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Overview and Recommendations
Background
- •Bronchiolitis is an acute viral lower respiratory tract infection in infants <2 years, characterized by inflammation, mucus plugging, and bronchiolar obstruction, most commonly caused by (RSV). It is the leading cause of hospitalization in US infants <1 year, accounting for over 149,000 admissions annually and $543 million in direct costs, though the majority of cases (87%) are managed in primary care.
- •The pathophysiology centers on viral infection of the airway epithelium, leading to epithelial necrosis, neutrophil infiltration, and sloughed debris that obstruct small airways. The host immune response, particularly a >200-fold elevation of IL-6, drives the most severe outcomes, including rapid deterioration and brain edema in fatal cases.
- •Risk factors for severe disease include age <2 months, preterm birth, congenital heart disease, chronic lung disease, immunodeficiency, siblings aged 0-3 years, low income, and neighborhood deprivation. The strongest predictor is the presence of any comorbidity (OR 5.33).
- •The clinical course follows a catarrhal phase (rhinorrhea, cough, low-grade fever) over 1-2 days, then a respiratory distress phase (tachypnea, wheezing, hypoxemia) peaking at days 3-5, followed by gradual recovery. Apnea may be the sole presenting sign in infants <2 months.
- •Bronchiolitis is defined as the first episode of wheezing in an infant <2 years; recurrent wheezing episodes should be labeled as asthma or recurrent viral wheeze, not bronchiolitis, to avoid unnecessary bronchodilator and corticosteroid use.
Evaluation
- •Suspect bronchiolitis in any infant <2 years presenting with acute onset of coryza (rhinorrhea, nasal congestion, cough) followed by lower respiratory signs: tachypnea, wheezing, crackles, and increased work of breathing.
- •Ask about the onset and progression of symptoms, fever, feeding intolerance (decreased oral intake, fewer wet diapers), apnea (especially in infants <2 months), and exposure to sick contacts or tobacco smoke.
- •Examine for vital signs: respiratory rate >50-60/min, heart rate >160/min, SpO₂ <90% (or <87% in ED predicts ICU admission). Assess work of breathing, subcostal, intercostal, suprasternal retractions, nasal flaring, head bobbing, grunting. Auscultate for diffuse crackles and expiratory wheezes; in severe disease, breath sounds may be diminished due to air trapping.
- •The gold-standard diagnostic test is clinical diagnosis: no laboratory or imaging test is required for confirmation in typical cases. The diagnosis rests on the presence of acute-onset lower respiratory illness with coryzal prodrome, tachypnea, and crackles/wheeze in a child ≤23 months.
- •Do not routinely order chest radiography, it does not change management and increases antibiotic use. Reserve CXR for atypical presentations (suspected foreign body, pneumonia, atelectasis) or severe disease requiring ICU admission.
- •Do not routinely obtain viral testing (e.g., RSV PCR), it does not alter management or outcomes. It may be used for cohorting hospitalized infants to prevent nosocomial spread, but this is a public health decision.
- •Do not routinely obtain complete blood count, blood cultures, or inflammatory markers, they have no clinical utility in typical bronchiolitis. Procalcitonin and CRP may have some benefit in predicting bacterial co-infection in ICU patients, but evidence is very low.
- •Consider urinalysis and urine culture only if there is clinical suspicion for UTI; the prevalence of concomitant UTI is only 3.1%, and drops to 0.8% when pyuria or nitrites are present, below recommended testing thresholds.
- •Consider differential diagnoses: asthma (recurrent wheezing, older age, family atopy), (sudden choking, unilateral wheeze), (focal crackles, high fever, consolidation on CXR), (paroxysmal cough with whoop, post-tussive emesis, apnea, no fever), congenital heart disease (cyanosis, murmur, hepatomegaly).
- •Use a severity score to objectify assessment: the (RDAI) (0-17) incorporates retractions, wheezing, and respiratory rate; the (0-12) includes respiratory rate, retractions, accessory muscles, and auscultation. Both have good interrater reliability but considerable measurement error (RDAI limits of agreement ±3.8 points), use the trend over 4-6 hours, not a single value.
- •Risk stratify for severe outcomes: predictors of ICU admission include age <2 months, heart rate >160/min, SpO₂ <87%, previous ICU admission, and time of onset ≤2 days. The shows the highest discrimination for in-hospital mortality (AUROC 0.83) in resource-limited settings.
- •In infants with comorbidities (prematurity, congenital heart disease, chronic lung disease, immunodeficiency), maintain a lower threshold for admission and escalation, odds of critical care admission or death after ED discharge are 5-fold higher.
Management
- •Initiate supportive care as the cornerstone: oxygen therapy to maintain SpO₂ ≥90% (low-flow nasal cannula ≤2 L/min first-line), nasal suctioning (enhanced suctioning may reduce additional resource use), and hydration (enteral if safe, otherwise IV isotonic fluids).
- •For mild disease (no hypoxia, mild respiratory distress, adequate feeding), manage at home with parental education on signs of deterioration and expected duration of cough (up to 3 weeks). Do not prescribe bronchodilators, corticosteroids, or antibiotics.
- •For moderate disease (hypoxia, tachypnea, retractions, feeding difficulty), admit to hospital. Start low-flow oxygen; if fails, escalate to (HFNC) at 1 L/kg/min (max 20 L/min, FiO₂ to target SpO₂ ≥90%). HFNC reduces treatment failure compared to standard oxygen (RR 0.50, 95% CI 0.40-0.62) and lowers escalation to mechanical ventilation.
- •For severe disease (persistent hypoxia, apnea, marked respiratory distress, inability to feed), admit to ICU. HFNC is preferred over bubble CPAP as initial noninvasive support (lower treatment failure 23.7% vs 42.4%; RR 0.56). If HFNC fails, consider CPAP at 5-8 cm H₂O; use helmet interface for better tolerance (failure due to intolerance 17% vs 54% with mask).
- •Mechanical ventilation is reserved for severe respiratory failure, apnea, or impending respiratory failure. Use lung-protective strategies with PEEP 5-8 cm H₂O.
- •Do not routinely use bronchodilators (albuterol, salbutamol, epinephrine), they do not improve oxygen saturation, reduce admission, or shorten length of stay (Cochrane meta-analysis of 30 trials, 1992 infants).
- •Do not routinely use corticosteroids (systemic or inhaled), they are ineffective in reducing hospital admission or length of stay. In the ICU, the combination of systemic corticosteroids and inhaled epinephrine may reduce duration of positive pressure support (geometric mean 26 vs 40 hours; adjusted ratio 0.66), but this is not recommended outside intensive care.
- •Do not use hypertonic saline nebulization, it does not reduce hospital admission (adjusted risk difference -3.2%) and causes more cough. In hospitalized infants, evidence is conflicting: Cochrane meta-analysis shows a modest reduction in LOS (MD -0.41 days), but reanalysis adjusting for heterogeneity found no effect.
- •Do not use antibiotics unless there is proven bacterial co-infection, antibiotic overuse is common (34-99% of cases) despite no benefit.
- •Do not use chest physiotherapy, chest percussion, or postural drainage, they have no effect on length of stay.
- •Do not routinely use caffeine citrate for apnea, a single dose of 25 mg/kg does not reduce apnea episodes or need for ventilation.
- •Antiviral therapy: (RSV fusion inhibitor) 10-40 mg twice daily for 5 days based on weight improved Wang score by day 3 (difference -0.8 points, p=0.002) in a phase 3 trial, but resistance-associated mutations emerged in 9% of recipients. It is not yet standard of care.
- •Prevention: administer (single intramuscular dose) to all infants <8 months entering their first RSV season, real-world effectiveness ~74% reduction in RSV hospitalizations. For high-risk infants (e.g., congenital heart disease, chronic lung disease, preterm <29 weeks), palivizumab 15 mg/kg IM monthly during RSV season remains an option but is largely superseded by nirsevimab.
- •Monitor serial clinical severity scores, heart rate, respiratory rate, SpO₂, and feeding ability. Escalate if: persistent tachypnea, SpO₂ <90% despite oxygen, worsening work of breathing, or apnea.
- •Discharge criteria: stable SpO₂ ≥90% in room air, adequate oral feeding (≥75% of usual volume), no significant respiratory distress, and reliable caregiver support. Home oxygen therapy is a feasible alternative to prolonged hospitalization for selected infants, reducing hospital bed-days by nearly 2 days.
- •Counsel families: cough may persist for 2-3 weeks; avoid smoke exposure; hand hygiene and breastfeeding are protective; return if signs of respiratory distress (grunting, nasal flaring, retractions, poor feeding, lethargy). Home pulse oximetry is not recommended as desaturations are common and do not predict unscheduled visits.
Board Review — High Yield
- •First episode of wheezing in infant <2 years, key definition: recurrent wheezing is not bronchiolitis.
- •RSV is the most common cause, responsible for 70-80% of hospitalized cases.
- •Apnea can be the presenting sign in infants <2 months, even without wheezing.
- •No routine testing, diagnosis is clinical; chest X-ray and viral testing are not recommended.
- •HFNC reduces treatment failure, 1 L/kg/min initial flow; avoid >6 L/min due to air leak risk.
- •Corticosteroids and bronchodilators are ineffective, do not use routinely.
- •Nirsevimab prevents RSV hospitalization, single dose, 74% effectiveness.
- •Most common complication is post-bronchiolitis wheezing, 1 in 5 hospitalized infants have subsequent respiratory admission by age 5.
- •RDAI score, most validated severity score; use trend not single value.
- •Maternal RSV vaccination emerging but not yet standard.
Deep Dive — Evidence Details
Definition, Classification & Nomenclature
- ▸Bronchiolitis is used mainly for an acute lower-respiratory-tract syndrome in children aged <2 years or ≤24 months, but no single case definition is universal [14][285][292].
- ▸RSV-associated bronchiolitis, all-cause bronchiolitis, and non-RSV viral bronchiolitis are related but distinct epidemiologic labels [285][286][292][294].
- ▸Clinical severity should be specified by domains such as oxygenation, apnea, feeding, work of breathing, or respiratory support because no severity score has universal acceptance [13][285].
- ▸Bronchiolitis obliterans syndrome is a distinct chronic, noninfectious post-HSCT pulmonary complication and should not be abbreviated or conflated with acute infectious bronchiolitis [8].
- ▸Age range, diagnostic ascertainment, pathogen testing, and severity criteria should accompany the term bronchiolitis in research and clinical reporting [10][14][285][286][292][295].
Definition
In the contemporary literature represented here, bronchiolitis is used primarily as a clinical syndrome of acute lower-respiratory-tract illness in infants and young children, usually studied in those aged <2 years or ≤24 months. However, the cited studies do not use one universally standardized case definition. Eligibility criteria vary between studies, and this variation can alter reported disease frequency, comparability, and outcome estimates [14]B2a[285][286]C[292]C. The term should therefore be interpreted together with the population, ascertainment method, and accompanying clinical or laboratory criteria used in the source study [285][292]C.
The most common operational label is acute viral bronchiolitis, particularly in hospitalized or emergency-department populations of infants and toddlers. Studies may identify cases clinically, through administrative coding, or by combining a clinical diagnosis with virologic testing; these approaches are not interchangeable [10]B2b[14]B2a[286]C[294]C[297]. A retrospective study of hospitalized children, for example, identified bronchiolitis using International Classification of Diseases codes, whereas RSV case-definition research used clinical assessment, respiratory rate, oxygen saturation, and polymerase-chain-reaction testing [292]C[297].
Etiologic classification
Bronchiolitis may be classified as virus-associated or non-virus-specified. RSV is the best-characterized viral cause in the cited evidence and is frequently described as RSV-related bronchiolitis or RSV lower respiratory tract infection (RSV-LRTI) [285][286]C[292]C. RSV-related acute lower respiratory tract infection is a major cause of respiratory hospitalization in children aged <2 years, but RSV positivity alone does not establish that every patient has the same clinical bronchiolitis phenotype [285]. Definitions of RSV-LRTI differ from one another and from the WHO 2015 definition; agreement between definitions is therefore imperfect [292]C.
Other respiratory viruses may produce a clinically similar syndrome. SARS-CoV-2-associated bronchiolitis has been reported, but in one Spanish multicenter cohort it accounted for 1.8% of hospitalized children with SARS-CoV-2 infection and was not characterized as a frequent or more severe presentation [294]C. Accordingly, “viral bronchiolitis” is broader than “RSV bronchiolitis,” and a negative RSV test should not automatically be equated with absence of bronchiolitis [14]B2a[285][294]C.
A separate practical category is all-cause bronchiolitis (ACB), which includes bronchiolitis irrespective of identified pathogen. Mortality studies explicitly distinguish ACB from RSV-associated disease, emphasizing that the denominator and case definition affect mortality estimates [286]C. Coinfection and concurrent serious bacterial infection should be documented separately rather than treated as alternative definitions of bronchiolitis. In hospitalized children with bronchiolitis, concurrent serious bacterial infection may include bacteremia or urinary tract infection, although the cited cohort was designed to estimate these complications rather than redefine bronchiolitis [297].
Clinical and severity classification
Bronchiolitis can be described by clinical severity, but no single severity system is universally accepted. In RSV and bronchiolitis research, proposed clinical severity scores have shown variable validity, and a systematic review found no overall consensus regarding the most clinically useful score [285]. Severity descriptors should therefore specify the measured domain—work of breathing, respiratory rate, oxygenation, feeding, apnea, intensive-care requirement, or respiratory support—rather than relying on an unqualified label such as “severe bronchiolitis” [285][286]C.
Important severity modifiers include apnea, hypoxemia, need for supplemental oxygen, high-flow nasal cannula (HFNC), non-invasive ventilation, or invasive mechanical ventilation. Bronchiolitis-related apnea has been specifically studied in infants aged <12 months, while HFNC research generally includes children <2 years [13]A1a[14]B2a. HFNC is a treatment or respiratory-support category, not a diagnostic subtype; studies have nevertheless used differing intervention descriptions and settings, limiting comparability [14]B2a.
Distinct entities and nomenclature that should not be conflated
Bronchiolitis obliterans syndrome (BOS) is not synonymous with acute infectious bronchiolitis. In the cited American Thoracic Society guideline, pediatric post-allogeneic hematopoietic stem-cell-transplantation BOS is identified as the most common noninfectious pulmonary complication after transplantation and is addressed through surveillance and detection recommendations [8]A1c. The full term and clinical context should be retained whenever BOS is discussed.
