Background: Cough is a common feature of asthma, which is often resistant to inhaled corticosteroids (ICSs). The pathophysiology of this refractoriness may differ between daytime and nighttime asthmatic cough. We sought to identify factors contributing to ICS-refractory daytime and nighttime asthmatic cough.
Methods: Sixty-seven patients with asthma presenting solely or predominantly with chronic cough were prospectively enrolled from April 2012 to December 2014. At baseline and 12 weeks after ICS treatment, the capsaicin cough threshold (C2, C5) and methacholine airway sensitivity and reactivity were examined. A visual analog scale (VAS) and numeric scores were used to evaluate daytime and nighttime cough symptoms separately. The Japanese version of the Leicester Cough Questionnaire was also completed. When either the VAS or numeric scores showed an improvement of ≥50% or ≥2 points, patients were considered responders to ICS treatment.
Results: Fifty-five patients were eligible for evaluation. Subjective cough indices improved significantly at 12 weeks after ICS treatment (P<.001). Multivariate analysis revealed that lower C2 significantly contributed to residual daytime cough (P=.04). Meanwhile, methacholine hyperreactivity and lower IgE levels were predictors of the nighttime residual cough (P=.002 and P=.03, respectively).
Conclusions: Heightened cough reflex sensitivity is an independent factor of daytime asthmatic cough that is refractory to ICSs. In contrast, airway hyperreactivity and less atopic status contribute to ICS-refractory nighttime cough.

Key words: Asthmatic cough, Cough sensitivity, Airway reactivity, IgE, Inhaled corticosteroids