Rationale: Structural alterations to airway smooth muscle (ASM) are a feature of asthma and cystic fibrosis (CF) in adults.
Objectives: We investigated whether increase in ASM mass is already present in children with chronic inflammatory lung disease.
Methods: Fiberoptic bronchoscopy was performed in 78 children (median age [IQR], 11.3 [8.5-13.8] yr): 24 with asthma, 27 with CF, 16 with non-CF bronchiectasis (BX), and 11 control children without lower respiratory tract disease. Endobronchial biopsy ASM content and myocyte number and size were quantified using stereology.
Measurements and main results: The median (IQR) volume fraction of subepithelial tissue occupied by ASM was increased in the children with asthma (0.27 [0.12-0.49]; P < 0.0001), CF (0.12 [0.06-0.21]; P < 0.01), and BX (0.16 [0.04-0.21]; P < 0.01) compared with control subjects (0.04 [0.02-0.05]). ASM content was related to bronchodilator responsiveness in the asthmatic group (r = 0.66, P < 0.01). Median (IQR) myocyte number (cells per mm(2) of reticular basement membrane) was 8,204 (5,270-11,749; P < 0.05) in children with asthma, 4,504 (2,838-8,962; not significant) in children with CF, 4,971 (3,476-10,057; not significant) in children with BX, and 1,944 (1,596-6,318) in control subjects. Mean (SD) myocyte size (mum(3)) was 3,344 (801; P < 0.01) in children with asthma, 3,264 (809; P < 0.01) in children with CF, 3,177 (873; P < 0.05) in children with BX, and 1,927 (386) in control subjects. In all disease groups, the volume fraction of ASM in subepithelial tissue was related to myocyte number (asthma: r = 0.84, P < 0.001; CF: r = 0.81, P < 0.01; BX: r = 0.95, P < 0.001), but not to myocyte size.
Conclusions: Increases in ASM (both number and size) occur in children with chronic inflammatory lung diseases that include CF, asthma, and BX.