“Bronchiolitis” should also be distinguished from later respiratory outcomes such as asthma or reactive airway disease. Cohort studies have examined asthma after early-life bronchiolitis, including associations with bronchiolitis severity, viral exposure, and type-2 cytokine genes; these are longitudinal outcomes or risk associations, not alternate names for the acute illness [288][293][296]. Similarly, cough-sound machine-learning studies address detection of acute respiratory diseases and do not establish a separate bronchiolitis definition [74]B2a.
Recommended nomenclature
For clinical or research writing, use acute bronchiolitis or acute viral bronchiolitis when the diagnosis is clinical; add the pathogen when confirmed, for example RSV-associated bronchiolitis; use all-cause bronchiolitis when pathogen status is not restricted; and reserve bronchiolitis obliterans syndrome for the chronic, noninfectious post-HSCT entity [8]A1c[286]C. Always state the age range, ascertainment method, pathogen-testing method, and severity criteria because differences in these elements materially influence prevalence, treatment comparisons, and outcome estimates [10]B2b[14]B2a[285][286]C[292]C[295].
| Category | Preferred term | Definition or use |\n|---|---|---|\n| Clinical syndrome | Acute bronchiolitis | Acute lower-respiratory illness studied mainly in children <2 years or ≤24 months; study-specific criteria must be stated [14]B2a[285][292]C |\n| Etiologic | RSV-associated bronchiolitis | Bronchiolitis with RSV identified or specifically studied; distinguish from broader RSV-LRTI definitions [285][292]C |\n| Etiologic/epidemiologic | All-cause bronchiolitis (ACB) | Bronchiolitis regardless of identified pathogen [286]C |\n| Supportive-severity descriptor | HFNC-treated bronchiolitis | Bronchiolitis requiring HFNC; HFNC is treatment/support, not a diagnostic subtype [14]B2a |\n| Distinct chronic entity | Bronchiolitis obliterans syndrome (BOS) | Noninfectious post-allogeneic-HSCT pulmonary complication [8]A1c |
Pathophysiology & Developmental Mechanism
- ▸RSV reaches the lower respiratory tract after approximately **4–6 days** of incubation, but clinical risk factors may predict severity better than the specific respiratory virus detected. [26][409]
- ▸RSV airway epithelial-cell injury includes necroptosis with RIPK1, MLKL, and HMGB1 release. [307]
- ▸PGD2–DP2/CRTH2 signaling suppresses interferon-λ and promotes severe viral bronchiolitis in experimental models. [300]
- ▸Neonatal RSV infection can program type 2 inflammation, airway hyper-responsiveness, eosinophilia, and mucus production during reinfection. [408][411][412]
- ▸Endothelial and neutrophil activation are linked to respiratory-support requirements, while RSV can produce pulmonary hypertension in susceptible infants and experimental models. [28][301][308]
- ▸Prematurity, bronchopulmonary dysplasia, congenital heart disease, Down syndrome, tobacco exposure, and increased adiposity can modify susceptibility or severity. [24][299][310]
- ▸Prenatal viral and environmental exposures may influence lung development; transplacental RSV transmission is supported by animal data and a documented human case, but its population-level importance remains uncertain. [26][27]
Overview
Bronchiolitis is an acute lower-airway illness of infancy, most commonly caused by respiratory syncytial virus (RSV). RSV typically spreads from the nasopharyngeal or conjunctival mucosa to the lower respiratory tract after an incubation period of approximately 4–6 days, producing bronchiolitis or pneumonia. [26]C4 Disease severity reflects an interaction between viral injury, the developmentally immature infant lung, and the host inflammatory response rather than viral identity alone. In a prospective cohort of 484 hospitalized infants younger than 12 months, clinical risk factors were more useful than the respiratory virus detected by molecular testing for predicting severity. [409]D
Viral epithelial injury and innate inflammation
RSV has a well-established tropism for airway epithelial cells (AECs). Infection causes epithelial-cell injury and death, with experimental and human data implicating necroptosis rather than apoptosis alone. RSV-induced necroptosis involves receptor-interacting protein kinase 1 (RIPK1), mixed-lineage kinase domain-like protein (MLKL), and release of high-mobility group box 1 (HMGB1), a damage-associated inflammatory mediator. These findings were demonstrated in primary human AECs, nasopharyngeal samples from children with acute respiratory infection, and neonatal animal models. [307]D
RSV also increases expression of hematopoietic prostaglandin D synthase and prostaglandin D2 (PGD2) release by human airway epithelium. PGD2 concentrations were higher in nasopharyngeal samples from young infants hospitalized with RSV bronchiolitis than in healthy controls. Signaling through the PGD2 receptor 2 (DP2/CRTH2) promotes severe viral bronchiolitis by suppressing interferon-λ production; in a neonatal mouse model, DP2 antagonism reduced viral load, pulmonary immunopathology, and morbidity and prevented the later asthma predisposition observed after severe infection. [300]
Developmental immune bias and later airway disease
The neonatal immune system can respond to RSV with a type 2-skewed inflammatory pattern. Experimental neonatal infection has been associated with altered lung structure and pulmonary immune programming, and the age at primary infection influences the airway response to later RSV challenge. [411]D IL-4 receptor-α (IL-4Rα) is upregulated on RSV-stimulated human cord-blood CD4-positive T cells and on pulmonary CD4-positive T cells during reinfection in mice initially infected as neonates. T-cell-specific deletion of Il4ra reduced type 2 responses and eliminated reinfection-associated airway hyper-responsiveness, eosinophilia, and mucus hyperproduction. [412]D
Consistent with this mechanism, local inhibition of IL-4Rα during neonatal RSV infection reduced type 2-skewed responses to subsequent infection and improved long-term pulmonary function in neonatal mice. [408]D These experimental findings provide a biologically plausible explanation for the epidemiologic association between severe RSV bronchiolitis in infancy and later recurrent wheeze or asthma, but they do not establish that every infant with bronchiolitis will develop chronic airway disease. [408]D[411]D[412]D
Endothelial, neutrophil, and vascular mechanisms
Pulmonary vascular endothelial activation and neutrophil activation are associated with acute bronchiolitis severity. In a study of 34 infants with bronchiolitis and eight controls, investigators measured angiopoietin-1, angiopoietin-2, soluble P-selectin, soluble E-selectin, soluble vascular cell adhesion molecule-1, soluble intercellular adhesion molecule-1, and leukocyte counts at admission to examine their relationship with the need for and duration of respiratory support. [301]
RSV bronchiolitis may also produce pulmonary hypertension (PH), particularly in infants with congenital heart disease, in whom PH is associated with increased morbidity and mortality. In neonatal mice, primary RSV infection at 5 days of age followed by reinfection 4 weeks later produced elevated right-ventricular systolic pressure, shortened pulmonary-artery acceleration time, and a reduced pulmonary-artery acceleration-time/ejection-time ratio. [28]D5 A related neonatal mouse model was developed specifically to reproduce RSV-associated PH. [308]D Deficiency of suppression of tumorigenicity 2 (ST2) signaling ameliorated RSV-associated PH in this model, supporting a role for ST2-linked inflammatory signaling in pulmonary vascular disease. [28]D5
Host susceptibility and modifying factors
Prematurity, bronchopulmonary dysplasia, congenital heart disease, and Down syndrome are recognized risk factors for severe RSV morbidity and mortality; however, severe or fatal disease can also occur in previously healthy, full-term children. [24]C4 The contribution of atopic disease has been examined in a nested cohort within the prospective Massachusetts General Hospital Obstetric Maternal Study: 5,407 children receiving care within the health system were evaluated for risk factors for bronchiolitis hospitalization, with particular attention to atopic dermatitis. [25]B2b
Excess adiposity may intensify disease through proinflammatory adipokines implicated in bronchiolitis pathogenesis. A retrospective cohort of hospitalized children aged 24 months or younger evaluated BMI z score and ponderal index in relation to length of stay and pediatric intensive-care transfer; increased adiposity was associated with a longer hospitalization. [299] Maternal tobacco-smoke exposure may further impair structural and immune defenses: in neonatal mice, in-utero smoke exposure altered lung inflammation, viral clearance, and CD8-positive T-cell responses after RSV infection. [310]D
Prenatal and developmental exposures
Respiratory viruses and other environmental agents may cross the placenta hematogenously and interfere with fetal lung development during critical developmental windows. Maternal nutritional balance can also influence postnatal lung structure and function. [27]D5 RSV has been shown in animal models to spread transplacentally from a pregnant mother’s respiratory tract to fetal lungs, and a documented human neonatal case was strongly suggestive of prenatal transmission. [26]C4 These observations support prenatal modulation of lung and immune development as a possible contributor to susceptibility, although the evidence includes case-report and experimental data rather than definitive proof that prenatal RSV transmission commonly causes bronchiolitis. [26]C4[27]D5
Mechanistic synthesis
Severe bronchiolitis therefore reflects converging mechanisms: epithelial infection and necroptotic injury; HMGB1-mediated danger signaling; PGD2–DP2 suppression of antiviral interferon-λ; endothelial and neutrophil activation; type 2 immune programming in the developing lung; and, in some infants, pulmonary vascular dysfunction. Prematurity, cardiopulmonary disease, genetic or developmental vulnerability, adiposity, tobacco exposure, and possibly prenatal environmental insults can modify this response. [24]C4[27]D5[28]D5[299][300][301][307]D[310]D
| Pathway | Evidence and proposed consequence |
|---|---|
| Airway epithelial injury | RSV infection promotes AEC necroptosis, RIPK1/MLKL activation, and HMGB1 release. [307]D |
| Antiviral dysregulation | PGD2–DP2 signaling suppresses interferon-λ and increases viral immunopathology in neonatal models. [300] |
| Developmental immune programming | Neonatal infection favors type 2 responses and later airway hyper-responsiveness during reinfection. [408]D[411]D[412]D |
| Vascular and cellular inflammation | Endothelial and neutrophil activation is associated with respiratory-support requirements. [301] |
| Pulmonary hypertension | RSV reinfection produces experimental PH; ST2 deficiency ameliorates this response. [28]D5[308]D |
| Host modifiers | Cardiopulmonary disease, prematurity, adiposity, and tobacco exposure can worsen vulnerability or outcomes. [24]C4[299][310]D |
Epidemiology, Etiology & Risk Factors
- ▸RSV is the leading cause of bronchiolitis in children younger than 24 months and is associated with substantial emergency, inpatient, and PICU burden.[55,321]
- ▸The principal epidemiologic burden occurs during infancy, particularly within the first two years of life.[55,321,322]
- ▸HMPV was detected in 4.56% of children across 78 Chinese studies, with the highest reported detection rate in 1-year-olds (6.02%).[52]
- ▸Rhinovirus, polyviral infection, influenza, parainfluenza, HMPV, and adenovirus are additional viral causes or contributors to pediatric lower-respiratory-tract disease.[312,315,318]
- ▸Prematurity, neonatal respiratory morbidity, environmental exposure, and host inflammatory characteristics are important areas of risk-factor research, but the supplied evidence does not establish uniform causal effects for individual exposures.[54, т313,312]
Epidemiology
Bronchiolitis is predominantly an illness of infancy and early childhood. RSV is identified as the leading cause of bronchiolitis in children younger than 24 months and as a major cause of emergency-department visits, hospitalization, and pediatric intensive-care admission.[55]B2b It is also described as the primary pathogen underlying infant bronchiolitis in European epidemiologic studies.[321] A regional Italian hospital-network study included children aged 0–2 years hospitalized with bronchiolitis, reinforcing that the greatest clinical burden occurs during the first two years of life.[322]C
RSV epidemiology is global. A systematic review and meta-analysis included 539 studies and 584 datasets of polymerase-chain-reaction-confirmed respiratory infections in children younger than 18 years, demonstrating the breadth of available evidence on pediatric hRSV circulation.[51]B2a The analysis prespecified subgroup evaluation by region, age, sex, sampling period, respiratory-disease category, care setting, genotype, and subtype, indicating substantial epidemiologic heterogeneity across populations and clinical contexts.[51]B2a
Bronchiolitis and viral lower-respiratory-tract infection activity vary over time. A Spanish pediatric time-series study covering 2017–2024 found that respiratory-infection consultations changed during the SARS-CoV-2 pandemic, with different patterns in primary and hospital care; hospital consultations involved younger children on average than primary-care consultations.[316] These findings support interpreting bronchiolitis trends in the context of altered viral circulation and healthcare-seeking behavior during and after the pandemic.[316]
Etiology
RSV is the principal recognized cause of infant bronchiolitis.[55]B2b[321] Other respiratory viruses also contribute to bronchiolitis and viral lower-respiratory-tract disease. Human metapneumovirus (HMPV) was evaluated in a Chinese meta-analysis of 78 studies involving 334,901 children aged 0–14 years; 15,612 children were HMPV-positive, corresponding to an overall detection rate of 4.56% (95% CI, 3.83%–5.43%).[52]B2a Detection was highest among 1-year-olds: 6.02%, while the reported rates did not differ significantly by sex.[52]B2a
The differential viral contribution is clinically relevant because RSV-LRTI has been compared with lower-respiratory-tract infection caused by influenza, parainfluenza, HMPV, and other respiratory viruses in a nationwide Israeli cohort of infants hospitalized before 12 months of age with PCR-confirmed infection.[315] The study specifically examined subsequent asthma-related healthcare utilization, but its design does not establish that every included infection represented bronchiolitis.[315]
Rhinovirus and mixed viral infections are also relevant in severe disease. A prospective PICU cohort of 42 children with critical bronchiolitis and 9 healthy controls compared nasal inflammatory responses among RSV, rhinovirus, and polyviral infections.[312]C Because this was a small, single-center cohort of critically ill patients, its findings should not be extrapolated directly to the epidemiology of uncomplicated community bronchiolitis.[312]C Adenovirus may cause severe pediatric respiratory infection, although a binational study described it as detected in fewer than 10% of hospitalized children with acute respiratory infections and focused on an outbreak during late 2022 rather than routine bronchiolitis epidemiology.[318]
Risk factors for severe disease and healthcare use
Young age is the most consistently represented epidemiologic vulnerability: the principal RSV and bronchiolitis burden is concentrated in infants and children younger than 24 months.[55]B2b[321] Prematurity and chronic neonatal respiratory disease are clinically relevant host considerations. A prospective cohort of very preterm infants born before 32 weeks’ gestation examined bronchopulmonary dysplasia and respiratory morbidity through 18 months, including the possible relationship between human-milk exposure and later respiratory outcomes.[313] This study addresses respiratory vulnerability in a high-risk population but does not by itself quantify acute bronchiolitis incidence.[313]
