Acute Bronchiolitis: Definition, Etiopathogenesis, and Management

I. Definition

Acute bronchiolitis is a clinical syndrome characterized by inflammation of the bronchioles (small airways).

• AAP Definition: The first episode of wheezing in a child <2 years of age, preceded by a viral respiratory prodrome (coryza/cough).
• It is the most common lower respiratory tract infection in infancy.

II. Etiopathogenesis

1. Etiology

• Respiratory Syncytial Virus (RSV): Cause of 50–80% of cases.
• Other Viruses: Human Metapneumovirus (hMPV), Rhinovirus, Parainfluenza, Adenovirus, Influenza.

2. Pathophysiology

The virus infects the terminal bronchiolar epithelial cells, causing direct damage.

1. Necrosis & Sloughing: Ciliated epithelium undergoes necrosis and sloughs into the lumen.
2. Inflammation & Edema: Submucosal edema thickens the airway wall.
3. Mucus Hypersecretion: Increased mucus production.
4. Obstruction: The combination of necrotic debris (plugs), mucus, and edema obstructs the narrow bronchioles.
   • Ball-Valve Effect: Air enters on inspiration but is trapped on expiration $\to$ Hyperinflation.
   • Complete Obstruction: Distal gas absorption $\to$ Atelectasis.
5. V/Q Mismatch: Leads to hypoxemia.

III. Characteristic Clinical Features

• Prodrome: 1–3 days of upper respiratory symptoms (rhinorrhea, mild cough, low-grade fever).
• progression: Worsening cough, tachypnea, and feeding difficulties (Day 3–5 is usually the peak).
• Examination:
  • Inspection: Tachypnea, subcostal/intercostal retractions, nasal flaring.
  • Auscultation:
    • Inspiratory Crackles: Widespread fine crepitations (hallmark).
    • Expiratory Wheeze: High-pitched, polyphonic.
    • Prolonged expiratory phase.
• Signs of Severity: Apnea (especially in premies/neonates), grunting, cyanosis, poor feeding (<50% normal).

IV. Diagnosis

Clinical Diagnosis (AAP 2014 Guidelines):

Diagnosis is strictly CLINICAL based on history and physical examination.

• Inclusion:
  • Age < 24 months.
  • First episode of wheezing.
  • Prodrome of URI (coryza, nasal congestion) followed by lower respiratory signs.
• Exclusion:
  • Recurrent wheezing (think early onset asthma).
  • Stridor (think Croup/Tracheitis).
  • Chronic lung disease history.

Routine investigations are NOT recommended for typical bronchiolitis.

• Chest X-Ray: Only if diagnosis is uncertain or suspicion of complications (pneumothorax/bacterial pneumonia).
  • Findings: Hyperinflation (flat diaphragm, horizontal ribs), peribronchial cuffing, patchy atelectasis (often confused with consolidation).
• Viral Antigen/PCR: Not routine; useful only for cohorting/infection control (RSV vs. Influenza).
• Blood Gas (ABG/VBG): Indicated only in severe respiratory distress to rule out hypercapnic respiratory failure.

Differential Diagnosis (High Yield)

• Viral-induced wheeze / Asthma: Recurrent episodes, family history of atopy, older age.
• Bacterial Pneumonia: High fever, toxic look, focal consolidation on CXR.
• Foreign Body Aspiration: Sudden onset, unilateral reduced air entry.
• Congestive Heart Failure: Hepatomegaly, murmur, cardiomegaly on CXR.
• Pertussis: Paroxysmal cough, "whoop", lack of fever/wheeze relative to cough.

V. Modified TAL scoring #scoring

Used to assess severity and guide management (admission vs. discharge).
Score is calculated by summing points from 4 parameters (Max Score = 12).

Interpretation of MTS:

• Mild (Score ≤ 5): Outpatient management with counseling.
• Moderate (Score 6 – 10): Admission, trial of nebulization (hypertonic saline), O2 support.
• Severe (Score ≥ 11): ICU admission, potential for HFNC/CPAP/Ventilation.

VI. Essential Steps in Management

Management is primarily Supportive. "Less is More."

1. Airway & Oxygenation:
   • Positioning: Head elevation to maintain airway patency.
   • Suctioning: Superficial nasal suctioning for secretions (especially before feeds).
   • Oxygen Therapy: Indicated if SpO2 persistently <90–92%. Administered via nasal prongs or face mask.
2. Hydration & Nutrition:
   • Maintain fluid balance (insensible losses are high due to tachypnea).
   • Oral/Enteral: Preferred. Small, frequent feeds.
   • NG Tube/IV Fluids: Indicated if respiratory rate >60–70/min, nasal flaring prevents sucking, or hydration status is compromised.
3. Monitoring:
   • Pulse oximetry, respiratory rate, and signs of exhaustion/apnea.
   • Isolation (contact precautions) to prevent nosocomial spread.

VII. Recent Advances in Management #recent

Current "advances" focus heavily on de-implementation of ineffective therapies and the use of advanced non-invasive support.

1. High Flow Nasal Cannula (HFNC)

• Role: Rescue therapy for moderate-to-severe distress to prevent intubation.
• Mechanism: Delivers heated, humidified oxygen at high flow rates (2 L/kg/min).
  • Washes out anatomical dead space ($C{O}_2$ washout).
  • Provides mild Positive End-Expiratory Pressure (PEEP ~4-6 cm $H_{2}O$).
  • Reduces work of breathing.

2. Nebulized Hypertonic Saline (3%)

• Mechanism: Osmotic draw of water into airway lumen rehydrates the sol layer + breaks non-covalent bonds in mucus $\to$ improves mucociliary clearance.
• Status: Considered in hospitalized patients (may reduce length of stay), though recent guidelines suggest weak/equivocal recommendation.

3. "Choosing Wisely" (Evidence-Based De-escalation)

Strict guidelines (AAP 2014, NICE) against unnecessary medications:

• NO Bronchodilators (Salbutamol/Epinephrine): Routine use is not recommended as bronchiolitis is primarily airway edema/debris, not bronchospasm. (Trial dose only if strong family history of atopy).
• NO Corticosteroids: Systemic or inhaled steroids do not reduce admission or length of stay.
• NO Antibiotics: Unless there is a documented secondary bacterial infection (rare).
• NO Chest Physiotherapy: Can increase distress.

4. Heliox Therapy (Helium-Oxygen mixture)

• Used in refractory cases in PICU. Lower density gas reduces airway resistance and work of breathing.

5. Novel Antivirals (Research Phase)

• Fusion inhibitors (e.g., Presatovir) are under investigation but not yet standard clinical practice.

6. Prophylaxis

• Palivizumab - Monoclonal antibody against RSV Fusion protein. especially in high risk population like preemies <29weeks, CLD, CHD
• Nirsevimab - longer acting monoclonal antibody. one dose is sufficient.

Comparison: Viral Pneumonia vs. Bacterial Pneumonia vs. Bronchiolitis

Key Diagnostic Criteria

• Bacterial Pneumonia: Defined by Alveolar inflammation. Gold standard sign is Consolidation (Clinical or Radiological).
• Viral Pneumonia: Defined by Interstitial inflammation. Diagnosis is often one of exclusion (failed antibiotic response) or viral PCR.
• Bronchiolitis: Defined by Small Airway (Bronchiolar) obstruction. Clinical triad: Infant (<2yr) + Viral Prodrome + First episode of Wheeze.