13.2 Paediatric Respiratory Illness: Croup, Bronchiolitis & Asthma

Paediatric Upper & Lower Airway Anatomy & Pathophysiology

Respiratory emergencies account for up to 30% of all paediatric presentations to Caribbean emergency departments. The paediatric respiratory system possesses distinct anatomical and physiological vulnerabilities compared to that of adults:

1. Narrow Airway Calibre & Poiseuille's Law: Resistance to laminar airflow is inversely proportional to the fourth power of the airway radius ($R \propto 1/r^4$). In infants and young children, whose normal subglottic diameter is only 4 to 5 mm, a mere 1 mm of circumferential mucosal edema reduces the cross-sectional area by over 60% and increases airway resistance by sixteen-fold. In turbulent flow (crying, agitation), resistance is inversely proportional to the fifth power of the radius ($R \propto 1/r^5$).
2. Funnel-Shaped Airway: The paediatric larynx is funnel-shaped (conical), with its narrowest non-distensible anatomical point located at the complete cartilaginous ring of the cricoid cartilage (unlike the adult larynx, where the narrowest point is the vocal cords / rima glottidis).
3. Compliant Chest Wall & Horizontal Diaphragm: The paediatric thoracic cage consists of cartilaginous ribs positioned horizontally, offering little mechanical rigidity. In respiratory distress, intense negative intrathoracic pressures cause severe sternal and intercostal retractions rather than expansive tidal breathing, placing high reliance on the diaphragm and predisposing infants to rapid muscular fatigue and respiratory failure.

Acute Viral Croup (Laryngotracheobronchitis)

Viral croup is the most common cause of acute infectious upper airway obstruction in young children, predominantly affecting toddlers between 6 months and 3 years of age (peak incidence at 1 to 2 years, with an autumn and winter predominance).

Etiology & Pathogenesis

• Causative Agents: Human Parainfluenza viruses account for over 75% of cases (Type 1 is the most common, followed by Type 2 and Type 3). Other viral triggers include Respiratory Syncytial Virus (RSV), Adenovirus, Influenza A and B, and Measles.
• Pathology: The infection begins in the nasopharynx and spreads downward into the larynx, trachea, and bronchi. Viral replication induces erythema, mononuclear inflammatory infiltration, and marked mucosal and submucosal edema strictly localized to the subglottic space within the rigid boundaries of the cricoid cartilage. Dynamic inspiratory collapse of the narrowed subglottic airway generates turbulent flow, manifesting clinically as harsh inspiratory stridor.

Clinical Presentation & The Westley Severity Score

Croup typically begins with a 1-to-3-day prodrome of mild fever, coryza, and rhinorrhea. It then transitions abruptly—frequently awakening the child in the middle of the night—into the classic clinical triad:

1. A harsh, barking, brassy, metallic cough (classically described as a "seal-bark" cough).
2. Inspiratory stridor (high-pitched musical sound generated by turbulent airflow through the narrowed subglottis).
3. Hoarseness of voice or cry (due to vocal cord edema).

Severity is objectively classified using the Westley Clinical Score, which evaluates five clinical variables:

• Mild Croup (Score <= 2): Barking cough, hoarseness, but NO stridor at rest and no or mild retractions.
• Moderate Croup (Score 3 to 7): Frequent barking cough, easily audible inspiratory stridor at rest, and moderate sternal/intercostal retractions, with normal sensorium.
• Severe Croup (Score 8 to 11): Prominent stridor at rest, severe chest wall indrawing, marked sternal retractions, decreased air entry, and agitation or distress.
• Impending Respiratory Failure (Score >= 12): Lethargy, exhaustion, depressed consciousness, dusky skin or cyanosis at rest, and paradoxical "quieting" of stridor due to critically diminished tidal volume.

Radiographic Hallmarks: The Steeple Sign

Croup is a clinical diagnosis. Radiographs are NOT required for typical presentations and should never delay treatment. If an anteroposterior (AP) soft-tissue neck radiograph is performed to rule out alternative pathologies, it reveals symmetrical, smooth, tapered subglottic narrowing—termed the "Steeple Sign" (or pencil-point sign)—representing mucosal edema of the subglottic trachea silhouetted against luminal air.

