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Abstract
BACKGROUND The aim of the present study was to compare the prevalence of asthma among schoolchildren in 1978, 1991, and 1998 in Patras, Greece.
METHODS The study populations of the three comparable cross sectional surveys comprised third and fourth grade public school children in Patras, Greece. Sample sizes in 1978, 1991, and 1998 were 3735, 2952 and 3397 children and response rates were 80.4%, 81.9%, and 90.6%, respectively. Prevalence of current, non-current, and lifetime asthma or recurrent wheezing was determined by parental questionnaire. Personal communication with the parents of asthmatic children in 1991 and 1998 provided data on lost schooldays.
RESULTS Prevalence rates of current asthma or wheezing in 1978, 1991, and 1998 were 1.5%, 4.6%, and 6.0%, respectively (1978–91: p=0.01, 1991–98: p=0.02, 1978–98: p=0.03). Lifetime prevalences of asthma or wheezing in 1991 and 1998 were 8.0% and 9.6%, respectively (p=0.03). Current diagnosed asthma increased proportionally to diagnosed wheezing during 1991–98. The number of schooldays lost in the previous 2 years because of asthma did not change (p>0.1) between 1991 (0.31 per child) and 1998 (0.34 per child).
CONCLUSIONS Our results support a true increase in the prevalence of current and lifetime asthma in the last 20 years among pre-adolescent children in Patras, Greece.
- asthma prevalence
- wheezing
- childhood
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Although unequivocal evidence is lacking,1 a large body of data supports the impression that the prevalence of asthma has increased in the last decades.2-4 Data from Greece on the prevalence of asthma are scarce5 and there are no longitudinal data published to date. In 1978 a cross sectional survey estimated the prevalence of asthma among schoolchildren in the city of Patras.6 In the present study, using identical methodology, we have compared the data of that survey with that of two surveys conducted in 1991 and 1998 in order to determine possible changes in the prevalence of asthma over the last 20 years.
Methods
The target populations of all three surveys were schoolchildren in the third and fourth grade of primary public schools—that is, children aged 8–10 years—in the city of Patras, Greece. In the 1978 survey there were 3735 children from 43 schools,7 in the 1991 survey there were 2952 children from 42 schools, and in the 1998 survey there were 3397 children from 44 schools. Forty schools were common in all three surveys and the rest were situated in neighbouring areas. Study populations included over 80% of the target populations. No school sample consisted of more than 10% from non-Greek ethnic origin groups.
The following standard parental questionnaire was distributed in all three surveys: (1) Has a physician stated that your child had asthma in the last 2 years? (2) Has a physician stated on two or more separate occasions that your child had wheezing in the last 2 years? (3) Has a physician stated that your child had asthma prior to 2 years ago? (4) Has a physician stated on two or more separate occasions that your child had wheezing prior to 2 years ago? Note: If in doubt about your answer, please check “NO”. Questions 3 and 4 were not included in the 1978 survey. Two collection attempts were made in each survey. In 1991 and 1998 the answers were confirmed by personal communication of one of the investigators with all parents who had a positive response to at least one question, all parents who returned questionnaires with missing values, and with 350 randomly selected controls (negative responses to all questions). During personal communication, additional information was obtained on hospital admissions for asthma and/or wheezing from all positive responders and on the number of school days lost because of asthma and/or wheezing during the previous 2 years from current asthmatic subjects. Prevalence was calculated for current (positive answer to questions 1 and/or 2), non-current (positive answer to questions 3 and/or 4), and lifetime asthma (positive answer to any of the four questions). All surveys were conducted in the months of January and February for comparability.
Confidence intervals and prevalence differences were calculated and significance tests were made using the χ2 test for comparison of two proportions.
Results
The response rates after the second collection of questionnaires in 1991 and 1998 were 81.9% (76.1–92.8%) and 90.5% (78.2–96.5%) respectively. Missing values represented less than 2% of returned questionnaires. After personal communication no missing values occurred. Seventy three of 77 positive responders to question 1 in 1991 (94.8%) and 126 of 131 in 1998 (96.2%) also responded positively to question 2. Respective values for questions 3 and 4 were 43 of 45 (95.5%) and 83 of 85 (97.6%).
