A Practical Overview of Pediatric Atopic Dermatitis, Part 1: Epidemiology and Pathogenesis

Age Considerations

Diagnosis of AD is made by 5 years of age in 85% to 90% of children who will develop the disease and by age 1 year in 60% to 65%.^6,19,20 Atopic dermatitis will persist into adulthood in up to one-third of children.^21,22 Infantile AD is characterized by erythematous, oozing, excoriated plaques on the cheeks (sparing the nose), scalp, trunk, and extensor surfaces. Pruritus is always seen in AD and can be a source of morbidity.^16-18 Seborrheic dermatitis may complicate or overlap with AD in infancy.²²

By 2 years of age, most children who are going to develop AD begin to show disease signs of childhood AD characterized by flexural lesions and lesions on the neck and in the postauricular area with sparing of the diaper area.²³ Adult AD often presents as eczema of the hands and/or feet. Hand eczema in adulthood is correlated with a prior history of childhood hand eczema and/or childhood AD as well as wet work and caring for small children.²⁴ Children with skin of color may manifest with follicular eczema as their primary disease phenotype. Facial and eyelid dermatitis are more common in Asian females, infants, and teenagers.^12,25 Other disease phenotypes that are common in patients with skin of color include lichenoid AD and postinflammatory hypopigmentation.¹²

Pathogenesis of AD

There are 2 theories on the pathogenesis of AD known as the inside-out and outside-in hypotheses.²⁶ The inside-out hypothesis suggests that allergic triggering leads to a weakened skin barrier that furthers allergen introduction and presentation, while the outside-in hypothesis suggests that the skin barrier is weakened in AD and allows for the presentation of allergens. Both theories have validity and biologic basis, and both may in fact be true in certain individuals.²⁶

The Skin Barrier: An Overview

The skin barrier is a complex set of factors present and functional at birth that seal the keratinocytes and the interkeratinocyte space so that the skin can perform key processes and functions including retention of fluid, exclusion of allergens, protection from UV light and solvents, and prevention of pathogen entry (eg, infections).^27-29 The superficial stratum corneum or the cornified envelope consists of keratinocytes with intercellular stripes of hydrophobic and hydrophilic substances formed by various intercellular lipids, largely ceramides, cholesterol, and free fatty acids.^30,31 Keratinocytes are the first responders to a variety of environmental insults with the production of IL-18, RANTES (regulated on activation, normal T-expressed, and presumably secreted), granulocyte-macrophage colony-stimulating factor, and thymic stromal lymphopoietin. These inflammatory substances produce acute and chronic inflammation, mast cell reactivity, and T-cell activation.¹⁴ Corneodesmosins link the keratinocytes. Peptidases released will cleave the corneodesmosins and allow normal desquamation or shedding of surface skin, which is replaced by division of stem cells in the basal layer.²⁹

The stratum granulosum is the layer beneath the stratum corneum that co-contributes to barrier activity. The stratum granulosum is absent or reduced histologically in ichthyosis vulgaris,³² a form of skin dryness linked to filaggrin mutations and AD. Filaggrin breakdown creates natural moisturizing factor, a series of hygroscopic compounds that attract water into the skin.³³ Histidine, a filaggrin breakdown product, is used by urocanic acid to process UV light insults.³⁴ Filaggrin also contributes to other barrier functions including pH and stratum corneum cohesion as well as paracellular permeability of the stratum corneum. Tight junctions in the stratum granulosum include claudin-1 and claudin-6 and provide another barrier feature.²⁹

The skin barrier is composed of lipids and keratinocytes. Ceramides, which represent one type of lipids, are reduced in AD, causing alteration in the lamellar pattern³⁵ and increased transepidermal water loss. Furthermore, the stratum corneum is thickened in AD, possibly in response to trauma, and hydration is reduced.³⁶ Filaggrin (chromosome arm 1q21.3) is formed from the 400-kDa⁺ precursor profilaggrin through dephosphorylation and cleavage, and it performs an essential function in the skin barrier through its differential cleavage and breakdown as well as release of natural moisturizing factor and other compounds.³⁷ Filaggrin mutations are linked to AD and ichthyosis vulgaris; however, barrier defects as evidenced by transepidermal water loss in the absence of filaggrin mutation are sufficient to allow for sensitization to allergens through the skin.²⁹ Filaggrin mutations have been associated with AD development and vary in prevalence worldwide. In the United Kingdom, a prevalence study of filaggrin mutations in patients aged 7 to 9 years (N=792) demonstrated an 18.4% carrier rate in AD patients versus 12.9% in controls.³⁴ A similar study in Sweden (N=3301) showed carrier rates of 13.5% versus 6.5%, respectively.³⁸ Although filaggrin mutations are lower in black patients,³⁹ ceramide content may be reduced in this population, demonstrating that a variety of skin barrier defects can result in AD. Carriers of filaggrin mutations are more likely to have eczema on skin exposed to environmental factors (eg, face, hands).⁴⁰