Small Airway-Predominant Asthma Revealed During Oscillometry Mannitol Challenge Despite Normal Spirometry

Objective confirmation of asthma commonly relies on spirometry, which may not capture distal airway dysfunction. A 58-year-old woman with episodic exertional dyspnoea and chest tightness had normal baseline spirometry, including forced expiratory volume in 1 s (FEV1), FEV1/forced vital capacity and forced expiratory flow at 25%-75% of vital capacity. Mannitol challenge was negative by the conventional FEV1 criterion, with no clear obstructive change in the flow-volume curves. Functional computed tomography demonstrated widespread lobular air trapping and ventilation heterogeneity, confirming small-airway involvement. At peak exposure, oscillometry showed abnormal resistance and reactance that improved after bronchodilator: R5 by 30%, R5-R20 by 56% and reactance area by 73%. After inhaled triple therapy, symptoms resolved and oscillometry normalised. The concordant clinical, imaging, physiological and longitudinal findings established small airway-predominant asthma. Oscillometry provided complementary physiological confirmation when standard spirometric indices remained normal.