Excerpt from Atopic Dermatitis

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Background

Atopic dermatitis (AD) is a pruritic disease of unknown origin that usually starts in early infancy and is typified by pruritus, eczematous lesions, xerosis (dry skin), and lichenification on the skin (thickening of the skin and increase in skin markings). AD is associated with other atopic diseases (eg, asthma, allergic rhinitis, urticaria, acute allergic reactions to foods, increased immunoglobulin E [IgE] production) in many patients. It is a disease of great morbidity, and the incidence appears to be increasing.

Pathophysiology

The pathophysiology of AD is poorly understood. Several cell types seem to be involved, including T lymphocytes, eosinophils, Langerhans cells, and keratinocytes. Other factors, including cytokines and IgE, are also implicated.

Laboratory findings suggest a number of different pathogenetic mechanisms. One invokes an immune defect involving an abnormality of T_H2 cells that interacts with Langerhans cells and results in increased production of interleukin (IL)–4, IL-5, IL-6, IL-10, and IL-13. This leads to increased IgE and decreased gamma interferon levels. The imbalance of T_H2 cells occurs in the acute process, with a swing toward T_H1 cells in the chronic stages of the disease. Another theory involves defective barrier function in the stratum corneum leading to the entry of antigens, which results in the production of various inflammatory cytokines.

Xerosis is known to be an associated sign in most AD patients. The xerosis is thought to involve defective lipid (particularly ceramide) production. A third mechanism involves environmental antigens from food (the gut), dust mites (the lungs), and other factors and portals of entry that react with antibodies to produce increased levels of IgE and, possibly, increased histamine reactions from mast dells. Superimposed with these mechanisms is a genetic predisposition to react to various environmental allergens.

Frequency

United States

The prevalence rate is 10-12% in children and 0.9% in adults.

International

The prevalence rate is as high as 18% and is rising, especially in developed countries. In China and Iran, the prevalence rate is approximately 2-3%. The frequency is increased in patients who immigrate to developed countries from underdeveloped countries.

Mortality/Morbidity

Incessant itch and work loss in adult life is a great financial burden. A number of studies have reported that the financial burden to families and government is similar to that of asthma, arthritis, and diabetes mellitus. In children, the disease causes enormous psychological burden to families and loss of school days. Mortality due to AD is unusual.

• Kaposi varicelliform eruption (eczema herpeticum) is seen with some frequency in patients with AD. It usually occurs with a primary herpes simplex infection, but it also may be seen recurrently. Vesicular lesions can begin at any location, but they are particularly common in areas of eczema. The virus spreads rapidly to involve all eczematous areas and healthy skin. Lesions may become secondarily infected. Although vaccination with the vaccinia vaccine for the prevention of small pox is now no longer mandatory, patients with AD can contract eczema vaccinatum either from the vaccination of themselves or their relatives. This condition had a high mortality rate (up to 25%). In the current climate of threats of bioterrorism, vaccination may once again become necessary and physicians should be aware of eczema vaccinatum in this setting.
• With regard to bacterial infection (eg, with Staphylococcus aureus or Streptococcus pyogenes), note that the skin of most patients with AD is colonized by S aureus. Clinical infection may occur and is worsened by scratching and occlusion from medications. Eczematous and bullous lesions on the palms and soles are often infected with beta-hemolytic group A Streptococcus.
• Urticaria and acute anaphylactic reactions to food occur with increased frequency in patients with AD. The food groups most commonly implicated include peanuts, eggs, milk, soya, fish, and seafood.
• Latex allergy is more common in patients with AD than in the general population.
• Of patients with AD, 30% develop asthma and 35% have nasal allergies.

Race

AD may be more common among whites, but it affects persons of all races.

Sex

The male-to-female ratio is 1:1.4.

Age

In 85% of cases, AD occurs in the first year of life; in 95% of cases, it occurs before age 5 years.

• Disease is most prevalent in early infancy and childhood. The disease may have periods of complete remission, particularly in adolescence, and may then recur in early adult life.
• In the adult population, the rate of AD frequency diminishes to 0.9%. Rarely, onset may be delayed until adulthood, when the disease is more difficult to control.

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