Small airways (internal diameter ≤2mm or internal perimeter of ≤6mm) are located beyond the 7th and 8th generation of the tracheobronchial tree and account for >98% of the cross-sectional area of the lung, ending in the alveolar sacs. They have no cartilage to support their structure and collapse easily on constriction. Small airways are narrowed in patients with asthma as a result of inflammation.
Postma et al describe SAD as ‘a complex and silent signature of asthma’. Small airways are considered ‘silent’ because of their relatively low impact on the whole resistance of the respiratory tree compared to larger airways.
In the AssessmenT of smalL Airways involvemeNT In aSthma (ATLANTIS) study, Postma et al investigated which combination of biomarkers, physiological tests and imaging markers best measure the presence and extent of SAD in asthma patients.
The study is the most comprehensive evaluation of SAD to date and included 773 patients with a confirmed diagnosis of asthma, and 99 controls without airway obstruction, normal spirometry as well as airways responsiveness, and smoked for a maximum of 10 pack-years.
Asthma patients’ disease had to be under control using any previous regular asthma treatment (including so-called rescue β2-agonists alone) at a stable dose for more than eight weeks before baseline, and they smoked for a maximum of 10 pack-years in their lifetime.
Diagnosis had to be supported by objective evidence at baseline or during the past five years.
All participants were assessed using spirometry, body plethysmography, impulse oscillometry and multiple breath nitrogen washout. Patients with asthma were also assessed using questionnaires about disease control, asthma-related quality of life, as well as health status. Selected participants also underwent computed tomography (CT) scans.
Postma and team found that SAD was present in asthma of all severities and showed proportionally more structural and functional abnormalities with increasing clinical severity.
The authors found that the difference between the two clinical SAD groups was particularly clear with SAD measurements related to spirometry and impulse oscillometry. They concluded that these two tests were the most effective in identifying SAD in clinical practice because they are easy to use. Postma et al concluded that the clinical classification of SAD into two groups using impulse oscillometry and spirometry is meaningful given its association with GINA severity stages, asthma control, quality of life and exacerbations.
According to Zinellu et al, evaluating the functional and inflammatory impairment of small airways should be part of the routine evaluation of all asthma patients, and not just to measure disease severity. Assessment could help in identifying the clinical phenotype and guide treatment decisions.
REFERENCES: Available on request.