Determinants of airway hyperresponsiveness in mild asthma

Patients with mild asthma may have coexisting severe airway hyperresponsiveness (AHR), although the reasons for this are uncertain.

To evaluate the factors that determine AHR in mild asthma.

We performed a retrospective database evaluation of two groups of patients with mild asthma with forced expiratory volume in 1 second (FEV1) of 80% or more than predicted. Group A (n = 92; mean inhaled corticosteroid dose, 491 microg) had moderate-to-severe AHR to methacholine (provocative dose causing a 20% decrease in FEV1 [methacholine PD20], < or = 100 microg), whereas group B (n = 92; mean inhaled corticosteroid dose, 509 microg) had borderline AHR (methacholine PD20, > or = 800 microg). Both groups were matched for age, sex, inhaled corticosteroid use, and FEV1.

From our database, we found 361 patients with an FEV1 of 80% or more than predicted of whom 123 (34%) had a methacholine PD20 of 100 microg or less and 138 (38%) had a methacholine PD20 of 800 microg or more. The methacholine PD20 geometric means (geometric SE) of groups A and B were 25 microg (3 microg) and 5,392 microg (295 microg), respectively. Despite matched mean values for FEV1, compared with group B, group A had a lower predicted forced expiratory flow between 25% and 75% (71% vs 81%, P = 0.007). A greater proportion of group A compared with group B patients were sensitized to house-dust mite (76% vs 54%, P = 0.002). No significant differences were found between groups in terms of presence of rhinitis and sensitization to other individual aeroallergens.

Increased sensitization to house-dust mite and reduced small airway caliber were associated with moderate-to-severe AHR in mild asthma. Skin prick testing to common aeroallergens, especially house-dust mite, should be a routine part in the evaluation of asthmatic patients, including those patients with mild disease.