Evidence-Based Medical Management

+-----------------------------------------------------------------------------------------+
|                        EVIDENCE-BASED CROUP PHARMACOTHERAPY MATRIX                      |
+-----------------------------------------------------------------------------------------+
| MILD CROUP (Westley <= 2):                                                              |
| - Single-dose Oral Dexamethasone: 0.15 mg/kg (max 16 mg)                                |
| - Reassurance, discharge home with clear worsening instructions                          |
+-----------------------------------------------------------------------------------------+
| MODERATE TO SEVERE CROUP (Westley >= 3):                                                |
| 1. Minimize agitation: keep child calm in caregiver's arms; avoid painful interventions |
| 2. Supplemental Oxygen: humidified blow-by O2 if SpO2 < 92%                             |
| 3. Single-dose Oral Dexamethasone: 0.15 to 0.60 mg/kg (max 16 mg)                       |
|    (Use IV/IM dexamethasone or nebulized budesonide 2 mg ONLY if child is vomiting)     |
| 4. Nebulized Epinephrine for Stridor at Rest:                                           |
|    * L-Epinephrine 1:1,000: 0.5 mL/kg (max 5.0 mL) diluted in normal saline, OR         |
|    * Racemic Epinephrine 2.25%: 0.05 mL/kg (max 0.5 mL) in 2.5 mL normal saline         |
| 5. Mandatory 2-to-4 Hour ED Observation Window to rule out "Rebound Stridor"           |
+-----------------------------------------------------------------------------------------+

• Systemic Corticosteroids (Dexamethasone):
  • Corticosteroids are the cornerstone of croup therapy, indicated in ALL children with croup, regardless of severity. Corticosteroids decrease capillary endothelial permeability, inhibit inflammatory cytokine cascades, and resolve subglottic edema.
  • Dose: Single dose of oral Dexamethasone at 0.15 to 0.60 mg/kg (maximum 16 mg). Clinical trials confirm that low-dose oral dexamethasone (0.15 mg/kg) is clinically equivalent to high-dose regimens (0.60 mg/kg) in mild-to-moderate croup. Dexamethasone's long biological half-life (36 to 72 hours) covers the entire natural window of subglottic swelling.
  • Oral administration is preferred because it is non-invasive and avoids needle-induced panic. If the child is actively vomiting, administer intramuscular/intravenous dexamethasone (0.6 mg/kg) or nebulized Budesonide (2 mg).
• Nebulized Epinephrine (Adrenaline):
  • Indicated for moderate to severe croup with stridor at rest or significant retractions.
  • Formulations: Nebulized L-Epinephrine 1:1,000 (0.5 mL/kg, maximum 5.0 mL) or Racemic Epinephrine 2.25% (0.05 mL/kg, max 0.5 mL in 2.5 mL normal saline) driven by 6 to 8 L/min of oxygen.
  • Mechanism: Stimulates subglottic alpha-1 adrenergic receptors, inducing rapid arteriolar vasoconstriction that shrinks mucosal edema and relieves airflow obstruction within 10 to 30 minutes.
  • The Rebound Phenomenon: Epinephrine's clinical effect is transient, dissipating within 90 to 120 minutes. Once vasoconstriction wanes, mucosal edema can re-expand, precipitating severe "rebound" stridor and airway obstruction. Therefore, any child treated with nebulized epinephrine must be observed in the emergency department for a minimum of 2 to 4 hours. Discharge is safe only if the child maintains no stridor at rest, normal air entry, no retractions, and normal pulse oximetry.

Acute Bronchiolitis in Infants

Acute bronchiolitis is the single most common lower respiratory tract infection and the leading cause of hospital admission among infants under 12 months of age worldwide.

RSV Pathophysiology & Vulnerable Cohorts

• Etiology: Respiratory Syncytial Virus (RSV) accounts for 70% to 80% of all cases. Other culprits include Human Rhinovirus, Human Metapneumovirus, Parainfluenza, and Adenovirus.
• Pathophysiology: Inoculation of respiratory epithelium leads to viral necrosis of ciliated epithelial cells in the bronchioles, peribronchiolar lymphocytic infiltration, and marked submucosal edema. Desquamated epithelial debris, fibrin, and thick mucus form dense intraluminal plugs. Because infant bronchioles are narrow, partial luminal occlusion creates a "ball-valve" mechanism: air enters during active inspiration but becomes trapped during passive expiration, producing hyperinflation and alveolar air trapping. Complete bronchiolar occlusion causes resorption of trapped air, producing patchy subsegmental microatelectasis and ventilation-perfusion ($V/Q$) mismatch with resultant hypoxemia.
• High-Risk Vulnerable Cohorts: Prematurity (< 35 weeks gestation), chronological age < 12 weeks, hemodynamically significant congenital heart disease, chronic lung disease of prematurity (bronchopulmonary dysplasia), neuromuscular disorders, and congenital or acquired immunodeficiencies. These infants have high risk for apnea and respiratory failure.

