Differences in sensitivity, maximal response and position of the concentration-response curve to methacholine between asthmatics, patients with allergic rhinitis and healthy subjects.

The aim of this study was to detect differences in maximal response and position of the concentration-response curves to methacholine between asthmatics and subjects with allergic rhinitis. A total of 228 adults (107 mild asthmatics, 96 allergic rhinitics and 25 healthy control subjects) were challenged with methacholine. The test was interrupted when FEV1 dropped by more than 40% or when the highest concentration of methacholine (200 mg ml-1) had been administered. Concentration-response curves were characterized by their PC20 (concentration of methacholine that produced 20% fall in FEV1 = airway sensitivity), and if possible, by their EC50 (concentration of methacholine that produced 50% of the maximal response = position) and level of plateau. The proportion of subjects with plateau was significantly lower in asthmatics (18.7%) than in either allergic rhinitics (57.3%) or healthy subjects (92%). It was also significantly lower in allergic rhinitics than in healthy subjects. The level of plateau for asthmatics was (means +/- SD) 31.5 +/- 5.5%, compared with 20.8 +/- 8.1% in allergic rhinitics and 13.7 +/- 6.7% in healthy subjects (P < 0.01). It was also higher in allergic rhinitics than in healthy subjects (P < 0.01). The EC50 values were decreased in asthmatics when they were compared with either allergic rhinitics or healthy subjects (geometric mean EC50: asthmatics = 2.7 mg ml-1, allergic rhinitics = 6.2 mg ml-1, healthy subjects = 8.7 mg ml-1; P < 0.01), but no significant differences were detected between allergic rhinitics and healthy subjects. These results demonstrate that in subjects with allergic rhinitis, the prevalence and level of the plateau on the methacholine concentration-response curve is intermediate between that of asthmatics and normals. Furthermore, while the asthmatic curves differ from normal in having both an increased maximal response and a leftward shift, the rhinitic curves differ only in terms of plateau level. These results suggest that airway responsiveness in asthma and allergic rhinitis may be a consequence of mechanisms that are at least partially different.