Study design and population
This retrospective observational study was conducted at the Pediatric Outpatient Continuity Respiratory Clinic of Songklanagarind Hospital, the university hospital in Southern Thailand. This research was approved by Human Research Ethic Committee (REC. 65-059-1-1), Faculty of Medicine, Prince of Songkla University, Thailand. The written informed consent was waived because of the restrospective nature of the data. Patient information was collected from the hospital records using the International Classification of Diseases, 10th revision codes. Medical records of children that attended the clinic between 2015 and 2020 with one of the following diagnoses were recruited for initial review: R06.2 (wheezing) and J45 (asthma). Based on the initial review, data of children who met all the following criteria were enrolled as study participants: (1) ≤ 5 years old with recurrent wheezing; (2) initial treatment with regular daily low dose ICS for at least 3 months (therapeutic trial) with good compliance (ICS used > 80% of treatment period); (3) symptoms disappeared without controller medication before the age of six; and (4) follow-up duration at least 1 year after discontinuation of ICS therapy. Children were excluded from the study if they met any of the following criteria: (1) pre-existing congenital airway anomalies such as vascular rings or, bronchial atresia (2) bronchopulmonary dysplasia or chronic lung disease; or (3) incomplete data on important variables (i.e., perinatal history, comorbidities, compliance, and medication) in the medical records.
Data collection
Patient information was collected from medical records at the initial and follow-up visits. The data collected included sex, age at onset of the first wheeze, perinatal use of oxygen, type of feeding (bottle or breast) at the age of six months, family history of asthma, environmental exposure before the age of six (for example, living in an industrial area, smoking exposure, pet ownership, and daycare attendance), wheezing exacerbation related to seasonal changes (seasonal wheezing), comorbidities (allergic rhinitis or, atopic dermatitis), blood eosinophil counts before the initiation of ICS therapy, results of the skin prick test performed at any age, history of respiratory syncytial virus (RSV) infection during the study period, and duration of ICS use.
Outcome measurement
After initial treatment with an ICS therapeutic trial, symptom responsiveness was evaluated and classified into two categories taking ICS prescription into account: (1) symptoms control and lack of wheezing exacerbations without ICS medication until the age of six (successful ICS cessation), or (2) presence of symptoms or wheezing exacerbations and the need for ICS to maintain symptomatic control (recommencement of ICS required). The primary outcomes were the potential factors associated with less successful ICS cessation after the initial therapeutic trial of ICS in preschool wheezers.
Sample size calculation
Sample size calculation was based on a previous study by Sitthisarunkul et al. [9], which reported the predictors of ICS cessation in preschool wheezers. The calculated sample size for a two-tailed alpha of 0.05 and predictive power of 80% was 81 subjects. Estimating that 20% of the data would be missing, a final sample size of 98 participants was determined.
Operational definitions
Recurrent wheezing was defined as wheezing episodes occurring more than three times per year. Allergic sensitization to aeroallergens was said to have occurred if the skin prick test was positive for least one aeroallergen. Perinatal oxygen use was defined as oxygen therapy within seven days after birth. Symptoms control was as per the information provided by their caregiver over the previous 4 weeks. Satisfactory symptom control was defined as the absence of the following: (1) daytime asthma symptoms (2) activity limitation due to asthma (3) use of a short acting beta-2-agonist (SABA) reliever more than once a week, and (4) night waking or night coughing due to asthma. Symptoms of an exacerbation included any of the following, (1) presence symptoms of a respiratory tract infection (2) an acute or subacute increase in wheeze and shortness of breath (3) an increase in coughing, especially while asleep (4) reduced exercise tolerance (5) impairment of daily activities, including feeding, and (6) poor response to reliever medication.
Statistical analysis
All statistical analyses were performed with R software, version 4.1.2 (R Foundation for Statistical Computing, Vienna, Austria). For continuous variables, normality was tested using the Shapiro-Wilk test. The Wilcoxon rank-sum test with continuity correction was used if continuous variables were not normally distributed (the age at onset of the first wheeze, duration of ICS use, blood eosinophil counts). Categorical variables were presented as frequency and percentage (sex, breast feeding, perinatal oxygen use, parental asthma, environmental exposures, allergic rhinitis, and atopic dermatitis). We used the Fisher exact test or a Chi-square test to compare characteristics of the preschool wheezers with and without the outcome. Multiple logistic regression was performed to identify the independent predictors of successful ICS cessation. The covariates with P < 0.2 on univariate analysis were included in the multivariate model. The variable remaining in each final multivariate model was chosen based on backward elimination. P < 0.05 was considered statistically significant.
Prediction model development
We chose the relevant factors associated with successful ICS cessation in transient wheezer using logistic regression. We estimated the impact of predictors using a nomogram [library(nomogramFormula)]. We recorded all potential predictors with more than two response categories into multiple binary variables. The final prediction model allowed the calculation of a predictive score and the probability of successful ICS cessation. To assess performance and determine the discriminative ability of the model, we plotted the receiver operating characteristic curve (ROC) and calculated the area under the curve (AUC) [library(pROC)]. The AUC can take on values from 0 to 1. Discrimination is considered not better than chance if the AUC is 0.5 (moderate discrimination if AUC 0.6-0.8, and good discrimination if AUC > 0.8). For internal validation of our model, we used bootstrap validation and constructed a calibration plot for visualization [library(rms)]. On the calibration plot, a perfect calibration curve would lie exactly on the diagonal line.