Pathophysiology of Croup
Croup results from viral infection causing inflammation, edema, and hypersecretion in the larynx, trachea, and bronchi, leading to upper airway obstruction that manifests as the characteristic barking cough, inspiratory stridor, and respiratory distress. 1
Viral Etiology and Mechanism
- Parainfluenza viruses (types 1 and 2) are the primary causative agents, accounting for the majority of croup cases and up to 11% of all hospitalizations in children under 5 years of age 1
- Other viral pathogens include respiratory syncytial virus (RSV), rhinoviruses, human coronaviruses, adenoviruses, influenza viruses, and human metapneumovirus 1
- Influenza-associated croup tends to follow a more severe clinical course than parainfluenza-induced croup and carries higher risk of bacterial tracheitis as a complication 1
Anatomic and Physiologic Changes
The viral infection triggers a cascade of pathophysiologic changes in the upper airway:
- Epithelial destruction or minimal epithelial changes occur, but all viral infections uniformly cause vasodilation and hypersecretion in the affected airways 1
- The inflammation and edema primarily affect the subglottic region, larynx, trachea, and bronchi, creating varying degrees of upper airway obstruction 2, 3
- The narrowing of the airway lumen produces the characteristic inspiratory stridor as turbulent airflow occurs through the constricted passage 4, 5
Clinical Manifestations of Airway Obstruction
The pathophysiologic changes translate into specific clinical features:
- Inspiratory stridor results from turbulent airflow through the narrowed upper airway during inspiration 4, 2
- Barking cough is produced by air passing through the inflamed, edematous larynx 5, 2
- Hoarseness occurs due to laryngeal involvement and vocal cord edema 4
- Respiratory distress develops when obstruction becomes severe enough to impair adequate ventilation 2
Age-Related Susceptibility
- Croup predominantly affects children 6 months to 6 years of age, with peak incidence in those 6 months to 3 years 6, 4, 2
- The anatomic characteristics of the pediatric airway—including smaller diameter, more compliant cartilage, and relatively larger tongue—make young children particularly vulnerable to symptomatic obstruction from even modest amounts of edema 2
Temporal Pattern
- Croup typically presents in late fall or early winter with seasonal variation corresponding to parainfluenza virus circulation 6
- The illness is usually preceded by upper respiratory infection symptoms, followed by the acute onset of barking cough and stridor 6, 5
- Most children experience symptom resolution within 2 days, though the clinical course can vary 5