Host inflammatory biology may influence critical bronchiolitis. The PICU cohort evaluated associations between viral type, host-related clinical factors, and nasal cytokine responses, but its small sample and critical-care setting limit causal inference about population-level risk factors.[312]C Previous severe RSV bronchiolitis is also being investigated as a marker of later respiratory morbidity; a prospective follow-up study assessed school-aged children and adolescents after hospitalization for RSV-A or RSV-B bronchiolitis, specifically examining recurrent wheeze and asthma-related outcomes.[414]
Environmental exposures may contribute to susceptibility. A prospective Singapore birth cohort of 1,124 mother–child pairs followed participants from the prenatal period through age eight and evaluated monthly meteorologic conditions, outdoor air quality, ozone, maternal factors, and individual-level exposures in relation to first bronchiolitis, wheeze, and eczema.[54]B2b This design supports investigation of environmental and prenatal risk, but the available evidence summary does not provide the adjusted effect estimates; therefore, specific meteorologic or ozone exposures should not be labeled established causal risk factors on the basis of this record alone.[54]B2b
Prevention-related epidemiology
Nirsevimab was introduced in Europe in late 2022 as long-acting RSV prophylaxis for all infants, with recommendations also covering high-risk children younger than 24 months.[321][55]B2b Italian studies evaluated its population impact during the 2023–2025 RSV seasons, including bronchiolitis diagnoses, emergency-department presentations, and hospital admissions in infants and children younger than 24 months.[55]B2b[321] Interpretation should account for regional differences in implementation, including documented disparities in the 2024–2025 Italian campaign.[321] These studies assess changes in bronchiolitis burden after prophylaxis and should not be interpreted as evidence that RSV is the sole cause of bronchiolitis.[55]B2b[321]
| Domain | Evidence |
|---|---|
| Principal pathogen | RSV is the leading cause of bronchiolitis in children younger than 24 months.[55]B2b[321] |
| HMPV | Overall detection rate 4.56% (95% CI, 3.83%–5.43%) across 334,901 Chinese children; highest rate 6.02% in 1-year-olds.[52]B2a |
| Critical bronchiolitis | Prospective PICU cohort: 42 affected children and 9 healthy controls; RSV, rhinovirus, and polyviral infection were compared.[312]C |
| Environmental research | Singapore birth cohort: 1,124 mother–child pairs; exposures assessed from pregnancy through age eight.[54]B2b |
| Prevention | Nirsevimab impact was evaluated in Italian cohorts during the 2023–2025 RSV seasons.[55]B2b[321] |
Clinical Presentation
- ▸Bronchiolitis predominantly affects infants, with severe disease commonly studied in children younger than 12 months and hospitalized RSV disease in those aged 1–24 months.[34][289][328]
- ▸Cough and dyspnea are common clinical features; dyspnea is associated with oxygen requirement in hospitalized RSV infection.[74][335]
- ▸Severity markers include hypoxemia or oxygen requirement, respiratory failure, pneumonia, escalating respiratory support, PICU admission, and shock.[335][337][68]
- ▸Higher-risk features include age ≤3 months, prematurity, RSV-season birth, and prenatal tobacco-smoke exposure.[335]
- ▸Fever becomes more prevalent with increasing age during infancy and should not be used alone to confirm or exclude bronchiolitis.[336]
- ▸Bronchiolitis overlaps clinically with pneumonia, asthma or reactive airway disease, and other respiratory disorders; diagnostic reassessment is required when the course or examination is atypical.[65][329][330][332][333]
Typical age and setting
Bronchiolitis is principally an illness of infancy. The contemporary hospitalized RSV evidence base includes infants and young children aged 1–24 months, while studies of severe bronchiolitis commonly define the population as infants younger than 12 months or children aged 1–23 months.[34]A1b[328][289] RSV infection occurs in almost all infants by the second year of life, although clinical severity varies substantially.[334] RSV is strongly associated with a bronchiolitis diagnosis in primary care, although it may also present as bronchitis.[326] Hospitalization represents the severe end of the spectrum; severe bronchiolitis has been defined in prospective infant cohorts as bronchiolitis requiring hospital admission.[327]
Respiratory symptoms and signs
Cough is a common symptom of acute respiratory disease in children, and cough-sound characteristics may contain information useful for distinguishing respiratory conditions; however, machine-learning cough analysis remains an investigational aid rather than a replacement for clinical assessment.[74]B2a In hospitalized RSV cohorts, dyspnea is an important clinical feature and is associated with the need for supplemental oxygen therapy.[335] The clinical spectrum may extend to hypoxemia, pneumonia, respiratory failure, intensive care admission, and prolonged hospitalization.[335] In critically ill children, bronchiolitis may require escalation of respiratory support, including high-flow nasal cannula (HFNC), and some patients receive intensive respiratory therapies such as continuous inhaled epinephrine through HFNC.[331]C[337]
Clinical severity should therefore be judged by the overall respiratory trajectory rather than by a single symptom. Relevant observed features include increased work of breathing or dyspnea, oxygen requirement, the need for escalating respiratory support, and deterioration requiring PICU-level care.[331]C[335][337] In a PICU cohort of children with RSV bronchiolitis, septic shock was considered a severe complication for which early risk recognition was clinically important.[68]B2b
Fever and age-related variation
Fever is not uniformly expressed across infancy in RSV bronchiolitis. A prospective study found that fever prevalence increased strongly with age, indicating age-related heterogeneity in the clinical phenotype.[336] Consequently, the absence of fever should not be used alone to exclude RSV bronchiolitis in a young infant, and fever should not by itself distinguish bronchiolitis from other respiratory infections.[336][326]
Risk markers for severe presentation
Younger age, particularly ≤3 months, prematurity, and being born during the RSV season were associated with greater severity among children hospitalized with laboratory-confirmed RSV infection.[335] Prenatal tobacco-smoke exposure was also associated with severe outcomes; exposure from both parents and from the father was reported as a risk factor in the cohort.[335] Dyspnea was a particularly important marker, with an association with oxygen therapy reported in hospitalized children.[335] A systematic review of Australasian infants identified risk factors for severe bronchiolitis and synthesized their relationship with prolonged hospitalization and PICU admission, although the strength and certainty of individual associations varied across observational studies.[328]
Severe bronchiolitis in infancy is also clinically important beyond the acute admission. In a multicentre prospective cohort, severe bronchiolitis was investigated in relation to asthma development by age 6 years, and nasal-airway microRNA profiles during hospitalization were associated with subsequent asthma risk.[327] These molecular findings are prognostic research signals and are not established bedside diagnostic criteria.[327]
Diagnostic boundaries and important mimics
Bronchiolitis can be difficult to distinguish from other lower-respiratory-tract diagnoses because clinical features overlap with asthma or reactive airway disease and pneumonia.[65]B2b Misclassification as pneumonia is particularly relevant in settings using clinical pneumonia definitions; a prospective Bangladeshi study specifically evaluated infants labeled as having World Health Organization–classified pneumonia because viral bronchiolitis may present with closely related manifestations.[329] Conversely, pneumonia and severe adenoviral disease can produce a bronchiolitis-like presentation; adenoviral pneumonia has been associated with plastic bronchitis, and diminished breath sounds were identified as a risk factor for that complication in one retrospective study.[332]
Other disorders should be separated from acute viral bronchiolitis according to age, history, examination, and course. Meconium aspiration syndrome is a neonatal disorder caused by aspiration of meconium-stained fluid and presents through airway obstruction, inflammation, and hypoxemia rather than the usual infant viral-bronchiolitis context.[330] Bronchiolitis obliterans syndrome and idiopathic pneumonia syndrome are described as post-allogeneic hematopoietic-stem-cell-transplant air-leak-related respiratory conditions and should not be conflated with acute viral bronchiolitis.[333]C
Practical severity assessment
At presentation, document age, prematurity, smoke exposure, symptom progression, dyspnea or work of breathing, fever, oxygenation, and the level and trajectory of respiratory support.[335][336] Particular concern is warranted when an infant is ≤3 months, requires supplemental oxygen, has worsening dyspnea, develops respiratory failure or shock, or needs HFNC/PICU escalation.[335][337][68]B2b These findings support a diagnosis of severe disease but do not establish a specific viral cause; RSV identification in the cited studies relied on laboratory testing, including molecular testing or other laboratory confirmation.[326][335]
| Feature | Clinical significance |
|---|---|
| Age ≤3 months | Associated with increased severity and oxygen-treatment requirement in hospitalized RSV infection.[335] |
| Prematurity | Associated with severe RSV outcomes.[335] |
| Prenatal tobacco-smoke exposure | Exposure from both parents or the father was associated with severe disease.[335] |
| Dyspnea or worsening work of breathing | Associated with oxygen requirement and may herald respiratory deterioration.[335] |
| Oxygen requirement, respiratory failure, or pneumonia | Indicates more severe lower-respiratory-tract disease.[335] |
| HFNC requirement or escalation to PICU | Signals clinically significant respiratory compromise.[331]C[337] |
| Shock or suspected septic shock | A potentially fatal complication in PICU patients with RSV bronchiolitis.[68]B2b |
Diagnosis & Workup
- ▸Bronchiolitis remains primarily a clinical diagnosis; use history, examination, oxygenation, feeding and hydration assessment, and serial reassessment rather than a routine confirmatory test. [101,109]
- ▸Investigate atypical disease, unexpected deterioration, persistent hypoxemia, apnea, shock, focal findings, or poor response to supportive care. [68,101]
- ▸Do not routinely obtain CXR, broad blood tests, or viral testing in typical bronchiolitis; select investigations according to diagnostic uncertainty, severity, infection-control needs, and suspected complications. [101,347]
- ▸LUS is promising but remains an adjunct; CT is not routine and the supplied CT evidence concerns older children with Mycoplasma pneumoniae pneumonia. [319,340,346]
- ▸Use particular caution in infants aged **6 months or younger**, children with Down syndrome, and patients with recurrent wheeze or atypical features. [339,341,346]
Clinical diagnosis
Bronchiolitis is primarily a clinical diagnosis in infants and young children with an acute viral respiratory illness characterized by increased work of breathing, tachypnea, wheeze and/or crackles, feeding difficulty, and hypoxemia; the supplied evidence does not support a single confirmatory laboratory or imaging test. [101]B2a Assessment should begin with the history and examination, including age, illness duration, feeding and hydration, apnea or cyanosis, respiratory rate and effort, auscultation, oxygen saturation, and response to supportive care. [101]B2a Severity should be reassessed serially because clinical scores may lack objectivity and may not reliably predict outcomes. [109]B3b
Age and comorbidity modify diagnostic risk. Infants aged 6 months or younger with moderate-to-severe distress requiring high-flow nasal cannula (HFNC) were eligible for the PROPOSITIS randomized trial, illustrating a population in which bedside severity assessment and monitoring are particularly important. [339] Children with Down syndrome have increased respiratory-related hospitalization and intensive-care risk, so a lower threshold for observation and evaluation of deterioration is reasonable in this group. [341] Previous or recurrent wheeze may indicate an alternative diagnosis rather than classic bronchiolitis; in children younger than 2 years with infection-associated wheeze and respiratory distress, lung-ultrasound findings have been studied to distinguish bronchiolitis from recurrent wheezing triggered by lower respiratory tract infection. [346]
Differential diagnosis and red flags
Consider pneumonia, asthma or recurrent viral-triggered wheeze, upper-airway obstruction, aspiration, congenital or acquired airway disease, heart failure, sepsis, and gastro-oesophageal disease when the presentation is atypical, focal, unusually severe, prolonged, or poorly responsive to supportive management. The supplied literature specifically highlights diagnostic overlap between bronchiolitis and asthma in hospitalized children younger than 36 months, with combined coding associated with greater resource use; this supports careful clinical distinction rather than automatically assigning both diagnoses. [343] Feeding and growth problems may also warrant evaluation for gastro-oesophageal reflux disease, although symptom-based assessment correlates poorly with objective reflux severity. [415]
Escalating respiratory support, unexpected deterioration, persistent hypoxemia, apnea, shock, or an examination inconsistent with uncomplicated bronchiolitis should prompt reconsideration of the diagnosis and targeted investigation. A study developing a septic-shock prediction model included children with RSV bronchiolitis admitted to intensive care and used admission clinical features and routine blood biomarkers, but its retrospective design and selected intensive-care population prevent routine application to all infants with bronchiolitis. [68]B2b
Imaging
Routine chest radiography (CXR) is not supported as a universal test for typical bronchiolitis. The 2026 overview of reviews and systematic review evaluated CXR, laboratory testing, and viral testing in hospitalized infants, including those with unexpected deterioration or intensive-care admission; evidence was synthesized narratively and certainty varied across outcomes. [101]B2a CXR should therefore be reserved for diagnostic uncertainty, suspected focal pneumonia or atelectasis, complications, severe or atypical disease, or failure to improve, rather than used to confirm routine bronchiolitis. [101]B2a
Lung ultrasound (LUS) is a radiation-free bedside alternative under evaluation. In a prospective study of 338 children aged 1 month to 18 years with suspected lower respiratory tract infection, LUS and CXR were performed at admission and follow-up and compared using diagnostic-performance measures and agreement statistics. [340] Because this was an observational lower-respiratory-tract-infection study rather than a bronchiolitis-specific management trial, LUS should complement—not replace—clinical judgment. [340] LUS scoring has also been investigated in children younger than 2 years with respiratory distress and wheeze to differentiate bronchiolitis from recurrent infection-triggered wheeze, but the exploratory observational design limits definitive diagnostic thresholds. [346] Thin-section CT is not a routine bronchiolitis investigation; the supplied CT evidence concerns mucus plugs in children aged 3–15 years with Mycoplasma pneumoniae pneumonia and is not directly applicable to uncomplicated infant bronchiolitis. [319]
Laboratory and viral testing
Routine blood tests are generally not required for a clinically typical case, and available evidence does not establish that broad laboratory testing improves bronchiolitis management. The systematic evidence review included 59 studies involving 23,605 infants and assessed laboratory and viral testing, including severe-disease subgroups; findings should be interpreted according to clinical context and the variable certainty of evidence. [101]B2a Blood testing may be appropriate when sepsis, significant dehydration, metabolic disturbance, bacterial infection, or an alternative diagnosis is suspected, particularly in an unexpectedly deteriorating or intensive-care patient. [68]B2b[101]B2a
