Figures
- Figure 1
A map of the world showing the location of many cohort studies (including intervention studies). ALSPAC: Avon Longitudinal Study of Parents and Children; BAMSE: Children, Allergy, Milieu, Stockholm, Epidemiological survey; CAPPS: Canadian Asthma Primary Prevention Study; CAPS: Childhood Asthma Prevention Study; COPSAC: Copenhagen Prospective Study on Asthma in Childhood; KOALA: Child, Parent and Health; Lifestyle and Genetic Constitution; MAAS: Manchester Asthma and Allergy Study; MAS: German Multicentre Asthma Study; NZACS: New Zealand Asthma and Allergy Cohort Study; PIAMA: Prevention and Incidence of Mite Allergy; PIAF: Perth Infant Asthma Follow-up; PREVASC: Prevention of Asthma in Children; PROBIT: Promotion of Breastfeeding Intervention Trial; SAGE: Study of Asthma Genes and the Environment; SIDRIA: Italian Studies of Respiratory Disorders in Childhood and the Environment.
Tables
- Table 1 Differences between the designs of asthma cohort studies. In one study [11] the majority of study participants were from an unselected population but additionally, an intervention given to high risk infants was nested within the study design
Category Examples Age at first data Pre conception cohorts [4], Birth cohorts [1], Childhood cohorts [12] Inclusion criteria Unselected cohorts [5, 11, 13–16], inclusion of “high risk infants” (e.g. at least one parent with asthma) [11, 17], inclusion of only patients with asthma [12] Respiratory specific? Respiratory and non-respiratory outcomes measured [1, 2], Respiratory/allergy specific [3, 4] Exposure Allergen exposure [11], Ambient air quality [18], Breast feeding [19], Maternal diet [20], Maternal illness during pregnancy [21], Smoking [22] - Table 2 Strengths and weaknesses of asthma cohort studies
Strengths Limitations Prospective data collection (no recall bias) Can be difficult to change study protocol, e.g. retrospectively add data, change outcome Hypothesis driven Still does not prove causation Essential for intervention studies Recruitment of representative population Longitudinal analysis can reduce sample size Drop out Analysis can minimise effect of drop out Often only considers one exposure Able to consider different exposures at different times Cost Follow up for >10 years often necessary Lack of gold standard outcome - Table 3 Results from systematic reviews relating relative risk for asthma to factors. This list is not intended to be exhaustive but serves to illustrate that each factor has a relatively modest association in isolation. What is unknown is whether relative risks are additive or multiplicative when more than one factor is present for an individual
Risk factor for developing childhood asthma Relative risk [95% CI] Comment Either parent smoke [25] 1.37 [1.15–1.64] Maternal asthma [26] 3.04 [2.59–3.56] Paternal asthma [26] 2.44 [2.14–2.79] Atopy [27] 1.5 to 4.8 Antenatal paracetamol exposure [28] 1.28 [1.16–41] Dog exposure [29] 1.14 [1.01–1.29] 1.39 [1.00, 1.95] for any furry pet Cat exposure [29] 0.72 [0.55–0.93] Breast feeding [19] 0.70 [0.60–0.81] Lower in atopic families (OR 0.52) Visible mould [30] 1.49 [1.28–1.72] Mould derived compounds may reduce risk High maternal vitamin D during pregnancy [20] 0.56 [0.42–0.73] Wheezing outcomes not asthma High maternal vitamin E during pregnancy [20] 0.68 [0.52–0.88] Wheezing outcomes not asthma Mediterranean diet during pregnancy [20] 0.22 [0.08–0.58] Wheezing outcomes not asthma - Table 4 Summarising relative weight placed on measured variables in children aged 3 years for asthma at 11–13 years (7–8 years for PIAMA)
Tucson [44] Isle of Wight [47] Leicester#[48] MAS¶ [45] PIAMA [49] Early recurrent wheeze +++ +++ 1.8 3.9 Parental asthma/atopy +++ 2.3 8.3 1.9 Eczema +++ 3.2 2.1 Early IgE sensitisation ++ 5.7 4.7 Rhinitis ++ 0.4 1.6 Wheeze without URTI ++ 2.9 Male gender 2.8 1.8 Recurrent chest infection 2.0 1.8 Ig: immunoglobulin. #: study also validated the Tucson Asthma Predictive Index (API) but concluded that early recurrent wheeze (i.e. at least three episodes by three years of age) was equivalent in precision to the API; ¶: study also described interactions between variables, odds ratios (OR) presented are for those with early recurrent wheeze. +++: greater weight; ++: intermediate weight.