Clinical Course & Radiographic Findings

• Clinical Presentation: Bronchiolitis follows a predictable timeline. It begins with 2 to 3 days of upper respiratory prodrome (clear rhinorrhea, sneezing, low-grade fever). Between days 3 and 5 (the clinical peak), infection descends into the lower airways, manifesting with:
  • Tachypnea (respiratory rate > 50 to 70 breaths/min).
  • Increased work of breathing: intercostal, subcostal, and suprasternal retractions, nasal flaring, and expiratory grunting.
  • Auscultation: Bilateral, diffuse, fine end-inspiratory crackles (crepitations) and prolonged expiration with high-pitched, musical, polyphonic expiratory wheezes across all lung fields.
  • Poor feeding and dehydration due to tachypnea and excessive respiratory work.
• Radiographic Findings: Routine chest radiography is NOT recommended for typical bronchiolitis. When obtained in severe or atypical cases, it demonstrates bilateral hyperinflation (flattened diaphragmatic leaflets, increased retrosternal lucency, visualization of > 9 posterior ribs), peribronchial thickening ("cuffing"), and patchy areas of atelectasis (frequently misdiagnosed by inexperienced clinicians as bacterial consolidation).

Supportive Care: The Evidence-Based Paradigm

International paediatric consensus guidelines (AAP, NICE) emphasize that the management of acute viral bronchiolitis is almost entirely supportive:

1. Oxygen Therapy: Give supplemental oxygen for persistent SpO2 <90% in otherwise healthy infants aged >=6 weeks, or <92% in younger infants or those with underlying conditions. In cyanotic congenital heart disease, use the child's individual baseline and cardiology-prescribed target rather than routinely aiming for 92%.
2. Gentle Nasopharyngeal Suctioning: Infants are obligate nasal breathers. Suctioning thick nasal secretions with a bulb syringe or soft suction catheter prior to feeds and sleep relieves upper airway resistance and improves feeding.
3. Hydration & Nutrition Support: If respiratory rate is 60 to 70 breaths/min, oral feeding may precipitate aspiration. Provide enteral feeds via a nasogastric (NG) tube in small, frequent boluses. If respiratory distress is severe or the child is exhausted, administer intravenous maintenance fluids (isotonic 0.9% Normal Saline with 5% Dextrose) at two-thirds to full maintenance rate, monitoring closely for inappropriate antidiuretic hormone secretion (SIADH).
4. High-Flow Nasal Cannula (HFNC): Heated, humidified high-flow nasal cannula therapy (1 to 2 L/kg/min) washes out anatomical nasopharyngeal dead space, delivers low-level positive end-expiratory pressure (PEEP of 2 to 4 $cmH_2O$), staves off alveolar collapse, and decreases the work of breathing, supporting infants with persistent hypoxemia or severe distress despite standard low-flow oxygen.

Interventions Proven Ineffective or Harmful

In CAMC and regional examinations, candidates must recognize medications that are NOT RECOMMENDED for routine acute bronchiolitis:

• Inhaled Bronchodilators (Salbutamol/Albuterol): Clinical trials show no significant impact on oxygen requirements, disease resolution, or length of hospital stay. Infants under 12 months possess immature bronchial smooth muscle and uncoupled beta-2 adrenergic receptors; the primary lesion is luminal plug occlusion and mucosal edema, not bronchospasm.
• Systemic Corticosteroids (Prednisolone, Dexamethasone): Completely ineffective in altering the clinical course of viral bronchiolitis; corticosteroids provide no benefit and expose infants to immunosuppressive and metabolic complications.
• Antibiotics: Viral etiology renders antibacterial agents useless. Secondary bacterial pneumonia occurs in < 1% of infants with bronchiolitis. Antibiotics should be withheld unless clear concurrent bacterial infection (e.g., acute otitis media or documented bacteremia) is identified.
• Chest Physiotherapy: Generates agitation, increases oxygen consumption, and does not accelerate clinical recovery.
• Prophylaxis (Palivizumab): A humanized monoclonal antibody directed against the RSV F (fusion) protein. Not used for acute treatment, but administered as monthly intramuscular injections (15 mg/kg) during the epidemic season to high-risk preterm infants (< 29-32 weeks) and infants with congenital heart disease or bronchopulmonary dysplasia.

