Acute Bronchiolitis

Acute bronchiolitis is a viral lower respiratory tract infection in infants younger than 2 years, defined by inflammation, edema, and mucus plugging of the small airways that produces respiratory distress, wheezing, and crackles ^{[4]B3b[10]A1a}.

Also Called

• Acute viral bronchiolitis
• RSV bronchiolitis (when caused by respiratory syncytial virus)
• Bronchiolitis (common shorthand)

Etiologic Classification

Respiratory syncytial virus (RSV) is the most common causative pathogen, followed by rhinovirus ^{[4]B3b[16]B3b}. Other viruses (e.g., human metapneumovirus, adenovirus, parainfluenza) account for a smaller fraction. The table below summarizes the two predominant viral etiologies.

Clinical Significance

Bronchiolitis is the most common lower respiratory tract infection among young children in the United States ^[4]B3b. Almost all children are exposed to RSV and other pathogens during the first 2 years of life; approximately 40% develop clinical bronchiolitis and up to 3% require hospitalization ^[4]B3b. In 2016, the incidence of bronchiolitis hospitalization was 13.5 per 1000 person-years, accounting for 18% of all infant hospitalizations and $734 million in direct costs ^[4]B3b. Most cases (87%) are managed in primary care, but 3 of every 100 children under 2 years are hospitalized, with the highest risk in infants aged 2 to 10 weeks ^[14]B3b. RSV hospitalization rates are highest in the first 3 months of life (48.9 per 1000) ^[17]B3b.

Pearl: Bronchiolitis is a clinical diagnosis restricted to infants <2 years with a first episode of viral lower respiratory tract infection; the term should not be applied to recurrent wheezing episodes in older children, which represent a different pathophysiology.

From the definition of bronchiolitis as a clinical syndrome of the small airways, the pathogenesis emerges as a dual process: direct viral cytopathology of the airway epithelium and a maladaptive, developmentally conditioned host immune response. RSV infects ciliated airway epithelial cells (AECs) after an incubation period of 4-6 days ^[27]C4. Viral replication is extensive by 4 days, causing degeneration, necrosis, and syncytial cell formation in the bronchioles ^[30]D5.

Epithelial Cell Death and Danger Signals

RSV-infected AECs die predominantly by necroptosis, a programmed form of necrosis, rather than apoptosis. Phosphorylated RIPK1 and MLKL are upregulated, while active caspase-3 is absent ^[33]D5. Necroptosis releases HMGB1, a damage-associated molecular pattern that amplifies inflammation. Levels of HMGB1 are elevated in nasopharyngeal aspirates of children with acute RSV infection ^[33]D5. Simultaneously, RSV upregulates hematopoietic prostaglandin D synthase in AECs, increasing PGD2 release; PGD2 is elevated in nasopharyngeal samples of infants with RSV bronchiolitis compared to healthy controls ^[24]B3b.

Immune Dysregulation and the Th2/Th17 Axis

PGD2 acts on DP2 receptors on type 2 effector cells, suppressing interferon-λ (IFN-λ, IL-28A/B) production, a critical antiviral cytokine. DP2 antagonism in a neonatal mouse model decreased viral load, immunopathology, and morbidity by restoring IFN-λ expression ^[24]B3b. The infant immune system mounts a Th2-skewed response to RSV, characterized by IL-4, IL-13, and IL-5, which promotes goblet cell hyperplasia, mucus hypersecretion, and airway hyperreactivity ^[39]D5. Local inhibition of IL-4Rα during primary infection in neonatal mice abolishes these Th2 responses and prevents pulmonary dysfunction upon reinfection ^[39]D5. IL-13 levels remain elevated nearly three months after initial infection, while TNF-α rises early ^[40]D5.

A Th17 response also contributes: interleukin-17 increases with severity of illness, and its neutralization decreases mucous production and the inflammatory response ^[23]B3b. Excess adiposity in infancy upregulates proinflammatory adipokines (leptin, TNF-α, IL-6) and promotes a Th17 bias, providing a mechanistic link between obesity and longer hospital stays ^[23]B3b.

Endothelial and Neutrophil Activation

Markers of endothelial activation, Ang-2, Ang-2/Ang-1 ratio, sE-selectin, and neutrophil activation (immature neutrophil count, neutrophil/lymphocyte ratio) are higher in acute bronchiolitis than in controls. Admission Ang-2 levels and NLR are positive predictors for the duration of respiratory support ^[25]B3b.

Developmental Susceptibility

Infants are uniquely vulnerable due to immature lung development, impaired innate immunity, and reduced antioxidant defenses. Prematurity, congenital airway anomalies, and low lung function heighten susceptibility ^[30]D5[32]D5. Oxidative stress and deficiencies in zinc, selenium, and magnesium exacerbate viral inflammatory responses ^[32]D5. Nanosize viruses can spread hematogenously across the placenta to interfere with fetal lung development, predisposing to aberrant cholinergic innervation and enhanced airway smooth muscle contractility ^[28]D5.

Pulmonary Hypertension and Long-term Sequelae

Up to 75% of infants with moderate to severe RSV bronchiolitis develop pulmonary hypertension (PH) ^[29]D5. ST2 signaling (IL-33 receptor) mediates type 2 immunity and induces NOS uncoupling, leading to increased right ventricular systolic pressure and pulmonary artery remodeling ^[29]D5[37]D5. RSV infection during the neonatal period alone can cause persistent airway hyperreactivity, peribronchial inflammation, and subepithelial fibrosis in adulthood, and these changes are exacerbated by subsequent allergen exposure ^[40]D5. Necroptosis inhibition in early life ameliorates asthma progression induced by later viral or allergen challenge ^[33]D5.

Pearl: The infant's immune response, not simply viral cytopathology, drives the severity of bronchiolitis; the necroptosis-HMGB1-PGD2 axis and the Th2/Th17 imbalance are emerging targets for future therapies that could interrupt the progression to asthma.

From the inflammatory cascade described above, the epidemiologic burden of acute bronchiolitis becomes clear. Respiratory syncytial virus (RSV) is the predominant pathogen, though rhinovirus and other respiratory viruses contribute ^{[41]A1c[53]B2a}. The disorder is the most common lower respiratory tract infection in infants under 2 years, and its incidence and risk profile have shifted over the past two decades.

