Bronchiolitis

Acute Bronchiolitis

Acute bronchiolitis is the single most common lower respiratory tract pathogen-driven illness during the first year of life. It is characterised by acute bronchiolar inflammation, epithelial necrosis, and dynamic airway plugging.

Pathophysiology and Aetiology

• Aetiological Agents: Respiratory syncytial virus (RSV) is the culprit in 70% to 80% of cases. Other pathogens include human rhinovirus, human metapneumovirus, parainfluenza, and adenovirus.
• Pathophysiology: Viral replication in the ciliated respiratory epithelial cells of terminal bronchioles results in epithelial necrosis, loss of cilia, peribronchiolar lymphocytic infiltration, and submucosal oedema. Excessive mucus production combined with shed necrotic debris forms dense luminal plugs. This produces widespread partial airway obstruction (causing air trapping and hyperinflation via a ball-valve mechanism) and complete obstruction (causing patchy resorption atelectasis). The resultant ventilation-perfusion ($V/Q$) mismatch leads to hypoxaemia.

Clinical Presentation and Disease Trajectory

• Typical Demographics: Infants $< 12\text{ months}$ of age, with peak severity between 2 and 6 months.
• Natural History: Begins with a 2- to 3-day viral upper respiratory tract prodrome (clear rhinorrhoea, sneezing, low-grade fever). Symptoms progress downward into the lower respiratory tract over days 3 to 5 (the peak of clinical severity), characterized by a dry, persistent cough, tachypnoea, and feeding difficulty. Resolution is gradual, with cough lasting a median of 10 to 14 days (up to 3 weeks in 20% of infants).
• Physical Examination:
  • Increased work of breathing: Subcostal and intercostal retractions, tracheal tug, and nasal flaring.
  • Auscultation: The diagnostic hallmark is bilateral widespread fine end-inspiratory crackles accompanied by high-pitched expiratory wheezes across all lung fields.
  • Feeding evaluation: In infants, tachypnoea ($RR > 60\text{ breaths/min}$) interferes with the suck-swallow-breathe reflex, leading to dehydration and fatigue.

High-Risk Cohorts for Severe Bronchiolitis

Infants with any of the following features carry an elevated risk of rapid decompensation, severe apnoea, and PICU admission:

• Age $< 6\text{ to }8\text{ weeks}$ of life.
• Prematurity ($< 37\text{ weeks}$ gestational age, particularly $< 32\text{ weeks}$).
• Congenital heart disease (especially lesions with pulmonary hypertension or left-to-right shunts).
• Chronic neonatal lung disease (bronchopulmonary dysplasia).
• Neuromuscular weakness or congenital immunodeficiency.

Diagnostic Stewardship

In accordance with the PREDICT (Paediatric Research in Emergency Departments International Collaborative) guidelines and Australian RCH practice:

• Bronchiolitis is a clinical diagnosis: Routine nasopharyngeal viral multiplex PCR swabs do not alter acute clinical management and are reserved for infection control cohorting.
• No Routine Chest X-Ray: CXRs in bronchiolitis invariably show hyperinflation, flattened diaphragmatic domes, and patchy peribronchial thickening or subsegmental atelectasis. These atelectatic areas are frequently misdiagnosed as bacterial consolidation, leading to inappropriate, harmful antibiotic prescribing.

Evidence-Based Supportive Management

Bronchiolitis is a self-limiting viral infection for which no curative pharmacotherapy exists. Routine management is mainly supportive; specific intensive-care exceptions do not justify routine ward treatment:

1. Minimal Handling: Physical agitation significantly increases metabolic rate and oxygen consumption. Unnecessary examinations, suctioning, and investigations must be minimized.

• Bronchiolitis oxygen (2025): Give oxygen for persistent saturation below 90% in otherwise healthy infants aged at least six weeks; use below 92% for infants under six weeks or with an underlying condition. Assess the whole clinical picture, not a fleeting desaturation.

1. Hydration and Nutritional Support:
   • If the infant takes $\ge 50\%$ of their normal feed volume and exhibits no severe respiratory distress, continue oral breast milk or formula in smaller, more frequent feeds.
   • If oral intake drops below $50\%$ of normal, or if the infant exhibits marked tachypnoea ($RR > 60\text{ to }70\text{ breaths/min}$) with severe indrawing, commence nasogastric (NG) tube feeds with expressed breast milk or formula. NG feeding is physiologically superior to intravenous fluids because it maintains enteral nutrition, avoids venepuncture distress, and preserves fluid/electrolyte balance.
   • Intravenous fluid therapy (isotonic $0.9\%$ sodium chloride $+ 5\%$ glucose at $80\%$ maintenance) is reserved for infants with severe abdominal distension, recurrent vomiting, or refractory distress requiring mechanical support.
   • Delivered at flow rates of $1\text{ to }2\text{ L/kg/min}$ with heated and humidified gas.
   • Provides low-level positive end-expiratory pressure (PEEP $\approx 2\text{ to }4\text{ cmH}_2\text{O}$), splints open micro-airways, washes out anatomical nasopharyngeal dead space, and reduces the work of breathing. Indicated for infants who fail low-flow oxygen or present with severe respiratory distress.
2. Ineffective Therapies (The "Do NOT" List):
   • Bronchodilators (Salbutamol / Ipratropium): Smooth muscle hyperplasia is not the primary mechanism of obstruction in infants under 12 months; multiple large randomized trials have shown salbutamol does not improve oxygenation, reduce admission, or shorten length of stay. Salbutamol causes tachycardia, tremors, and ventilation-perfusion mismatch.

• Bronchiolitis medicines: Routine bronchodilators, corticosteroids, adrenaline and antibiotics are not recommended for uncomplicated ward/ED bronchiolitis. The 2025 guideline allows specialist consideration of combined corticosteroid and adrenaline in severe disease requiring ICU care. This exception must not become routine ward treatment.
  • Antibiotics: Viral aetiology renders antibacterial agents ineffective; secondary bacterial pneumonia in bronchiolitis occurs in $< 1\%$ of cases.
  • Chest Physiotherapy: Increases infant distress without clinical benefit.

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