Bronchial Hyperresponsiveness in the Assessment of Asthma Control : Airway Hyperresponsiveness in Asthma: Its Measurement and Clinical Significance

The two key pathophysiologic features of asthma are bronchial hyperresponsiveness (BHR) and airway inflammation. Symptoms and lung function are the most accessible clinical markers for the diagnosis of asthma as well as for assessing asthma control using the most effective treatment of asthma, inhaled corticosteroids (ICS). However, BHR and inflammation usually take longer to resolve using ICS compared with symptoms and lung function. BHR can be assessed using “direct” stimuli that act on the airway smooth muscle (eg, methacholine) or “indirect” stimuli that require the presence of airway inflammation (eg, exercise, osmotic stimuli). Although there are practical limitations in using BHR to assess asthma control, efforts have been made to make BHR more accessible and standardized. Some studies have demonstrated that treatment aimed to decrease BHR with direct stimuli can lead to improved asthma control; however, it often results in the use of higher doses of ICS. Furthermore, BHR to direct stimuli does not usually resolve using ICS because of a fixed component. By contrast, BHR with an indirect stimulus indicates a responsive smooth muscle that occurs only in the presence of inflammation sensitive to ICS (eg, mast cells, eosinophils). BHR to indirect stimuli does resolve using ICS. Because ICS target both key pathophysiologic features of asthma, assessing indirect BHR in the presence of ICS will identify resolution or persistence of BHR and airway inflammation. This may provide a more clinically relevant marker for asthma control that may also lead to improving the clinical usefulness of ICS.