Seasonal risk factors for asthma exacerbations among inner-city children

Abstract

Background: Asthma exacerbations remain common, even in children and adolescents, despite optimal medical management. Identification of host risk factors for exacerbations is incomplete, particularly for seasonal episodes.

Objective: We sought to define host risk factors for asthma exacerbations unique to their season of occurrence.

Methods: This is a retrospective analysis of patients aged 6 to 20 years who comprised the control groups of the Asthma Control Evaluation study and the Inner City Anti-IgE Therapy for Asthma study. Univariate and multivariate models were constructed to determine whether patients' demographic and historical factors, allergic sensitization, fraction of exhaled nitric oxide values, spirometric measurements, asthma control, and treatment requirements were associated with seasonal exacerbations.

Results: The analysis included 400 patients (54.5% male; 59.0% African American; median age, 13 years). Exacerbations occurred in 37.5% of participants over the periods of observation and were most common in the fall (28.8% of participants). In univariate analysis impaired pulmonary function was significantly associated with greater odds of exacerbations for all seasons, as was an exacerbation in the previous season for all seasons except spring. In multivariate analysis exacerbation in the previous season was the strongest predictor in fall and winter, whereas a higher requirement for inhaled corticosteroids was the strongest predictor in spring and summer. The multivariate models had the best predictive power for fall exacerbations (30.5% variance attributed).

Conclusions: Among a large cohort of inner-city children with asthma, patients' risk factors for exacerbation vary by season. Thus information on individual patients might be beneficial in strategies to prevent these seasonal events.

Keywords: Asthma; IgE; allergy; asthma exacerbations; biomarkers; eosinophils; exhaled nitric oxide; pulmonary function; seasons.

Figure 1

Univariate associations of baseline predictors (total IgE, specific IgE's, number of positive skin tests, and eosinophils) and previous season predictors (FEV₁/FVC, FeNO, exacerbation in the previous season and ICS step requirement) with the probability of exacerbation by season. Solid lines are for marginally significant associations (p<0.10), and dashed lines are for non-significant associations.

Figure 2

Multivariate models for associations of baseline and previous season predictors of exacerbations. Baseline predictors were allergy (skin tests and total IgE) and age. Previous season predictors were FEV₁/FVC ratio, FeNO, exacerbation history and ICS step requirement. Bars length and color indicates the strength of the association between the predictor and seasonal exacerbation as measured by variance explained. The total predictive power of a multivariate model containing all 7 predictors is annotated in the bottom right hand corner. Negative associations are indicated with an asterisk.

Figure 3

Seasonal exacerbation prevalence by risk levels. Each participant had an exacerbation risk level calculated for each season using a simple index of 7 risk factors (allergy, age, FEV₁/FVC ratio, FeNO, exacerbation in the previous season, and ICS step requirement). Seasonal risk levels were then divided in to tertiles (labeled “Low”, “Medium” and “High” risk) which are plotted against observed exacerbation rates in ACE and ICATA. An additional “Very High” risk group was added for participants with a risk index in the top 10 percent.