Incidence and Temporal Trends

The incidence of bronchiolitis hospitalizations in US children <2 years declined from 17.9 to 13.5 per 1000 person-years between 2000 and 2016, a 25% decrease (P trend <.001) ^[4]B3b. Despite this, the proportion of all hospitalizations attributable to bronchiolitis rose from 16% to 18%, and the proportion of children with complex chronic conditions increased from 6% to 13% (117% increase) ^[4]B3b. Mechanical ventilation use more than doubled, from 2% to 5% (184% increase), and inflation-adjusted national hospital costs rose from $449 million to $734 million (63% increase) ^[4]B3b. Simultaneously, diagnostic labeling shifted: the proportion of lower respiratory tract conditions diagnosed as bronchiolitis increased from 55.4% to 79.6% (adjusted incidence rate ratio [aIRR] 1.022 per year) ^[47]B3b.

Seasonal Variation

Bronchiolitis follows RSV seasonality, typically peaking in winter. In Tampere, Finland, the highest incidence rate ratios compared with low-incidence months were between December (22.5) and February (25.5) in 2000-2006, shifting to February-April in later years ^[58]B3b. After the COVID-19 pandemic, RSV circulation became highly atypical: in Central New York, season onset shifted to week 27 in 2021 and 2022, compared with historical patterns ^[59]B3b. During the 2020-2021 winter, a marked decrease in bronchiolitis diagnoses and near disappearance of the RSV winter epidemic were observed across Europe and Israel ^[55]C4.

Risk Factors for Severe Disease

Several independent predictors of critical care unit admission or death after emergency department discharge have been identified. The strongest are summarized in the table below.

Infants with comorbidities plus ≥2 additional predictors had 25-fold higher odds of subsequent critical care unit admission or death (OR 25.1, 95% CI 11.4-55.3) ^[51]B3b. Bronchiolitis hospitalization in infancy also confers later risk: children with a prior admission have a threefold to fivefold increased hazard of subsequent respiratory hospital admissions by age 5 years (adjusted HR 2.82 for any respiratory admission; HR 4.35 for asthma; HR 5.02 for wheezing) ^[54]B3b.

Etiology

RSV remains the most commonly identified pathogen, detected in 82% of hospitalized infants in one Finnish cohort ^[58]B3b. Rhinovirus is also prevalent, identified in 13%-59% of children with respiratory disease, but its attributable role is difficult to establish because of high detection rates in asymptomatic individuals (6%-50%) ^[53]B2a. During the COVID-19 pandemic, rhinovirus became the most common pathogen detected, while RSV was identified in only one case of bronchiolitis in a European-Israeli cohort ^[55]C4.

Pearl: The combination of comorbidities (especially congenital heart disease, Down syndrome, or chronic lung disease) plus younger age, prematurity, or low income identifies a subgroup with >25-fold odds of critical care admission or death after ED discharge, these infants should be considered for inpatient observation regardless of initial severity.

Controversies and Guideline Disagreement

The clinical features of bronchiolitis emerge from the viral infection's pathophysiologic effects on the small airways: inflammation, edema, and mucus plugging lead to airflow obstruction, hyperinflation, and ventilation-perfusion mismatch. The presentation follows a characteristic timeline, though age, pathogen, and host factors shape the phenotype.

Presenting Symptoms and Timeline

Bronchiolitis typically begins with 2-3 days of upper respiratory prodrome: rhinorrhea, nasal congestion, cough, and low-grade fever. Lower respiratory tract signs then develop, with cough becoming more frequent and paroxysmal, and tachypnea, wheezing, and retractions appearing. The illness peaks around day 3-5 of lower respiratory symptoms, with gradual resolution over 7-14 days. In a large multicenter cohort of infants hospitalized with bronchiolitis, the mean length of stay was 3.6 days ^[1]A1a. The time from illness onset to hospital presentation is a predictor of severity: presentation within ≤2 days of symptom onset is associated with higher risk of ICU admission (OR 0.78 per day, 95% CI 0.65-0.94) ^[60]A1b.

Symptoms vary by pathogen. In rhinovirus-associated bronchiolitis, cough (81.5%), runny nose (76.8%), and fever (68.9%) are most common, with wheezing in 29.1% and crepitations in 18.5% ^[71]B3b. RSV bronchiolitis, the most common cause, presents with respiratory distress, hypoxemia, and fever ^[72]B3b. Human metapneumovirus (hMPV) causes bronchiolitis in 65.7% of infected children, with coinfection with RSV leading to longer hospitalization and greater oxygen requirement ^[73]B3b. SARS-CoV-2-related bronchiolitis is rare and typically mild, with only 3% of SARS-CoV-2-positive infants developing bronchiolitis and most not requiring oxygen ^[55]C4.

Physical Examination Findings

General appearance: Infants may appear irritable or lethargic. Signs of respiratory distress include nasal flaring, intercostal and subcostal retractions, head bobbing, and grunting. Tachypnea is universal; a respiratory rate >60 breaths/min in infants is common.

Vital signs: Tachycardia (heart rate >160/min) is a predictor of ICU admission (OR 1.80, 95% CI 1.23-2.63) ^[60]A1b. Hypoxia, defined as SpO2 <87%, also predicts ICU admission (OR 0.91 per % SpO2, 95% CI 0.86-0.95) ^[60]A1b. Fever is present in about two-thirds of cases but is typically low-grade; high fever may suggest bacterial coinfection.

Chest examination: Auscultation reveals diffuse wheezing, prolonged expiration, and fine crackles (crepitations). Hyperinflation is evident on percussion. In severe disease, breath sounds may be diminished due to mucus plugging and atelectasis. The presence of focal signs (e.g., localized crackles or dullness) should raise suspicion for pneumonia or foreign body aspiration ^[64]C4.

Other systems: Nasal congestion and rhinorrhea are nearly universal. Conjunctivitis and otitis media may be present. Dehydration can occur due to increased insensible losses from tachypnea and poor oral intake.

