Define Acute Bronchiolitis. Describe its aetiology, pathophysiology, clinical features and investigations.

Definition

• Acute bronchiolitis is an acute viral lower respiratory tract infection of infants (usually < 2 years, peak 2–6 months), characterised by inflammation and obstruction of the small airways (bronchioles), producing wheeze and respiratory distress.
• It typically occurs in winter epidemics.

Etiology

• Respiratory Syncytial Virus (RSV) — the commonest cause (~70%).
• Others — human metapneumovirus, parainfluenza, influenza, adenovirus, rhinovirus.
• Spread is by droplets and contact; highly contagious (hospital & household spread).

Pathophysiology

Viral infection of bronchiolar epithelium → Inflammation, oedema & mucus + sloughed cells → Small-airway obstruction (a 'ball-valve' effect) → Air trapping + hyperinflation and patchy collapse → V/Q mismatch → hypoxia and increased work of breathing

Clinical Features

• A preceding coryza (runny nose, mild fever) for 1–3 days, then lower-respiratory signs.
• Cough, wheeze, tachypnoea and feeding difficulty (breathlessness interrupts feeds).
• Respiratory distress — chest indrawing, nasal flaring, grunting; a hyperinflated chest.
• Auscultation — fine end-inspiratory crepitations with widespread wheeze.
• Apnoea may be the presenting feature in young/preterm infants.

Risk Factors for Severe Disease

• Age < 3 months; prematurity; low birth weight.
• Congenital heart disease, chronic lung disease, immunodeficiency, and neuromuscular disease.

Diagnosis & Investigations

• A clinical diagnosis; investigations are usually unnecessary.
• Pulse oximetry to assess hypoxia; chest X-ray (if atypical) shows hyperinflation, patchy atelectasis.
• Nasopharyngeal aspirate for RSV (rapid antigen/PCR) — mainly for cohorting in hospital.

Management — Mainly Supportive

• Oxygen for hypoxia (SpO2 < 90–92%); gentle nasal suction to clear secretions.
• Maintain hydration & feeding — small frequent feeds; NG or IV fluids if unable to feed.
• Minimal handling; monitor for apnoea and worsening distress.
• Respiratory support (high-flow nasal cannula / CPAP / ventilation) for severe disease.

Epidemiology

• Chiefly affects infants < 2 years, peaking at 2–6 months; occurs in winter epidemics.
• A leading cause of infant hospitalisation for respiratory illness.

Assessment of Severity

• Mild: feeding well, minimal distress, SpO2 normal.
• Moderate: some feeding difficulty, chest indrawing, tachypnoea.
• Severe: poor feeding, marked distress/grunting, SpO2 < 92%, apnoea, exhaustion.

Criteria for Hospital Admission

• Poor feeding (< 50% of normal) or dehydration; apnoea.
• Hypoxia (SpO2 < 92%); severe respiratory distress; age < 3 months or high-risk infant.

Differential Diagnosis

Prognosis

Most infants recover fully within 1–2 weeks. Some have recurrent viral wheeze in early childhood; severe RSV bronchiolitis in infancy is associated with later wheezing tendency.

Clinical Course

• Illness typically peaks around days 3–5 and then improves over 1–2 weeks; cough may linger longer.
• Close monitoring during the peak is important, especially in young infants at risk of apnoea.

Key Supportive Measures

• Ensure adequate hydration — offer small, frequent feeds; use NG/IV fluids if feeding is inadequate.
• Gentle nasal saline and suction before feeds; nurse with the head slightly elevated.
• Reassess frequently for hypoxia, apnoea and worsening work of breathing.

Management is supportive — bronchodilators are of little value.