Why does my infant with croup develop coarse breath sounds?

Why Coarse Breath Sounds Occur in Croup

Coarse breath sounds in croup result from turbulent airflow through inflamed, narrowed upper airways—specifically the larynx, trachea, and proximal bronchi—where viral infection causes mucosal edema and increased secretions that create audible vibrations during breathing.

Pathophysiology of Airway Sounds in Croup

Primary Mechanism: Upper Airway Inflammation

• Croup causes airway inflammation and edema primarily affecting the larynx, trachea, and bronchi, leading to obstruction at these levels 1
• The viral infection produces mucosal swelling that narrows the airway diameter, forcing air to move through a restricted passage 2
• This narrowing creates turbulent rather than laminar airflow, generating the characteristic coarse sounds heard on auscultation 3

Sound Generation

• Inspiratory stridor is the hallmark finding—a high-pitched, harsh sound caused by air rushing through the narrowed subglottic region during inspiration 4, 3
• The "barking seal" cough results from vibration of inflamed vocal cords and tracheal mucosa 3, 2
• Coarse breath sounds represent turbulent airflow through airways partially obstructed by edema and secretions, audible throughout the respiratory cycle but typically worse on inspiration 5

Clinical Presentation Pattern

Typical Findings on Examination

• Low-grade fever accompanies the respiratory symptoms 4, 2
• Hoarseness develops from laryngeal involvement 2
• Progressive disease shows both inspiratory and expiratory stridor as obstruction worsens 3
• Agitation may signal hypoxemia rather than anxiety—a critical distinction requiring oxygen supplementation 6

Severity Assessment

• Mild croup presents with stridor without intercostal retractions 5
• Moderate-to-severe croup demonstrates increased work of breathing with accessory muscle use, tracheal tug, and chest wall recession 6, 5
• Life-threatening signs include silent chest, cyanosis, fatigue/exhaustion, or poor respiratory effort 6

Important Clinical Pitfalls

What Coarse Sounds Do NOT Mean in Croup

• Coarse breath sounds in croup do not indicate lower respiratory tract infection or pneumonia—the pathology is upper airway obstruction 1
• The sounds originate from the larynx and trachea, not from alveolar or bronchiolar disease 2
• Chest physiotherapy offers no benefit and should not be performed, as the problem is not secretion clearance but rather airway edema 6

Critical Differential Diagnoses

• Bacterial tracheitis can present similarly but with higher fever, toxicity, and purulent secretions 6
• Foreign body aspiration must be excluded, especially with sudden onset and unilateral findings 6
• Laryngomalacia causes chronic positional stridor from infancy, not acute illness 7
• Up to 68% of infants with persistent stridor have concomitant lower airway abnormalities, warranting flexible bronchoscopy if symptoms are severe, persistent, or atypical 6, 7

Management Implications

Why Understanding the Sounds Matters

• The presence of stridor at rest indicates at least moderate severity requiring nebulized epinephrine in addition to corticosteroids 6, 5
• Worsening stridor or development of silent chest signals impending respiratory failure 6, 3
• Oxygen should be administered to maintain SpO₂ ≥94% even in the presence of stridor, as hypoxemia can develop rapidly 6

Treatment Does Not Target the Sounds Directly

• Corticosteroids (dexamethasone 0.15-0.6 mg/kg) reduce mucosal edema, which secondarily improves airway caliber and reduces turbulent flow 6, 5
• Nebulized epinephrine provides temporary vasoconstriction of inflamed mucosa, widening the airway for 1-2 hours 6, 5
• Humidified air and cool mist lack evidence for benefit and do not alter the underlying pathophysiology 6, 5