Airway hyperresponsiveness: the usefulness of airway hyperresponsiveness testing in epidemiology,in diagnosing asthma and in the assessment of asthma severity

The present PhD thesis was conducted at the Respiratory Research Unit at the Pulmonary Department L in Bispebjerg Hospital, Copenhagen, Denmark and describes airway hyperresponsiveness in asthma patients in four studies. The first study concerned risk factors for the development of asthma in young adults in a 12‐year prospective follow‐up study of a random population sample of 291 children and adolescents from Copenhagen, who were followed up from the age of 7–17 years (1986) until the age of 19–29 years (1998). During follow‐up, 16.1% developed asthma, and in these subjects, the most important predictor of asthma development was airway hyperresponsiveness to histamine at baseline. Airway hyperresponsiveness is associated with more severe asthma and a poorer prognosis in terms of more exacerbations and less chance of remission of the disease. The second study described the relation between airway hyper‐responsiveness to methacholine and the quality of life in 691 asthma patients: In asthma patients with airway hyperresponsiveness to methacholine, the quality of life measured with a validated questionnaire (Junipers Asthma Quality of Life Questionnaire) was significantly reduced compared to asthma patients who did not respond to bronchial provocation with methacholine. Airway hyperresponsiveness is not uncommonly observed in non‐asthmatics, and the response to bronchial provocation with methacholine is therefore relatively non‐specific. The mannitol test is a relatively new bronchial provocation test that acts indirectly on the smooth airway muscle cells through the release of mediators from inflammatory cells in the airways; the mannitol could consequently be a more specific test compared with methacholine. The third study showed that out of 16 non‐asthmatics with airway hyperresponsiveness to methacholine, 15 did not respond to bronchial provocation with mannitol Because of the mechanism of action of mannitol, it seems plausible that the response to mannitol is more closely correlated to airway inflammation in asthma compared with the response to methacholine. The fourth study showed that in 53 adult asthma patients, who did not receive treatment with inhaled steroids, there was a positive correlation between the degree of airway inflammation and the degree of airway responsiveness to mannitol as well as to methacholine. The mannitol does, however, have the advantage of being a faster and simpler test to perform, requiring no additional equipment apart from a spirometer. Conclusions: Airway hyperresponsiveness in children and in adolescents without asthma predicts asthma development in adulthood. Asthma patients with airway hyperresponsiveness to methacholine have a poorer quality of life as well as more severe disease and a poorer prognosis compared with asthma patients without airway hyperresponsiveness. Bronchial provocation with mannitol as well as with methacholine were useful for evaluating the severity of asthma and the degree of airway inflammation, and accordingly for determining the need for steroid statement. The mannitol test does, however, have practical advantages over the methacholine test that make it preferable for clinical use.