Abstract
In several countries it has been observed that there has been no further increase in the prevalence of asthma. This study aimed to look at asthma prevalence in children during the past 15–30 years in The Netherlands.
Both general practice registration (Continuous Morbidity Registration, Nijmegen) and surveys of the public health service in Limburg (Southern parts of the Netherlands) were used for this purpose.
The general practice registration showed that after a five-fold increase in asthma prevalence during the 1980s and 1990s a downward trend seemed to occur around the turn of the century. The public health service confirms a clear decrease in wheezing and dyspnoea in children during the late 1990s. Thus far, there has been no satisfactory explanation for this observation.
Since the 1980s a strong increase in the prevalence of asthma has taken place 1, 2. In several countries, an increase in the prevalence of asthma has been reported on the basis of several morbidity indicators, such as hospital admissions 3, general practitioner (GP) diagnosis 4 and asthma symptoms, as well as objective features of asthma, such as atopy or hyperresponsiveness 5, 6. The average increase in asthma symptoms was ∼5% per year 7. The increase may have been caused by increasing awareness in doctors and patients, but there is general consensus that at least part of the increased registered morbidity is attributable to an actual rise in the number of asthma cases.
The general practice registrations provide a fair reflection of the morbidity presented to a GP. However, these do not necessarily represent the total morbidity in the open population as diseases are often under-reported. The advantage of epidemiological population studies is that these studies use standardised methods to measure aspects of asthma morbidity. However, the disadvantage is that these studies are often based on questionnaires, without clinical interpretation of the symptoms by a doctor. In order to obtain a good impression of the actual prevalence data sources contribute relevant information.
Recently, the first publications have appeared that indicate that in several countries, no further increase (and possibly even a decrease) is being observed in the prevalence of asthma among children 8–15. Thus far, it is unclear what might have caused this recent change 16. In the present study, recent trends in asthma morbidity in the Netherlands from GP registrations and repeated cross-sectional surveys are reported.
REVERSING THE TREND OF ASTHMA PREVALENCE IN CHILDREN IN THE NETHERLANDS
Data from the general practice registrations of the Dutch Nijmegen Continuous Morbidity Registration (CMR; the Netherlands) have shown a steep rise in asthma prevalence since the 1980s 17. The CMR recording is based on the Dutch healthcare system, where all patients are registered with a GP and all access to care is via the physician 18. The CMR consists of 12,000 subjects and began in 1967. In the database, all morbidity presented in general practice is recorded in a standardised way, and the diagnosis is based on the International Classification of Health Problems in Primary Care 19. Asthma is defined as recurrent episodes of acute bronchial obstruction with either pulmonary function tests showing variable obstruction relieved by bronchodilators, or the presence of respiratory symptoms, such as wheeze, dry cough and prolonged expiratory phase. The increase in asthma prevalence in the CMR was greatest in children and lower in older age groups. The prevalence of 10 per 1,000 children in 1983 had increased at least five-fold in 1999, making asthma one of the diseases that have increased most during the past 10 yrs. It is striking, however, that a recent downward trend seems to have set in around the late 1990s, especially among children (fig. 1⇓).
Furthermore, recent data from repeated cross-sectional surveys suggest a possible recent declining trend in asthma prevalence among children in the southern part of the Dutch province of Limburg (the Netherlands). Since 1989, the regional public health service of this area has investigated the prevalence of respiratory complaints every 4 yrs. As the survey was combined with normal periodic medical examination, >95% of the invited children participated. The studies in 1989, 1993, 1997 and 2001 included 1,794, 1,526, 1,670 and 1,102 children aged 8–9 yrs, respectively. All four surveys used the same questionnaire, in which questions were asked about coughing, productive cough, chest wheezing, shortness of breath and medication relating to respiratory complaints over the past 12 months. Age and sex distributions were similar in each of the four successive surveys, so there appears to have been no selection bias in the study population 20. It was striking that the presence of one or more respiratory complaints shows a clear and steady decline from 17.4% in 1989 to 13.5% in 2001, whereas the use of (bronchodilator and inhaled steroid) medication by wheezing children has clearly increased from almost 40% in 1989 to 56% in 2001 (fig. 2⇓).
Percentage of children in Limburg, the Netherlands, with one or more respiratory symptom (–––), and percentage of wheezing children with bronchodilator and inhaled steroid medication (– – –) 20.
DISCUSSION
These data suggest that in the Netherlands, after an initial increase, the prevalence of respiratory complaints in children is now stabilising or even declining. The causes of this trend can only be speculated. It has been suggested that the underlying cause of the increase in asthma cases could be due to changes towards a western lifestyle 21–23. These lifestyle factors would play a dominant role just before and after birth, which could be the explanation as to why an increase is observed especially in young children 24. Interestingly, children who were born after the re-unification in former East-Germany showed an increase in asthma prevalence 25. However, other explanations, such as (confounding) behaviour by parents, might also contribute to these observations 26. Moreover, it seems unlikely that the recent observed decline would be due to a stabilisation in a western lifestyle. Besides, it cannot be explained that the increase in childhood asthma prevalence mainly took place in the 1980s and 1990s, while the change towards a western lifestyle with an increased hygiene started years before that period. It is not unlikely that a prevalence plateau of all genetically predisposed children has been reached, i.e. that all children with genetic predisposition become asthmatic, due to relevant exposure. In addition, these children were detected earlier because of increased alertness and improved diagnostics.
It seems more likely that a possible decline in asthma prevalence is related to improved diagnostics followed by correct treatment (fig. 2⇑). It is obvious that the observed changes in symptoms and medication use are not necessarily causally related, but it is not unlikely that these factors are associated. This suggestion is further strengthened by the observed clear decrease in wheezing and shortness of breath between 1997 and 2001 20. This period coincides with the publication by the Dutch College of General Practitioners of the guidelines for the diagnosis and treatment of childhood asthma 27. These guidelines underline the importance of early and adequate treatment with inhaled corticosteroids. Interestingly, in England 28 and the USA 12 no increase was found in the number of GP visits for asthma complaints in the 1990s. This finding was probably directly related to improved asthma management 28. In Finland, “primary care teams” were established and trained, resulting in a 75% increase in the use of inhaled corticosteroids, accompanied by an 80% decrease in hospitalisation and mortality due to asthma 29. In the Netherlands, mortality as a result of asthma among patients <35 yrs of age has also shown a striking, sharp decrease in the early 1990s (fig. 3⇓), which is likely to be related to improved treatment.
Asthma mortality among males (– – –) and females (–––) aged 5–34 yrs in the Netherlands in the period 1979–2001 (per 100,000 people).
Conclusion
In conclusion, it can be determined that in the Netherlands a steep rise in the prevalence of asthma since the 1980s is being followed by a levelling off or a decreasing trend, especially among children. Given that a clear explanation for the increasing trends has still not been established, it is too early to draw conclusions on the newly developing reversing trend.
Acknowledgments
The authors would like to thank R. Gijsen and H. Van den Hoogen for making data of the Continuous Morbidity Registration of Nijmegen University (The Netherlands) available.
- Received February 21, 2005.
- Accepted June 15, 2005.
- © ERS Journals Ltd
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