5.5 Bronchiolitis, Croup & Epiglottitis

5.5 Bronchiolitis, Croup & Epiglottitis

Young children have a smaller airway, relatively large tongue and occiput, a more cephalad larynx, and a long often omega-shaped epiglottis. Modern imaging shows the lumen is not a simple circular funnel with one universal narrowest point: the glottic and subglottic dimensions vary by plane, age, patient, and cuff state. Even modest edema can therefore produce a large proportional reduction in usable lumen.

According to Poiseuille's Law, laminar airflow resistance ($R$) is inversely proportional to the fourth power of the internal airway radius ($r$):

$R \propto \frac{1}{r^4}$

As an illustrative geometric model, 1 mm of circumferential edema reduces a 4 mm lumen to 2 mm and cuts cross-sectional area by 75%. Poiseuille's fourth-power relationship predicts a large resistance rise under laminar rigid-tube conditions; the living upper airway is compliant and often turbulent, so do not treat “16-fold” as a measured universal response.

---

Viral Bronchiolitis (RSV & Viral Pathogens)

Bronchiolitis is an acute viral lower respiratory tract infection affecting infants and children under 2 years of age, with peak incidence between 2 and 6 months of age.

Etiology & Pathogenesis

• Etiological Agents: Respiratory Syncytial Virus (RSV) accounts for 70% to 80% of all cases. Other culprits include Human Rhinovirus (HRV), Human Metapneumovirus (hMPV), Parainfluenza, Adenovirus, and Coronavirus.
• Pathological Sequence:
  1. Viral inoculation of the nasopharynx spreads down the tracheobronchial tree into terminal bronchioles ($<2\text{ mm}$ diameter).
  2. Extensive cytopathic destruction and necrosis of ciliated bronchiolar epithelial cells occurs.
  3. Dense peribronchiolar lymphocytic and mononuclear cellular infiltration develops with profound submucosal edema.
  4. Proliferation of non-ciliated secretory cells and sloughed necrotic debris, combined with mucus hypersecretion, forms dense intraluminal mucus plugs.
  5. Complete bronchiolar plugging results in distal gas absorption and patchy microatelectasis; partial plugging creates a one-way ball-valve obstruction causing hyperinflation and dynamic air trapping.

Clinical Presentation & Vulnerable Populations

• Clinical Progression: Initial upper respiratory infection (rhinorrhea, sneezing, low-grade fever) over 1 to 3 days, progressing to lower tract distress peaking between days 3 and 5. Signs include tachypnea ($RR\ 60\text{ to }80+\text{ breaths/min}$), paroxysmal coughing, prolonged expiratory phase, intercostal/subcostal retractions, nasal flaring, and diffuse bilateral fine inspiratory crackles and expiratory wheezes.
• High-Risk Vulnerable Infants: Infants born premature ($<35\text{ weeks}$), those with Bronchopulmonary Dysplasia / Chronic Lung Disease, Congenital Heart Disease (hemodynamically significant shunts or pulmonary hypertension), and neuromuscular disorders. In neonates $<2\text{ months}$ and former preterms, central and obstructive apnea is a common presenting sign that can precede audible pulmonary signs.

AAP Evidence-Based Clinical Practice Guidelines

The American Academy of Pediatrics (AAP) bronchiolitis guidelines emphasize diagnostic stewardship and avoidance of ineffective therapies:

• DO NOT Routinely Administer Inhaled Bronchodilators (Albuterol / Epinephrine): Bronchiolar obstruction is caused by cellular sloughing, intraluminal plugs, and mucosal edema—not smooth muscle bronchospasm. Extensive randomized trials confirm albuterol does not improve oxygenation, reduce admission rates, or shorten hospital stay.
• Do not routinely administer systemic corticosteroids: They are not recommended for typical viral bronchiolitis; reassess the diagnosis when asthma, croup, or another steroid-responsive condition is suspected.
• DO NOT Perform Routine Chest Physiotherapy (CPT): CPT increases infant agitation, oxygen consumption, and stress without improving mucus clearance.
• DO NOT Routinely Administer Antibiotics: Bronchiolitis is viral; secondary bacterial pneumonia occurs in $<1%$ of hospitalized infants.
• DO NOT Obtain Routine Radiographs: Chest X-rays show non-specific peribronchial cuffing, hyperinflation, and patchy subsegmental atelectasis, which are frequently misinterpreted as bacterial pneumonia, leading to inappropriate antibiotic use.

