Bed-sharing and childhood asthma: from associations to causal modelling.

One can hypothesise diverse influences of bed-sharing practices on respiratory disease. First, the close contact between bed-sharing parents and their offspring could lead to increased transmission of infectious agents and, thus, more virus-induced wheeze in the children. Conversely, the hygiene hypothesis suggests that contact with common pathogens in early childhood might protect from later allergic disease [2]. Lastly, parents of children with nocturnal asthma could tend to bed-share as a way to monitor their children. LUIJK et al. [1] took the challenge of investigating these associations by studying the early-life trajectories of 6160 children from the Generation R study, a population-based cohort from the Netherlands. They assessed bed-sharing twice, at ages 2 and 24 months, and wheezing at ages 1, 2, 3, 4 and 6 years. Data were analysed using generalised estimating equation (GEE) models, which allow analysis of longitudinal outcome data with several measurements per subject. The authors found no association between bed-sharing in infancy (at age 2 months) and later wheeze. In contrast, bed-sharing in toddlerhood (at age 2 years) was associated with more subsequent wheeze. The authors then used cross-lagged modelling to investigate the directions of the association. This approach compares regression coefficients between variables measured at subsequent time-points and estimates the strength of the causal effects between them. It confirmed that bed-sharing in infancy was not associated with subsequent wheezing but showed that bed-sharing in toddlers was, and the association became stronger for wheeze occurring at later ages. It found no evidence suggesting that wheezing in infancy leads to bed-sharing at age 2 years.