Red Flags and Predictors of Severe Disease

Several clinical features identify infants at risk for deterioration and ICU admission:

• Age <2 months (OR 0.98 per week, 95% CI 0.96-0.99) ^[60]A1b
• Heart rate >160/min ^[60]A1b
• SpO2 <87% ^[60]A1b
• Previous ICU admission (OR 2.16, 95% CI 1.07-4.40) ^[60]A1b
• Time from illness onset to presentation ≤2 days ^[60]A1b
• Comorbidities (OR 5.33, 95% CI 2.82-10.10 for critical care admission/death after ED discharge) ^[51]B3b
• Apnea (especially in infants <2 months or those born preterm)
• Poor feeding (less than half of usual intake)
• Lethargy

Infants with comorbidities (e.g., prematurity, bronchopulmonary dysplasia, congenital heart disease, immunodeficiency, neuromuscular disease, cystic fibrosis, Down syndrome ) are at significant risk for severe bronchiolitis ^[67]A1c. The absolute risk of critical care admission or death after ED discharge is 1.5% in infants with comorbidities versus 0.26% in those without ^[51]B3b.

Atypical Presentations

Bronchiolitis can be mimicked by other conditions. Foreign body aspiration should be considered in a wheezing infant with sudden onset of symptoms, especially if unilateral findings or failure to improve with standard therapy ^[64]C4. Adenovirus infection can cause severe disease with high fever, conjunctivitis, and gastrointestinal symptoms, and may lead to pneumonia, acute kidney injury, or liver failure in immunocompromised children ^[70]B3b. SARS-CoV-2 bronchiolitis, when it occurs, is typically mild ^[55]C4.

Pearl: The combination of age <2 months, heart rate >160/min, SpO2 <87%, and presentation within 2 days of symptom onset identifies infants at highest risk for ICU admission; these factors should guide disposition decisions even in the absence of severe distress at initial evaluation ^[60]A1b.

Building on the clinical presentation, the diagnosis of acute bronchiolitis is established at the bedside: a focused history and physical examination are sufficient, and no routine laboratory or imaging test is required ^[75]A1c. The American Academy of Pediatrics (AAP) defines bronchiolitis as a clinical syndrome in children 1-23 months of age, and its guidelines explicitly recommend against the routine use of chest radiography, viral testing, and blood studies ^{[75]A1c[41]A1c}. Overuse of low-value services remains common, with 56% of children receiving at least one non-recommended diagnostic or therapeutic intervention ^[5]B3b.

History and Physical

Key historical features include the first episode of wheezing in an infant <2 years, preceding coryza and cough, and presentation during the RSV season. Physical examination reveals tachypnea, intercostal or subcostal retractions, nasal flaring, fine crackles, and expiratory wheezing ^[75]A1c. The typical clinical course evolves over 2-5 days, peaking in respiratory distress around day 3-4. The diagnosis becomes less certain when wheezing is recurrent, there is no viral prodrome, or focal findings suggest consolidation or lobar atelectasis.

Gold-Standard Test

There is no single laboratory or imaging gold standard. The reference standard is a clinical diagnosis made by a clinician based on the above criteria. Confirmatory testing is not recommended for diagnosis ^{[75]A1c[41]A1c}.

Laboratory Studies

Routine laboratory testing is not indicated. Specific considerations include:

• Urinalysis and urine culture: The prevalence of concomitant UTI in bronchiolitis is 3.1% (95% CI, 1.8%-4.6%) from 18 studies. When a positive urinalysis (pyuria or nitrites) is added as a diagnostic criterion, the prevalence drops to 0.8% (95% CI, 0.3%-1.4%), a rate below recommended testing thresholds ^[3]B2a. AAP therefore advises against routine UTI screening in bronchiolitis ^[75]A1c.
• Viral testing (RSV, influenza, SARS-CoV-2): Not required for diagnosis but may be used for infection control or cohorting ^[75]A1c. Despite this, 26% of children still receive RSV testing, a form of overuse ^[5]B3b.
• Chest radiography: Not recommended ^[75]A1c because it does not alter management and exposes children to radiation. Data from 2014-2016 show 246 radiographs per 1000 bronchiolitis encounters among Pennsylvania Medicaid beneficiaries ^[83]B3b. Only consider if atypical features (e.g., focal signs, high fever, rapid deterioration, or concern for foreign body).

Differential Diagnosis

The following conditions should be considered when the presentation is atypical:

In one illustrative case, a 2-year-old with wheezing initially treated for bronchiolitis and pneumonia was found on rigid bronchoscopy to have a foreign body; a chest CT with contrast eventually led to the correct diagnosis ^[64]C4.

Diagnostic Algorithm

The following flowchart outlines the diagnostic approach from presentation to confirmation of acute bronchiolitis or identification of an alternative diagnosis.

Step 1: Assess for the typical triad, first episode of wheezing, viral prodrome (coryza, cough), and age <2 years during RSV season. Step 2: If all features are present, the diagnosis is clinical and no further testing is needed. Step 3: If any feature is absent (e.g., recurrent wheezing, no prodrome, older age, or focal signs), pursue a targeted evaluation: chest radiograph to rule out pneumonia or foreign body, viral testing for RSV/influenza/ COVID-19 if it will change management, and consider asthma or aspiration. Step 4: If an alternative diagnosis is confirmed, treat accordingly; if not, the diagnosis remains clinical bronchiolitis and supportive care is initiated.

Pearl: The diagnosis of bronchiolitis is made clinically; if an infant has recurrent wheezing, no viral prodrome, or focal auscultatory findings, broaden the differential to include foreign body, asthma, or pneumonia before accepting the diagnosis ^{[64]C4[75]A1c}.

With the diagnosis confirmed, the next step is to stratify disease severity using validated clinical scores and risk factors that predict the need for escalation of care. Standardized assessment tools convert clinical findings into reproducible risk tiers, guiding monitoring intensity, respiratory support decisions, and disposition.

Clinical Scoring Systems

The Wang bronchiolitis clinical score (range 0-12, higher scores indicate greater severity) is the most widely validated tool for acute bronchiolitis. It was used as the primary endpoint in the phase 3 ziresovir trial, where the mean reduction from baseline at day 3 was -3.4 points with ziresovir versus -2.7 points with placebo (difference -0.8 points; 95% CI -1.3 to -0.3) ^[49]B2b. The same score (WBSS) is incorporated into nurse-driven high-flow nasal cannula (HFNC) weaning protocols, where structured assessments guide timely step-down ^[101]A1b. A score ≥8 identifies infants at highest risk for intensive care and is a common threshold for escalation.

