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Keywords:

  • allergy;
  • asthma;
  • child;
  • epidemiology;
  • risk factor

Abstract

  1. Top of page
  2. Abstract
  3. Background
  4. Participants and methods
  5. Prevalence and remission by age
  6. Risk factors by age
  7. Prevalence and risk factors by time
  8. References

Background:  Childhood is the most important age for asthma development. Recent reports indicate that the prevalence of asthma in children has plateaued after having increased for decades.

Aims:  To study prevalence and risk factor patterns of asthma by age and by time.

Methods:  In 1996, all children in grade 1–2 (age 7–8) in three cities in Northern Sweden were invited to an expanded International Study of Asthma and Allergies in Childhood (ISAAC) questionnaire. A total of 3430 children (97%) participated yearly until 2000 (age 11–12). A subset (n = 2454) was invited to skin-prick testing in 1996 and 2000 with 88% and 90% participation. In 2006, another cohort (n = 2704) was identified and studied by identical methods with 96% participation. A total of 1700 children (90% of invited) were skin-prick tested.

Results and comments:  From age 7–8 to 11–12, the prevalence of physician-diagnosed asthma increased, 5.7%–7.7% (P < 0.01) while current wheeze decreased, 11.7%–9.4% (P < 0.01), indicating a less diverse spectrum of symptoms with age. The yearly remission from asthma was 10% (lasting remission 5%), largely determined by allergic sensitisation. Allergic sensitisation (OR 5) and a family history of asthma (OR 3) were important risk factors for asthma at age 7–8 and 11–12. However, several other significant risk factors at age 7–8 (low birth weight, respiratory infections and house dampness) lost importance until age 11–12. Maternal and paternal asthma were equally important risk factors (OR 3–4) at age 7–8. Sibling asthma was only a marker of parental disease.

Future perspectives:  Through comparison with the 2006 cohort, trends in prevalence and in risk factors from 1996 to 2006 will be studied.

Please cite this paper as: Bjerg A and Rönmark E. Asthma in school age: prevalence and risk factors by time and by age. The Clinical Respiratory Journal 2008; 2: 123–126.


Background

  1. Top of page
  2. Abstract
  3. Background
  4. Participants and methods
  5. Prevalence and remission by age
  6. Risk factors by age
  7. Prevalence and risk factors by time
  8. References

The prevalence of asthma has increased worldwide (1) and an estimated 300 million people are affected (2). Incidence, but also remission peaks during childhood (3) and these events are related to a number of risk and prognostic factors (3–6). Using population-based surveys, school age has not been studied to the same extent as infancy. Recently, there have been reports that the prevalence of asthma is no longer increasing in some regions (7–9). This trend break seems to have occurred preferably in westernised countries, but not exclusively in the regions with the highest prevalence. The factors accounting for this development have not been studied.

This is a summary of recent studies within the Obstructive Lung Disease in Northern Sweden (OLIN) Studies (6, 10), which were included in a thesis for doctoral degree in 2008. We have investigated prevalent asthma and its risk factors by age from age 7–8 to 11–12 years in one cohort of school children identified in 1996, and by time through comparison with a similar cohort identified 10 years later.

Participants and methods

  1. Top of page
  2. Abstract
  3. Background
  4. Participants and methods
  5. Prevalence and remission by age
  6. Risk factors by age
  7. Prevalence and risk factors by time
  8. References

The OLIN pediatric study has been described previously (5, 11, 12). In 1996 all school children in first and second grade (age 7–8 years, n = 3525) in Kiruna, Luleå and Piteå, Northern Sweden, were invited to a parental questionnaire. The cohort was then followed by annual questionnaires until age 18–19 years, initially with 97% yearly participation. This article however covers the study ages 7–8 to 11–12 years (1996–2000). Skin-prick testing for sensitization to aeroallergens was performed in Kiruna and Luleå (n = 2454) in 1996 and 2000 with 88% and 90% participation, respectively.

Prevalence and remission by age

  1. Top of page
  2. Abstract
  3. Background
  4. Participants and methods
  5. Prevalence and remission by age
  6. Risk factors by age
  7. Prevalence and risk factors by time
  8. References

From age 7–8 to 11–12 years, the prevalence of physician-diagnosed asthma increased while the prevalence of current wheeze decreased (Table 1). At age 11–12 years, one-third had ever experienced wheeze. This demonstrates that all wheeze is not asthma and how this ratio varies with age, explaining some of the variability in prevalence of ‘asthma’ across studies. It also reflects how the wide spectrum of wheeze in children shifts towards clinically recognizable asthma symptoms in pre-teenagers. The yearly remission was 10% of which half relapsed, reflecting the variability of asthma at this age. Remission was strongly negatively related to allergic sensitisation (P < 0.01), in keeping with previous findings (3).

Table 1.  Prevalence (%), life-time prevalence and asthma remission from age 7–8 to 11–12 years. Reprinted with permission from the journal and Bjerg et al. 2006 Allergy; 61: 549–55 (6)
 Age (years)
7–8 n = 34308–9 n = 34539–10 n = 344610–11 n = 340611–12 n = 33957–8 vs 11–12 P-value
  • *

    Measured only in children in Kiruna and Luleå, n = 2148 in 1996 and n = 2156 in 2000.

