What's new in atopic dermatitis?

There is a widespread recognition that the prevalence of atopy in children in Western countries has increased over recent decades; it has recently been shown to have reached 30%. The prevalence of atopic dermatitis (AD) in children in the U.K. has been estimated at 15±20%, with prevalences of 17 ́2%, 21 ́3% and 30 ́8% having been reported from Oregon, Denmark and Australia, respectively. Although hereditary factors are important for the risk of AD, the increase in prevalence and variation within and between countries suggest that non-hereditary risk factors also play a substantial part. AD has been found to be commoner in older siblings, in less crowded households, with late entry into nursery care, with increased maternal education, and probably in higher social class families. The inverse relationship between the incidence of atopy and sibship size and the risk of atopy being linked to higher birth order led Strachan to propose the `hygiene hypothesis' which suggests that infections in early childhood may reduce the risk of AD. Unlike earlier studies, a recent study surprisingly failed to show a protective effect of breastfeeding in infancy and the subsequent development of AD. Interesting observations have come about through the `natural experiment' that resulted from the reunification of Germany in 1990. The prevalence of atopic disease rose substantially in Leipzig in the 6 years following reunification, as a more Western lifestyle became more prevalent. However, in contrast to atopy in general, this relationship with respect to AD is more complex. The prevalence of AD was found to be consistently higher in East Germany, in contrast to the higher prevalence of hay fever and asthma in West Germany, suggesting that AD follows a risk pattern that is different from respiratory allergies and sensitization. However, when this relationship was examined in more detail, extrinsic-type AD (allergic sensitization, as assessed by positive skin prick test) was consistently and significantly higher in children in West Germany than in those in East Germany, suggesting that factors other than allergy are responsible for the higher prevalence of AD in East Germany. Factors which may contribute to the higher manifestation of intrinsictype AD in East Germany may include air pollution, nutritional factors and infections. The marked variation in the prevalence of AD between countries is also observed within countries. In the U.K., in a study of 7-year-old children, there was a tendency for a higher prevalence of AD in the south and eastern parts of the country, even after corrections for affluence and other confounding factors. Explanations for this observation may include higher annual temperatures in southern areas acting as a direct trigger for eczema, higher humidity giving rise to increased house dust mite (HDM) populations or increased rainfall in the north and west of Britain resulting in lower pollen counts in the air. The role of HDMs in both the initiation and continuance of AD in children has been proposed for some time but remains controversial. Surprisingly, a recent study suggested that increased exposure to HDMs in early life appeared to offer a protective effect against AD. However, in another study of young children with extrinsic-type AD, HDM allergen avoidance measures led to a significant improvement in AD, especially in the autumn and winter seasons when HDMs are at their highest level. The latter study suggests that the use of allergen avoidance measures such as encasing mattresses and pillows, removal of soft toys and carpets and frequent vacuuming of carpets should be suggested to all families with children suffering from extrinsic AD. The introduction of HDM avoidance measures in the first months of life to children at risk may help to prevent HDM sensitization and particularly reduce the risk of asthma. While environmental factors may be important, they must be considered in conjunction with genetic