Introduction and objectives Cough is generally considered a protective airway reflex, however emerging evidence suggests chronic coughing is provoked by noxious sensations from the airway and serves to relieve these sensations. We have found patients with unexplained chronic coughing identify sensations of irritation, tickle (throat) and the urge to cough (UTC) as important sensations provoking coughing. We hypothesised that inhaling low dose tussive agents would evoke similar sensations and could provide a model for investigating chronic cough.
Methods Twelve chronic cough patients (mean age 61.4 yrs, 75% female, median cough duration 7 yrs) and 10 healthy volunteers (mean age 48.8 yrs, 40% female) inhaled increasing concentrations of citric acid from a dosimeter (0.01–4 M, 18 ascending concentrations). Following each inhalation subjects rated irritation, tickle, UTC and taste on 100 mm visual analogue scales (VAS; 0mm = none and 100 mm = worst). The experiment continued until subjects coughed at least twice on any concentration of citric acid (C2). Somatosensory amplification score (SSAS) and State Trait Anxiety Index (STAI) were also collected. For the analysis, VAS data were aligned by the C2 concentration and sensation VAS scores compared using Mann-Whitney U tests. STAI scores were also compared with Mann-Whitney U tests and SSAS with an independent T test.
Results The chronic cough patients had a much lower C2 than healthy controls (median 0.094 vs. 0.5M, p = 0.009). The UTC VAS and coughs evoked were similar at C2 and for the preceding concentrations in both groups, Figure 1. However, tickle, irritation and taste were rated more highly in healthy volunteers compared with chronic cough patients at C2 and for several preceding concentrations. For example, at C2, irritation VAS was significantly higher in healthy controls (dp = 0.035) and tickle VAS was borderline significant (p = 0.052) compared with chronic cough patients, however taste differences were not significant (p = 0.29). SSAS, STAI state and trait were not significantly different between the groups (p = 0.23, p = 0.096 and p = 0.62 respectively).