What is the usual cause of croup?

Causes of Croup

Parainfluenza viruses (types 1 and 2) are the most common cause of croup (laryngotracheobronchitis), followed by other respiratory viruses including influenza, respiratory syncytial virus, human metapneumovirus, adenovirus, and coronaviruses. 1

Primary Viral Etiologies

Croup is predominantly caused by viral infections that affect the upper airway, with specific patterns:

• Parainfluenza viruses:

  • Types 1 and 2 are most frequently associated with the classic croup syndrome 1
  • Account for the majority of cases 2, 3
  • Type 3 is more commonly associated with bronchiolitis and pneumonia 1

• Other significant viral causes:

  • Influenza viruses (typically causing more severe croup) 1, 4
  • Respiratory syncytial virus (RSV)
  • Human metapneumovirus 1
  • Adenoviruses 1
  • Coronaviruses 1
  • Enteroviruses 1

Clinical Presentation and Pathophysiology

Viral infection of the upper respiratory tract leads to:

• Inflammation and edema of the larynx, trachea, and bronchi
• Narrowing of the subglottic region
• Characteristic barking cough and inspiratory stridor
• Often preceded by upper respiratory symptoms (rhinorrhea, congestion)

The viral infection causes:

• Vasodilation and hypersecretion in the upper airway 1
• Increased sensitivity of airway sensory receptors 1
• Inflammation-mediated narrowing of the subglottic space

Severity Considerations

The severity of croup varies by viral etiology:

• Influenza-associated croup tends to be more severe than parainfluenza-associated croup 1, 4
• Influenza croup is more likely to be complicated by bacterial tracheitis 1

Epidemiology

• Affects approximately 3% of children 6 months to 3 years of age 5
• Accounts for up to 15% of emergency department visits for respiratory illness in children 2
• Most common in children under 5 years, with peak incidence between 6 months and 3 years 5

Important Clinical Distinctions

When evaluating a child with suspected croup, it's essential to consider other potentially serious conditions that may present similarly:

• Bacterial tracheitis
• Epiglottitis
• Foreign body aspiration
• Peritonsillar or retropharyngeal abscess
• Angioedema

Management Implications

Understanding the viral etiology of croup guides management:

• Dexamethasone is recommended for all patients with croup regardless of severity 2, 5
• Nebulized epinephrine is reserved for moderate to severe cases 2, 3
• Most cases are mild and self-limiting, with symptoms typically resolving within 48 hours 2
• Only 1-8% of croup cases require hospitalization 2

The viral nature of most croup cases explains why antibiotics are not routinely indicated unless bacterial superinfection is suspected.