discouraged by European guidelines for routine management. Repeated use causes severe rebound flares upon cessation, hypothalamic-pituitary-adrenal (HPA) axis suppression, osteoporosis, and cataracts. Reserved exclusively for exceptional, short bridging crises (<7 days) during acute hospitalization.