Acute Bronchiolitis - Clinical Notes

Definition

• Rosen's Emergency Medicine

Epidemiology

• Age: Most common and most severe in infants < 12 months; almost all children are infected by the causative viruses at least once by 24 months
• Seasonality: Peaks November to April in temperate climates (winter and early spring)
• Incidence: 1-3% of all children in the first 12 months will be hospitalized for severe RSV lower respiratory tract disease; highest rates in the first 6 months of life
• Transmission: Via fomites (hand-to-nose) and respiratory droplets from sneezing or coughing; viral shedding begins before clinical symptoms and continues for 2-3 weeks
• Incubation period: 2-8 days

Etiology

• Red Book 2021; Rosen's Emergency Medicine

Pathophysiology

1. Viral replication begins in epithelial cells of the upper airway, then spreads to the mucosal surfaces of the lower respiratory tract
2. Infected epithelial cells are destroyed by lysis or apoptosis - desquamation of cells + release of host inflammatory mediators
3. Histology shows:
   • Epithelial cell necrosis
   • Monocytic inflammation and edema of peribronchial tissues
   • Mucus and fibrin plugging of distal airways
4. This causes wheezing and lower airway obstruction
5. Small caliber of distal airways in younger infants + lack of active immunity = more severe clinical symptoms
6. Severe lower airway obstruction → air trapping and atelectasis → ventilation-perfusion mismatch → hypoxemia
7. Young infants are at risk for fatigue → hypercarbia and respiratory failure

• Rosen's Emergency Medicine; Swanson's Family Medicine Review

Clinical Features

Prodrome

• Nasal congestion, copious rhinorrhea (URI symptoms for 2-3 days)

Progression

• Tight cough (often with difficulty feeding)
• Audible wheezing (reported by parents)
• Tachypnea, increased work of breathing

Physical Examination

• Fever - in approximately one-third of patients
• Tachycardia, tachypnea, hypoxia
• Nasal flaring, retractions (intercostal, subcostal)
• Lung auscultation: decreased air movement, rales, rhonchi, wheezing (often with shifting crackles), prolonged expiratory phase
• Irritability or lethargy in young infants - indicates more severe disease
• Dehydration signs (decreased oral intake + increased insensible losses)

Important special presentations

• Very young infants (preterm): May present with lethargy, irritability, poor feeding with minimal respiratory symptoms; high risk for apnea even without other respiratory symptoms
• Apnea may precede the onset of typical symptoms in young infants

Disease Course

• Worst phase: first few days
• Median hospital stay (if admitted): 2-3 days
• Total illness duration: median 12 days
• Cough and noisy breathing may persist >4 weeks

Severity Assessment

• Rosen's Emergency Medicine, Table 163.4

Risk Factors for Severe Disease

• Prematurity (especially <29 weeks gestation)
• Low birth weight
• Chronic lung disease of prematurity (formerly bronchopulmonary dysplasia)
• Hemodynamically significant congenital heart disease (especially with pulmonary hypertension)
• Immunodeficiency states
• Neurologic and neuromuscular conditions
• Young age (<6 months)

• Maternal smoking during pregnancy / secondhand smoke exposure
• Family history of atopy
• Lack of breastfeeding
• Household crowding

Diagnosis

• History of prodromal URI symptoms in an infant/young child
• Physical examination findings: wheezing (often with shifting crackles) + increased work of breathing

Investigations - Generally NOT recommended routinely:

• Chest X-ray: Not routinely recommended; may show hyperinflation, peribronchial thickening; ordered when diagnosis is uncertain or severe disease is suspected
• ABG: Reserved for severe disease with impending respiratory failure to assess hypercarbia and respiratory acidosis (pulse oximetry adequate for most)
• Viral testing (RT-PCR multiplex): In most outpatient settings, specific viral testing has little effect on management and is not recommended; may help with hospital cohorting in admitted patients
  • RT-PCR can detect coinfection with 2+ viruses in up to 30% of symptomatic children
• Blood cultures / CSF: For febrile infants <1 month regardless of RSV/bronchiolitis status (SBI risk up to 12% in all febrile infants <8 weeks)
• Urinalysis and urine culture: For febrile infants 1-3 months who are RSV-positive or have clinical bronchiolitis (most concomitant SBIs in this group are UTIs)

Management

Supportive Care (cornerstone of treatment)

1. Hydration assessment:
   • Mild disease: oral feeds appropriate if respiratory rate <60-70 breaths/min and feeding not compromised
   • IV fluids for: retractions, nasal flaring, coughing (risk of aspiration), poor oral intake, dehydration
2. Supplemental oxygen: Only when SpO2 persistently <90% in a previously healthy infant
3. Nasal suctioning: Provides temporary benefit; routinely recommended
4. Respiratory monitoring: Careful assessment of respiratory status

Respiratory Support

• High-flow nasal cannula (HFNC): May have utility in preventing endotracheal intubation; reserve for moderate-to-severe disease
• CPAP: Limited evidence; useful in severe cases; use in consultation with critical care/pulmonary specialist
• Intubation and mechanical ventilation: For respiratory failure; in consultation with critical care specialist

Pharmacologic Treatments - Evidence and Recommendations

Febrile Infant with Bronchiolitis - Special Consideration

• Age <1 month: Full SBI workup + empirical antibiotics regardless of RSV/bronchiolitis status
• Age 1-3 months: Urinalysis + culture if febrile and RSV-positive/clinical bronchiolitis; individualized decision for blood and CSF cultures
• Age >3 months: Low risk for SBI (<1% bacteremia/meningitis in viral bronchiolitis)

Complications

• Acute bacterial otitis media: Most common - prevalence up to 60%; treat accordingly
• Apnea (especially in preterm, young infants)
• Respiratory failure / need for mechanical ventilation
• Dehydration
• Long-term: Increased risk of asthma later in life (whether causal or reflecting common genotype remains debated; immunoprophylaxis has not reduced this risk)

Indications for Hospitalization

• Young age
• Prematurity
• Significant hypoxemia (SpO2 <92-94%)
• Severe tachypnea (RR >70)
• Poor feeding / dehydration
• Apnea
• Severe increased work of breathing

Prevention

Palivizumab (RSV Immunoprophylaxis)

• Palivizumab = humanized monoclonal antibody against RSV F-protein
• Dose: 15 mg/kg/dose IM monthly during RSV season (max 5 doses per season)
• Cost-benefit: Cost analyses consistently show palivizumab cost exceeds benefit, even in high-risk groups

Eligibility for Prophylaxis (AAP/Red Book):

• Season: typically October/November to March/April (based on local RSV surveillance data)
• No more than 5 monthly doses recommended in the continental US

General Prevention

• Hand hygiene (handwashing before handling infants)
• Avoiding exposure to infected individuals
• Breastfeeding (protective)
• Avoiding secondhand smoke

Key Teaching Points

1. Bronchiolitis = clinical diagnosis - do not rely on imaging or labs routinely
2. RSV causes 70% of cases; seasonal (winter)
3. Pathophysiology: viral epithelial destruction → edema + mucus plugging → air trapping → VQ mismatch → hypoxemia
4. Management = supportive - the evidence does NOT support routine bronchodilators, steroids, antibiotics, or chest physio
5. Supplemental O2 only if SpO2 persistently <90%
6. Apnea risk in young/preterm infants - may precede typical respiratory symptoms
7. Febrile infants <1 month need full SBI workup regardless of RSV status
8. AOM complicates up to 60% of cases
9. Palivizumab for high-risk groups (15 mg/kg/month IM, max 5 doses)
10. RSV bronchiolitis in infancy is associated with increased asthma risk later in life