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- Material and methods
Background: A cross-sectional study was performed among 7–8-year-old schoolchildren during the winter of 1996 in three municipalities in the most northern province of Sweden, Norrbotten. The study was the starting point of a longitudinal study of asthma, rhinitis, eczema, and type-1 allergy, and provided data on prevalence and risk factors for these conditions. The aim of the present study was to validate the classification of asthma based on a parental questionnaire, and to examine risk factors for atopic and nonatopic asthma.
Methods: The ISAAC questionnaire with additional questions was distributed by the schools to the parents. The response rate was 97%, and 3431 completed questionnaires were returned. The children in Kiruna and Luleå were also invited to be skin tested, and 2149 (88%) were tested with 10 common airborne allergens. A structured interview was administered by pediatricians in stratified samples of the children to test the validity of the diagnosis of asthma based on the questionnaire.
Results: After the validation study, the prevalence of “ever asthma” was estimated to be 8.0%. The specificity of the question, “Has your child been diagnosed as having asthma by a physician?”, was high, >99%, while the sensitivity was around 70%. The strongest risk factor for “ever asthma” was a positive skin test (OR 3.9). Risk factors for asthma in the asthmatics who were not sensitized were family history of asthma, OR 3.6; breast-feeding less than 3 months, OR 1.8; past or present dampness at home, OR 1.8; smoking mother, OR 1.7; and male sex, OR 1.6. Among the sensitized asthmatics, only a family history of asthma was a significant risk factor (OR 3.0), while breast-feeding less than 3 months was not associated with an increased risk (OR 1.0). A synergistic effect between genetic and environmental factors was found especially in the nonatopic asthmatics; the children with a family history of asthma who had a smoking mother and past or present dampness at home had an OR for “ever asthma” of 13.
Conclusions: Different risk-factor patterns were found for asthma and type-1 allergy. In addition, the risk factors for atopic or allergic asthma diverged from those for nonatopic asthma.
In many countries, asthma and allergic diseases now comprise the largest group of diseases among children, teenagers, and young adults. An increasing prevalence of asthma and allergic diseases has been shown all over the world ( 1–3), but understanding of the cause of the increase is still limited. The high prevalence rates of asthma today are probably due partly to an increased diagnostic intensity and altered criteria for the disease. Postal questionnaires are commonly used in studies of asthma prevalence and in risk-factor analyses. For comparison with other studies, it is an advantage to use similar questionnaires ( 4); however, it is necessary to validate the questions used.
It is well known that a family history of asthma is a risk factor for the disease. This has been shown in cross-sectional studies in both adults ( 3) and children ( 5, 6), as well as in prospective longitudinal studies ( 3, 7). Family history is also a major risk factor for type-1 allergy ( 6, 8–10). The other major risk factor for asthma is allergic sensitization ( 11, 12). The relevant allergens vary, but they are predominantly indoor allergens ( 6, 12–16). In addition to genetic factors, studies have suggested that the environment ( 17–19) and lifestyle in industrialized countries ( 8, 20) are associated with an increased risk of asthma and allergic sensitization.
In northern Sweden, a longitudinal intervention study of allergic diseases in schoolchildren started in 1996. The design of this study makes it possible to measure the prevalence and the incidence of asthma as well as to identify risk factors for the condition ( 6). The aim of the present study was to examine risk factors for sensitized and nonsensitized asthma, and to validate the asthma classification derived from parental responses to a questionnaire.
Material and methods
- Top of page
- Material and methods
As the first step in a longitudinal study of asthma, rhinitis, eczema, and type-1 allergy among 7–8-year-old schoolchildren, a cross-sectional study was performed during the winter of 1996 in northern Sweden ( 6). All 3525 children enrolled in classes one and two in three municipalities, Kiruna, Luleå, and Piteå, were invited to participate. The study was approved by the ethical committee at the University and the University Hospital of Northern Sweden in Umeå.
The ISAAC questionnaire ( 4), with additional questions ( 6) about symptoms, diagnosis of asthma, rhinitis, and eczema, use of medicines, family history of the diseases under study, past and present living conditions, indoor and outdoor environmental factors, smoking habits of the parents, and whether pets were kept at home, was distributed by the schools to the parents of the children. Completed answers were received from 3431 (97%) children. Asthma diagnosed by a physician, “physician-diagnosed asthma”, was reported by 5.7% of respondents, 7.1% had used asthma medicines during the last 12 months, and 12% reported wheezing during the last year ( 6).
The 2454 children in Kiruna and Luleå were invited to be skin tested, and 2149 (88%) participated. The skin tests were carried out at the schools by two trained nurses. The tests were performed according to the standards developed by the European Academy of Allergology and Clinical Immunology (EAACI) ( 21); however, single tests on one arm were used. The allergen extracts used were Soluprick from ALK, Denmark, and included a Swedish standard panel: birch, timothy, mugwort, dog, cat, horse, two mites (Dermatophagoides farinae and D. pteronyssinus), and two molds (Cladosporium and Alternaria). Histamine 10 mg/ml and glycerol were used as positive and negative controls, respectively, and the potency of the extracts was 10 HEP, except for the two molds, which had a potency of 1:20 w/v. A positive reaction was recorded if the mean diameter of the wheal was ≥3 mm. The results showed that 21% had a positive test to at least one allergen. Of the children with physician-diagnosed asthma reported in the questionnaire, 49% had at least one positive skin test ( 6).