The ROX index (SpO₂/FiO₂ divided by respiratory rate) provides an objective, continuous measure of oxygenation efficiency. In the nurse-driven weaning trial, the ROX index was used alongside the WBSS to determine readiness for HFNC weaning, with high protocol adherence (92%) and no increase in adverse outcomes ^[101]A1b.

Other scores such as the modified Woods clinical asthma score and Silverman-Andersen score have been applied in bronchiolitis research, but in a prospective study of 44 infants, peak tidal inspiratory flow (PTIF) indexed to weight (mean 1.68 L/kg/min) did not correlate with these scores or with markers of disease severity (respiratory rate, FiO₂, PCO₂) ^[95]B3b.

Risk Factors for Severe Disease

Several patient-level factors independently predict a more severe course and should be incorporated into risk stratification:

• Vitamin D deficiency: In a multicenter cohort of 1016 hospitalized infants, total 25(OH)D <20 ng/mL was associated with a higher risk of requiring intensive care (aOR 1.72; 95% CI 1.12-2.64) and longer hospital length of stay (adjusted rate ratio 1.39; 95% CI 1.17-1.65) compared with levels ≥30 ng/mL ^[93]B2b.
• Viral coinfection: Detection of two or more respiratory viruses in nasopharyngeal aspirates was associated with deterioration of the severity score after HFNC initiation (OR 1.361; 95% CI 1.036-1.786), longer HFNC treatment (OR 1.018; 95% CI 1.006-1.029), and a higher likelihood of atelectasis (OR 2.923; 95% CI 1.049-8.148) ^[100]B3b.
• Neighborhood opportunity: Children from neighborhoods with lower Child Opportunity Index (COI) scores had a higher proportion requiring critical care, and this disparity persisted after adjusting for race/ethnicity and payor status ^[94]B3b.
• Rhinovirus C with IgE sensitization: Infants hospitalized with rhinovirus C bronchiolitis who also had IgE sensitization (to food or aeroallergens) had a significantly higher risk of developing recurrent wheeze by age 3 years (HR 3.03; 95% CI 1.20-7.61) and asthma by age 4 years (HR 4.06; 95% CI 1.17-14.1) ^[89]B2b. While this is a long-term outcome, it identifies a subgroup that may benefit from closer follow-up.

Severity-Based Monitoring and Admission Decisions

Admission rates for bronchiolitis vary more than threefold across US children's hospitals (19%-65% after severity adjustment), highlighting the need for standardized criteria ^[92]B3b. For mild-to-moderate disease, an expert panel recommended intermittent vital sign and oximetry measurement only; continuous cardiorespiratory monitoring is reserved for severe disease ^[96]D5. Observation-status stays (median cost $2559) cost on average $260 less than short inpatient stays, but cost distributions overlap substantially, and observation use varies from 2% to 45% across hospitals ^[86]B3b.

Pearl: The Wang bronchiolitis clinical score (0-12) is the most widely validated tool for assessing severity and response to therapy; a score ≥8 identifies infants at highest risk for intensive care and should prompt escalation of monitoring and respiratory support.

Once severity is graded, management proceeds along a supportive pathway with escalation of respiratory support as needed. The core principle is that no pharmacotherapy has proven benefit for most infants; the mainstay is oxygen, hydration, and airway clearance.

Step 1: Initial Assessment and Disposition Decision

Severity classification (mild, moderate, severe) guides the care setting. Infants with apnoea, persistent SpO₂ < 95%, tachypnoea, chest recession, nasal flaring, or feeding difficulty require hospital admission ^[114]A1c. Those with risk factors, age < 3 months, prematurity, chronic lung disease, haemodynamically significant congenital heart disease, neuromuscular disorders, or immunodeficiency, warrant particularly close monitoring and early referral to secondary care ^[114]A1c. Mild cases (no distress, SpO₂ ≥ 95%, feeding well) can be managed at home with clear return precautions.

Step 2: Supportive Care (First-Line Intervention)

• Oxygen: Administer via low-flow nasal cannula to maintain SpO₂ ≥ 90% (or ≥ 92% per some guidelines). The Finnish 2023 guidelines recommend a threshold of SpO₂ < 95% for considering oxygen ^[114]A1c.
• Hydration: If the infant cannot feed orally, provide nasogastric or intravenous fluids. Avoid overhydration; isotonic solutions are preferred.
• Nasal suctioning : Gentle suction of nasal secretions before feeds or as needed improves work of breathing. Deep suctioning is not routinely recommended.
• Chest physiotherapy: Not effective and should not be used ^[41]A1c.

Step 3: Respiratory Support Escalation (Second-Line)

When low-flow oxygen fails to correct hypoxemia or respiratory distress worsens, escalate to non-invasive respiratory support.

Treatment failure protocol: If on low-flow oxygen → escalate to HFNC. If on HFNC and not improving within 1-2 hours → escalate to CPAP. If CPAP fails (persistent hypoxia, rising CO₂, apnoea) → intubate and ventilate.

Step 4: Pharmacotherapy, Limited Role

Most medications have no proven benefit and should be avoided.

Step 5: Monitoring and Discharge Criteria

• Monitor respiratory rate, SpO₂, work of breathing, feeding tolerance, and urine output every 2-4 hours.
• Discharge when: off oxygen for ≥ 12 hours, feeding adequately, no significant respiratory distress, and family can return if worsening.
• Home oxygen therapy is a feasible alternative for selected children who still require low-flow oxygen but are otherwise stable, reducing hospital bed time by ~42 hours (p=0.001) ^[112]A1b.

What NOT to Do

• Do not use bronchodilators (albuterol, salbutamol) routinely; they do not improve oxygenation or reduce LOS ^[114]A1c.
• Do not use systemic or inhaled corticosteroids for mild-moderate bronchiolitis ^[114]A1c.
• Do not use antibiotics unless bacterial coinfection is confirmed.
• Do not perform chest physiotherapy ^[41]A1c.
• Do not use hypertonic saline as first-line therapy; evidence is conflicting ^{[106]A1b[107]A1b[111]A1b}.

Controversies and Guideline Disagreement

Pearl: Supportive care with oxygen and hydration is the cornerstone; escalate to HFNC or CPAP based on clinical severity, and reserve pharmacotherapy (epinephrine plus corticosteroids) only for severe bronchiolitis in the ICU where it may shorten ventilation duration ^[77]A1b.

