Clinical meaning
Bronchiolitis (primarily RSV) involves viral invasion of bronchiolar epithelial cells causing necrosis, edema, mucus hypersecretion, and small airway obstruction. In infants with bronchiolar lumens less than 1 mm, even 1 mm of edema reduces cross-sectional area by 75%, creating a ball-valve mechanism with air trapping, hyperinflation, and hypoxemia. Croup (laryngotracheobronchitis, primarily parainfluenza virus) involves subglottic inflammation below the cricoid cartilage, the narrowest portion of the pediatric airway. Subglottic edema of 1 mm reduces the infant airway cross-sectional area by approximately 60%, producing the classic triad of barking (seal-like) cough, inspiratory stridor, and hoarseness.
Exam relevance
Risk factors: - Age less than 12 months for bronchiolitis (peak 2-6 months) - Age 6 months to 3 years for croup (peak incidence) - Prematurity or bronchopulmonary dysplasia - Congenital heart disease - Daycare attendance and older siblings - Exposure to secondhand smoke - Lack of breastfeeding - Fall and winter seasonality (RSV and parainfluenza)