Prevalence of serious eosinophilia and incidence of Churg-Strauss syndrome in a cohort of asthma patients

Background: Some leukotriene receptor antagonists, such as zafirlukast and montelukast, have been associated with systemic eosinophilia, with interest focused on Churg-Strauss syndrome (CSS). Objective: To calculate the background incidence rate of CSS and prevalence of eosinophilia among people with asthma who have not used leukotriene receptor antagonists. Methods: We conducted a cohort study in the setting of three geographically diverse UnitedHealthcare health plans. We identified 36,230 people who received a diagnosis of asthma during the period October 1994 through September 1997. We identified 241 potential cases from the claims data. Using a standardized hospital record abstract form, nurses abstracted relevant clinical data from the hospital charts of potential cases. We applied several a priori case definitions to the abstracted clinical data and computed incidence rates of CSS among patients with asthma. We additionally used these data to compute the prevalence of serious eosinophilia. Results: Incidence rates of definite CSS among asthma patients ranged from zero (90% confidence interval 0.0 to 23.0) to 67 (90% confidence interval 22.5 to 160.6) cases per 1,000,000 person-years, depending on the definition used. All patients who met the criteria for CSS expressed symptoms consistent with mild asthma. Conclusions: This report is the first direct measurement of the incidence rate of CSS among asthma patients. We believe that the prevalence and incidence information that we report is a useful description of population rates in the United States for these conditions.