Methacholine-induced cough as an indicator of bronchodilator-responsive cough

Background: Cough variant asthma (CVA) is the most common cause of chronic cough and responds well to bronchodilator therapy. Previous studies on methacholine-induced cough have shown that heightened cough response due to bronchoconstriction is a feature of CVA. The aim of this study was to assess Mch-induced cough as an indicator of bronchodilator-responsive cough (BRC). Methods: This was a single-center retrospective study of prolonged/chronic cough cases who underwent evaluation via spirometry, FeNO and bronchial challenge testing using Mch and capsaicin (C5). Resultant bronchoconstriction after Mch challenge was assessed by flow-volume curves measuring the expiratory flow of the partial flow-volume curve 40% above residual volume (PEF40) and FEV1. BRC was defined as a decrease in cough with bronchodilator therapy by 30% or more on a visual analog scoring scale. Results: Of the 100 patients evaluated, 63 were diagnosed with BRC. Mch-induced cough at a decrease in PEF40 of 35% (PC35-PEF40) was predictive of BRC on AUROC analysis with an AUC of 0.82 (95% CI 0.73-0.90) and cut-off of 24. The AUC for C5, FeNO and PC20-FEV1 were 0.65, 0.47, and 0.58, respectively. Conclusion: Compared to C5, FeNO and PC20-FEV1, Mch-induced cough better supports a diagnosis of BRC.