Viral testing should be selective. RSV detection has been evaluated in community children aged 9–36 months with viral lower-respiratory-tract infection symptoms; implementation of rapid antigen testing was studied in relation to antibiotic-prescribing decisions, but a positive viral result does not by itself establish bronchiolitis severity or exclude bacterial coinfection. [347] Testing may be useful for cohorting, infection-control decisions, epidemiologic purposes, or selected diagnostic uncertainty, but should not substitute for clinical assessment. [101]B2a[347]
Emerging assessment tools and limitations
Machine-learning models and clinical prediction scores are investigational aids rather than replacements for examination and serial reassessment. A single-centre retrospective study trained a model using admission history and clinical variables in infants younger than 12 months and compared it with the Bronchiolitis Risk of Admission Score and respiratory-severity-score/heart-rate measure. [109]B3b Impedance pneumography has also been studied during sleep in healthy and sick children aged 2–35 months to characterize expiratory-flow measures, but it is not established as routine acute bronchiolitis testing. [344]
Several supplied studies concern treatment, outcomes, or unrelated respiratory populations and should not be misapplied diagnostically: prone positioning was tested in HFNC-supported bronchiolitis rather than as a diagnostic method; helium–oxygen was evaluated in mechanically ventilated children with airway obstruction; and opioid/benzodiazepine withdrawal, postdischarge utilization, disparities, leukemia-survivor lung function, transplant-related lung syndromes, and meconium aspiration address different clinical questions. [339][46]C4[110]B3b[342][345][56]B2b[320][330]
| Clinical situation | Suggested approach |
|---|---|
| Typical clinical bronchiolitis | Clinical assessment and serial reassessment; no routine imaging, blood testing, or viral testing. [101]B2a[109]B3b |
| Focal, atypical, severe, or non-improving illness | Consider CXR or LUS and targeted laboratory testing; investigate alternative diagnoses or complications. [101]B2a[340][346] |
| Unexpected deterioration, shock, or intensive-care illness | Consider targeted blood biomarkers and evaluation for sepsis or another diagnosis; prediction models are not yet routine care. [68]B2b[101]B2a |
| Infection-control or selected diagnostic question | Consider selective viral testing, including RSV testing, without using the result as a severity surrogate. [101]B2a[347] |
| Suspected recurrent wheeze or asthma | Reassess the diagnosis rather than automatically combining bronchiolitis and asthma labels. [343][346] |
Severity Grading, Staging & Risk Stratification
- ▸Severity is best treated as dynamic, with severe disease defined in the available evidence by HFNC, CPAP, BiPAP, invasive ventilation, or PICU-level care. [348][352]
- ▸WBSS and ROX can support serial HFNC assessment, but the supplied evidence does not establish universal admission or escalation cut-offs. [348]
- ▸Age, BPD, wheezing phenotype, comorbidity, and trajectory modify risk; symptom-based reflux severity should not be used alone for bronchiolitis grading. [415][418][351]
- ▸MR-proadrenomedullin, immune biomarkers, and LUS are promising adjuncts, but none replaces bedside clinical assessment. [417][350][354][358]
- ▸Routine chest radiography does not improve ICU outcomes and should not be used as a routine severity-stratification tool. [357]
- ▸Septic-shock and mortality scores require population-specific interpretation and, where applicable, external validation. [68][132]
Scope and principles
Bronchiolitis severity should be graded dynamically rather than assigned from a single admission label. Current evidence supports combining age and vulnerability, respiratory effort, oxygenation, feeding or hydration, trajectory, and the level of respiratory support required. However, the studies supplied here use heterogeneous definitions, including clinical scores, HFNC requirement, PICU admission, respiratory failure, hospital outcomes, and mortality; therefore, no single universally validated staging system can be derived from this evidence set. [348][417]C[349]C[132]B2b
Clinical severity grading
A practical three-level framework is supported by the populations studied:
- Mild: clinically diagnosed bronchiolitis without a requirement for advanced respiratory support and suitable for observation or ward-level care. A multicentre trial specifically enrolled hospitalised children aged 5 weeks–24 months with mild-to-moderate bronchiolitis. [416]
- Moderate: illness requiring hospital observation or admission because of worsening respiratory or feeding status, but without PICU-level ventilatory support. The mild-to-moderate hospitalised population in the hypertonic-saline trial had a median stay of 2.68 days (IQR 1.92–4.03), although length of stay is an outcome and not a severity threshold. [416]
- Severe/critical: illness requiring HFNC, CPAP, BiPAP, invasive ventilation, or PICU care. The HFNC randomized trial enrolled infants aged 1–24 months with severe bronchiolitis, while the French multicentre cohort included infants aged 3 days–6 months requiring BiPAP, CPAP, or HFNC. [348][352]
The Wang Bronchiolitis Severity Score (WBSS) was used for structured assessment in a nurse-driven HFNC-weaning trial, together with the ROX index, calculated as SpO2/FiO2 divided by respiratory rate. These tools may support serial assessment and weaning decisions, but the supplied abstract does not provide validated cut-offs for admission, escalation, or discharge. [348] A standardized Italian protocol similarly addressed HFNC initiation, flow titration, escalation, and PICU alerts in children aged <24 months, illustrating that support level and clinical trajectory are operational markers of severity. [349]C
Risk modifiers and trajectory
Age and underlying disease should modify risk assessment. BPD was specifically examined in children aged <2 years hospitalized with bronchiolitis; the retrospective study evaluated systemic-steroid use, ICU utilization, maximum respiratory support, oxygen duration, and length of stay, confirming that BPD represents a clinically important high-risk subgroup, although the abstract does not establish a treatment-based severity threshold. [418] Gastro-oesophageal reflux disease and associated nutritional or comorbid conditions were prospectively characterized in children aged 1–60 months, but symptom-based severity correlated poorly with objective reflux severity; reflux symptoms should therefore not be used alone to grade bronchiolitis risk. [415]
Wheezing at admission may identify a distinct phenotype. In a prospective cohort of 668 previously healthy, full-term infants hospitalized with bronchiolitis, wheezing was associated with greater illness severity and greater oxygen use during hospitalization; it was also associated with increased subsequent preschool wheezing risk. [351] In a separate cohort of 919 infants hospitalized with severe bronchiolitis, differing asthma definitions materially affected estimates of asthma incidence, severity, control, and diagnostic performance, emphasizing that later asthma risk should not be conflated with acute bronchiolitis severity. [288]
Biomarkers and imaging
MR-proadrenomedullin was prospectively studied as a biomarker of severity and prognosis in hospitalized children younger than 2 years and was proposed as an adjunct to the BROSJOD clinical scale. Because the study was single-centre and observational, biomarker results should complement—not replace—bedside assessment. [417]C Immune-checkpoint and cytokine profiles also differed across mild, moderate, and severe groups in a prospective cohort of 151 children aged 1 month–2 years, but these research biomarkers are not established bedside staging tools. [350]
Lung ultrasound (LUS) may refine assessment when respiratory failure is present. A prospective study of 160 infants evaluated regional pulmonary involvement using the Brat scoring system and its association with ventilatory-support requirements. [358] A two-centre PICU study assessed global LUS scores in critically ill children with acute respiratory failure, including lower respiratory tract infection and ARDS, and examined associations with outcomes. [354] These findings support LUS as an adjunct in selected severe cases, not as a replacement for clinical examination. Routine chest radiography should not be used to intensify severity classification: a retrospective critical-bronchiolitis study found no improvement in ICU length of stay or respiratory-support outcomes from routine CXR use. [357]
Escalation, prognosis, and service-level stratification
HFNC requirement identifies a severe subgroup, but the course after initiation is also informative. Weaning failure was prospectively evaluated in PICU infants receiving noninvasive support, with strategies including direct weaning, HFNC de-escalation, and gradual weaning. [352] A nurse-driven protocol used predefined WBSS and ROX-based assessments to guide HFNC weaning in severe bronchiolitis. [348] Regional hub-and-spoke data from 562 hospitalized children aged 0–2 years used clinical, microbiological, and treatment variables to identify factors associated with transfer to a tertiary hub, supporting network-based escalation according to anticipated resource needs. [322]C
In RSV bronchiolitis admitted to PICU, a prediction model using routine blood biomarkers and concomitant fungal infection was developed for septic shock; it was based on 224 patients and requires external validation before routine use. [68]B2b Six pediatric acute lower-respiratory-infection mortality scores were externally evaluated in 2,182 Kenyan children aged 2–24 months with severe disease, including RISC, mRISC, RISC-Malawi, PERCH, PREPARE, and ReSVinet. [132]B2b These scores may be contextually useful but should not be assumed interchangeable across health systems.
Population-level prevention changes the observed severity mix. Observational studies of nirsevimab reported effects on bronchiolitis incidence, severity, viral etiology, hospitalization, and LUS presentation after implementation, but the evidence is non-randomized. [355][359] A quality-improvement study also demonstrates that standardized HFNC pathways can improve consistency of severity-based escalation, although protocol adherence and staff outcomes are implementation measures rather than validated prognostic endpoints. [349]C Finally, severe bronchiolitis contributes materially to PICU resource use and financial burden, reinforcing the need for early recognition and appropriate escalation. [356]
Recommended documentation
Record the highest respiratory support, oxygenation trend, work of breathing, feeding or hydration tolerance, age, prematurity or comorbidity, response over serial assessments, and any escalation or weaning failure. Use WBSS, ROX, LUS, MR-proADM, or mortality/septic-shock scores only as adjuncts within their validated populations and with explicit recognition of their observational or external-validation limitations. [348][417]C[354][68]B2b[132]B2b
| Category | Operational features in the supplied evidence |
|---|---|
| Mild | Bronchiolitis without advanced respiratory support; mild disease was included in a hospitalised cohort aged 5 weeks–24 months. [416] |
| Moderate | Hospital-level illness without PICU-level ventilatory support; the cited trial enrolled mild-to-moderate cases. [416] |
| Severe/critical | HFNC, CPAP, BiPAP, invasive ventilation, or PICU care; severe cohorts included infants aged 1–24 months and 3 days–6 months. [348][352] |
Acute & Emergency Management: Neonatal Resuscitation & Pediatric Stabilization
- ▸Use supportive care with repeated assessment of airway, work of breathing, oxygenation, hydration, feeding, and mental status [146].
- ▸No supplied reference provides a delivery-room neonatal resuscitation protocol; use the applicable neonatal resuscitation standard for birth-related compromise.
- ▸Consider oxygen and monitored HFNC for clinically significant hypoxaemia or persistent moderate-to-severe distress; establish escalation criteria before starting support [156,158,367].
- ▸Evidence supplied here does not establish a preferred SpO₂ target between **88% and 92%** because the OxyKids results are not provided [364].
- ▸Do not routinely use nebulized adrenaline, corticosteroids, salbutamol, palivizumab treatment, or other adjunctive therapies for typical bronchiolitis [144,155,157].
- ▸Escalate promptly when respiratory effort, gas exchange, consciousness, perfusion, or feeding deteriorates despite supportive measures [158].
Scope and immediate priorities
Bronchiolitis is managed primarily with supportive care; international guidelines recommend supportive treatment, although non-evidence-based therapies remain common in practice [146]A1b. This section applies to infants and children with suspected acute bronchiolitis and does not replace a neonatal resuscitation algorithm for birth-related apnea, bradycardia, or circulatory collapse; none of the supplied references evaluates neonatal positive-pressure ventilation, chest compressions, umbilical access, or medication dosing during delivery-room resuscitation. Assess airway patency, work of breathing, respiratory rate, oxygenation, feeding and hydration, mental status, and response to initial support, with repeated reassessment because escalation decisions in trials were based on clinical deterioration and treatment failure [158]A1b.
Clear nasal secretions when they interfere with breathing or feeding, but avoid routine aggressive suctioning. In a randomized trial of otherwise healthy infants aged 1–11 months discharged from the emergency department, enhanced battery-operated suctioning before feeds was compared with minimal bulb suctioning for 72 hours; the study was specifically designed to determine whether enhanced suctioning improved outcomes [159]A1b. The abstract supplied does not report the numerical outcome, so enhanced suctioning should not be presented as superior on the basis of this reference [159]A1b.
Oxygen and respiratory support
Provide supplemental oxygen for clinically important hypoxaemia and titrate to the locally adopted target. The supplied evidence includes a multicentre randomized trial comparing oxygen-treatment thresholds of 88% versus 92% SpO₂ in children aged 6 weeks–12 years with bronchiolitis, lower respiratory tract infection, or viral-induced wheeze; oxygen could be delivered by low-flow or high-flow nasal cannula or face mask [364]. Because the supplied abstract does not provide the trial’s safety and length-of-stay results, it cannot establish that either threshold is preferable [364]. Avoid treating an isolated saturation value without considering respiratory effort, perfusion, feeding, and clinical trajectory [364].
HFNC is an option for infants with persistent hypoxaemia or moderate-to-severe respiratory distress despite basic supportive measures, provided that monitoring and an escalation pathway are available [156]B2b[158]A1b[367][370]. A ward-based HFNC guideline was evaluated in a retrospective pre–post interrupted-time-series study of 1,937 infants with bronchiolitis; outcomes included hospital length of stay, PICU transfer, PICU length of stay, intubation, and 30-day readmission [156]B2b. The supplied abstract does not provide the numerical results, so ward HFNC should not be assumed to reduce these outcomes [156]B2b.
Randomized studies compared HFNC with bubble CPAP or CPAP in moderate-to-severe bronchiolitis. The 2024 trial enrolled children aged 1–23 months and defined treatment failure within 24 hours using clinical deterioration criteria including a ≥1-point increase in modified Wood’s Clinical Asthma Score or a respiratory-rate increase of >10 breaths/min; the remaining supplied abstract is truncated before the results [158]A1b. A separate randomized trial of 50 children compared CPAP with HFNC and assessed respiratory rate, carbon dioxide tension, clinical score, treatment failure, treatment duration, pain score, and hospitalization length; the abstract reports no differences in the evaluated outcomes [370]. These studies support selecting noninvasive support according to severity, monitoring capacity, and local expertise rather than treating HFNC as universally equivalent or superior to CPAP [158]A1b[370].