Validation study –“ever asthma”
The validation study took place during the winter of 1997. According to the answers from the questionnaire study, five stratified samples of symptomatic children were invited to the validation study ( Table 1). The five strata included 215 children, 117 (54%) boys and 98 (46%) girls, as follows:
Table 1. Results of validation study for five groups of symptomatic children identified from questionnaires
| || || || ||Classification|
|Sample||Invited n||Participated n (%) ||Diagnostic instrument ||Current asthma n (%) ||Ex-asthma n (%) ||Other diagnoses n (%) ||Nonspecific respiratory symptoms n (%) |
|Physician-diagnosed||65||60 (92)||Criteria||51 (85)|| 8 (13)|| 1 (2)|| –|
| asthma|| || ||Assessment||49 (82)|| 8 (13)|| 2 (3) || 1 (2) |
|Medicine||69||65 (94)||Criteria||38 (58)|| 9 (14)||11 (17)|| 7 (11)|
| users|| || ||Assessment||32 (49)||10 (15)||12 (19)||11 (17)|
|Severe||22||19 (86)||Criteria|| 7 (37)|| –|| 9 (47)|| 3 (16)|
| symptoms|| || ||Assessment|| 4 (21)|| –||11 (58)|| 4 (21)|
|Wheeze >||15||12 (80)||Criteria|| 5 (42)|| 1 (8) || 5 (42)|| 1 (8) |
| 4 times|| || ||Assessment|| 5 (42)|| 1 (8) || 5 (42)|| 1 (8) |
|Wheezing||44||38 (86)||Criteria||14 (37)|| –||11 (29)||13 (34)|
| apart from colds|| || ||Assessment|| 8 (21)|| 2 (5) ||11 (29)||17 (45)|
Physician-diagnosed asthma: one-third of the 197 children who had been diagnosed as having asthma by a physician
Users of asthma medicines: all children who had used asthma medicines during the last 12 months, without having physician-diagnosed asthma
Severe symptoms: all children who during the last 12 months reported sleep disturbances one or more nights per week due to wheezing, or reported that wheezing had been severe enough to limit speech to only one or two words at a time between breaths, and who were not included in the samples above
Wheeze >4 times: all children who during the last 12 months reported wheeze at least four times, and who were not included in the samples above
Wheezing apart from colds: all children who during the last 12 months reported wheezing apart from colds, and who were not included in the samples above.
In addition, we invited a sixth control group comprising a random sample of 104 children without symptoms associated with asthma who also did not report either rhinitis or eczema.
The participation rate among the symptomatic groups was 88% in boys and 92% in girls; overall, 194 children participated. The participation rate within the different samples was in the range 80–94%. Of the nonsymptomatic children, 64 (62%) participated.
The study consisted of a structured interview of the child and the parents, usually the mother, including questions about symptoms, year of onset, use of health care, and past and current medication. The interviews were administered by five pediatricians at the pediatric clinics in Kiruna, Luleå, and Piteå, respectively. The pediatricians did not know to which group each child had been assigned.
For the diagnosis of asthma in the validation study, the following criteria were used: wheezing or attacks of shortness of breath in combination with provoking factors such as allergens, irritants, cold air, physical exertion, or food; or wheezing and attacks of shortness of breath during colds, and at least two such periods over 12 months. Also classified as asthmatic were children not fulfilling either of these two criteria but who had asthma previously diagnosed by a physician, and used asthma medicines, and had a previous history of asthma.
In addition to these diagnostic criteria for asthma, the pediatricians made their own “overall assessment” based on their clinical examinations.
If these criteria were present during the last 12 months, asthma was classified as “current”, and if asthma had been present previously, it was classified as “ex-asthma”. “Ever asthma” was defined as current asthma or ex-asthma. “Atopy” was defined as at least one positive skin test. The subjects with “ever asthma” and at least one positive skin test were classified as having atopic asthma, and those with “ever asthma” who did not have any positive skin test were classified as having nonatopic asthma.
Statistical analyses were performed with the Statistical Package for the Social Sciences (SPSS) at the National Institute for Working Life in Umeå (Sweden). When examining prevalence rates in relation to different determinants of asthma by bivariate models, Student's t-test, the chi-square test, and one-way analysis of variance (ANOVA) were used. Multiple regression analysis was performed when assessing the simultaneous influence of possible determinants of positive skin test, “ever asthma”, atopic asthma, and nonatopic asthma, respectively. In calculating risks of “ever asthma”, the whole study sample from Kiruna, Luleå, and Piteå was used in the analyses. In calculating risks of atopic and nonatopic asthma, the skin-tested children from Kiruna and Luleå were included. The independent variables included family history of asthma, family history of allergic disease, sex, smoking mother, dampness at home, breast-feeding, and area of domicile (urban or rural, and which of the three municipalities).