Abstract / Summary
Asthma exacerbations cause substantial morbidity, healthcare use, and long-term lung function decline, yet no diagnostic gold standard exists. Current assessment relies mainly on symptoms and spirometry, which may not capture early pathological changes. Small airway dysfunction (SAD) is increasingly recognised in asthma, particularly in exacerbation-prone patients, and may reflect distal inflammation and ventilation heterogeneity. In this narrative review, we summarise evidence on physiological tests of SAD, including impulse oscillometry (IOS), multiple breath washout (MBW), and forced expiratory flow at 25–75% of forced vital capacity (FEF25–75%), and examine whether these measures may detect exacerbation-related physiological abnormalities that are not captured by forced expiratory volume in one second (FEV1). Existing studies suggest that SAD abnormalities may accompany exacerbations, identify physiological impairment not captured by conventional spirometry, and change during recovery. However, the evidence remains limited by heterogeneous populations, test methods, thresholds, and timing of measurements, and no study has established that SAD testing improves exacerbation diagnosis or management. Large, standardised longitudinal studies are therefore needed to define clinically meaningful changes and determine whether SAD assessment provides added value beyond conventional clinical and spirometric measures.