The prevalence of current and lifetime asthma in 1978, 1991, and 1998 is shown in table 1. There were significant consecutive increases in the prevalence of asthma. Statistical significance of differences (D) and 95% confidence intervals (95% CI) between surveys were as follows: Dcurrent 1978–91: 3.1% (95% CI 2.6 to 4.0), p=0.01; Dcurrent 1991–98: 1.4% (95% CI 0.2 to 2.6), p=0.02; Dcurrent 1978–98: 4.5% (95% CI 3.6 to 5.4), p=0.03; Dnon-current 1991–98: 0.2% (95% CI –0.8 to 1.2), p>0.1); Dlifetime 1991–98: 1.6% (95% CI 0.1 to 3.1), p=0.03.
Total number and prevalence (%) of current, non-current and lifetime asthma, hospital admissions, and lost schooldays due to asthma or wheezing in 1978, 1991, and 1998 surveys
The ratio of “current physician diagnosed asthma” over “⩾2 episodes of current physician diagnosed wheezing not identified as asthma” was 2.2 (77/35) and 2.5 (131/53) in 1991 and 1998, respectively (p>0.1). Respective ratios for “non-current physician diagnosed asthma” over “non-current physician diagnosed wheezing” were 1.2 (45/37) and 3.1 (85/27), (p=0.04).
History of “ever been hospitalised” (lifetime asthma) did not change significantly between 1991 and 1998 (D: 0.6% (95% CI –0.26 to 1.46), p>0.1). The mean number of school days lost because of asthma in the previous 2 years among current asthmatics and in the total sample did not change significantly between 1991 and 1998 (D: 1.05 (95% CI –1.7 to 3.8), p>0.1 and D: 0.03 (95% CI –0.6 to 0.6), p>0.1, respectively).
Discussion
Large sample sizes and high response rates were achieved in the three surveys. The four schools that varied between the surveys did not differ in social or environmental aspects. The age group sampled remained constant and the racial, socioeconomic, and cultural structure of the samples remained essentially unchanged.
The prevalence of current asthma or wheezing has increased in Patras approximately threefold in the 1978–91 period from 1.5% to 4.6% (mean yearly rate 0.24%). It continued to increase until 1998 to 6.0%, albeit at a slower rate (0.20% per year).
A standard questionnaire was used in the three surveys. Written questionnaires are probably the method of choice for comparing prevalence.7 Others have shown that questions on physician diagnosed lung disease are exceptionally specific.8 The labelling of two or more episodes of wheezing as asthma will tend to overestimate asthma prevalence, but less so among current asthmatics. Our results may be subject to bias because of the increasing awareness of both asthma and wheezing in the community and could be influenced by parental recall or acceptance of these labels, access to health services, and physician attitudes. We expect that the last sentence of our written questionnaire and the fact that there were no questions on recurrent or persistent cough have significantly limited false positive answers.9 This is supported by the fact that the majority of physician diagnosed asthmatic subjects were also recurrent wheezers. Access to health care is unrestricted to all children and changing consultation patterns are unlikely to have influenced trends of physician diagnosed asthma since paediatricians have remained, almost exclusively, the primary and secondary care physicians of children during the last 20 years in Greece. Other studies have shown increasing prevalence of objective parameters of the disease.2 The cause for this worldwide increase is unclear2-4 and the present study did not address this issue.
An increase in true current asthma is supported by the essentially unchanged ratio of “current physician diagnosed asthma” over “current physician diagnosed wheezing not identified as asthma” during the period 1991–8. These results counter the concern about diagnostic transfer from bronchitis to asthma. The ratio of “non-current diagnosed asthma” over “non-current wheezing not identified as asthma” significantly increased during 1991–8. This should probably be interpreted as increased labelling of wheezing of younger children (under 6–8 years) by physicians as asthma as “total” non-current asthma (diagnosed asthma plus wheezing not identified as asthma) did not change over the same period.
The mean number of lost school days due to asthma during the previous 2 years decreased by approximately one day (15.5%) among current asthmatics and increased by 9.7% in the general population during the period 1991–8. These differences, however, were not significant.
In conclusion, our results show a continuing increase in the prevalence of asthma over a 20 year period and support a true increase in the prevalence of current asthma. Physicians may have become more willing to diagnose wheezing as asthma in younger children during the period 1991–8. There has been no change in the burden of asthma among asthmatics and in the general population during this period in the city of Patras, although hospital admissions for asthma or wheezing have become more common in recent years.