Airway hyperresponsiveness

AHR is a defining feature of asthma, where an external stimulus causes bronchospasm and airway narrowing, limiting airflow. This narrowing arises from interactions between airway inflammation and changes in airway smooth muscle reactivity, resulting in excessive contractility. AHR is assessed with direct bronchial provocation tests acting directly on airway smooth muscle, or alternately assessed with indirect tests, acting via triggering release of bronchoconstricting mediators from inflammatory cells. A positive direct test with methacholine typically indicates asthma, but is not very specific, whereas a negative test suggests alternative diagnoses. A positive indirect test with mannitol strongly suggests active asthma, whereas a negative test may be seen in milder or well-controlled ICS-treated asthma, and hence does not exclude a diagnosis of asthma. Beyond diagnostics, bronchial provocation tests assess specific pathological asthma traits, extending the utility to assessing disease activity, phenotyping and prognosticating patients as well as evaluating treatment response.