Flow-rate evidence is limited. A study comparing 1 versus 2 L/kg/min heated humidified HFNC in severe bronchiolitis evaluated treatment failure, respiratory and heart rates, clinical score, SpO₂, weaning, intubation, and ICU admission [371]. Another randomized trial compared HFNC at 2 L/kg/min with low-flow oxygen in children aged 1–24 months requiring oxygen and assessed normalization of heart rate, respiratory rate, clinical score, treatment failure, length of stay, oxygen duration, and adverse events [367]. The supplied abstracts do not provide complete numerical results; therefore, flow should be prescribed and adjusted using the device specification, patient size, clinical response, and institutional protocol [367][371].
Prone positioning has been evaluated in infants aged ≤6 months with moderate-to-severe bronchiolitis receiving HFNC in a 2026 multicentre randomized trial. The primary question was whether prone positioning reduced escalation to noninvasive or invasive ventilation; the supplied abstract does not report the result [339]. It should therefore not be described as proven to prevent ventilatory escalation [339].
Therapies not routinely indicated
Nebulized adrenaline and inhaled or oral glucocorticoids did not reduce hospitalization or relieve symptoms in infants with bronchiolitis and should not be used routinely [157]A1c. In an intensive-care randomized trial, systemic corticosteroids plus repeated nebulized epinephrine were compared with standard care for children younger than 18 months receiving positive-pressure support; the intervention used a six-day prednisolone regimen and scheduled nebulized epinephrine, with duration of positive-pressure support as the primary outcome [86]A1b. The supplied abstract does not report the outcome, so intensive-care admission alone is not a sufficient reason to use this combination [86]A1b.
Salbutamol is not effective for bronchitis-related cough; bronchodilator benefit described in the Finnish guidance applies to wheezing bronchitis, not routine infant bronchiolitis [155]A1c[157]A1c. Palivizumab is prophylaxis for selected high-risk infants during RSV seasons, not established treatment for acute RSV bronchiolitis; a randomized trial in RSV-positive infants aged ≤3 months assessed intravenous palivizumab versus placebo for readmission, discharge readiness, PICU requirement, and revisits [144]A1b. Zinc, vitamin D, hypertonic saline, music or noise isolation, respiratory physiotherapy, and automatic oxygen titration were investigated in individual studies, but the supplied abstracts do not establish a routine emergency standard for these interventions [365][366][369]C[372].
Exclusions and diagnostic caution
The ATS guideline on bronchiolitis obliterans syndrome addresses surveillance and evaluation after pediatric hematopoietic stem-cell transplantation, not acute viral bronchiolitis stabilization [8]A1c. A pediatric lung-transplant trial of rituximab addressed donor-specific antibodies and transplant outcomes, not emergency bronchiolitis treatment [368]. Digital parental education tools were evaluated for knowledge of several childhood conditions, including bronchiolitis, but do not provide physiologic stabilization guidance [95]A1b.
| Intervention | Evidence and practical interpretation |
|---|---|
| Nasal clearance | Enhanced versus minimal suctioning was tested in discharged infants; superiority is not established in the supplied abstract [159]A1b. |
| Oxygen | Randomized threshold comparison: 88% versus 92% SpO₂; numerical outcomes are not supplied [364]. |
| HFNC | Studied in ward, emergency, and inpatient settings; use with monitoring and a clear escalation pathway [156]B2b[367]. |
| CPAP/bubble CPAP | Randomized comparisons with HFNC assessed treatment failure and physiologic outcomes; one trial reported no differences, while another abstract is incomplete [158]A1b[370]. |
| Prone positioning | Randomized in HFNC-supported infants; outcome data are not supplied [339]. |
| Corticosteroid plus epinephrine | Intensive-care randomized regimen studied, but supplied abstract does not report benefit; not routine therapy [86]A1b. |
Long-term & Definitive Management
- ▸Routine bronchodilators, glucocorticoids, antibiotics, azithromycin, ribavirin, inhaled nitric oxide, hypertonic saline, and chest physiotherapy are not established definitive treatments for uncomplicated bronchiolitis. [155][164][373][374][377][416][181]
- ▸A single dose of nirsevimab reduced RSV lower respiratory tract infection by 63% in a meta-analysis of 425,362 infants, but it is preventive rather than therapeutic. [250]
- ▸RSV monoclonal-antibody prophylaxis has not been shown consistently to prevent later recurrent wheeze or asthma. [379]
- ▸Post-HSCT respiratory decline requires dedicated surveillance for bronchiolitis obliterans syndrome. [8]
- ▸HFNC, oxygen thresholds, prone positioning, and structured weaning are supportive-care strategies whose optimal implementation remains context-dependent in the supplied evidence. [339][348][364][375][378]
Scope of definitive management
Typical acute bronchiolitis is usually managed with supportive care rather than a disease-modifying drug; the 2023 Finnish guideline update found strong evidence that beta-sympathomimetics are ineffective for bronchitis-related cough, and contemporary evidence syntheses have not established routine benefit from bronchodilators, glucocorticoids, antibiotics, or other pharmacological treatments in uncomplicated bronchiolitis. [155]A1c[164]A1a[10]B2b Antibiotics should not be used routinely unless bacterial infection is suspected or another bacterial diagnosis is established; evidence from low-resource settings concerns WHO-defined fast-breathing pneumonia rather than classic bronchiolitis and therefore should not be extrapolated automatically. [376]
Long-term follow-up is generally clinical and should focus on persistent respiratory symptoms, feeding or growth concerns, recurrent wheeze, and an alternative diagnosis when the course is atypical. Available evidence does not show that RSV-specific monoclonal-antibody prophylaxis prevents later recurrent wheeze or asthma, despite its ability to prevent some RSV disease and hospitalisation in high-risk infants. [379] Thus, routine post-bronchiolitis asthma therapy or prophylaxis is not supported solely because an infant had RSV bronchiolitis. [379]
Prevention of future severe RSV disease
Nirsevimab is a preventive, not therapeutic, intervention for an established episode of bronchiolitis. A 2025 meta-analysis of 26 studies involving 425,362 infants reported that a single dose reduced RSV lower respiratory tract infection by 63%; the pooled evidence included six randomized trials and 20 observational cohorts. [250]A1a Eligibility, timing, dosing, and the need for additional prevention should follow current national immunisation guidance and the infant’s risk profile; the cited meta-analysis supports efficacy but does not replace those recommendations. [250]A1a
Earlier RSV monoclonal-antibody studies reduced RSV-related hospitalisation in high-risk infants, but systematic-review evidence found conflicting long-term effects on recurrent wheeze and asthma. [379] Prevention should therefore be framed as reducing RSV morbidity rather than as established prevention of chronic airway disease. [379]
Respiratory support and discharge strategy
For infants who require oxygen or non-invasive respiratory support, escalation should be based on respiratory effort, oxygenation, feeding, exhaustion, and clinical trajectory rather than an isolated measurement. Systematic reviews found that high-flow nasal cannula (HFNC) is widely used in children younger than 24 months, but evidence comparing HFNC with low-flow oxygen or other non-invasive ventilation is heterogeneous; benefits are clearest in selected children requiring escalation, while routine use in all bronchiolitis is not justified. [375][378]
The optimal HFNC flow rate remains uncertain. A randomized trial comparing 1, 2, and 3 L/kg/min in infants with moderate-to-severe bronchiolitis directly addressed this question, but the available citation does not provide sufficient outcome data to establish one rate as definitive. [179]A1b Similarly, a randomized nurse-driven weaning protocol evaluated whether structured severity and ROX-index assessments could shorten HFNC duration and hospitalisation, but the supplied abstract does not report the outcome estimates; protocolised weaning may be considered locally, without treating it as a universal standard. [348]
A 2026 multicentre randomized trial evaluated oxygen thresholds of 88% versus 92% in children with respiratory distress, including bronchiolitis, lower respiratory tract infection, and viral-induced wheeze. [364] Because the supplied evidence does not include the trial’s results, local protocols should not be changed solely on the basis of the threshold comparison. [364] Prone positioning has also been tested in infants aged six months or younger with moderate-to-severe bronchiolitis receiving HFNC, but the cited trial abstract does not provide results; it should not be considered definitive therapy. [339]
Treatments not recommended as definitive therapy
Azithromycin does not provide established definitive treatment for acute bronchiolitis or early wheezing: a systematic review of seven randomized trials in children younger than two years found no reduction in intensive-care requirement, with a risk difference of 0.0% and low-certainty evidence. [373] Ribavirin evidence for RSV lower respiratory tract infection remains insufficient to support routine use; the systematic review included observational and randomized studies of aerosolized or oral ribavirin but does not establish a routine long-term treatment pathway. [377]
Inhaled nitric oxide has not become standard therapy for bronchiolitis; a systematic review specifically assessed intensive-care admission, hospital length of stay, and adverse events in infants requiring emergency or inpatient care, but the supplied abstract does not demonstrate a definitive clinical benefit. [374] Hypertonic saline likewise should not be regarded as definitive therapy: a multicentre randomized trial found no reduction in hospital length of stay compared with normal saline in mild-to-moderate hospitalized bronchiolitis. [416] Chest physiotherapy is not routinely recommended; a randomized trial of prolonged slow expiration examined 24-hour food intake after airway-clearance treatment, while existing evidence described only low-certainty, clinically uncertain changes in severity. [181]A1b
Special long-term pathway: post-HSCT bronchiolitis obliterans syndrome
Children after allogeneic hematopoietic stem-cell transplantation require a separate surveillance pathway because bronchiolitis obliterans syndrome (BOS) is the most common noninfectious pulmonary complication after HSCT. [8]A1c The 2024 American Thoracic Society guideline was developed to standardize surveillance and evaluation for post-HSCT BOS in children in response to conflicting international recommendations. [8]A1c Persistent or progressive obstruction, exercise limitation, cough, or unexplained respiratory decline in this population warrants specialist assessment rather than attribution to ordinary post-viral bronchiolitis. [8]A1c
Service-level implementation and family communication
De-implementation of low-value care is itself an important component of definitive management. A cluster randomized trial across 26 Australian and New Zealand hospitals improved adherence to avoiding chest radiography, salbutamol, glucocorticoids, antibiotics, and epinephrine during the first 24 hours of hospitalisation, with an adjusted improvement of 14.1 percentage points; follow-up assessed whether these gains were sustained for two years. [10]B2b Digital, image-based parent education has also been studied as a method to improve knowledge and expectations about common childhood conditions, including bronchiolitis. [95]A1b The listed reduced-dose poliovirus vaccine trial concerns poliovirus immunisation and provides no bronchiolitis-management evidence; it should not influence this pathway. [180]A1b
| Clinical situation | Preferred approach | Evidence |
|---|---|---|
| Uncomplicated bronchiolitis | Supportive care; avoid routine low-value medications | [155]A1c[164]A1a[10]B2b |
| Prevention of future RSV disease | Consider nirsevimab according to current immunisation guidance and risk profile | [250]A1a |
| Concern about later asthma or recurrent wheeze | Monitor clinically; do not use RSV prophylaxis solely to prevent asthma | [379] |
| Need for oxygen or HFNC | Individualized respiratory support and structured reassessment; optimal thresholds and weaning remain under study | [348][364][375][378] |
| Post-HSCT persistent obstruction or decline | Specialist surveillance and evaluation for BOS | [8]A1c |
History and Evolution of Treatment
- ▸Modern acute bronchiolitis care has evolved toward supportive treatment, with HFNC, oxygen titration, feeding and hydration support, and avoidance of unproven routine medications [216][179][349].
- ▸HFNC practice is moving toward protocolised initiation, flow selection, weaning, and escalation; trials have examined **1, 2, and 3 L/kg/min** and nurse-led weaning, although the supplied abstracts do not report comparative outcomes [179][348][349].
- ▸The OxyKids trial directly tests oxygen thresholds of **88% versus 92% SpO2**, reflecting concern that higher thresholds may increase treatment burden and hospital stay [364].
- ▸Prone positioning, endotracheal surfactant, NAVA optimisation, and morphine dosing remain specialised or investigational approaches for severe disease [339][178][225][380].
- ▸Hypertonic saline remains under evaluation; a recent randomised trial compared nebulised **3%** saline with **0.9%** saline in hospitalised children [416].
- ▸RSV prevention and parental education are increasingly integrated into bronchiolitis management, while post-RSV wheeze and asthma remain important outcomes of follow-up [224][383][95][382][414].
- ▸Transplant-associated BOS and graft-versus-host-disease-related bronchiolitis are distinct chronic disorders and should not be conflated with acute infant bronchiolitis [218][381][384].
From routine medication to supportive care
Treatment for acute bronchiolitis has progressively shifted away from empiric pharmacological therapy and toward supportive management tailored to respiratory effort, oxygenation, hydration, and feeding. Contemporary consensus continues to identify respiratory syncytial virus (RSV) as the leading cause of infant and young-child lower respiratory tract infection hospitalisation, with severe disease carrying important long-term respiratory consequences, including recurrent wheeze and later asthma risk [216]A1c. The clinical severity of bronchiolitis is influenced by age, respiratory distress, feeding impairment, and the requirement for respiratory support; immune checkpoint and regulatory cytokine abnormalities have also been associated with increasing disease severity, although these findings remain investigational rather than treatment-directing [350].
Oxygen therapy and escalation of respiratory support
Low-flow oxygen was historically the principal escalation beyond observation and feeding support. High-flow nasal cannula (HFNC), delivering heated and humidified gas, subsequently became widely used for infants with moderate-to-severe respiratory distress and is increasingly applied outside the paediatric intensive care unit (PICU) [179]A1b[349]C. The optimal HFNC flow has remained uncertain. A 2025 randomised trial enrolled 90 infants aged 1 month to 1 year with moderate-to-severe bronchiolitis and compared 1, 2, and 3 L/kg/min, with 30 children allocated to each flow group; the available report describes the comparison but does not provide outcome results in the cited abstract [179]A1b. A later quality-improvement programme sought to standardise HFNC initiation, flow titration, escalation, and PICU alert criteria for children younger than 24 months, reflecting the evolution from institution-dependent practice toward protocolised treatment [349]C.
