Small Airway Dysfunction Assessed by Impulse Oscillometry in Children with Congenital Adrenal Hyperplasia: A Preliminary Case–Control Study

Background: Congenital adrenal hyperplasia (CAH) is associated with chronic hormonal dysregulation and prolonged glucocorticoid exposure, which may contribute to alterations in pulmonary physiology. However, small airway function in children with CAH has not been adequately investigated. This study aims to evaluate small airway function in children with congenital adrenal hyperplasia (CAH) using impulse oscillometry system (IOS) and to compare the findings with those of healthy controls. Methods: This cross-sectional case–control study included 27 children with CAH and 27 age- and sex-matched healthy controls. IOS measurements, including resistance at 5 Hz (R5), resistance at 20 Hz (R20), peripheral airway resistance (R5–R20), reactance at 5 Hz (X5) and 20 Hz (X20), reactance area (AX), and resonance frequency (Fres), were obtained in all participants. Demographic, clinical, laboratory, and treatment-related characteristics were also recorded. Results: The median age of the CAH group was 14 years, and 66.7% were female. Compared with controls, children with CAH demonstrated significantly higher R5 values [0.55 vs. 0.38 kPa·s/L, p = 0.031], increased R5–R20 values [0.21 vs. 0.12 kPa·s/L, p = 0.009], and elevated AX values [2.05 vs. 1.02 kPa·s/L, p = 0.001]. In addition, X5 values were significantly more negative in the CAH group [−0.16 vs. −0.10 kPa·s/L, p = 0.001]. No significant differences were observed in R20, X20, and Fres parameters. Based on the z-scores calculated using the reference equations implemented in the device software, abnormal oscillometric findings were most frequently observed for X5 (29.6%), followed by R5–R20 (25.9%) and R5 (22.2%). Conclusions: Children with CAH exhibited IOS abnormalities suggestive of subclinical small airway dysfunction despite the absence of overt respiratory disease. IOS may serve as a sensitive tool for the early detection of small airway involvement in pediatric CAH. Further prospective studies with larger sample sizes are warranted.