Thickened airway walls, which correlate with airway obstruction and/or occlusion, are greatly increased in patients with severe asthma, according to study results published in the American Journal of Respiratory and Critical Care Medicine.
Researchers conducted a study (ClinicalTrials.gov Identifier: NCT02351141) to determine whether computed tomography (CT) total airway count (TAC) could be a reliable indicator of airway structure and function in asthma.
The study included patients with asthma between the ages of 18 and 70 years with a <1 pack-year smoking history (N=70). The researchers performed postbronchodilator inspiratory CT, as well as pre- and post-bronchodilator spirometry and hyperpolarized 3He magnetic resonance imaging (MRI). The sum of airways in the segmented airway tree was used to measure CT TAC in all patients. In addition, the investigators measured airway wall area percent and lumen area. Finally, MRI ventilation abnormalities were also assessed using the ventilation defect percent.
Patients were stratified by asthma severity based on Global Initiative for Asthma (GINA) treatment steps, including GINA 1-3 (mild asthma), as well as GINA 4 and GINA 5 (severe asthma). Compared with patients with mild asthma, patients with GINA 4 asthma had significantly worse forced expiratory volume in 1 second (FEV1;88%±20%pred vs 64%±19% pred, respectively; P =.003) and FEV1/forced vital capacity (FVC; 74%±11% vs 58%±17%, respectively; P =.006). In a similar fashion, patients with GINA 5 asthma also had significantly worse FEV1 (65%±22%pred vs 88%±20%pred; P =.004) and FVC (80%±18%pred vs 94%±14%pred; P =.03), wall area percent (68.2%±1.7% vs 66.7%±1.5%; P =.006), and ventilation defect percent (10%±8% vs 3%±2%; P =.02) vs patients in the mild asthma subgroup (GINA 1-3).
In the majority of patients (n=69), sub-subsegmental airways were missing or invisible on CT, with 10 being the most common number of missing sub-subsegments in the overall cohort. Patients with ≥10 missing subsegments demonstrated significantly worse wall area percent (68.6%±1.4% vs 66.8%±1.3%; P <.0001), lumen area (9.1±1.9 mm2 vs 11.8±2.2 mm2; P <.0001), and ventilation defect percent (7%±7% vs 11%±9%; P =.03) relative to patients with <10 missing subsegments.
A multivariable model found TAC (β=0.50; P =.001) to be an independent predictor of FEV1 (R2=0.27; P =.003). In addition, TAC (β=-0.53; P <.0001) was an independent predictor of airway wall area percent in a separate model (R2=0.32; P =.0001).
A limitation of the study included the convenience sampling of patients with more severe disease, rather than a population-based sample that included patients with a range of disease severities. Therefore, the study was underpowered to individually evaluate asthma considered GINA 1-3.
The researchers acknowledged that their study offers more questions than it answers, stating “[h]ow TAC may change with treatment or over time in patients with asthma remains to be determined.”
Eddy RL, Svenningsen S, Kirby M, et al. Is computed tomography airway count related to asthma severity and airway structure-function? [published online January 2, 2020]. Am J Respir Crit Care Med. doi:10.1164/rccm.201908-1552OC