Methods
Study design and subjects – This study is a single-center, retrospective cohort study conducted at the Amsterdam University Medical Center (UMC), a tertiary medical center. The PICU has a 12–bed facility providing intensive care treatment for the northwestern parts of the Netherlands. The demographic, social, and clinical data were extracted from the electronic hospital records of all children admitted to the PICU of the Amsterdam UMC between 2018 and 2021 with a diagnosis of SAA. The diagnosis of SAA was made by a pediatrician and based on the following definition: “Severe aggravation of bronchial obstruction due to asthma that does not improve after a few doses of a bronchodilator by inhalation”. Wheezing children aged less than two years were excluded as these children will more likely present with bronchiolitis, and differentiation between bronchiolitis and asthma is extremely difficult. Additionally, the total number of children admitted to the PICU of the Amsterdam UMC was also extracted from the electronic patient file database.
Institutional Review Board approval – The study design was reviewed by the ethical committee of the Amsterdam UMC. Owing to the retrospective and anonymous nature of the current study it was deemed that informed consent was not necessary.
Data collection – The exact dates and levels of governmental COVID-19 restrictions in the Netherlands were obtained from the Oxford COVID response tracker16. The key stringency criteria examined within the current study were the complete closure of schools, a complete working from home order, or both being in effect. Environmental exposures were defined as the amount of ambient air pollution and pollen exposure in the Netherlands. Ambient air pollution was represented by particulate material with an aerodynamic diameter of less than 2.5 microns (PM2.5) which has previously been associated with respiratory (and other) diseases17. Also, high concentrations of PM2.5 have been associated with an increase in asthma exacerbations, and children with asthma were at a higher risk of requiring an ED visit for an exacerbation18. Hourly concentrations of ambient PM2.5 were measured at the official monitoring stations of the Dutch National Institute for Public Health and the Environment (RIVM) (in concentrations of µg/m3)19. As the residential information of the patients was not available, the “Stadhouderskade” station, which was the closest measuring station to the Amsterdam UMC (with a distance of 11 km) was considered the proxy for exposures in the area. The daily amount of total pollen in the Netherlands (pollen index) was obtained from the pollen monitoring station of the Leiden University Medical Center20, located approximately 35 km. from the Amsterdam UMC. Pollen grains were collected at the roof-top level (approx. 20 m. above ground level) and counted following the requirements of the European Aerobiology Society21. The total daily values of all pollen types (in pollen/m3) were summed to give the pollen index used in this study, and then collapsed to monthly and annual averages.
Analysis – Statistical analysis was conducted using IBM SPSS Statistics for Windows version 28.0.1.1 (SPSS, Chicago IL, USA). Because of the small study population, only descriptive analyses were executed. Subgroups, based on period of time/years, were described by the means with standard deviation (for variables with a normal distribution) or median with 25th-75th percentiles (for variables not normally distributed) or by frequency with percentages for quantitative variables. Spearman correlations were calculated to evaluate the relationship between admission numbers and PM2.5 as well as pollen concentrations. Graphical representations of the collected data were generated using R version 4.2.222.