Croup

Croup, Bronchiolitis, Paediatric Asthma & Anaphylaxis

Acute respiratory distress is the most frequent reason for emergency paediatric presentation and hospital admission in Australia. Because infant and child airways are anatomically narrower, more compliant, and possess fewer collateral ventilation pathways than adult airways, minor luminal narrowing produces dramatic increases in airflow resistance ($R \propto 1/r^4$ according to Poiseuille's law). Rapid phenotypic differentiation between upper airway obstruction, lower airway disease, and systemic allergic emergencies is essential to deliver time-critical interventions and prevent respiratory fatigue and arrest.

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Croup (Acute Laryngotracheobronchitis)

Croup is an acute viral infection of the upper respiratory tract causing inflammation and mucosal oedema of the larynx, trachea, and subglottic airway (the conus elasticus). The rigid cricoid cartilage forms a complete unyielding ring around the subglottis; swelling here disproportionately restricts laminar airflow, generating high-velocity turbulent flow manifested clinically as inspiratory stridor.

Aetiology and Epidemiology

• Primary Pathogen: Human parainfluenza virus type 1 accounts for over 70% to 80% of cases, followed by parainfluenza types 2 and 3, respiratory syncytial virus (RSV), adenovirus, and influenza viruses.
• Demographics: Predominantly affects children aged 6 months to 3 years, with a peak incidence in the second year of life. It exhibits a distinct seasonal peak during late autumn and early winter.

Clinical Presentation and Severity Stratification

The illness typically begins with an unremarkable 1- to 3-day viral coryzal prodrome (low-grade fever, rhinorrhoea, mild cough) followed by the abrupt nocturnal onset of:

• A harsh, metallic "barking" cough (classically likened to a barking seal).
• Hoarseness or raspy dysphonia due to vocal cord inflammation.
• Inspiratory stridor, which initially occurs only with agitation, crying, or exertion, but progresses to rest in moderate-to-severe disease.
• Increased work of breathing: Suprasternal and sternal indrawing, intercostal recession, and tracheal tug.

Clinical severity is classified according to the Westley Croup Score and Australian Clinical Practice Guidelines (such as the Royal Children's Hospital Melbourne guidelines):

• Mild: Clinical Features: Barking cough with little respiratory effort; Work of Breathing: A cough alone without stridor may not require steroid; use the RCH risk-based pathway
• Moderate: Clinical Features: Frequent barking cough; stridor audible at rest; Work of Breathing: Moderate sternal and subcostal indrawing; Management Strategy: Single dose oral dexamethasone $0.15\text{ to }0.6\text{ mg/kg}$; observe in ED
• Severe: Clinical Features: Continuous prominent stridor at rest; agitated or distressed; Work of Breathing: Marked sternal recession, tachypnoea, lethargy; Management Strategy: Nebulised adrenaline $1:1000$ stat $+$ oral/IV dexamethasone $0.6\text{ mg/kg}$
• Bronchiolitis support: Start low-flow oxygen when indicated. High-flow therapy is considered when low-flow fails, with escalation according to response; it is not routine for a non-hypoxaemic infant. Hydration may be oral/NG or IV according to work of breathing and tolerance.

Diagnostic Pitfalls and Imaging

• Clinical Diagnosis: Croup is an exclusively clinical diagnosis. Laboratory investigations and diagnostic imaging are unnecessary in typical presentations and should be avoided.
• Avoid Agitation: Agitation markedly increases negative intrathoracic pressures, worsening dynamic subglottic collapse. The child should be allowed to remain in the parent's arms; tongue depressors, painful venepuncture, and separation from caregivers are contraindicated.
• Radiography: An anteroposterior soft-tissue neck radiograph demonstrating subglottic tapering ("steeple sign") is classic but has poor sensitivity and must never delay treatment. X-rays are reserved solely for children with atypical features, suspected foreign body inhalation, or poor response to treatment.
• Life-Threatening Differentials:
  • Acute Epiglottitis: Caused by Haemophilus influenzae type b (Hib) in unimmunised children or Streptococcus pyogenes. Presents with abrupt high fever, toxic appearance, absence of barking cough, severe odynophagia, inability to swallow, drooling, and the "tripod position" (sitting upright, leaning forward with chin thrust out). Never inspect the pharynx or place a tongue blade, as this can trigger immediate fatal laryngospasm; transfer directly to the operating theatre for gaseous induction and endotracheal intubation by a senior anaesthetist and ENT surgeon.
  • Bacterial Tracheitis: Secondary bacterial superinfection (usually Staphylococcus aureus or Streptococcus pneumoniae) following viral croup. Manifests as high fever, toxic prostration, copious thick purulent secretions, and severe stridor that fails to respond to nebulised adrenaline.

Evidence-Based Management of Croup

1. Systemic Corticosteroids:
   • Croup steroid: RCH does not require steroids for barking cough alone without stridor. Stridor/risk factors support a suitable dexamethasone regimen; severe disease needs higher-acuity care and nebulised adrenaline. Review response and observe after adrenaline.
   • Alternative: Oral prednisolone ($1\text{ mg/kg}$, repeated after 12-24 hours if needed) if oral dexamethasone liquid is unavailable, or nebulised budesonide ($2\text{ mg}$ stat) if the child is actively vomiting.
2. Nebulised Adrenaline (Epinephrine):
   • Indication: Moderate-to-severe croup with stridor at rest and significant chest wall recession.
   • Dose: Adrenaline 1:1000 ($1\text{ mg/mL}$) at $0.5\text{ mL/kg}$ (maximum single dose $5\text{ mL}$), nebulised with $6\text{ to }8\text{ L/min}$ of supplemental oxygen.
   • Mechanism: Stimulates alpha-1 adrenergic receptors on mucosal arterioles, causing rapid vasoconstriction and immediate reduction in subglottic capillary pressure and oedema within 10 to 30 minutes.

• Croup observation: Nebulised adrenaline can improve airway obstruction rapidly but the benefit wears off. Symptoms may return, which is different from evidence of a rebound worse than baseline. Observe for at least three hours after the last dose and discharge only when clinically suitable.

1. Humidified Air / Steam Therapy: Clinical trials have consistently demonstrated that warm mist, humidified tents, or bathroom steam offer no objective clinical benefit over room air and carry a risk of thermal burns; routine use is not recommended in Australian guidelines.

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Primary references (checked 7 October 2026): RCH croup; 2025 Australasian bronchiolitis guideline; ASCIA device availability.