Atopic Dermatitis

Detailed Overview

AD is the cutaneous manifestation of 'atopy', often preceding asthma and allergic rhinitis in the 'atopic march'. A fundamental defect involves mutations in the filaggrin gene, which compromises the stratum corneum, leading to transepidermal water loss and increased penetration of allergens. The intense itching triggers a 'itch-scratch cycle' that further damages the skin, promoting colonization and infection by Staphylococcus aureus. Management requires a dual approach: aggressive barrier restoration and suppression of local inflammation.

Epidemiology & Demographics

Affects up to 20% of children and 1-3% of adults globally. Onset usually occurs before age 5, with many cases presenting within the first 6 months of life.

Etiological Mechanism

A combination of genetic epidermal barrier defects (e.g., null mutations in the FLG gene encoding filaggrin) and dysregulation of the innate and adaptive immune systems, skewed toward Th2 inflammation (IL-4, IL-13, IL-31).

Primary Causes

Triggered by environmental factors (harsh soaps, dry winter weather, wool clothing), psychological stress, and exposure to specific allergens (dust mites, pet dander) acting on a genetically susceptible barrier.

• Family History: Strongest risk factor; having one or both parents with AD, asthma, or hay fever.
• Filaggrin Mutation: FLG mutations strongly predispose to severe, early-onset AD and peanut allergies.
• Urban Environment: Higher incidence in urban settings and lower humidity environments.

Filaggrin deficiency causes a leaky epidermal barrier. Antigens penetrate the skin and are captured by Langerhans cells, which migrate to lymph nodes and stimulate naive T cells into Th2 cells. Th2 cells migrate back to the skin, releasing IL-4 and IL-13 (which further inhibit filaggrin production, worsening the barrier) and IL-31 (the 'itch cytokine' that directly stimulates cutaneous sensory nerves). The scratched skin bleeds, excoriates, and releases damage-associated molecular patterns (DAMPs) that sustain chronic inflammation (lichenification).

Characteristic Clinical Presentation

• Pruritus: Severe, uncontrollable itching, worse at night. If it doesn't itch, it is generally not atopic dermatitis.
• Xerosis: Generalized, profound dryness of the skin.
• Lichenification: Thickened, leathery skin with exaggerated skin lines resulting from chronic rubbing.

Physical Examination Signs

• Infants: Erythematous, weeping, crusted plaques on the face, scalp, and extensor surfaces of limbs.
• Adults/Older Children: Lichenified plaques localized to the flexural areas (antecubital and popliteal fossae), neck, and wrists.
• Dennie-Morgan Folds: Prominent extra fold of skin beneath the lower eyelid.

• Bacterial Superinfection: Over 90% of AD skin is colonized with S. aureus. Flares frequently involve impetiginization, requiring antibiotics.
• Eczema Herpeticum: A life-threatening, disseminated HSV infection in atopic skin, presenting with punched-out erosions.

Diagnostic Criteria & Guidelines

Clinical diagnosis based on the Hanifin and Rajka criteria or UK Working Party criteria: requires an itchy skin condition plus 3 or more of: 1) history of flexural involvement, 2) history of asthma/hay fever, 3) history of generalized dry skin, 4) onset before age 2, and 5) visible flexural dermatitis.

Differential Diagnosis

• Contact Dermatitis (Irritant or Allergic) - usually sharply demarcated at site of exposure.
• Seborrheic Dermatitis - affects groin/axillae/scalp with greasy scales.
• Psoriasis - sharply demarcated, silvery scales on extensor surfaces.
• Scabies - burrowing tracts, severe night itch, affects web spaces of fingers.

Laboratory Tests & Biomarkers

• Clinical Diagnosis: Labs are generally not required.
• Serum IgE: Elevated in up to 80% of patients but non-specific.
• Skin Swab Culture: Used to confirm S. aureus superinfection and guide antibiotic choice if MRSA is suspected.

Imaging Modalities & Findings

• None:

• Acute
  Intensely itchy, erythematous papules and vesicles with exudate and crusting.
• Subacute
  Erythematous, excoriated, scaling papules and plaques.
• Chronic
  Thickened, hyperkeratotic skin with prominent skin markings (lichenification) and fibrotic nodules (prurigo nodularis).

Second-Line & Adjunctive Therapy

Topical Calcineurin Inhibitors (Tacrolimus 0.1% ointment) or PDE4 inhibitors (Crisaborole 2% ointment) for face/eyelids to avoid steroid atrophy. For severe, recalcitrant systemic disease: Dupilumab (anti-IL-4Rα biologic) 600 mg SQ induction then 300 mg SQ every 2 weeks, or oral JAK inhibitors (e.g., Upadacitinib 15 mg PO daily).

Surgical & Procedural Management

Not applicable.

Recommended Lifestyle Changes

• Bathe daily in lukewarm water for 5-10 minutes using a gentle, soap-free synthetic detergent (syndet).
• Wear loose, 100% cotton clothing. Avoid wool and synthetic fibers.
• Use a humidifier in the bedroom during winter months.

Patient Counseling & Advice

Reassure parents that while steroids have a bad reputation ('steroid phobia'), short bursts of appropriate-strength topical steroids are incredibly safe and necessary to prevent misery and infection.

Follow-Up & Monitoring Schedule

Assess control using tools like EASI (Eczema Area and Severity Index). Monitor patients on chronic topical steroids for skin atrophy, striae, and telangiectasias.

Preventive Strategies

Daily, relentless moisturization of high-risk infants starting from birth may delay or prevent the onset of clinical eczema.

Approximately 60-70% of children outgrow their severe symptoms by early adolescence. However, many retain dry, sensitive skin for life, and a subset experiences severe, lifelong disease.