Clinical and pathophysiological roles of lower lobe–dominant mucus plugs on computed tomography in patients with asthma with and without bronchiectasis

Background: Despite the clinical relevance of mucus plugging, the role of spatial distribution of mucus plugs remains unclear in patients with asthma. Objective: We sought to examine whether greater lower lobe mucus plug dominance is associated with more clinical and pathophysiological impairments in 2 cohorts, including patients with and without bronchiectasis. Methods: Patients with asthma without and with clinical diagnosis of bronchiectasis underwent chest computed tomography at Kyoto University Hospital (Kyoto cohort) and Japanese multicenters (bronchiectasis and asthma [BEXAS] cohort), respectively. Mucus plugs in airways were visually scored on computed tomography, and the difference in mucus plug score between lower and upper-middle lobes (Δ mucus plug score) was calculated. Results: Among 176 (Kyoto) and 42 (BEXAS) enrolled patients, 82 and 33 exhibited mucus plug scores greater than or equal to 1, respectively. Higher Δ mucus plug score was associated with lower percentage of the predicted FEV₁ and the presence of exacerbation history in both cohorts. Higher Δ mucus plug score was associated with luminal narrowing of the fifth-generation, but not the third- or fourth-generation, lower lobe airways in the Kyoto cohort, and bronchiolitis score in the BEXAS cohort. In the multivariable model, higher Δ mucus plug score was associated with symptoms and exacerbations, independent of whole-lung mucus plug score in the Kyoto cohort. Conclusions: Lower lobe–dominant mucus plugs were associated with lower lung function and exacerbations in patients with asthma, irrespective of comorbid bronchiectasis. The spatial distribution of mucus plugs additionally to whole-lung mucus plug score may help to understand clinical roles of mucus plugging in asthma.