Acute Paediatric Asthma Exacerbations

Asthma is the most prevalent chronic non-communicable childhood disease in the Caribbean. Acute exacerbations are characterized by acute, reversible bronchospasm, mucosal edema, and mucous plugging triggered by viral respiratory infections, aeroallergens, or sudden changes in temperature.

Pathophysiology & Severity Stratification (PRAM Score)

Acute airway narrowing leads to expiratory airflow limitation, air trapping, hyperinflation, and severe $V/Q$ mismatching. Clinical assessment relies on validated scoring instruments such as the Preschool Respiratory Assessment Measure (PRAM) or Pediatric Respiratory Assessment Measure:

• Mild Exacerbation (PRAM 0 to 3): Alert, talks in sentences, mild expiratory wheeze, $SpO_2$ >= 95%.
• Moderate Exacerbation (PRAM 4 to 7): Talks in phrases, tachypneic, moderate retractions, loud expiratory wheeze, $SpO_2$ 92% to 94%.
• Severe Exacerbation (PRAM 8 to 12): Agitated, talks in single words, marked accessory muscle use, inspiratory and expiratory wheezing (or ominously absent breath sounds / "silent chest"), $SpO_2$ < 92%.

First-Line Emergency Pharmacotherapy

1. Inhaled Short-Acting Beta-2 Agonist (Salbutamol / Albuterol):
   • Stimulates beta-2 adrenergic receptors, activating adenylate cyclase, elevating intracellular cAMP, and inducing rapid bronchial smooth muscle relaxation.
   • Delivery: Administered via a pressurized Metered-Dose Inhaler (pMDI) with a valved holding chamber (spacer) and tight-fitting face mask (or mouthpiece in children >= 5 years). Systematic reviews demonstrate that pMDI with spacer is as effective as, or superior to, wet nebulization in reversing bronchospasm, with shorter emergency stays and fewer side effects (less tachycardia and tremor).
   • Dosing: 4 to 10 puffs (100 mcg/puff) every 20 minutes for 3 doses in the first hour. In severe exacerbations, deliver via oxygen-driven wet nebulizer (2.5 mg for children < 5 years; 5.0 mg for children >= 5 years).
2. Inhaled Anticholinergic (Ipratropium Bromide):
   • Blocks muscarinic M3 receptors on bronchial smooth muscle, inhibiting cholinergic bronchomotor tone and reducing mucus hypersecretion.
   • Indication: Added to salbutamol in the first hour for all moderate and severe asthma exacerbations.
   • Dosing: 4 to 8 puffs via pMDI with spacer, or 250 mcg (< 5 years) / 500 mcg (>= 5 years) nebulized every 20 minutes for 3 doses alongside salbutamol. Combination therapy significantly reduces hospital admission rates.

Systemic Corticosteroids & Inhaled Anticholinergics

Systemic corticosteroids must be administered within the first hour of presentation for all moderate and severe attacks. Corticosteroids suppress eosinophilic and lymphocytic airway inflammation, reduce mucosal edema, and upregulate downregulated beta-2 receptors, taking 2 to 4 hours to manifest clinical benefit:

• Oral Prednisolone: 1 to 2 mg/kg once daily (maximum 50 to 60 mg) for 3 to 5 days. Routine taper is unnecessary for courses lasting <= 5 days.
• Alternative: Intravenous Methylprednisolone (1 mg/kg every 6 hours) or IV Hydrocortisone (4 to 5 mg/kg every 6 hours) if the child is vomiting or in severe respiratory distress.

