Confounding with familial determinants affects the association between mode of delivery and childhood asthma medication
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ALLERGY, ASTHMA & CLINICAL IMMUNOLOGY
RESEARCH
Open Access
Confounding with familial determinants affects the association between mode of delivery and childhood asthma medication – a national cohort study Lennart Bråbäck1,2,8*, Cecilia Ekéus3, Adrian J Lowe1,4,5 and Anders Hjern6,7
Abstract Background: Mode of delivery may affect the risk of asthma but the findings have not been consistent and factors shared by siblings may confound the associations in previous studies. Methods: The association between mode of delivery and dispensed inhaled corticosteroid (ICS) (a marker of asthma) was examined in a register based national cohort (n=199 837). A cohort analysis of all first born children aged 2-5 and 6-9 years was performed. An age-matched sibling-pair analysis was also performed to account for shared genetic and environmental risk factors. Results: Analyses of first-borns demonstrated that elective caesarean section was associated with an increased risk of dispensed ICS in both 2-5 (adjusted odds ratio (aOR)=1.19, 95% confidence interval (CI) 1.09-1.29) and 6-9 (aOR=1.21, 1.09-1.34) age groups. In the sibling-pair analysis, the increased risk associated with elective caesarean section was confirmed in 2-5 year olds (aOR=1.22, 1.05-1.43) but not in 6-9 year olds (aOR=1.06, 0.78-1.44). Emergency caesarean section and vacuum extraction had some association with dispensed ICS in the analyses of first-borns but these associations were not confirmed in the sibling-pair analyses. Conclusions: Confounding by familial factors affects the association between mode of delivery and dispensed ICS. Despite this confounding, there was some evidence that elective caesarean section contributed to a modestly increased risk of dispensed ICS but only up to five years of age. Keywords: Asthma, Caesarean section, Child, Epidemiology, Inhaled corticosteroids, Sib pair analysis
Introduction A moderate (20%) increase in asthma prevalence has been described in children delivered by caesarean section [1,2]. A number of studies have also suggested that instrumental vaginal delivery (vacuum extraction and forceps) may contribute to the development of asthma [3-5]. This represents a major public health concern, as there has been a substantial increase in deliveries by caesarean sections and vacuum extractions in high and middle-income countries all over the world [6]. * Correspondence: [email protected] 1 Occupational & Environmental Medicine, Department of Public Health and Clinical Medicine, Umeå University, Umeå, Sweden 2 Department of Research and Development, Västernorrland County Council, Sundsvall, Sweden Full list of author information is available at the end of the article
Most previous studies have used a simple dichotomised categorisation of deliveries as being vaginal or caesarean, not taking into account the important differences between elective and emergency caesarean sections, and unaided and assisted vaginal deliveries. It has been hypothesed that the increased risk of asthma in children born by caesarean section is due
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