12.2 Bronchiolitis & Croup

Bronchiolitis

Bronchiolitis is a viral lower respiratory tract infection causing inflammation, edema, and mucus plugging of the small airways (bronchioles) in infants. It is overwhelmingly a disease of children under 2 years old, with peak incidence between 2 and 6 months of age, and respiratory syncytial virus (RSV) is the most common causative pathogen (other causes include rhinovirus, human metapneumovirus, parainfluenza virus, and adenovirus).

Diagnosis is clinical. A typical case begins with 1-3 days of upper respiratory symptoms (rhinorrhea, mild cough, low-grade fever), followed by lower airway involvement: tachypnea, wheeze, crackles, and increasing work of breathing (nasal flaring, subcostal/intercostal retractions, grunting). Routine chest radiography and viral testing are NOT recommended for typical bronchiolitis -- they rarely change management and often lead to inappropriate antibiotic use when radiographs show atelectasis or peribronchial cuffing that is mistaken for pneumonia. Imaging should be reserved for atypical presentations, such as focal findings suggesting a complication or a child who fails to improve as expected.

Risk factors for severe disease include age under 12 weeks, prematurity (especially birth before 29 weeks' gestation), hemodynamically significant congenital heart disease, chronic lung disease of prematurity, and immunodeficiency.

Management: Supportive Only

Bronchiolitis management is a favorite exam topic precisely because the correct answer is usually "do less," reflecting evidence-based guidelines:

Most infants recover with supportive care over 1-2 weeks, though cough can persist longer. Palivizumab, a monoclonal antibody against RSV, has historically been used for monthly prophylactic dosing in the highest-risk infants during RSV season. Nirsevimab, a longer-acting monoclonal antibody given as a single dose, is now recommended more broadly for infants entering their first RSV season (and select high-risk children in their second season), reflecting a major recent shift in RSV prevention policy.

When to Admit

Hospitalization is indicated when an infant with bronchiolitis has significant hypoxia (oxygen saturation persistently below 90% despite supplemental oxygen), inability to maintain hydration orally, apnea or cyanotic spells, marked tachypnea with retractions, age under 3 months (especially under 6 weeks), or an underlying high-risk condition such as prematurity, hemodynamically significant congenital heart disease, chronic lung disease, or immunodeficiency. Young and premature infants should be monitored closely because apnea can be the dominant presentation.

Croup (Laryngotracheobronchitis)

Croup is inflammation and edema of the subglottic airway, most commonly caused by parainfluenza virus (types 1-3). It typically affects children 6 months to 3 years old, peaks in fall and winter, and presents with a 1-2 day viral prodrome followed by the classic triad: a barking (seal-like) cough, inspiratory stridor, and hoarseness, often worse at night and with agitation or crying.

Diagnosis is clinical. A frontal (anteroposterior) neck radiograph, if obtained, classically shows the "steeple sign" -- symmetric subglottic narrowing of the tracheal air column -- but imaging is not required for typical presentations and should not delay treatment.

Severity Assessment: The Westley Croup Score

The Westley croup score is the standard tool for grading severity and guiding disposition, scored from 0 to 17 based on five components:

A score under 4 indicates mild croup, 4-6 indicates moderate croup, and above 6 indicates severe croup.

Management

• Mild croup (barking cough without stridor at rest): a single dose of oral dexamethasone (typically 0.15-0.6 mg/kg) is recommended even for mild disease -- it reduces symptom severity, return visits, and hospitalization; most children can be discharged home.
• Moderate-to-severe croup (stridor at rest, retractions): dexamethasone PLUS nebulized epinephrine (racemic epinephrine or L-epinephrine), which rapidly reduces airway edema through alpha-adrenergic vasoconstriction. Because epinephrine's effect wears off faster than the underlying inflammation resolves, observe the child for at least 2-4 hours after nebulized epinephrine for "rebound" worsening of stridor before discharge.
• Cool mist or humidified air therapy is commonly used but lacks strong evidence of benefit.
• Most croup resolves within 3-7 days; children with recurrent or atypical croup (e.g., prolonged, afebrile, or poorly responsive to treatment) should be evaluated for an underlying structural airway abnormality such as subglottic stenosis.

Croup vs. Epiglottitis -- Quick Distinction

Croup is distinguished from epiglottitis (covered in detail in the Emergencies chapter) mainly by tempo and toxicity: croup has a gradual viral prodrome with a barking cough in a non-toxic-appearing child, while epiglottitis has an abrupt onset, high fever, drooling, a toxic or anxious appearance, and absence of cough, and is a true airway emergency requiring controlled airway management rather than direct examination of the throat.

Croup vs. Bacterial Tracheitis

Bacterial tracheitis (also called pseudomembranous croup) should be suspected when a child who initially appeared to have viral croup becomes progressively more toxic, febrile, and unresponsive to dexamethasone and nebulized epinephrine, often with copious purulent secretions that require repeated suctioning. Unlike viral croup, these children look ill. Staphylococcus aureus is the usual pathogen (group A streptococcus and Moraxella are less common). Management requires close airway monitoring, IV antibiotics with coverage for S. aureus (including MRSA where prevalent), and often endotracheal intubation, because the thick purulent membrane can abruptly obstruct the airway.