Once the infant is stabilized and admitted for ongoing care, the focus shifts to optimizing respiratory support while avoiding unnecessary interventions. The definitive management of acute bronchiolitis is supportive care, as no pharmacologic therapy has consistently demonstrated benefit in reducing disease duration or severity. The following evidence-based protocol outlines the stepwise approach for hospitalized infants.

Step 1: Initial Assessment and Disposition

Classify severity using clinical signs (e.g., respiratory rate, work of breathing, oxygen saturation, feeding ability). Infants with mild disease (no hypoxia, adequate feeding) may be discharged with close follow-up. Those with moderate disease (hypoxia, mild-moderate respiratory distress) are admitted to a general pediatric ward. Severe disease (persistent hypoxia, severe distress, apnea, inability to feed) requires intensive care unit (ICU) admission. The AAP 2014 guideline recommends inpatient management for infants with oxygen saturation persistently <90% ^[75]A1c.

Step 2: First-Line Supportive Care

• Oxygen therapy: Administer humidified oxygen to maintain saturations ≥90%. For infants with moderate hypoxia, standard low-flow nasal cannula (up to 2 L/min) is first-line. The duration of oxygen therapy is the primary driver of length of stay.
• Hydration: Ensure adequate hydration, either orally or via nasogastric tube if respiratory distress prevents feeding. Intravenous fluids are reserved for those unable to tolerate enteral feeds.
• Suctioning : Nasal suctioning prior to feeds and as needed for visible secretions improves comfort and feeding. Deep suctioning of the lower airway is not recommended.

Step 3: Respiratory Support Escalation

• High-flow nasal cannula (HFNC): For infants with moderate bronchiolitis who fail to maintain saturations ≥90% on low-flow oxygen, high-flow oxygen therapy (heated, humidified, up to 1 L/kg/min, max 20 L/min) reduces the risk of treatment failure. The landmark RCT by Franklin et al. (2018) showed that HFNC, compared with standard oxygen therapy, reduced escalation of care from 23% to 12% (risk difference, -11 percentage points; 95% CI, -15 to -7; P<0.001) ^[124]A1b. However, it did not shorten duration of oxygen therapy or hospital stay. High-flow therapy is also effective as a rescue therapy: among infants who failed standard oxygen, 61% responded to HFNC rescue ^[124]A1b.

Step 4: Pharmacotherapy - What Works and What Does Not

No pharmacologic agent has proven benefit for all infants with bronchiolitis. The 2014 AAP guideline advises against routine use of bronchodilators (albuterol, epinephrine) and corticosteroids ^[75]A1c. The table below summarizes the evidence for key interventions.

What NOT to do:

• Do not use chest physiotherapy; it has no benefit.
• Do not use systemic corticosteroids (except in the asthma-risk subgroup noted above).
• Do not use bronchodilators routinely; they do not reduce hospital stay or admission rate.
• Do not use heliox via nasal cannula.
• Do not use palivizumab for treatment of acute RSV infection.

Controversies and Guideline Disagreement

Step 5: Monitoring and Discharge

• Discharge criteria: The infant should have saturations >90% on room air for at least 6-8 hours, be feeding adequately, and have no significant respiratory distress. For infants with borderline hypoxia, home oxygen therapy is a viable option after a period of observation. In a randomized trial, 70% of infants with initial saturations ≤87% were successfully discharged on home oxygen after an 8-hour observation period, with high caregiver satisfaction and only one readmission (2.7%) ^[122]A1b. This strategy requires close outpatient follow-up.

Pearl: For infants hospitalized with bronchiolitis, supportive care, oxygen, hydration, and nasal suctioning, remains the mainstay; high-flow nasal cannula reduces escalation of care but not length of stay, and no pharmacotherapy (including hypertonic saline, epinephrine, or corticosteroids) has consistent evidence of benefit across all patients.

The therapeutic approach to acute bronchiolitis has undergone a fundamental shift from polypharmacy to evidence-based supportive care, driven by a series of landmark trials that dismantled long-held practices. The history is largely one of de-implementation: the most important interventions were often the ones that were stopped.

The Evidence Vacuum and the Rise of Guidelines (1980s-2006)

From 1980 to 2000, the hospitalization rate for bronchiolitis rose by nearly 250%, while mortality remained constant ^[128]A1b. This paradox was partly attributed to the increasing use of pulse oximetry : a landmark survey of pediatric emergency physicians found that a 2% difference in SpO₂ (94% vs 92%) changed the admission recommendation from 43% to 83% ^[128]A1b. The 2006 American Academy of Pediatrics (AAP) guideline marked the first systematic effort to curb this variation, recommending supportive care as the mainstay and discouraging routine bronchodilators, corticosteroids, chest physiotherapy, and antibiotics ^[41]A1c. The 2014 update reinforced these recommendations ^[75]A1c.

The Fall of Routine Pharmacotherapy

The Emergence of Respiratory Support: High-Flow Nasal Cannula

The most consequential advance came from the 2018 multicenter PARIS trial (n=1472), which demonstrated that high-flow oxygen therapy via nasal cannula, delivered outside the intensive care unit, reduced the rate of escalation of care due to treatment failure from 23% to 12% (risk difference, -11 percentage points; 95% CI, -15 to -7; P<0.001; NNT = 9) ^[124]A1b. Among the 167 infants in the standard-therapy group who met criteria for treatment failure, 61% responded to rescue high-flow therapy ^[124]A1b. This trial transformed the respiratory management of moderate-to-severe bronchiolitis.

Prevention and De-implementation

Palivizumab prophylaxis was established by a 1998 trial showing a reduction in RSV hospitalization from 10.6% to 4.8% (original trial, updated guidance ^[43]A1c); NNT = 17. Subsequent data refined the indication to infants at highest risk ^[43]A1c. Targeted de-implementation interventions, including audit-and-feedback and train-the-trainer workshops, improved compliance with avoidance of chest radiography, albuterol, glucocorticoids, antibiotics, and epinephrine from 73.0% to 85.1% (adjusted risk difference 14.1%; 95% CI, 6.5%-21.7%) ^[105]A1b.