  • Physician-diagnosed asthma and either symptoms or use of medications the previous year, and no symptoms or use of medications the present year.

  • No symptoms or medications for the rest of the study period.

Prevalence
 Physician-diagn asthma5.76.57.17.77.7<0.001
 Current wheeze11.710.710.29.79.40.001
 Allergic sensitisation*20.6   30.9<0.001
Life-time prevalence
 Ever wheeze21.027.130.833.034.7<0.001
Remission10.210.37.511.8
Lasting remission4.06.25.2

Risk factors by age

  1. Top of page
  2. Abstract
  3. Background
  4. Participants and methods
  5. Prevalence and remission by age
  6. Risk factors by age
  7. Prevalence and risk factors by time
  8. References

At age 7–8 years, there were several significant risk factors for asthma, many of which lost importance with increasing age (Table 2). Allergic sensitisation and hereditary asthma, however, retained their strength of association also at age 11–12 years (Table 2). Moreover, these factors have previously been shown to decrease remission probability (3, 13), which suggests late-onset asthma may have lower remission probability.

Table 2.  Risk factors for current asthma at age 7–8 and 11–12 years, by multivariate analysis. All displayed variables were included in the model, and the study population was limited to the skin-prick tested children. Statistically significant (P < 0.05) associations in bold text. Reprinted with permission from the journal and Bjerg et al. 2006 Allergy; 61: 549–55 (6), and the journal and Bjerg et al. 2007 Pediatrics 120: 741–8 (10)
Risk factorAge 7–8Age 11–12
OR95% CIOR95% CI
  • *

    Corrected for damp house, birth weight <2500 g, male sex and respiratory infections.

Male sex1.61.1–2.31.20.8–1.7
Breast-fed ≤3 months1.30.8–2.01.10.7–1.7
Birth weight <2500 g2.61.2–5.40.90.4–2.3
Respiratory infections2.11.4–3.32.20.9–5.4
Maternal smoking1.51.0–2.31.41.0–2.1
Cat ever at home0.70.4–1.20.50.3–0.9
Living in damp house2.21.5–3.30.90.4–2.1
Allergic sensitisation4.93.3–7.25.63.9–8.2
Family history of asthma3.02.1–4.52.82.0–3.9
 Mother*2.81.9–4.1
 Father*3.72.6–5.4
 Mother + Father*10.04.4–22.9

For asthma at age 7–8 years, a family history of asthma was a considerably stronger risk factor than a family history of allergic rhinitis/eczema (Table 2, Fig. 1) (10). Sibling asthma was not a significant risk factor in absence of parental asthma and should not be used to identify children at risk. There was no statistically significant (P = 0.51) difference in the effect of maternal or paternal asthma (Table 2). It seems that in younger children, maternal asthma may be more important (14, 15), but results are inconclusive(16). More than one third of children with asthma in both parents have asthma at age 7–8 years (Fig. 1) (10), and the effect of two asthmatic parents is multiplicative (Table 2).

image

Figure 1. Prevalence (%) of current asthma in relation to parental asthma, parental allergy (allergic rhinitis/eczema), child's skin-prick test (SPT) result, and combinations of these. Reprinted with permission from the journal and Bjerg et al. 2007 Pediatrics 120: 741–8. (10)

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Prevalence and risk factors by time

  1. Top of page
  2. Abstract
  3. Background
  4. Participants and methods
  5. Prevalence and remission by age
  6. Risk factors by age
  7. Prevalence and risk factors by time
  8. References

To adequately study time trends, repeated surveys of similar (age, residence) populations using identical methods at different time points, are needed. Using the 1996 cohort as reference, a new cohort was identified 10 years later, in 2006. This cohort likewise consisted of all school children (n = 2704) in first and second grade, from the same towns. The same questionnaire as in 1996 was distributed and 96% of the children participated. The children in Kiruna and Luleå were invited to skin-prick tests using the same method as in 1996, and 90% participated.

This study will address whether the prevalence of asthma and wheeze has ceased to increase in Sweden from 1996 to 2006. In the worldwide International Study of Asthma and Allergies in Childhood (ISAAC) study (9), no increase in current wheeze was found in the Swedish center, but only 64% participated. Moreover, other asthma indices, such as diagnoses or medication use, were not studied.

Further, risk factors for wheeze and asthma will be compared between the 1996 and 2006 cohorts. The prevalence of each factor as well as the strength of association (risk/odds ratio) will be used to calculate the adjusted population attributable fraction, an estimate of the impact of each risk factor. Thus, trends in risk factors parallel to trends in disease can be studied. At present, these data are being analyzed, some of the findings were included in the thesis. Manuscripts are planned for submission during 2008.

References

  1. Top of page
  2. Abstract
  3. Background
  4. Participants and methods
  5. Prevalence and remission by age
  6. Risk factors by age
  7. Prevalence and risk factors by time
  8. References