+-------------------------------------------------------------------------------------------------------------+
|                          AAP BRONCHIOLITIS GUIDELINES: WHAT TO AVOID VS. WHAT TO DO                         |
+------------------------------------+------------------------------------------------------------------------+
| INEFFECTIVE (DO NOT DO)            | - Routine inhaled albuterol or levalbuterol                            |
|                                    | - Systemic corticosteroids (dexamethasone / prednisolone)              |
|                                    | - Routine chest physiotherapy (percussion / postural drainage)          |
|                                    | - Empiric antibiotics without confirmed co-infection                   |
|                                    | - Routine diagnostic chest radiographs and blood work                  |
+------------------------------------+------------------------------------------------------------------------+
| EVIDENCE-BASED (CORNERSTONES)      | - Gentle superficial nasal suction when secretions impair feeding      |
|                                    | - Oxygen and escalation to HFNC/CPAP when clinically indicated          |
|                                    | - Supportive hydration (enteral NG tube or IV fluids if RR > 60-70)    |
|                                    | - Supplemental oxygen titrated to SpO2 >= 90%                          |
|                                    | - Monoclonal antibody prophylaxis (Nirsevimab / Palivizumab) for high-risk|
+------------------------------------+------------------------------------------------------------------------+

Supportive Respiratory Interventions

1. Nasal Suctioning: Gentle superficial suctioning using a bulb syringe or suction catheter relieves high upper-airway resistance from copious secretions, especially prior to oral/NG feeding.
2. Heated Humidified High-Flow Nasal Cannula (HFNC):
   • Flow titrated at $1.0\text{ to }2.0\text{ L/kg/min}$.
   • Delivers optimal humidity ($37^\circ\text{C}$, $44\text{ mg/L }\text{H}_2\text{O}$) to thin secretions and prevent mucosal drying.
   • Washes out anatomical dead space in the nasopharynx, lowering work of breathing.
   • Generates low-level distending positive airway pressure (PEEP $\approx 2\text{ to }4\text{ cmH}_2\text{O}$), stenting open small airways and recruiting microatelectasis.
3. Continuous Positive Airway Pressure (CPAP): Bubble or mechanical CPAP at $5\text{ to }7\text{ cmH}_2\text{O}$ is indicated for refractory severe retractions, recurrent apnea, or severe hypoxemia failing HFNC.

---

Upper Airway Obstruction: Croup vs. Acute Epiglottitis

Upper airway infections can rapidly cause catastrophic airway obstruction. Differentiating Viral Croup (Laryngotracheobronchitis) from Bacterial Epiglottitis (Supraglottitis) is a core clinical competency for the respiratory specialist.

