Acute Bronchiolitis

Definition

Etiology

Pathophysiology

1. Viral replication begins in the upper airway epithelium, then spreads to the lower respiratory tract
2. Bronchiolar epithelial necrosis and sloughing → mucosal edema + submucosal infiltrate (lymphocytes, plasma cells)
3. Peribronchiolar inflammation and increased mucus secretion
4. Partial or complete obstruction of small airways → air trapping and hyperinflation
5. Ball-valve mechanism: air enters during inspiration (airways slightly dilate) but is trapped during expiration (airways collapse) → emphysema or atelectasis
6. V/Q mismatch → hypoxemia; CO₂ retention in severe cases

The infant's bronchioles are especially vulnerable — small caliber means even mild mucosal swelling causes significant resistance (resistance ∝ 1/r⁴).

Epidemiology & Risk Factors

• Peak age: 2–6 months; almost all children infected by age 2
• Seasonality: November–April (winter epidemics)
• Risk factors for severe disease (Nelson's):
  • Age < 12 weeks
  • Prematurity (< 35 weeks)
  • Congenital heart disease (especially hemodynamically significant)
  • Chronic lung disease / bronchopulmonary dysplasia
  • Immunodeficiency
  • Low birth weight
  • Non-breastfed infants
  • Crowded living conditions, day care

Clinical Features

Prodromal Phase (Days 1–3)

• Rhinorrhoea (coryza), nasal congestion
• Low-grade fever (rarely high)
• Cough (dry, brassy)

Established Disease (Days 3–5)

• Tachypnea (most sensitive sign; RR > 50–60/min)
• Expiratory wheeze ± inspiratory crackles (shifting crackles)
• Respiratory distress: nasal flaring, subcostal/intercostal retractions, use of accessory muscles
• Hyperinflation: barrel-shaped chest, liver and spleen pushed down (palpable)
• Prolonged expiration
• Cyanosis in severe cases
• Poor feeding, irritability, lethargy

On Auscultation

• Fine inspiratory crackles + high-pitched expiratory wheeze
• Reduced air entry in very severe obstruction ("silent chest" — ominous)

Severity Assessment

Investigations

Generally NOT required for diagnosis (clinical diagnosis)

Nelson's principle: Routine investigations (CXR, labs) have limited value and should not be done routinely. Diagnosis is clinical.

Chest X-ray Features

CXR showing bilaterally prominent peribronchial/perivascular markings (cuffing) at the hila — typical of bronchiolitis

CXR showing chest hyperinflation with horizontal ribs and air trapping consistent with bronchiolitis

Management

1. Supportive Care (Mainstay)

Oxygen

• Target SpO₂ ≥ 90–92% (some guidelines 94%)
• Deliver via nasal prongs, face mask, or high-flow nasal cannula (HFNC)

Hydration

• Oral/NG feeds preferred if respiratory rate < 60–70/min
• IV fluids if unable to feed, tachypnea > 70, or SpO₂ < 90%
• Use 2/3 maintenance to avoid SIADH (Nelson's caution: risk of hyponatremia)

Positioning

• Semi-upright (30–40°), head slightly extended

Nasal Suction

• Clear nasal secretions with saline drops + suction — improves feeding and breathing

2. Medications

Critical Nelson's & OP Ghai point: Most pharmacological therapies are NOT consistently effective in bronchiolitis.

3. Respiratory Support

4. Indications for Admission (Nelson's / OP Ghai)

• SpO₂ persistently < 92–94% on room air
• RR > 70/min
• Unable to maintain oral hydration / poor feeding
• Severe retractions, accessory muscle use
• Apnea
• Age < 2 months, prematurity
• Hemodynamically significant CHD / chronic lung disease
• Toxic/lethargic appearance
• Parental inability to manage at home

5. Indications for ICU / Mechanical Ventilation

• SpO₂ < 90% despite HFNC/CPAP
• PaCO₂ > 55 mmHg (rising CO₂ — fatigue)
• Apneic episodes
• Altered consciousness
• "Silent chest"

Prophylaxis

Palivizumab (Synagis)

• Monoclonal antibody against RSV F-protein
• Given monthly IM injection during RSV season
• Recommended for high-risk infants:
  • Premature infants < 35 weeks with chronic lung disease
  • Hemodynamically significant CHD
  • Severe immunodeficiency
• Not a treatment — strictly prophylactic; does not treat active RSV
• Reduces hospitalization by ~55% in high-risk groups

Nirsevimab (Beyfortus) — 2023 Update

• Long-acting monoclonal antibody (single dose, season-long protection)
• Now preferred over palivizumab for most infants per AAP 2023 update

Complications

• Apnea (especially in infants < 2 months and premature neonates)
• Bacterial superinfection (uncommon)
• Respiratory failure (rare in healthy term infants)
• Long-term: Increased risk of recurrent wheeze and asthma in subsequent years (mechanism debated — may reflect underlying atopy rather than RSV causing asthma)
• ALTE (apparent life-threatening events)

Differential Diagnosis

Key Nelson's & OP Ghai Teaching Points (Exam Highlights)

1. Most common cause: RSV (70%); most common cause of hospitalization in infants
2. Pathology: bronchiolar epithelial necrosis → edema + mucus → air trapping → hyperinflation
3. Diagnosis is CLINICAL — no routine investigations
4. Management is SUPPORTIVE — no drug is consistently effective
5. Bronchodilators: try once; continue only if response seen
6. Steroids: NOT recommended
7. 3% NaCl nebulization: mild benefit in hospitalized patients
8. Palivizumab: prophylaxis in high-risk infants, NOT treatment
9. Most sensitive sign of severity: tachypnea
10. Most common complication: apnea (especially in young/premature infants)
11. Long-term: recurrent wheeze / asthma association

Recent Evidence Update

• PMID 38902454 — Evidence-based management in high-risk ICU infants (2024)
• PMID 39690333 — HFNC use in bronchiolitis management (2024)
• PMID 41906981 — Investigations for bronchiolitis in infants: overview of reviews (2026) — supports limited utility of routine investigations

Library note: Nelson's Textbook of Pediatrics and OP Ghai's Essential Pediatrics are not directly available in this textbook library. This answer is compiled from my medical knowledge of these textbooks, supplemented by content from Rosen's Emergency Medicine (available in library) which closely parallels Nelson's approach on bronchiolitis management.