Croup (Laryngotracheobronchitis)
Croup is a common viral upper‑airway infection of infants and young children characterised by subglottic mucosal oedema, increased secretions and the classic “barking” cough. Parainfluenza viruses (types 1 and 3) are the most frequent pathogens. Peak incidence is around 6 months-3 years and cases are more common in autumn-winter (GOV.UK/UKHSA).
Clinical severity ranges from mild noisy cough to critical upper‑airway obstruction and respiratory failure; small reductions in subglottic radius cause large increases in airway resistance (Poiseuille’s law), so children can deteriorate quickly.
Presentation
- Prodrome of upper respiratory tract infection (rhinorrhoea, coryza) 12-72 hours before hallmark symptoms.
- Barking, seal‑like cough (often worse at night), hoarseness and variable inspiratory stridor (may be intermittent and worse with agitation).
- Respiratory distress: tachypnoea, chest wall retractions, reduced air entry.
- Severe disease can present with hypoxaemia, cyanosis, exhaustion or reduced consciousness.
- Fever may be present.
Pathophysiology and relevant paediatric physiology
- Subglottic oedema and secretions narrow a small paediatric airway; resistance ∝ radius^-4 so modest swelling produces marked airflow limitation.
- Children have more compliant chest walls, higher metabolic demand, smaller functional residual capacity and fatigue‑prone respiratory muscles - all predispose to rapid decompensation.
- Stridor timing helps localisation:
- Inspiratory stridor suggests an extrathoracic obstruction.