Background: For patients with idiopathic chronic cough, a subgroup is recognised with respiratory symptoms induced by scents and chemicals. The diagnosis of sensory hyperreactivity (SHR) has been suggested for this group of patients and can be made using a capsaicin inhalation test. The aim of the present study was to compare the results of inhaling capsaicin by tidal breathing with those obtained by the dosimeter method regarding repeatability, agreement, and ability to distinguish patients with SHR from healthy controls.
Methods: A total of 15 patients with chronic cough due to SHR and 15 healthy control subjects underwent a randomised cross-over protocol and were provoked in a double-blind, randomised fashion with vehicle and two concentrations of inhaled capsaicin, using either the tidal breathing or dosimeter method, in a total of four challenges opportunities, two with each method.
Results: Patients coughed more and showed more respiratory symptoms than healthy controls with each dose of capsaicin. Compared with tidal breathing, inhalation of capsaicin with the dosimeter method caused a significantly greater number of coughs and respiratory symptoms in both patients and controls. Among the patients, the mean number of coughs after inhalation of 1 mL of capsaicin 0.4 micromol/L from the first provocation with tidal breathing was 12 (95% CI: 7; 17) and after inhalation from the first provocation with the dosimeter method 32 (95% CI: 19; 46) (P < 0.05). Both methods showed good repeatability and similar ability to distinguish patients with SHR from healthy control subjects.
Conclusions: For patients with SHR, capsaicin cough sensitivity is increased and repeatable. The dosimeter method caused more coughs and other respiratory symptoms than the tidal breathing method, indicating that the methods cannot be used interchangeably. Knowledge of the type of inhalation device used, the particle size, the airflow rate and the inspiratory flow rate are essential when comparing different studies of capsaicin-induced cough.