What is the primary cause of croup?

Primary Cause of Croup

Parainfluenza viruses are the primary cause of croup (laryngotracheobronchitis), with types 1 and 2 most commonly associated with the croup syndrome. 1

Viral Etiology of Croup

• Parainfluenza viruses (types 1-3) are the most common viral cause of croup, accounting for the majority of cases 1
• Parainfluenza viruses are responsible for up to 11% of all hospitalizations in children under 5 years of age 1
• The clinical course of croup caused by influenza appears to be more severe than croup caused by the more common parainfluenza virus and is more likely to be complicated by bacterial tracheitis 1

Other Viral Causes

• Other viruses that can cause croup include:
  • Respiratory syncytial virus (RSV) 1
  • Rhinoviruses 1
  • Human coronaviruses 1
  • Adenoviruses 1
  • Influenza viruses 1
  • Human metapneumovirus 1

Pathophysiology

• Croup is a manifestation of upper airway obstruction resulting from swelling of the larynx, trachea, and bronchi 2
• The viral infection causes inflammation and edema of the subglottic region, leading to the characteristic symptoms 2
• The histologic effects of viral infection vary from epithelial destruction to minimal changes, but all cause vasodilation and hypersecretion 1

Clinical Presentation

• Croup typically presents with:
  • Barking cough (characteristic "seal-like" barking) 3
  • Inspiratory stridor 2
  • Hoarseness 2
  • Low-grade fever (though not always present) 2
  • Respiratory distress of varying severity 3
• Symptoms usually start like an upper respiratory tract infection, with low-grade fever and coryza (runny nose) followed by the barking cough and respiratory distress 4
• In most children, symptoms subside quickly with resolution of the cough within two days 4

Epidemiology

• Croup affects approximately 3% of children between six months and three years of age 2
• It accounts for up to 15% of emergency department visits due to respiratory disease in children in the United States 4
• It accounts for 7% of hospitalizations annually for fever and/or acute respiratory illness in children younger than five years 2
• Most episodes of croup are mild, with only 1-8% of patients requiring hospital admission 4

Differential Diagnosis

When evaluating a child with suspected croup, physicians should consider other diagnoses including:

• Bacterial tracheitis 5, 4
• Epiglottitis 3
• Foreign body aspiration 5, 3
• Peritonsillar abscess 4
• Retropharyngeal abscess 4
• Angioedema 4

Management Considerations

• A single dose of dexamethasone (0.15 to 0.60 mg per kg usually given orally) is recommended in all patients with croup, including those with mild disease 4, 5
• Nebulized epinephrine is indicated for moderate to severe croup with stridor at rest or respiratory distress 5, 2
• Humidification therapy has not been proven beneficial 4
• Children requiring two or more epinephrine treatments should be considered for hospitalization 6, 5

Recurrent Croup

• Recurrent croup (defined as two or more episodes per year) should be considered a symptom of an underlying structural or inflammatory airway abnormality 7
• Children with recurrent croup should undergo evaluation for underlying causes 7

In summary, parainfluenza viruses are the primary cause of croup in children, with types 1 and 2 most commonly associated with the characteristic croup syndrome. While other respiratory viruses can also cause croup, parainfluenza remains the predominant etiologic agent.