Monitoring and Post-Discharge Care

Two randomized trials confirmed that intermittent pulse oximetry (every 4 hours) is noninferior to continuous monitoring for nonhypoxemic hospitalized infants, with no difference in length of stay or safety ^{[123]A1b[129]A1b}. Among parents of hospitalized infants, as-needed follow-up was noninferior to scheduled follow-up for reducing anxiety at 7 days (mean anxiety score 3.9 vs 4.2; difference -0.3 points, within prespecified noninferiority margin of 1.1) ^[130]A1b.

Pearl: The evolution of bronchiolitis treatment is a story of de-implementation: the single most important intervention has been the removal of ineffective therapies, driven by rigorous trials that repeatedly failed to confirm benefit for bronchodilators, corticosteroids, montelukast, hypertonic saline, and heliox.

Beyond the acute respiratory management, bronchiolitis and its treatment can perturb growth, neurodevelopment, and nutritional status, particularly in critically ill infants. The metabolic stress response to severe viral infection, characterized by increased endogenous cortisol and a shift toward catabolism, directly threatens protein balance and linear growth ^[138]D5.

Nutritional Impact and Metabolic Stress

Infants with respiratory failure due to viral bronchiolitis are in a catabolic state. A double-blind RCT in 18 mechanically ventilated infants demonstrated that increasing protein and energy intakes promotes anabolism: infants receiving a protein- and energy-enriched formula (3.1 ± 0.3 g protein/kg/24 h, 119 ± 25 kcal/kg/24 h) achieved a significantly higher whole-body protein balance (0.73 ± 0.5 vs 0.02 ± 0.6 g/kg/24 h) compared with those receiving standard formula (1.7 ± 0.2 g protein/kg/24 h, 84 ± 15 kcal/kg/24 h) ^[132]A1b. This improvement resulted from increased protein synthesis (9.6 vs 5.2 g/kg/24 h) despite a parallel increase in protein breakdown (8.9 vs 5.2 g/kg/24 h) ^[132]A1b.

A subsequent systematic review and meta-analysis of 8 RCTs, mostly in infants with bronchiolitis or after cardiac surgery, confirmed that high enteral protein intake (~3 g/kg/day) was associated with significantly higher nitrogen balance (effect size 0.59; 95% CI 0.18-1.01; p = 0.0055) compared with recommended intakes (~1.6 g/kg/day) ^[139]A1a. However, high protein intake had no impact on PICU length of stay and was associated with increased blood urea levels; among 4 RCTs assessing gastrointestinal tolerance, 2 reported increased diarrhea or gastric retention ^[139]A1a. These findings suggest that while protein-energy enrichment can reverse catabolism, clinicians must monitor for tolerance and metabolic side effects.

Growth and Linear Growth Monitoring

Long-term growth does not appear to be adversely affected by bronchiolitis or its standard treatments. In a 6-year follow-up of an RCT comparing high-dose inhaled beclomethasone (3 months) with placebo in 185 infants hospitalized for RSV bronchiolitis, there were no differences in linear growth between groups ^[113]A1b. Similarly, in a trial of bovine lactoferrin supplementation in low-birth-weight infants, growth outcomes (weight, length, head circumference) at 24 months corrected age were similar between the lactoferrin and placebo groups, although the lactoferrin group had significantly less bronchiolitis (rate ratio 0.34; 95% CI 0.14-0.86) ^[46]A1b.

Neurodevelopmental Outcomes

Neurodevelopment after severe bronchiolitis appears largely preserved. In the same lactoferrin trial, the mean age-adjusted normalized Mullen composite score at 24 months was 83.3 ± 13.6 in the bovine lactoferrin group vs 82.6 ± 13.1 in the placebo group, with no significant difference ^[46]A1b. Rehospitalization rates during 2-year follow-up were also similar ^[46]A1b. These data suggest that, in the absence of severe complications, bronchiolitis does not independently impair neurodevelopment.

Complications Affecting Nutrition

Rarely, severe RSV bronchiolitis can be complicated by necrotizing enterocolitis (NEC), even in previously healthy term and late-preterm infants. In a case series of 4 infants admitted to the PICU with respiratory failure due to RSV bronchiolitis, all developed progressive abdominal distention, typical radiographic findings, and ascites shortly after admission ^[134]C4. Management required broad-spectrum antibiotics , bowel rest, and in some cases surgical intervention (colon resection, peritoneal drainage); one infant died of septic shock ^[134]C4. Enteral feeding intolerance, septic ileus, and complex ascites are clinical corollaries that should prompt evaluation for NEC in infants with severe RSV disease ^[134]C4.

Pearl: In critically ill infants with bronchiolitis, aim for protein intake of 2-3 g/kg/day to promote anabolism, but monitor for gastrointestinal intolerance and elevated blood urea; long-term growth and neurodevelopment are generally preserved, but severe disease rarely triggers necrotizing enterocolitis requiring bowel rest and surgical consultation.

Beyond the acute nutritional and growth challenges, bronchiolitis carries a spectrum of complications that range from immediate life-threatening events to long-term respiratory morbidity. The following sections map these complications by mechanism and timing, providing a management playbook for the ICU and ward.

Acute Respiratory Complications

• Air leak syndrome associated with heated, humidified high-flow nasal cannula (HHFNC) therapy has been reported in infants with RSV bronchiolitis. In a case series, a 2-month-old male developed a right pneumothorax on day 5 of illness at 8 L/min, requiring intubation and ventilation for 14 days ^[145]C4. The authors emphasize caution when using HHFNC at flows exceeding the patient's minute ventilation ^[145]C4.
• Nasogastric hydration in infants under 2 months carries a 27.4% adverse event rate, predominantly desaturations (21.9%), with no pulmonary aspirations reported. Apneas and bradycardias occurred at similar rates to intravenous hydration ^[146]B3b.
• Treatment escalation is reduced with HHFNC compared to low-flow oxygen: risk ratio 0.55 (95% CI 0.39 to 0.79; moderate-certainty evidence) ^[10]A1a.

