Sensory hyperreactivity – a possible mechanism underlying cough and asthma-like symptoms

Background Investigations of patients referred for suspected asthma have revealed a little-known group with symptoms suggesting hyperreactive airways in whom provocation with methacholine does not lead to bronchial obstruction. The underlying mechanisms are not known, and no objective diagnostic method has been available.
Methods Provocations by inhalation of capsaicin solutions in stepwise increasing concentrations were used. Ten patients with asthma-like symptoms after exposure to nonspecific irritating stimuli, but without IgE-mediated allergy or demonstrable bronchial obstruction, were compared to 10 patients with verified bronchial asthma and 28 healthy controls. Results The patients with asthma-like symptoms reacted with cough in a dose-dependent way. The number of coughs was significantly greater than in asthmatic patients and healthy controls. The latter two groups did not differ significantly.
Conclusions The capsaicin provocation test may be a valuable method for showing not only a greater cough sensitivity, but also asthma-like symptoms. The pathophysiology underlying the symptoms may be related to increased sensitivity of free, overactive nerve endings in the respiratory mucosa. Therefore, we suggest that this overreaction in the lower airways be called "sensory hyperreactivity".