Second-Line Escalation for Status Asthmaticus

If the child fails to respond to intensive first-hour therapy (persistent severe distress, $SpO_2$ < 92%, exhaustion):

1. Intravenous Magnesium Sulfate ($MgSO_4$):
   • Potent physiological calcium channel blocker; prevents calcium influx into bronchial smooth muscle cells, triggering immediate bronchodilation.
   • Dose: 40 to 50 mg/kg IV (maximum 2.0 g) infused over 20 to 30 minutes with continuous cardiac and blood pressure monitoring. Highly effective in reducing intensive care admissions.
2. High-Flow Nasal Cannula (HFNC) or Non-Invasive Positive Pressure Ventilation (BiPAP/CPAP) to reduce diaphragmatic fatigue.
3. Continuous IV Salbutamol Infusion (1 to 5 mcg/kg/min) or IV Aminophylline in a pediatric ICU setting.

Acute Epiglottitis: The Life-Threatening Supraglottic Emergency

Acute epiglottitis (supraglottitis) is a rapidly progressive, life-threatening bacterial cellulitis of the epiglottis, arytenoids, aryepiglottic folds, and false vocal cords.

Epidemiology & Microbiological Etiology

Historically, Haemophilus influenzae type b (Hib) accounted for over 95% of cases in toddlers aged 2 to 6 years. Widespread Caribbean implementation of the pentavalent/hexavalent Hib conjugate vaccine has reduced epiglottitis by > 98%. However, cases still occur in unimmunized or under-immunized children, or may be caused by Streptococcus pyogenes (Group A Strep), Streptococcus pneumoniae, or Staphylococcus aureus (including MRSA).

The Toxic Clinical Triad & Strict Examination Rules

Unlike croup, epiglottitis has an abrupt, fulminant onset, escalating over a few hours without a preceding viral prodrome:

• Clinical Presentation: High fever (> 39.0°C), toxic appearance, severe sore throat, and the classic "Three Ds":
  1. Dysphagia (inability to swallow liquids or saliva due to intense throat pain).
  2. Drooling (constant saliva pooling and dribbling from the mouth).
  3. Distress (severe inspiratory distress, marked anxiety, tachycardia).
• The Tripod Posture: The child instinctively sits bolt upright, leaning forward with arms braced on the knees, neck hyperextended, and chin thrust forward to maximize anatomical airway patency.
• Voice & Cough: The voice is muffled and thick ("hot potato voice"). The barking seal-like cough characteristic of croup is completely ABSENT.
• THE GOLDEN SAFETY DIRECTIVE: If acute epiglottitis is suspected, NEVER examine the posterior pharynx with a tongue depressor or perform painful venipuncture in an unmonitored setting. Agitating the child or stimulating the sensitive supraglottis can provoke catastrophic, immediate, complete laryngospasm and cardiac arrest. Keep the child upright in the caregiver's lap, administer humidified blow-by oxygen gently, and summon anesthesia and ENT surgery immediately.

Radiographic Assessment: The Thumbprint Sign

Radiographs are strictly secondary and should never be obtained if the child is in overt respiratory distress. If performed in a stable setting with airway personnel present, a lateral soft-tissue neck radiograph demonstrates an enlarged, rounded, edematous epiglottis protruding into the hypopharynx—termed the "Thumbprint Sign" (contrasting with the normal slender, finger-like epiglottis)—with loss of the vallecular space.

Emergency Airway Securing & Antimicrobial Regimens

• Airway Securing: Immediate transfer to the operating room under the joint care of a pediatric anesthesiologist and an otolaryngologist. The child undergoes inhalation induction with sevoflurane in the sitting position, followed by gentle endotracheal intubation using a tube 0.5 to 1.0 mm smaller than calculated for age. Direct laryngoscopy confirms a swollen, beefy, cherry-red epiglottis.
• Antimicrobial Therapy: Once the definitive airway is secured, blood cultures and epiglottic swabs are taken, and intravenous broad-spectrum antibiotics are initiated: Ceftriaxone (100 mg/kg/day IV divided q12h) or Cefotaxime (150 mg/kg/day IV divided q8h) PLUS Vancomycin (60 mg/kg/day IV) for 7 to 10 days. Extubation is typically achieved within 48 to 72 hours once direct laryngoscopy confirms resolution of supraglottic swelling and an audible endotracheal cuff leak is demonstrated.

Comparative Differential Diagnosis of Paediatric Stridor and Wheeze