Oscillometry-defined small airway dysfunction in steroid-naïve adult bronchial asthma: association with eosinophilic and non-eosinophilic phenotypes.

Objective: Small airway dysfunction (SAD) is a common feature of bronchial asthma. However, its association with asthma phenotypes remains poorly understood. This study aimed to assess the prevalence of oscillometry-defined SAD in steroid-naïve adult bronchial asthma and to explore its association with asthma phenotypes based on peripheral blood eosinophil count (BEC).

Methods: A total of 320 consecutive cases of bronchial asthma patients were enrolled. The severity of impairment in oscillometry parameters was expressed in z-scores. SAD in oscillometry was defined as R5-19 > upper limit of normal and/or X5 < lower limit of normal. The cohort was categorized into eosinophilic asthma (EA) and non-eosinophilic asthma (NEA) using a BEC cutoff of 300 cells/µL.

Results: The mean age of the cohort was 37.5 ± 12.5 years, and 58.1% were male. The median BEC was 350 cells/µL. The mean FEV₁ was 66.7 ± 18.4 %predicted. Oscillometry-defined SAD was observed in 54.4% (95% CI: 48.1-59.4). Patients with SAD exhibited significantly lower spirometric indices compared to those without SAD. The proportion of EA was 58.4% (95% CI: 53.4-64.7). Spirometric parameters did not differ significantly between the EA and NEA. The severity of impairment in R5 and X5 was less in EA compared to NEA, though the difference was not statistically significant. The proportion of impaired R5-19 was significantly less in EA than NEA (47.6% vs. 57.9%; p = .04).

Conclusions: Half of steroid-naïve bronchial asthma patients exhibited SAD at the time of diagnosis. NEA phenotypes are associated with higher impairment in oscillometry.