Significance of small airway involvement in asthma

Asthma, a chronic inflammatory airway disease, exhibits heterogeneity and is characterized by bronchial hyperresponsiveness to various stimuli. This hyperresponsiveness can resolve spontaneously or with treatment. In the past, asthma was commonly perceived as a condition mainly affecting the larger airways with the small airways termed a “quiet zone” believed to account for only less than  10% of the airway resistance. Emerging evidence now suggests that airway inflammation in asthma extends to the small airways, particularly in individuals with severe asthma. The small airways within the lungs refer to bronchial passages with a diameter of less than 2 mm, positioned beyond the 7th or 8th generation of the tracheobronchial tree.  The involvement of small airways disease (SAD) has significant implications for asthma control, severity, and the risk of exacerbations. The evaluation of the impact of small airways disease on severe asthma is hindered by the absence of a practical gold-standard measure for the small airways. Nonetheless, numerous published studies utilize surrogate markers, such as physiologic assessments and imaging techniques, to implicate small airways dysfunction in heightened symptoms, exacerbation risk, and asthma severity. The diagnosis of SAD can be done using several techniques, including spirometry, plethysmography, nitrogen washout, impulse oscillometry, and cross-sectional imaging. Patients with SAD seem to benefit from inhaled medications that possess ultrafine particles capable of reaching the distal airways effectively. When designing treatment strategies, the extent of SAD involvement and the use of devices with optimized particle delivery to the small airways should be taken into consideration.