Extrapulmonary Complications

• Necrotizing enterocolitis (NEC) is a rare but life-threatening extrapulmonary complication of severe RSV bronchiolitis. In a case series, 4 previously healthy term and late-preterm infants admitted to the PICU with respiratory failure developed NEC shortly after admission, with progressive abdominal distention, typical radiographic findings, and ascites . One required colon resection and ileostomy, two had peritoneal drainage, and one died of septic shock ^[134]C4. The authors hypothesize that a dysregulated proinflammatory response alters intestinal blood flow and compromises barriers to bacterial translocation ^[134]C4.
• Encephalopathy and sudden death have been linked to extreme cytokine elevation. In 2 full-term children who died within half a day of symptom onset, postmortem examination revealed extensive bronchiolitis, complete brain edema, and IL-6 levels >200-fold above normal despite a normal C-reactive protein level ^[21]C4. The authors suggest that extreme IL-6 elevation may predict risk for sudden death ^[21]C4.

Long-Term Respiratory Sequelae

• Subsequent respiratory hospital admissions are significantly increased after infant bronchiolitis. In a birth cohort of 613,377 infants, 21.7% of those with a bronchiolitis admission had a further respiratory hospital admission by age 5 years, compared to 8% without (adjusted HR 2.82, 95% CI 2.72 to 2.92). The association was strongest for asthma (HR 4.35) and wheezing (HR 5.02) ^[54]B3b.
• Inhaled corticosteroids have no proven benefit for subacute cough following bronchiolitis. A Cochrane review found no significant difference in the proportion of children not cured at follow-up (pooled OR 0.61, 95% CI 0.24 to 1.55) ^[150]A1a.

Hospital-Acquired Complications

• Secondary bacterial pneumonia is a concern, but antibiotics are not recommended for uncomplicated bronchiolitis. A Cochrane review of 7 studies (824 participants) found no difference in length of hospital stay, duration of oxygen, or readmission between antibiotics and placebo ^[149]A1a. Despite this, antibiotic use rates of 34% to 99% are reported in uncomplicated cases ^[147]A1a.
• Variability in care leads to different complication profiles. In a comparison of two PICUs, the Canadian site used invasive ventilation in 26% of infants vs 3% in France, with higher rates of chest radiographs, blood tests, and antibiotic use (72% vs 33%), yet similar durations of ventilatory support and length of stay ^[66]B3b.

Complication Table

Pearl: The most actionable complication to anticipate is air leak syndrome when using HHFNC at high flow rates, keep flows below the infant's minute ventilation and have a low threshold for chest imaging if respiratory status deteriorates ^[145]C4.

The preceding discussion of complications, apnea, respiratory failure, bacterial superinfection, delineates the severe end of the bronchiolitis spectrum. For the vast majority of infants, however, the natural history is one of gradual self-resolution over 1 to 2 weeks, with outcomes modified by patient factors and, to a lesser extent, by therapeutic interventions.

Natural History and Recovery

Most previously healthy infants recover completely. Among those hospitalized, median length of stay (LOS) is typically 2-4 days; in one trial, LOS was 2.6±1.9 days with nebulized 3% hypertonic saline versus 3.5±2.9 days with normal saline, a 26% reduction ^[111]A1b. Home oxygen therapy is feasible and safe: in a systematic review of 1257 infants, adverse events occurred in 0.6% with 0 mortality ^[151]B2a. After discharge, an as-needed follow-up strategy is noninferior to scheduled follow-up for parental anxiety, with no difference in readmission rates or symptom duration ^[130]A1b. Even during the COVID-19 pandemic, when bronchiolitis admissions declined by 80.1%, hospital-level outcomes (LOS, intensive care use, mortality) remained similar to pre-pandemic periods ^[78]B3b.

Predictors of Severe Disease

These factors, combined with clinical assessment, stratify risk at presentation. Age <2 months, tachycardia, hypoxemia, previous ICU admission, and brief illness duration independently predict ICU admission ^[60]A1b.

Impact of Interventions on Prognosis

Palivizumab prophylaxis reduces RSV hospitalization from 10.6% to 4.8% in high-risk infants ^[43]A1c. For acute treatment, combined epinephrine and dexamethasone reduced 7-day admission from 26.4% to 17.1% (unadjusted RR 0.65, 95% CI 0.45-0.95), though significance was lost after adjustment for multiple comparisons ^[108]A1b. High-flow oxygen did not shorten time on oxygen but reduced treatment failure from 33% to 14% (p=0.0016) ^[109]A1b. In severe bronchiolitis requiring intensive care, systemic corticosteroids plus inhaled epinephrine reduced the duration of positive pressure support from 40 hours to 26 hours (adjusted ratio 0.66, 95% CI 0.51-0.84) ^[77]A1b. Nebulized hypertonic saline showed a 26% reduction in LOS in one trial ^[111]A1b, but trial sequential analysis suggests that conventional meta-analyses may be false-positives due to inadequate information size ^[152]B2a.

Long-Term Outcomes

Neonatal bovine lactoferrin supplementation was associated with significantly less bronchiolitis in the first 2 years of life (rate ratio 0.34, 95% CI 0.14-0.86) ^[46]A1b. Palivizumab prophylaxis may also affect later wheezing, though the magnitude is not reported in the available evidence ^[42]A1c.

Pearl: Age <2 months, heart rate >160/min, and SpO₂ <87% at presentation are the three strongest independent predictors of ICU admission, use them to guide disposition decisions ^[60]A1b.

The preceding section detailed the natural history of bronchiolitis; however, outcomes and management decisions vary significantly across special populations, particularly among infants with prematurity, comorbidities, and those from indigenous communities.

Pediatrics

Bronchiolitis is predominantly a disease of infants under 2 years, and most evidence comes from this group. Within this population, several subgroups demand tailored attention.

Prematurity: A history of preterm birth (gestation <37 weeks) was present in 14% of infants in a large cluster RCT ^[105]A1b. Preterm infants are at increased risk for severe disease and ICU admission. In the same trial, 5% of infants had comorbidities - congenital heart disease, chronic lung disease, chronic neurological disorder, or failure to thrive - which further elevate risk ^[105]A1b.

Indigenous and impoverished communities: Infants from indigenous and impoverished backgrounds are disproportionately affected ^[105]A1b. In the Australasian study, 12% identified as Māori and 8% as Aboriginal/Torres Strait Islander ^[105]A1b. Structural inequities increase both incidence and severity.

Age <2 months: Independent predictors of ICU admission include age <2 months (OR 0.98 per week, 95% CI 0.96-0.99), pre-enrolment heart rate >160/min (OR 1.80, 95% CI 1.23-2.63), and SpO2 <87% (OR 0.91 per % point, 95% CI 0.86-0.95) ^[60]A1b. These factors should prompt earlier escalation of care.