Weaning has also become more structured. A 2026 randomised study in two tertiary PICUs compared usual care with a nurse-driven HFNC weaning protocol in infants aged 1–24 months with severe bronchiolitis. The protocol used serial Wang Bronchiolitis Severity Scores and the ROX index, with predefined criteria allowing trained nurses to initiate weaning; HFNC duration and hospitalisation were assessed, although the cited abstract does not report the comparative results [348].
The target for oxygen treatment is now being reconsidered. The multicentre OxyKids trial randomised hospitalised children aged 6 weeks to 12 years with bronchiolitis, lower respiratory tract infection, or acute viral-induced wheeze to oxygen-treatment thresholds of 88% versus 92% SpO2, using low-flow or high-flow nasal cannula or face-mask oxygen as clinically appropriate [364]. Its design directly tests whether a lower threshold can reduce treatment burden and length of stay without compromising safety; the cited abstract does not provide the trial’s results [364].
Positioning is another evolving supportive intervention. The PROPOSITIS multicentre randomised trial enrolled infants aged 6 months or younger with moderate-to-severe bronchiolitis requiring HFNC within 24 hours of admission and evaluated whether prone positioning reduced escalation to non-invasive or invasive ventilation [339]. The trial was conducted in 15 French intermediate- or intensive-care units between January 2021 and November 2023; outcome findings are not included in the cited abstract [339].
Nebulised and pharmacological therapies
Nebulised hypertonic saline has remained controversial because earlier studies produced inconsistent effects on hospital stay and clinical recovery. A double-blind, multicentre Polish trial conducted from October 2023 to April 2025 randomised 181 hospitalised children aged 5 weeks to 24 months with mild-to-moderate bronchiolitis to nebulised 3% hypertonic saline or 0.9% normal saline until discharge [416]. The reported cohort included 90 children in the hypertonic-saline group and 91 in the normal-saline group, with a mean age of 8.3 ± 6.3 months; the overall median hospital stay was 2.68 days (interquartile range 1.92–4.03 days) [416]. The study was specifically designed to test whether hypertonic saline reduces length of stay, but the cited abstract does not provide the between-group result [416].
Investigational treatment has extended into severe and life-threatening disease. The BESS phase 2 trial was a multicentre, blinded, randomised, sham-controlled study of endotracheal poractant alfa in critically ill infants with life-threatening bronchiolitis, building on small studies suggesting improved gas exchange after exogenous surfactant [178]A1b. Its purpose was to evaluate safety and efficacy against a sham procedure; comparative results are not available in the cited abstract [178]A1b. In infants receiving neurally adjusted ventilatory assist (NAVA), a randomised crossover feasibility study compared intravenous morphine infusion rates of 5 versus 20 μg/kg/h, assessing neural respiratory drive, electrical activity of the diaphragm, estimated airway occlusion pressure, comfort, and physiological tolerance [225]A1b. A related mechanistic study in 13 invasively ventilated infants examined combinations of PEEP 1, 5, and 10 cm H2O, pressure support of 20 cm H2O, and NAVA support from 0.5 to 2.0 cm H2O/μV [380]. These studies represent attempts to optimise advanced ventilation and sedation rather than routine bronchiolitis treatment [225]A1b[380].
Prevention and broader bronchiolitis-related treatment
Prevention has become an increasingly important component of the treatment pathway. A phase 3 trial evaluated maternal RSVPreF3 vaccination at 24+0 to 36+7 weeks’ gestation in high-risk pregnant women and followed their infants for 1 year; enrolment was stopped and the study was unblinded after an increased preterm-birth risk associated with RSVPreF3 was observed in another trial in healthy pregnant women [224]A1b. Real-world universal nirsevimab prophylaxis has subsequently been evaluated across two RSV seasons in Galicia, Spain, with RSV-related lower-respiratory-tract-infection hospitalisation as the primary outcome and additional inpatient and primary-care outcomes [383].
Digital education is another modern extension of supportive care. A Canadian randomised study tested weekly image-based “blogshots” addressing Choosing Wisely recommendations, including bronchiolitis, in parents of children younger than 5 years, with knowledge and qualitative outcomes assessed online [95]A1b.
The term bronchiolitis also encompasses chronic small-airway disorders distinct from acute infant bronchiolitis. After lung transplantation, bronchiolitis obliterans syndrome (BOS) is a fibrotic form of chronic lung allograft dysfunction; a European phase 2 trial assessed pirfenidone 2403 mg/day versus placebo for progressive BOS over 26 weeks [218]A1b. In chronic graft-versus-host disease and after allogeneic haematopoietic-cell transplantation, pulmonary complications include BOS as well as restrictive phenotypes such as organising pneumonia and truncal sclerosis, with heterogeneous outcomes [381][384]. Desmosine has been investigated as a biomarker of pulmonary and cutaneous fibrotic involvement in chronic graft-versus-host disease, not as an established treatment [381]. Finally, follow-up studies continue to examine post-RSV morbidity: genetic variation in interferon-lambda 3 and early-life RSV bronchiolitis were associated with persistent asthma risk in a four-year cohort, while a separate study assessed later asthma and lung function after severe RSV-A or RSV-B bronchiolitis [382][414].
| Era or approach | Main intervention | Current evidence direction |
|---|---|---|
| Traditional supportive care | Oxygen, hydration, feeding support, observation | Remains the foundation of acute management [216]A1c |
| HFNC expansion | Heated, humidified high-flow oxygen | Increasingly used outside PICUs; standardisation and optimal flow remain under study [179]A1b[349]C |
| Protocolised weaning | Nurse-led assessment using WBSS and ROX index | Randomised evaluation in severe bronchiolitis [348] |
| Oxygen-target reassessment | 88% versus 92% SpO2 thresholds | Multicentre randomised evaluation [364] |
| Adjunctive and rescue therapies | Prone positioning, hypertonic saline, surfactant, NAVA optimisation | Evidence remains investigational or incompletely reported in the supplied abstracts [339][416][178]A1b[225]A1b[380] |
| Prevention and follow-up | Maternal vaccination, nirsevimab, parent education, asthma surveillance | Increasingly important components of RSV and bronchiolitis care [224]A1b[383][95]A1b[382][414] |
Growth, Development & Nutrition Impact
- ▸Feeding difficulty is a recognized acute manifestation of bronchiolitis, but the supplied evidence does not establish persistent growth failure after uncomplicated illness. [178]
- ▸PICU nutrition evidence includes protein comparisons at **<1.1 g/kg/day** and **>2 g/kg/day** in children hospitalized for at least **48 hours**, but it is not bronchiolitis-specific. [241]
- ▸GERD may affect feeding, growth and quality of life in children aged **1–60 months**, while symptom severity correlates poorly with objective reflux severity. [415]
- ▸Early proton-pump-inhibitor exposure during the first **6 months** was studied in relation to respiratory infections through **24 months**; it does not support routine acid suppression for bronchiolitis. [242]
- ▸Severe bronchiolitis and wheezing at admission are associated with greater acute severity and later preschool wheezing or asthma risk, but direct neurodevelopmental effects are not established by the supplied evidence. [351][243]
Acute feeding and nutritional effects
Bronchiolitis commonly presents with coryza and feeding difficulty; respiratory distress, tachypnoea, fatigue and nasal obstruction can reduce oral intake and increase the risk of dehydration, particularly in young infants and those requiring hospital or intensive-care support. [178]A1b The supplied evidence does not provide a bronchiolitis-specific caloric or fluid target, nor does it establish that uncomplicated bronchiolitis causes persistent growth failure. Nutritional assessment should therefore be individualized using hydration status, recent intake, weight trajectory and illness severity rather than a single disease-specific threshold. [178]A1b
For critically ill children receiving enteral nutrition in a PICU, the 2025 systematic review evaluated higher versus lower protein intake while accounting for energy intake. Eligible trials included children hospitalized for at least 48 hours and compared protein intakes of <1.1 g/kg/day or >2 g/kg/day with internationally recommended intakes. [241]A1a This evidence is relevant to infants with life-threatening bronchiolitis who require prolonged intensive care, but it is not bronchiolitis-specific and should not be extrapolated automatically to infants managed on the ward or at home. [241]A1a
Enteral nutrition during severe disease
When oral feeding is inadequate during severe bronchiolitis, enteral nutrition may be considered within the broader PICU nutrition strategy; however, the supplied references do not establish superiority of gastric versus post-pyloric feeding, define an optimal energy target for bronchiolitis, or demonstrate that a high-protein regimen improves growth after discharge. [241]A1a The protein evidence concerns critically ill children as a heterogeneous population, and the abstract supplied does not provide sufficient results to support a universal high-protein prescription. [241]A1a
The BESS phase 2 trial evaluated endotracheal poractant alfa versus a sham procedure in infants with life-threatening bronchiolitis across 15 PICUs. [178]A1b Although this intervention addresses respiratory physiology rather than nutrition, any treatment that changes respiratory support requirements could influence the feasibility of oral or enteral feeding; the supplied abstract does not report post-discharge growth, developmental outcomes, feeding outcomes or nutritional effects. [178]A1b
Infants requiring invasive ventilation may also receive opioids and benzodiazepines, and a retrospective cohort of 407 ventilated children with bronchiolitis examined withdrawal during intravenous opioid weaning using WAT-1 scores. [110]B3b Withdrawal and prolonged sedative exposure can complicate comfort, participation in feeding and recovery, but the supplied study does not establish effects on weight gain, neurodevelopment or long-term feeding ability. [110]B3b
Reflux, feeding symptoms and medication considerations
Gastro-oesophageal reflux disease in children aged 1–60 months is associated with troublesome symptoms and complications affecting growth, feeding and quality of life; symptom-based assessment correlates poorly with objective reflux severity. [415] These findings support distinguishing suspected reflux-related feeding problems from bronchiolitis-associated feeding reduction and avoiding attribution of poor intake to reflux without appropriate clinical assessment. [415]
Early proton-pump-inhibitor exposure is relevant when evaluating recurrent respiratory symptoms or feeding complaints. In a Chilean birth cohort, early exposure was defined as any proton-pump-inhibitor use during the first 6 months, with follow-up through 24 months; the study assessed subsequent respiratory tract infections. [242]B2b This observational evidence does not demonstrate that proton-pump inhibitors treat bronchiolitis, improve feeding, or protect growth, and it should not be used to justify routine acid suppression for bronchiolitis-related feeding difficulty. [242]B2b
Developmental and longer-term respiratory implications
Severe bronchiolitis can mark infants at increased risk of later respiratory morbidity rather than directly causing a defined developmental delay. In a prospective cohort of 668 previously healthy, full-term infants hospitalized with bronchiolitis and followed for 4 years, wheezing at admission was associated with greater illness severity and oxygen use during hospitalization and with increased subsequent preschool wheezing or asthma risk. [351] A separate cohort of children hospitalized with acute respiratory infections found that the timing of early-life respiratory syncytial virus or human rhinovirus infection was examined in relation to later asthma, but the supplied abstract does not provide effect estimates. [243]B3b
Follow-up data after severe RSV bronchiolitis also address longer-term respiratory function: a 2026 observational study evaluated asthma and lung function in children and adolescents aged 5–18 years after hospitalization for RSV-A or RSV-B bronchiolitis. [414] The supplied abstract does not provide comparative results, so it cannot establish a difference between RSV subtypes or quantify developmental impact. [414]
The Australasian systematic review identified risk factors associated with severe bronchiolitis, prolonged hospitalization and intensive-care admission, but the supplied abstract does not report nutrition-specific outcomes. [328] Studies of nasopharyngeal microbiome–epigenome relationships in 504 infants with severe bronchiolitis, immune checkpoint dysregulation in 151 children aged 1 month–2 years, and mortality prediction in severe respiratory disease investigate biological or prognostic mechanisms rather than growth or development. [385][350][132]B2b Similarly, studies of serious bacterial infection in infants younger than 90 days, post-infectious bronchiolitis obliterans and bronchiectasis describe complications or disease burden but do not establish routine nutritional or developmental consequences of acute bronchiolitis. [388][390][391]
Practical interpretation
Monitor feeding tolerance, hydration and serial weight in infants with bronchiolitis, with heightened attention to those hospitalized for at least 48 hours, requiring intensive care, invasive ventilation or prolonged sedation. [178]A1b[110]B3b[241]A1a Persistent poor intake, faltering weight, recurrent choking, suspected reflux disease or ongoing respiratory symptoms after recovery warrants assessment beyond acute bronchiolitis, because the supplied evidence links reflux with feeding and growth concerns and severe bronchiolitis with later respiratory morbidity, while evidence for direct long-term neurodevelopmental harm remains insufficient. [415][351][414]
| Clinical domain | Evidence | Interpretation |
|---|---|---|
| Acute intake | Feeding difficulty occurs among bronchiolitis symptoms. [178]A1b | Assess hydration, intake and weight trajectory. |
| PICU nutrition | Trials included enterally fed children hospitalized ≥48 h and compared <1.1 g/kg/day or >2 g/kg/day with recommended intake. [241]A1a | Use as contextual PICU evidence, not a bronchiolitis-specific prescription. |
| Reflux and growth | GERD in children 1–60 months may affect growth and feeding; symptoms correlate poorly with objective severity. [415] | Investigate persistent or disproportionate feeding problems. |
| Longer-term outcomes | Wheezing at admission predicted greater severity and increased preschool wheezing/asthma risk in a 4-year cohort. [351] | Consider respiratory follow-up when clinically indicated. |
Complications
- ▸The major acute complications are hypoxaemic respiratory distress, hospitalisation, prolonged HFNC requirement, and PICU admission, particularly in young infants and high-risk children.[179][252][348][394]
- ▸Severe bronchiolitis is associated with later recurrent wheezing, asthma-related morbidity, and possible lung-function abnormalities, but the supplied studies do not prove causation for every episode.[385][414][315][243]
- ▸Down syndrome and very preterm birth identify populations with increased respiratory vulnerability, although several available studies evaluated respiratory illness broadly rather than bronchiolitis specifically.[341][313]
- ▸Nirsevimab reduced RSV lower respiratory tract infections by 63% in a meta-analysis of 425,362 infants, supporting prevention of RSV-related bronchiolitis complications.[250]
- ▸Maternal RSV vaccine safety requires attention because an increased risk of preterm birth was observed in another trial of healthy pregnant women.[224]
Overview
Bronchiolitis is usually self-limited, but severe disease can produce clinically important acute and longer-term complications. The principal acute burden is lower respiratory tract illness requiring hospitalisation, with the greatest severity observed in young infants, particularly those aged under 6 months.[252]B2b RSV is a leading cause of bronchiolitis-related hospitalisation in infants, and severe episodes may require high-flow nasal cannula (HFNC) support or paediatric intensive care.[250]A1a[348][394]
Acute respiratory complications
Severe bronchiolitis may progress to hypoxaemic respiratory distress requiring escalating respiratory support. In a randomised trial of infants aged 1–24 months with severe bronchiolitis requiring HFNC, duration of HFNC and hospitalisation were evaluated as clinically important outcomes, reflecting the burden of prolonged respiratory support in this population.[348] A separate randomised trial in infants aged 1 month to 1 year with moderate-to-severe bronchiolitis compared HFNC flow rates of 1, 2, and 3 L/kg/min, demonstrating that flow-rate selection is a relevant management issue in severe disease, although the supplied evidence does not establish a universally optimal rate.[179]A1b
PICU admission is an important marker of severe bronchiolitis. National and regional observational data evaluating RSV prophylaxis specifically tracked RSV-related hospitalisation and PICU admission, indicating that these are central severe-disease outcomes.[394] Children with Down syndrome have increased risks of respiratory-related hospitalisation and intensive care, although the supplied study examined respiratory infections overall rather than bronchiolitis alone.[341] Very preterm infants also have substantial baseline respiratory vulnerability; bronchopulmonary dysplasia and later respiratory morbidity remain relevant comorbid conditions in this group.[313]
Feeding, nutritional, and associated complications
Respiratory illness in infancy can coexist with feeding and growth problems, but the supplied references do not quantify feeding intolerance or dehydration specifically in bronchiolitis. A prospective study of children aged 1–60 months with suspected gastro-oesophageal reflux disease found that reflux-related symptoms and complications may affect feeding, growth, and quality of life; these findings describe an important comorbidity rather than a proven complication caused by bronchiolitis.[415]
Longer-term respiratory sequelae
Severe bronchiolitis has been associated with later recurrent wheezing and asthma-related morbidity. A multicentre cohort of 504 infants under 1 year with severe bronchiolitis investigated relationships among nasopharyngeal microbiota, host DNA methylation, and potential chronic respiratory sequelae, supporting investigation of biological pathways linking severe early-life disease with later respiratory outcomes.[385] Follow-up of children aged 5–18 years who had previously been hospitalised for severe RSV bronchiolitis assessed subsequent asthma, recurrent wheezing, and lung function, including whether outcomes differed between RSV-A and RSV-B; the supplied abstract does not provide the outcome estimates.[414]
Population-based studies also support an association between early viral lower respiratory tract infection and later asthma-related healthcare use. A nationwide cohort of infants hospitalised before 12 months with PCR-confirmed viral lower respiratory tract infection compared RSV with other respiratory viruses and evaluated subsequent asthma-related healthcare utilisation.[315] A separate cohort of 2,628 children with acute respiratory infections examined the timing of early-life rhinovirus and RSV infection in relation to incident asthma.[243]B3b These studies address association and healthcare burden; they do not establish that every bronchiolitis episode causes asthma.
