Response to mannitol in asymptomatic subjects with airway hyper-responsiveness to methacholine

BACKGROUND: Bronchial provocation using methacholine, a cholinergic agonist, causes airway narrowing directly by contraction of bronchial smooth muscle. While methacholine has a high sensitivity for identifying airway hyper-responsiveness (AHR), it does not have a high specificity to diagnose asthma and false-positive responses may be observed in non-asthmatics. Mannitol is an osmotic stimulus that acts indirectly to cause airway narrowing by release of endogenous bronchoconstricting mediators.

OBJECTIVES: We tested the hypothesis that subjects with asymptomatic AHR to methacholine would not have AHR to mannitol.

METHODS: Sixteen subjects with a methacholine PD(20) <8 micro mol were challenged with mannitol. A positive response to mannitol was defined as a 15% decline in forced expiratory volume in 1 s (FEV(1)) after <635 mg (PD(15)). Expired nitric oxide (eNO) and blood eosinophils were also measured.

RESULTS: The GM PD(20) for methacholine was 2.25 micro mol [95% confidence interval (CI): 2.19-5.29], the mean eNO was 14.7 p.p.b. (CI: 10.1-19.4) and the eosinophil count was 0.20 x 10(-9)/L (CI: 0.14-0.27 x 10(-9)/L). Only one subject (a smoker, 10 pack-years, FEV(1) 76% pred, non-allergic rhinitis, normal eNO and eosinophil count) also had a mild positive response to mannitol (PD(15): 451 mg).

CONCLUSIONS: The response to mannitol was within the normal range in asymptomatic subjects with AHR to methacholine. Further evidence on the responsiveness to mannitol compared with methacholine in a random population sample is required to elucidate whether mannitol is a more specific test for diagnosing asthma.