Management modifications: For infants with severe bronchiolitis requiring intensive care, the combination of systemic corticosteroids and nebulized epinephrine reduced the duration of positive pressure support from 40 to 26 hours (adjusted ratio 0.66, 95% CI 0.51-0.84) ^[77]A1b. For apnea of bronchiolitis, a single dose of caffeine citrate (25 mg/kg) did not reduce apnea episodes compared with placebo (geometric mean time to 24-hour apnea-free period: 28.1 vs 29.1 hours) ^[110]A1b.

Home oxygen therapy: For selected infants who remain oxygen-dependent after initial stabilization, home oxygen therapy is feasible and safe - 0% mortality in 1257 patients, 7 adverse events (0.6%) ^[151]B2a. It reduces hospital bed time from 96.9 hours to 55.2 hours (P=0.001) ^[112]A1b.

Prophylaxis: Nirsevimab, administered to all infants during the RSV season, reduced acute bronchiolitis episodes by 59.2% (95% CI 57.9-60.4), hospital admissions by 63.1%, and PICU admissions by 63.1% ^[155]B3b. Extended catch-up strategies (immunizing infants <6 months at season start) showed the greatest effectiveness ^[155]B3b.

Pregnancy

No evidence was identified in the provided literature regarding bronchiolitis in pregnancy. The condition is exceedingly rare in adults, and management should follow standard supportive care principles for respiratory infections in pregnancy. No data on teratogenicity of treatments (e.g., ribavirin, palivizumab) relevant to pregnancy were available.

Elderly

Bronchiolitis is uncommon in the elderly; the evidence base does not address this population. In older adults, respiratory syncytial virus infection typically presents as a lower respiratory tract illness but is not termed bronchiolitis. No specific management modifications can be derived from the provided studies.

Immunocompromised

Immunocompromised infants were not specifically studied in the included trials. Theoretically, they may have a more severe course, but no evidence-based recommendations for altered dosing or thresholds can be made from these data.

Pearl: For infants with bronchiolitis, the strongest predictors of ICU admission are age <2 months, heart rate >160/min, and SpO2 <87% - these should trigger early escalation of monitoring and respiratory support ^[60]A1b. Home oxygen therapy is a safe alternative to prolonged hospitalization for selected infants, reducing length of stay without increasing adverse events ^{[112]A1b[151]B2a}.

For infants at highest risk, prevention begins before the first encounter with the virus. The American Academy of Pediatrics (AAP) 2006 guideline emphasizes that prevention of respiratory syncytial virus (RSV) infection with palivizumab and control of nosocomial spread are key components of bronchiolitis management ^[41]A1c. Since then, the prevention landscape has expanded dramatically.

Immunoprophylaxis

Palivizumab, a monoclonal antibody requiring five monthly doses, is recommended only for high-risk infants: those born at <29 weeks' gestation, with chronic lung disease of prematurity, or hemodynamically significant congenital heart disease ^[41]A1c. Its short half-life (≈20 days) and high cost limit broader use.

Nirsevimab, a long-acting monoclonal antibody targeting the prefusion RSV-F protein, provides durable protection with a single intramuscular dose. In a pooled analysis of 2350 preterm and term infants, a weight-banded dose led to a 79.5% relative risk reduction (RRR) for medically attended RSV lower respiratory tract infection (LRTI) and a 77.3% RRR for hospitalization ^[140]A1b. Real-world effectiveness data from Catalonia (15,341 infants) showed nirsevimab reduced RSV bronchiolitis hospitalizations by 74% (adjusted HR 0.26, 95% CI 0.17-0.39) and PICU admissions by 85% (HR 0.15, 95% CI 0.07-0.28) ^[163]B3b. A test-negative case-control study in Lombardy reported 82% effectiveness against RSV infection, 78% against hospitalization, and 84% against intensive care admission ^[165]B3b. The CDC and ACIP now recommend nirsevimab for all infants <8 months entering their first RSV season, ideally before hospital discharge ^[140]A1b. Parental acceptance is influenced by trust in the pediatrician's recommendation and fear of RSV; misinformation about nirsevimab being a “new understudied vaccine” is a barrier that prenatal counseling can address ^[158]D5.

Maternal Vaccination

Maternal RSV vaccination (RSVpreF, administered at 32-36 weeks' gestation) offers passive protection via transplacental antibody transfer. In a phase 3 trial, vaccine efficacy against medically attended RSV LRTI at 90 days was 57.1% and against RSV-associated hospitalization 67.7% (99.17% CI 15.9%-89.5%), though efficacy declined to 51.3% and 56.8% by 6 months ^[140]A1b. Another candidate, RSVPreF3-Mat, showed robust immunogenicity in high-risk pregnant women (geometric mean ratio at delivery 8.87-fold for RSV A, 8.21-fold for RSV B) with a placental transfer ratio of 1.33 and no increased preterm birth in this population ^[168]A1b.

General Preventive Strategies

Breastfeeding , adequate immunization, and reduction of indoor air pollution are foundational ^[167]A1b. Bovine lactoferrin supplementation (200 mg/kg/day for 8 weeks) in very low birth weight infants did not reduce sepsis but was associated with significantly less bronchiolitis (rate ratio 0.34, 95% CI 0.14-0.86) ^[46]A1b. Children whose vaccination status is documented as not up-to-date have higher odds of undergoing laboratory testing during respiratory illness visits, underscoring the importance of routine immunization ^[157]B3b.

Surveillance and Screening

RSV seasonality has become unpredictable after the COVID-19 pandemic, with atypical circulation patterns ^[59]B3b. Local surveillance networks are essential for timing prophylaxis ^[59]B3b. High altitude (>2500 m) increases RSV hospitalization risk by 25% in infants <1 year and 53% in children 1-4 years per 1000-m elevation, warranting heightened prevention efforts in mountainous regions ^[160]D5. No routine screening for bronchiolitis is recommended; diagnosis remains clinical.

Pearl: A single dose of nirsevimab given before or shortly after birth reduces RSV-related hospitalization by approximately 74-85% in real-world settings, making it the most impactful preventive intervention for all infants entering their first RSV season ^{[163]B3b[165]B3b}.