High-risk groups and special complications
Children with Down syndrome may have increased respiratory hospitalisation and intensive-care risk because of co-occurring conditions, although bronchiolitis-specific effects were not isolated in the available study.[341] Very preterm infants may experience respiratory morbidity beyond bronchopulmonary dysplasia; a prospective cohort followed respiratory outcomes to 18 months and compared predominant maternal versus donor human milk exposure, but the supplied abstract does not report the comparative estimates.[313] Alloimmune lung syndromes such as idiopathic pneumonia syndrome and bronchiolitis obliterans syndrome are important complications after allogeneic haematopoietic-cell transplantation, but they are distinct transplant-related conditions and should not be attributed to ordinary bronchiolitis.[320]
Prevention-related implications
Prevention of severe RSV disease can reduce bronchiolitis-related complications. A meta-analysis including 425,362 infants from 26 studies found that a single dose of nirsevimab reduced RSV lower respiratory tract infections by 63%; the intervention was designed to prevent RSV infection and related complications.[250]A1a Population-based surveillance after universal nirsevimab prophylaxis evaluated RSV-LRTI hospitalisation, all-cause LRTI hospitalisation, and acute primary-care outcomes across two RSV seasons, although the supplied abstract does not report the numerical results.[383] Italian national observational data likewise assessed regional prophylaxis implementation and RSV-related hospitalisation and PICU admission during the 2023–2025 seasons.[394]
Maternal RSV vaccination is another prevention strategy, but safety must be considered. In a phase 3 trial among high-risk pregnant women and their infants, enrolment was stopped and the study was unblinded after an increased risk of preterm birth associated with RSVPreF3 vaccination was observed in another trial of healthy pregnant women.[224]A1b This finding concerns vaccine safety and does not represent a complication of bronchiolitis itself.
Prognostic assessment
Clinical severity assessment remains central to identifying infants at risk of complications. A prospective study evaluated adrenomedullin as a possible marker of bronchiolitis severity and prognosis in hospitalised children under 2 years, alongside the BROSJOD clinical scale; the supplied abstract does not provide diagnostic-performance estimates.[417]C Current evidence therefore supports monitoring respiratory severity, oxygenation, feeding, and need for escalating support, while recognising that biomarker-based prognostication remains investigational.[417]C[348]
| Domain | Clinical manifestation or outcome | Evidence |
|---|---|---|
| Acute severe disease | Hospitalisation, HFNC requirement, prolonged respiratory support, and PICU admission | [179]A1b[348][394] |
| Longer-term respiratory morbidity | Recurrent wheeze, asthma-related utilisation, and lung-function assessment after severe viral LRTI or RSV bronchiolitis | [243]B3b[315][385][414] |
| High-risk populations | Increased respiratory-care burden in Down syndrome and very preterm infants | [313][341] |
| Prevention | Reduced RSV-LRTI incidence after single-dose nirsevimab | [250]A1a |
| Prognosis | Investigational use of adrenomedullin with clinical severity scoring | [417]C |
Prognosis & Natural History
- ▸The supplied references do not quantify the complete untreated natural history, typical peak severity, or symptom-resolution timeline of bronchiolitis. [339][348][357][358]
- ▸Escalation to noninvasive or invasive ventilation is a principal acute prognostic outcome in severe bronchiolitis. [339][397]
- ▸Randomized evidence supplied here evaluates prone positioning, HFNC weaning, and **88% versus 92% SpO₂** oxygen thresholds, but the abstracts do not provide their outcome estimates. [339][348][364]
- ▸Lung ultrasound regional abnormalities were studied as potential markers of ventilatory-support need, whereas routine chest radiography was reported not to improve critical-bronchiolitis outcomes. [357][358]
- ▸Hospitalized early-life respiratory viral infection and bronchiolitis have been studied in relation to later asthma and mental-health outcomes, but the supplied abstracts do not provide sufficient estimates to quantify individual risk. [243][323]
Overall prognosis
The supplied evidence base focuses primarily on infants and young children with moderate-to-severe or critical bronchiolitis requiring hospital-level respiratory support; it does not provide a complete description of the untreated illness trajectory, expected day of peak severity, or time to symptom resolution. Prognosis is therefore best understood in terms of respiratory-support escalation, duration of oxygen or high-flow nasal cannula (HFNC) therapy, intensive-care utilization, postdischarge healthcare use, and possible later respiratory or neurodevelopmental associations. [339][348][357][358]
Acute respiratory deterioration and ventilatory support
Infants aged 6 months or younger with moderate-to-severe bronchiolitis requiring HFNC were enrolled in the multicentre PROPOSITIS randomized trial to determine whether prone positioning reduced escalation to noninvasive or invasive ventilation. The supplied abstract identifies this as the primary clinical question but does not report the trial’s outcome data; consequently, prone positioning cannot be described as prognostically beneficial or harmful from the provided evidence. [339]
A 2026 systematic review and meta-analysis evaluated HFNC against continuous positive airway pressure (CPAP) and conventional oxygen therapy in children with acute respiratory failure caused by bronchiolitis, pneumonia, or acute severe asthma. Its prespecified outcomes included treatment failure and safety, but the supplied abstract does not provide pooled estimates; therefore, it does not establish the magnitude of any prognostic advantage of HFNC for bronchiolitis. [397]
A nurse-driven HFNC-weaning randomized trial enrolled infants aged 1–24 months with severe bronchiolitis in two tertiary pediatric intensive-care units. The protocol used structured Wang Bronchiolitis Severity Scores and the ROX index, defined as SpO₂/FiO₂ divided by respiratory rate, to guide weaning; the primary outcome was total HFNC duration, with hospitalization and time to first wean among secondary outcomes. The supplied abstract does not report results, so the effect of protocolized weaning on recovery time or length of stay remains undetermined here. [348]
The OxyKids randomized trial compared oxygen-treatment thresholds of 88% versus 92% SpO₂ in children aged 6 weeks–12 years requiring oxygen for bronchiolitis, lower respiratory tract infection, or acute viral-induced wheeze. Oxygen could be delivered by low-flow or high-flow nasal cannula or face mask, and length of hospital stay was the principal clinical outcome. The supplied abstract does not state whether the lower threshold was noninferior or reduced hospitalization; it therefore cannot support a definitive prognostic threshold recommendation from these data alone. [364]
Markers of severe disease and intensive-care course
In a prospective study of 160 infants with bronchiolitis, lung ultrasound was performed within 12 hours of admission using regional Brat scoring to examine the distribution of pulmonary abnormalities and their relationship to the need for ventilatory support. The supplied abstract confirms that the study compared favorable-outcome and more severe clinical groups but does not provide the regional scores, effect estimates, or predictive performance. [358]
Routine chest radiography was examined retrospectively in children younger than 3 years admitted to pediatric intensive-care, step-down, or cardiac intensive-care settings for critical bronchiolitis. The study compared patients who underwent chest radiography with those who did not, assessing intensive-care length of stay and level of respiratory support; its title reports that routine radiography did not improve outcomes. [357]
Among mechanically ventilated children with bronchiolitis, a retrospective cohort study of 407 patients evaluated iatrogenic withdrawal during intravenous opioid weaning using the Withdrawal Assessment Tool-1. Withdrawal was defined as a score of ≥3; the study examined dose reductions and clinical characteristics, but the supplied abstract does not report the quantitative dose–withdrawal associations. [110]B3b
Longer-term outcomes
A retrospective cohort of 2,628 children hospitalized with acute respiratory infections and tested for respiratory viruses assessed subsequent asthma after early-life human rhinovirus or respiratory syncytial virus infection. The supplied abstract establishes the study’s objective and population but does not provide virus-specific hazard ratios or absolute asthma risks; therefore, it supports concern about possible later asthma associations without quantifying individual prognosis. [243]B3b
A South Korean national cohort included 985,957 children born in 2002–2003, including 25,550 hospitalized for acute bronchiolitis and 102,220 matched unexposed children, followed through December 2021 for 24 predefined mental-health disorders. The supplied abstract indicates that both early- and late-onset outcomes were evaluated but does not report adjusted hazard ratios; no causal mental-health effect should therefore be inferred from the information provided. [323]
Postdischarge burden and family communication
A single-center pediatric study introduced the 30-day ED/READ ratio—the ratio of postdischarge emergency-department visits to readmissions—to characterize diagnosis-specific hospital reuse. Although bronchiolitis was among the diagnoses considered, the supplied abstract does not provide bronchiolitis-specific ratios, so the magnitude of postdischarge reuse cannot be stated. [342]
A Canadian randomized study tested weekly image-based “blogshots” addressing common pediatric conditions, including bronchiolitis, in parents or guardians of children younger than 5 years. The intervention was designed to improve knowledge and manage expectations; the supplied abstract does not report bronchiolitis-specific knowledge or outcome changes. [95]A1b
Prognostic interpretation
Current evidence supplied here supports assessing prognosis dynamically according to respiratory distress, oxygen requirement, need for HFNC or escalation, and response to weaning, while recognizing that several contemporary randomized studies have not supplied outcome data in their abstracts. [339][348][364][397] Long-term associations with asthma and mental-health outcomes remain observational and incompletely quantified in the available summaries. [243]B3b[323]
| Domain | Population or comparison | Outcome addressed | Interpretation from supplied abstract |
|---|---|---|---|
| Ventilatory escalation | Infants ≤6 months on HFNC; prone versus usual positioning | Escalation to noninvasive or invasive ventilation | Trial question reported; results not supplied. [339] |
| Oxygen strategy | Children 6 weeks–12 years; 88% versus 92% SpO₂ thresholds | Length of hospital stay and safety | Trial design reported; results not supplied. [364] |
| HFNC recovery | Infants 1–24 months with severe bronchiolitis | HFNC duration and hospitalization | Protocol and outcomes specified; results not supplied. [348] |
| Disease severity assessment | 160 infants; regional lung-ultrasound scoring | Need for ventilatory support | Association was investigated; effect estimates not supplied. [358] |
| Long-term association | Early-life HRV or RSV infection | Subsequent asthma | Cohort study reported; virus-specific risks not supplied. [243]B3b |
| Long-term association | 25,550 children hospitalized with bronchiolitis versus matched controls | Mental-health disorders through 2021 | Outcomes were assessed; hazard ratios not supplied. [323] |
Special Populations
- ▸Infants aged ≤6 months with moderate-to-severe bronchiolitis requiring HFNC were the target population for a randomized prone-positioning trial, but outcome data are not supplied. [339]
- ▸Evidence addressing oxygen thresholds compares 88% with 92% in children aged 6 weeks–12 years, but the supplied abstract does not report results. [364]
- ▸Children with Down syndrome have increased respiratory hospitalization and intensive-care risk; specific comorbidity predictors are not available in the supplied abstract. [341]
- ▸Hypertonic saline did not reduce hospital length of stay according to the randomized-trial title, despite enrollment of children aged 5 weeks–24 months. [416]
- ▸Biomarkers, echocardiography, cytokine profiling, and septic-shock prediction models remain investigational in the cited observational studies. [417][399][312][68]
Young infants and age-related risk
Bronchiolitis predominantly affects infants, and the most recent interventional evidence includes children from 5 weeks to 24 months with mild-to-moderate hospitalized disease and infants aged 6 months or younger with moderate-to-severe disease requiring high-flow nasal cannula (HFNC). [416][339] Infants aged 1–24 months with severe bronchiolitis have also been studied in trials of HFNC weaning protocols. [348] Age should therefore be considered alongside respiratory distress, oxygen requirement, feeding ability, and comorbidity when determining observation intensity and escalation needs. [339][348]
Severe disease and respiratory support
For infants with moderate-to-severe bronchiolitis already receiving HFNC, the multicentre PROPOSITIS randomized trial evaluated whether prone positioning reduced escalation to noninvasive or invasive ventilation; it enrolled infants aged ≤6 months admitted for 24 hours or less in 15 French intermediate or intensive care units. [339] The available reference information describes the trial design and objective but does not provide its outcome data; prone positioning should therefore not be presented as proven to prevent ventilation escalation on the basis of this citation alone. [339]
A randomized trial in infants aged 1–24 months with severe bronchiolitis evaluated a nurse-driven HFNC weaning protocol based on structured Wang Bronchiolitis Severity Score assessments and the ROX index, defined as oxygen saturation divided by inspired oxygen fraction and respiratory rate. [348] The protocol was designed to guide initiation of weaning using predefined criteria and to assess HFNC duration, time to first wean, and hospitalization outcomes; the supplied abstract does not report the comparative results. [348]
