What causes bronchiolitis (inflammation of the bronchioles)?

Causes of Bronchioles Inflammation (Bronchiolitis)

Bronchiolitis is primarily caused by viral respiratory infections, with respiratory syncytial virus (RSV) being the most common causative agent, accounting for the majority of cases in infants and young children. 1

Primary Viral Causes

• Respiratory Syncytial Virus (RSV)

  • Most common etiology (40-90% of cases)
  • Highest incidence between December and March in North America (with regional variations)
  • 90% of children are infected with RSV in first 2 years of life
  • Up to 40% develop lower respiratory tract infection during initial infection
  • Does not grant permanent immunity; reinfections common throughout life 1

• Other Viral Causes 1, 2

  • Human rhinovirus
  • Human metapneumovirus
  • Influenza virus
  • Adenovirus
  • Coronavirus
  • Parainfluenza viruses

Pathophysiology

Bronchiolitis involves several pathological processes in the small airways:

• Acute inflammation
• Edema
• Necrosis of epithelial cells lining small airways
• Increased mucus production 1

Clinical Patterns by Viral Etiology

Research suggests bronchiolitis may comprise distinct clinical entities based on viral etiology:

1. RSV-induced bronchiolitis:

   • Typically affects younger infants
   • Characterized by mechanical obstruction due to mucus and cell debris
   • Associated with increased risk of recurrent wheezing 3

2. Rhinovirus-induced wheezing:

   • Associated with atopic predisposition
   • Higher risk of subsequent asthma development
   • May respond differently to treatments (possibly to systemic corticosteroids in severe cases) 3

3. Other viral causes:

   • Generally less frequent and severe 3

Risk Factors for Severe Disease

Certain populations are at higher risk for severe bronchiolitis:

• Premature infants
• Children with chronic lung disease
• Children with hemodynamically significant congenital heart disease
• Immunocompromised children 1, 4

Transmission

• RSV is transmitted through:
  • Direct contact with respiratory droplets from infected persons
  • Self-inoculation by contaminated secretions on surfaces 4

Clinical Progression

The typical progression of bronchiolitis follows a pattern:

1. Initial upper respiratory symptoms (2-4 days): fever, rhinorrhea, congestion
2. Progression to lower respiratory symptoms: increasing cough, wheezing, increased respiratory effort 4

Prevention

Prevention strategies include:

• Hand hygiene before and after patient contact
• Avoiding exposure to tobacco smoke
• Encouraging breastfeeding
• Palivizumab prophylaxis for high-risk infants 5, 4

Understanding the specific viral etiology of bronchiolitis is increasingly important, as it may guide both treatment approaches and predict long-term outcomes such as risk of developing asthma later in childhood.