What is bronchiolitis?

Definition of Bronchiolitis

Bronchiolitis is a viral lower respiratory tract infection in infants and young children (typically under 2 years of age) characterized by acute inflammation, edema, and necrosis of epithelial cells lining the small airways, along with increased mucus production and bronchospasm. 1

Clinical Characteristics

Pathophysiology

• The disease involves extensive inflammation of the small airways with accompanying mucus production and epithelial cell necrosis 1
• Bronchospasm contributes to airway obstruction 1
• These pathologic changes result in the characteristic clinical presentation of respiratory distress in young infants 1

Typical Clinical Presentation

• Initial phase: Viral upper respiratory prodrome with rhinitis, congestion, and possibly fever 1
• Progression to lower respiratory tract: Symptoms evolve to include tachypnea, wheezing, cough, and crackles/rales 1
• Signs of respiratory distress: Use of accessory muscles, nasal flaring, and retractions indicate increased work of breathing 1
• The clinical diagnosis is characterized by tachypnea and wheeze and/or crepitations/crackles in children under 2 years following an upper respiratory illness 1

Epidemiology and Etiology

Viral Causes

• Respiratory syncytial virus (RSV) is the most common causative agent, with peak incidence between December and March in North America (regional variations exist) 1
• Other viral pathogens include human rhinovirus, human metapneumovirus, influenza, adenovirus, coronavirus, and parainfluenza viruses 1
• Approximately 90% of children are infected with RSV within the first 2 years of life, and up to 40% will develop lower respiratory tract infection during their initial infection 1

Age Distribution

• Bronchiolitis is the most common lower respiratory tract infection in infants and young children under 2 years of age 1
• The highest incidence occurs in infants aged 1-6 months 2
• It is the most common cause of hospitalization in children under 1 year of age 1

Important Clinical Distinctions

Diagnostic Approach

• Bronchiolitis is a clinical diagnosis based on history and physical examination 1
• Routine laboratory testing and radiologic studies are not recommended for diagnosis 1
• The diagnosis should be differentiated from other causes of wheezing in young children, particularly asthma (which typically presents with recurrent episodes) 1

Severity Assessment

• Severe disease is characterized by signs of poor feeding, respiratory distress with tachypnea and nasal flaring, and hypoxemia 1
• Risk factors for severe disease include age less than 12 weeks, prematurity, underlying cardiopulmonary disease, or immunodeficiency 1