ASTHMA IN ADULTS
Identifying people with asthma
Comparing the prevalence and severity of asthma across countries is difficult because asthma is poorly defined and is hard to identify with certainty. The study of asthma in young adults, however, has fewer problems than in older adults or in children. This is because conditions that can be easily confused with asthma – wheezy bronchitis in childhood and the chronic obstructive lung disease of older age groups – are much less common between the ages of 20 and 45 years.
Early population surveys
In the 1980s The Union developed methods to survey populations for asthma using both questionnaires 1-4 and measures of airway responsiveness.5,6From these questionnaires further definitions have been derived, though these are not yet commonly reported.7,8
The European Community Respiratory Health Survey (ECRHS)
Range of prevalence for common outcomes related to asthma from the first European Community Respiratory Health Survey
to common allergens
|PD20 <1mg methacholine||3.4||13.0||27.8||8|
|Waking with breathlessness||1.5||7.3||11.4||8|
|On current asthma medication||0.6||3.5||9.8||16|
Following on from the earlier work, the European Community Respiratory Health Survey (ECRHS) assessed the prevalence of atopy, symptoms of airway disease, diagnoses and airway responsiveness in the general population aged 20 to 44 years at a number of sites, mostly in Western Europe.9 The initial survey showed wide variations in the prevalence of common conditions related to asthma (see Table).
The most common condition was sensitivity to allergens with a median of 33% and a 3-fold range from 16% to 45%. Hay fever symptoms were slightly less prevalent (21%) with a similar variation. Signs and symptoms associated with asthma all had an 8-fold range in prevalence with median values of 21% (wheeze) 13%, (airway response to less than 1 mg of methacholine) and 7% (waking with breathlessness). Current treatment for asthma was less commonly reported than any of these conditions and was far more variable. Only 3.5% of participants said that they were currently taking asthma medication, and there was a 16-fold difference between the lowest and highest report.10-12 This is twice the variation in the prevalence of symptoms and signs of asthma.
The principal limitation of the ECRHS data has been that it is almost entirely confined to the richer countries with the exception of Tartu (Estonia), Mumbai (India) and, for the initial survey, Algiers (Algeria). Although others have collected data using the same or similar methods,13-21 this has not been systematic.
Where surveys have been done in the same place, there is a good association between the prevalences reported by the International Study on Asthma and Allergies in Childhood (ISAAC) in children and by ECRHS in adults,22 but it is unwise to rely on an extrapolation of this association beyond those centres in which it was measured.
Figure 1: Prevalence of wheeze and asthma according to Gross National Income (GNI)
Figure 2: Proportions of participants with any asthma whose asthma was graded Severe, Moderate or Mild in the AIRE studies
Figure 3: Proportions of participants with persistent asthma whose asthma was graded Severe, Moderate or Mild in the AIRE studies
World Health Survey reports on the prevalence of asthma and wheeze
The best source of information on asthma in adults in low-income countries comes from the World Health Survey.23 This shows that there are very wide variations in the prevalence of wheeze and asthma regardless of overall national income. The mean prevalence of wheeze was highest in the poorest countries (13.3%) followed closely by that in the richest countries (13%) (Figure 1); the mean prevalence of diagnosed asthma was highest in the richest countries (9.4%) followed closely by the poorest countries (8.2%). The middle-income countries had the lowest prevalence in each case (wheeze: 7.6%; asthma: 5.2%). In each case, these averages disguise a wide variation between countries.
Severity of disease
Because of the rather poorly defined threshold at which asthma is diagnosed, it is difficult to assess relative severity and its distribution. The Asthma Insights and Reality (AIRE) studies categorised people identified from a telephone survey as having asthma. They categorised them according to the Global Initiative on Asthma (GINA) definitions of severity 24 as either intermittent or persistent and, if persistent, as either severe, moderate or mild. Severe persistent disease was relatively more common in Eastern and Central Europe (32%) and less common in the Asia-Pacific region (11%) (Figure 2).
This was also true if the intermittent disease was excluded (Figure 3). The proportion of patients who had persistent disease ranged from 41% in Central and Eastern Europe to 23% in the Asia-Pacific Region.25 The proportion in Latin America was close to that for Central and Eastern Europe (22% of all “asthma” identified, and 41% of all persistent asthma identified).26
Trends in disease over time
The clearest evidence on time trends comes from data in the ECRHS on sensitisation (presence of specific IgE) to common allergens. This has demonstrated a continuous increase in sensitisation among people born in successive decades from the 1940s to the 1970s.27 Another study has shown that this trend goes back at least to people born in the 1930s.28
Evidence on symptoms and reported diagnoses are more difficult to interpret. Within the ECRHS cohort, the prevalence of reported asthma increased markedly between the early 1990s and the turn of the millennium, but there was little increase in symptoms.29 This could easily be explained as an artifact due to the greater willingness to diagnose asthma, a trend that has been well documented.30 However, in the light of the rising prevalence in sensitisation and the strong association between sensitisation and airway disease, it might be premature to conclude that this is the whole explanation. Other changes may have altered the prevalence of symptoms, including a reduction in cigarette smoking31 and an increase in the use of inhaled steroids, and this may have been enough to offset the increase in symptoms due to increasing allergy.