Prevalence of asthma

A broad consensus exists that in most Western countries the prevalence of asthma increased over the last four decades of the 20th century. This is based largely on repeat studies of school age children. Evidence is emerging that in recent years this trend has flattened or fallen in some countries. For example, as part of the UK arm of the international study of asthma and allergies in childhood (ISAAC), repeat studies found that self reported symptoms of asthma in 13-14 year old children had fallen by about 20% in the United Kingdom between 1995 and 2002.¹ This trend was also observed in the health survey for England between 1996 and 2001.² Over the same period a similar fall in symptoms of asthma in 6-7 year old children reported by parents was seen in Melbourne.³ On the other hand, the only available repeat survey of preschool children noted a major increase in prevalence between 1990 and 1998.⁴ A global picture of recent trends in children will soon be provided by the results of ISAAC phase 3, which has obtained trends in prevalence between 1995 and 2002 in more than 100 centres in 58 countries.⁵ Little information exists about long term trends in adults, but recent trends in the UK seem to be flat.²,⁶

These data are limited by the lack of an objective measure of asthma in large populations and the reliance on surveys to elicit symptoms of wheezy breathlessness, which are likely to represent a heterogeneous group of disorders.⁷ Questions about lifetime prevalence are subject to serious recall bias, and the usual compromise is to rely on the 12 month period of prevalence in an attempt to capture the intermittent nature of symptoms experienced by most people with asthma, while limiting the recall entailed and avoiding problems with labelling. Trends in the cultural perception and naming of symptoms might explain the trends observed in prevalence studies,⁸ and while it remains true that without objective measures we cannot be certain how much is real and how much is artefact it seems unlikely that artefact would completely explain the observed trends.

If the trend is indeed flattening or decreasing, might it be explained by trends in health care? The use of inhaled corticosteroids, an effective treatment, has increased in most countries for which data are available. Over the 1990s, sales of inhaled corticosteroids in the UK increased from 1.9 to 4.3 patient months per 10 000 population and by the late 1990s about two thirds of patients with asthma treated by their general practitioner were receiving inhaled corticosteroids alone or in combination.⁹ This is one plausible explanation for the reduction in attacks of severe asthma as reflected in the fall in hospital admissions, primary care contacts for asthma attacks, and mortality that has occurred in the UK and some other Western countries over the past decade.¹⁰ Although this may also have had some effect on prevalence by affecting prognosis or severity, it is unlikely to explain the whole of the recent changes in prevalence, which have also been observed in milder cases and in associated atopic conditions such as hay fever and atopic eczema, which are not treated with inhaled corticosteroids.¹

More likely, the recent changes in the prevalence of asthma reflect changes in the underlying cause of asthma or its exacerbating factors. While theories abound, we must admit that neither the rise nor the recent flattening or fall in the prevalence of asthma can be explained on the basis of current knowledge. Any single explanation would need to account for both the rise and fall of the prevalence of asthma. A substantial change in prevalence would require a large change in an important risk factor to which a large proportion of the population was exposed.¹¹ Various explanations have been proposed to explain trends and geographical variations in asthma, including air pollution, tobacco smoke, aeroallergens, diet, and infections in early life, but none has been shown by epidemiological studies to fit the above requirements. At an individual level, the most powerful risk factor for asthma is a history of atopic disease such as atopic eczema or allergic rhinitis. However, one promising idea, the hygiene hypothesis, has not explained the epidemiology of atopy or asthma.¹¹ Since there is some evidence that atopic diseases tend to track together over time in populations, trends in asthma may partly reflect changes in the incidence of atopy in the population; but since the proportion of asthma cases attributable to atopy is less than one half,¹² factors unrelated to atopy must also be important. Any advance in our understanding of trends is likely to depend on the development of new theories of causation together with better methods of measuring and classifying asthma in population studies.