Bronchiolitis

Definition

Bronchiolitis is an acute viral lower respiratory tract infection primarily affecting infants and young children, characterized by inflammation, edema, and mucus accumulation in the small airways (bronchioles), leading to wheezing, cough, and respiratory distress.

Epidemiology

• Most common in infants under 2 years, peak incidence at 3–6 months
• Seasonal occurrence: winter and early spring in temperate climates
• RSV (respiratory syncytial virus) is the most common cause worldwide
• Other viral causes: rhinovirus, parainfluenza, influenza, adenovirus, human metapneumovirus
• Hospitalization rates highest in preterm infants, those with chronic lung disease or congenital heart disease

Etiology

• Viral infection: RSV (primary), rhinovirus, parainfluenza, influenza, adenovirus, metapneumovirus
• Risk factors: prematurity, congenital heart disease, chronic lung disease, immunodeficiency, exposure to tobacco smoke
• Transmission: primarily via respiratory droplets and direct contact with contaminated surfaces
• Seasonal outbreaks common in pediatric populations

Pathophysiology

• Viral infection of bronchiolar epithelium leads to cell death, edema, and mucus production
• Airway obstruction results from inflammation, debris, and mucus plugs
• Air trapping causes hyperinflation, atelectasis, and impaired gas exchange
• Hypoxia and increased work of breathing develop due to ventilation-perfusion mismatch
• Immune response contributes to airway inflammation and symptoms
• Severe cases may progress to respiratory failure, particularly in high-risk infants