+-------------------------------------------------------------------------------------------------------------+
|                             DIAGNOSTIC COMPARISON: CROUP VS. ACUTE EPIGLOTTITIS                             |
+---------------------+------------------------------------------+--------------------------------------------+
| Diagnostic Feature  | Croup (Laryngotracheobronchitis)         | Acute Epiglottitis (Supraglottitis)        |
+---------------------+------------------------------------------+--------------------------------------------+
| Primary Etiology    | Viral: Parainfluenza type 1 (most common)| Bacterial: Haemophilus influenzae type b   |
|                     | Parainfluenza types 2/3, RSV, Influenza. | (Hib), S. pneumoniae, S. aureus, GAS.      |
+---------------------+------------------------------------------+--------------------------------------------+
| Patient Age Range   | 6 months to 3 years (peak 1 to 2 years). | 2 to 7 years (unvaccinated or older children|
+---------------------+------------------------------------------+--------------------------------------------+
| Onset & Progression | Slow, insidious onset over 1 to 3 days,  | Rapid, fulminant progression over hours;   |
|                     | preceded by typical viral URI prodrome.  | rapid toxic decompensation.                |
+---------------------+------------------------------------------+--------------------------------------------+
| Clinical Signs      | Barking, 'seal-like' cough, hoarseness,  | The 4 Ds: Dysphagia, Drooling, Dysphonia   |
|                     | inspiratory stridor, low-grade fever.    | ('hot potato' voice), Distressed stridor.  |
|                     | Child looks non-toxic.                   | ABSENCE of barking cough. High toxic fever.|
+---------------------+------------------------------------------+--------------------------------------------+
| Characteristic      | Prefers sitting upright in parent's lap; | Classic 'Tripod' or 'Sniffing' posture:    |
| Posture             | agitated crying worsens stridor.         | sitting upright, leaning forward, chin out,|
|                     |                                          | neck hyperextended, mouth open drooling.   |
+---------------------+------------------------------------------+--------------------------------------------+
| Radiographic Sign   | AP Neck View: 'Steeple Sign'             | Lateral Neck View: 'Thumbprint Sign'       |
|                     | (subglottic conical tapered narrowing).  | (swollen, rounded, cherry-red epiglottis). |
+---------------------+------------------------------------------+--------------------------------------------+
| Airway precaution  | Avoid unnecessary invasive agitation.    | Avoid forceful pharyngeal examination      |
|                      |                                          | outside a prepared airway setting.          |
+---------------------+------------------------------------------+--------------------------------------------+
| Definitive          | - Oral/IM Dexamethasone 0.6 mg/kg single | - Emergency OR transfer with ENT/Anesthesia|
| Management          | - Nebulized Racemic Epinephrine 2.25%    | - Inhalation induction, secure ETT (0.5-1.0|
|                     |   (observe 2-3 hours for rebound).       |   mm smaller than normal), IV antibiotics. |
+---------------------+------------------------------------------+--------------------------------------------+

Clinical Pharmacology of Croup Management

1. Systemic Corticosteroids:
   • Dexamethasone: A single dose is standard croup therapy. Common pathways use $0.15\text{ to }0.6\text{ mg/kg}$ orally, IM, or IV, with the maximum dose and route set locally; $0.6\text{ mg/kg}$ is a common traditional regimen.
2. Nebulized Racemic Epinephrine:
   • Formulation & Dosing: Racemic Epinephrine 2.25% solution, dosed at $0.05\text{ mL/kg}$ (standard dose $0.5\text{ mL}$ diluted in $3.0\text{ mL}$ normal saline). If unavailable, $5\text{ mL}$ of standard $1:1,000$ L-epinephrine can be nebulized.
   • Mechanism: Powerful stimulation of local $\alpha_1$-adrenergic receptors on mucosal vasculature causes rapid vasoconstriction, fluid reabsorption, and immediate reduction of subglottic mucosal edema.
   • Observation after epinephrine: Benefit wanes over roughly 2 hours. Observe long enough to confirm sustained improvement without recurrent stridor at rest; the exact discharge interval and criteria follow the local pediatric pathway.

Emergency Safety Protocol for Acute Epiglottitis

Acute epiglottitis is a supraglottic cellulitis causing massive edema of the lingual surface of the epiglottis, aryepiglottic folds, and arytenoids.

Critical Emergency Warning: Epiglottitis Airway Safety

DO NOT insert a tongue depressor, swab, or laryngoscope into the child's pharynx in the emergency department. Direct mechanical stimulation of the hypersensitive, inflamed supraglottic structures triggers violent reflex laryngospasm, converting a precarious partial airway into a complete, irreversible obstruction that cannot be ventilated by bag-valve-mask.

Protocol:

1. Maintain the child in an undisturbed position of comfort (in parent's lap). Do not start peripheral IVs, draw blood, or force the child supine.
2. Deliver gentle blow-by oxygen if tolerated.
3. Mobilize the airway emergency team: Senior Anesthesiologist, Otolaryngologist (ENT), and Critical Care RT.
4. Transfer the child immediately to the Operating Room (OR).
5. Induction: Gentle inhalation induction with sevoflurane in 100% oxygen while maintaining spontaneous respiration. Secure the airway with an endotracheal tube sized $0.5\text{ to }1.0\text{ mm}$ smaller than age-predicted diameter due to intense subglottic/glottic edema.
6. Have a rigid bronchoscope and emergency tracheostomy tray opened and primed at the surgical bedside.

---