A retrospective cohort of children younger than 24 months who received HFNC at ≥0.5 L/kg/min on a pediatric ward examined factors associated with PICU transfer and escalation beyond HFNC. [400] Because the supplied reference does not include the identified predictors or effect estimates, it supports risk stratification as an important clinical question but does not establish specific transfer thresholds. [400] A separate Italian quality-improvement project addressed HFNC initiation, flow titration, escalation, and PICU-alert processes for patients younger than 24 months treated outside the PICU. [349]C
Oxygen thresholds
The OxyKids randomized trial enrolled children aged 6 weeks–12 years requiring oxygen for bronchiolitis, lower respiratory tract infection, or acute viral-induced wheeze and compared an oxygen-treatment threshold of 88% with 92% using low-flow or high-flow nasal cannula or face-mask oxygen. [364] The study was specifically designed to determine whether the lower threshold could safely reduce hospital length of stay; the supplied abstract does not provide outcome results, so these thresholds should not be interpreted as definitive evidence of equivalence. [364]
Hospitalized infants and supportive treatment
A double-blind multicentre trial randomized 181 hospitalized children aged 5 weeks–24 months with mild-to-moderate bronchiolitis to nebulized 3% hypertonic saline or 0.9% normal saline until discharge. [416] The groups included 90 children assigned to hypertonic saline and 91 to normal saline, with a mean age of 8.3 months; the reported median hospital stay for the study population was 2.68 days (interquartile range 1.92–4.03 days). [416] The reference title states that hypertonic saline did not reduce length of stay, supporting continued caution about routine use for this outcome. [416]
Children with Down syndrome and other comorbidities
Children with Down syndrome have increased risk of hospitalization and intensive care for respiratory illness; a retrospective cohort of 2,327 children with Down syndrome, followed in a large specialist clinic from 2011–2023, examined which co-occurring diagnoses were associated with respiratory-related hospitalization and intensive-care needs. [341] The supplied abstract does not identify the individual diagnoses or quantify their risks, so children with Down syndrome should be managed as a higher-risk population without relying on a specific comorbidity-based prediction from this citation. [341]
Gastro-oesophageal reflux disease (GERD) may coexist with feeding, growth, or respiratory concerns in young children, but symptom-based diagnosis correlates poorly with objective reflux severity. [415] A prospective study of children aged 1–60 months assessed clinical features, radionuclide reflux scintigraphy, nutritional status, and confirmed comorbidities; its available abstract does not provide bronchiolitis-specific treatment recommendations. [415] Early proton-pump-inhibitor exposure should not be assumed to prevent respiratory disease: in a birth cohort followed to 24 months, the study examined associations between PPI exposure during the first 6 months or first 24 months and respiratory tract infections, but the supplied abstract does not report the effect estimates. [242]B2b
Prognosis, biomarkers, and follow-up
Several observational studies explored markers of severe or prolonged disease but do not establish routine testing. A single-centre cohort of hospitalized children younger than 2 years evaluated mid-regional pro-adrenomedullin alongside the BROSJOD severity scale as a possible predictor of severity and prognosis. [417]C A pilot study of 46 PICU patients younger than 12 months assessed right-ventricular systolic function using TAPSE echocardiography at admission in relation to length of stay and respiratory-support duration. [399]C A prospective PICU cohort measured nasal cytokines in 42 children with critical bronchiolitis and nine healthy controls, examining differences among RSV, rhinovirus, and polyviral infection and associations with clinical risk factors. [312]C A retrospective model in 224 PICU patients with RSV bronchiolitis used routine blood biomarkers and concomitant fungal infection to predict septic shock. [68]B2b These approaches remain investigational based on the observational designs and the information supplied. [417]C[399]C[312]C[68]B2b
Children with previous severe RSV bronchiolitis were followed at ages 5–18 years in a prospective cross-sectional study comparing RSV-A and RSV-B with respect to recurrent wheeze, asthma, and lung function; the available abstract does not provide comparative results. [414]
Prevention, communication, and health-system context
Nirsevimab introduction was evaluated in a single-centre Italian cohort of children younger than 24 months across the 2023–2024 and 2024–2025 RSV seasons, with bronchiolitis emergency-department presentations as the epidemiological outcome. [55]B2b The supplied abstract does not report the admission estimates, so the study supports assessment of population impact rather than a numerical effect size here. [55]B2b In Canada, a randomized trial among parents or guardians of children younger than 5 years evaluated weekly image-based “blogshots” covering Choosing Wisely recommendations, including bronchiolitis, over three weeks; the intervention targeted parental knowledge and expectations rather than infant clinical outcomes. [95]A1b
Environmental and epidemiological context may modify respiratory disease burden. A Singapore birth cohort of 1,124 mother-child pairs assessed meteorological conditions, ozone, maternal factors, and individual-level exposures in relation to first bronchiolitis through age 8 years. [54]B2b A Spanish regional analysis of 71,918 pediatric respiratory-infection consultations from 2017–2024 found that hospital-attending children were younger on average than primary-care attendees, although the supplied abstract does not provide bronchiolitis-specific estimates. [316] In community emergency departments, a retrospective study of children aged 28 days–2 years without comorbidities examined guideline-discordant testing and treatment, including bronchodilators and chest radiography, among children discharged home. [420] Lung ultrasound was prospectively compared with chest radiography in 338 children aged 1 month–18 years with suspected lower respiratory tract infection, but the study was not limited to bronchiolitis. [340]
| Population or issue | Evidence | Clinical interpretation |
|---|---|---|
| Infants ≤6 months on HFNC | Prone-positioning RCT designed to assess ventilation escalation | Do not infer benefit because outcome data are not supplied. [339] |
| Children 6 weeks–12 years requiring oxygen | RCT comparing 88% versus 92% thresholds | Await reported outcomes before changing local thresholds. [364] |
| Children 5 weeks–24 months hospitalized | Hypertonic saline versus normal saline RCT | Trial title reports no reduction in length of stay. [416] |
| Children with Down syndrome | Cohort of 2,327 children | Treat as higher risk; individual predictors are not reported here. [341] |
| PICU patients with critical bronchiolitis | Biomarker, cytokine, echocardiographic, and prediction-model studies | These tests are not established routine care from the cited evidence. [417]C[399]C[312]C[68]B2b |
Prevention, Screening & Surveillance
- ▸Prioritise seasonal RSV prevention, high-risk infant identification, and measurement of prophylaxis coverage and breakthrough disease. [421][394]
- ▸Nirsevimab effectiveness is supported by multiple observational studies, but regional implementation and eligibility strongly influence outcomes. [55][321][355][406]
- ▸Maternal RSV vaccination offers passive infant protection but requires careful safety and regulatory assessment because of the preterm-birth signal described in the RSVPreF3-Mat evidence. [224]
- ▸Surveillance should include RSV, HMPV, adenovirus, PICU/ICU outcomes, prolonged hospitalisation, mortality, and longer-term sequelae. [51][52][57][284][318][323]
- ▸The OxyKids trial evaluated **88%** versus **92%** oxygen thresholds and provides direct evidence for monitoring oxygen-policy safety and hospital utilisation. [364]
Prevention priorities
Respiratory syncytial virus (RSV) is a major cause of bronchiolitis and lower-respiratory-tract disease in infancy, with the greatest risk of severe disease in young infants and children with prematurity, chronic lung disease, congenital heart disease, Down syndrome, immunocompromise, or other comorbidities. [421][396][57]B2b RSV-related disease is seasonal, although timing varies geographically; Mexican data describe a predominant September–March season. [421] Prevention should therefore combine seasonal planning, identification of high-risk infants, and timely passive or maternal immunisation where locally recommended. [421]
Nirsevimab is a long-acting monoclonal antibody used for RSV prophylaxis in infants and selected high-risk children younger than 24 months. [55]B2b Observational evidence after its introduction is consistently supportive but heterogeneous: Spanish implementation was associated with a reported 60%–80% reduction in bronchiolitis hospital admissions, while post-prophylaxis admissions were enriched by infants ineligible for immunisation, including those older than 6 months without comorbidities. [406] Italian studies reported reductions in bronchiolitis burden after implementation, including emergency-department admissions, primary-care diagnoses, and hospitalisations, but the magnitude varied according to regional delivery strategy and coverage. [55]B2b[321][355][324] A national Italian multicentre evaluation found substantial regional heterogeneity in rollout timing, organisational models, logistics, hospitalisation outcomes, and paediatric intensive-care-unit (PICU) admissions. [394] Methodologically standardised regional estimates are important because RSV admissions are concentrated in infancy and follow a marked seasonal pattern. [405]
Incomplete implementation can undermine population benefit. In an Italian pre-implementation cohort of 780 newborns, 9.2% developed bronchiolitis, 5.8% had RSV-positive bronchiolitis, 5.6% were hospitalised, and 0.9% required PICU admission; among hospitalised infants, 70% had RSV infection. [403] These data support high coverage, coordinated delivery, and monitoring of eligibility, uptake, breakthrough disease, hospitalisation, and PICU admission rather than relying solely on overall bronchiolitis counts. [403][394] Nirsevimab studies remain primarily observational and may be affected by regional policy, season-to-season variation, and differences in case ascertainment. [55]B2b[321][355][394]
Maternal RSV vaccination is another prevention strategy because transplacental antibody transfer can protect infants. [224]A1b[421] However, the phase 3 RSVPreF3-Mat trial in high-risk pregnant women was stopped and unblinded after an increased risk of preterm birth associated with the vaccine was observed in another trial in healthy pregnant women. [224]A1b Maternal vaccination should therefore be assessed using current regulatory and national recommendations, with explicit consideration of gestational age, maternal risk, and the availability of infant monoclonal-antibody prophylaxis. [224]A1b[421]
Risk assessment and screening
At every seasonal contact, clinicians should identify infants and children with factors associated with severe RSV outcomes, including young age, prematurity, chronic lung disease of prematurity, major congenital heart disease, Down syndrome, and other relevant comorbidities. [396][57]B2b In children aged 12–23 months hospitalised with RSV, these risk factors were specifically evaluated against prolonged hospitalisation and PICU admission. [396] A nationwide Swedish cohort additionally assessed RSV-associated death, ICU admission, and prolonged hospitalisation of ≥7 days, supporting surveillance of both mortality and resource-intensive outcomes rather than hospitalisation alone. [57]B2b
Routine pathogen testing should be targeted to situations in which the result changes isolation, cohorting, antimicrobial stewardship, outbreak investigation, or epidemiological surveillance; the supplied evidence does not establish universal viral testing for every clinically diagnosed case of bronchiolitis. RSV remains the dominant pathogen, but global meta-analytic data include 539 studies and 584 datasets of paediatric respiratory infections, demonstrating the broad and variable burden of laboratory-confirmed RSV. [51]B2a Human metapneumovirus (HMPV) is also relevant: a Chinese meta-analysis of 78 studies including 334,901 children found an overall detection rate of 4.56% and the highest rate in 1-year-olds, 6.02%. [52]B2a Testing strategies should therefore reflect local epidemiology and outbreak concerns rather than assume that all bronchiolitis is RSV.
Surveillance and service monitoring
Surveillance systems should track age, comorbidity, immunisation or prophylaxis status, viral aetiology, oxygen requirement, hospital length of stay, ICU/PICU admission, and death. RSV PICU disease has a substantial clinical and family burden; the prospective Dutch BRICK study was designed to capture the full burden of laboratory-confirmed RSV-related PICU admission in infants, including adverse admission events and parental post-traumatic-stress symptoms. [284]B2b Longer-term follow-up is also relevant: a large South Korean national cohort examined mental-health outcomes after hospitalisation for bronchiolitis, indicating that surveillance may need to extend beyond the acute respiratory episode. [323]
Surveillance should include non-RSV viruses and unusual clusters. Adenovirus-associated severe respiratory infection surveillance identified outbreaks in Brazil and Colombia during late 2022, with clinical and genomic characterisation of circulating strains. [318] Such systems can detect changing viral dynamics, regional outbreaks, and shifts in disease severity. HMPV surveillance is similarly justified by its measurable paediatric burden and age-specific variation. [52]B2a
For hospital oxygen policies, SpO2 thresholds should be monitored as a safety and utilisation outcome. The OxyKids randomised trial compared an 88% versus 92% oxygen threshold in children aged 6 weeks–12 years requiring oxygen for bronchiolitis, lower-respiratory-tract infection, or viral-induced wheeze. [364] The trial directly addresses whether a lower threshold can reduce hospital stay without compromising safety; local protocols should interpret its results alongside age, comorbidity, measurement quality, respiratory support, and clinical trajectory. [364]
| Domain | Indicators to monitor | Evidence |
|---|---|---|
| RSV prophylaxis | Eligibility, uptake, timing, breakthrough bronchiolitis, hospitalisation, PICU admission, regional variation | [55]B2b[321][394] |
| Maternal prevention | Gestational timing, maternal risk, infant passive protection, safety outcomes including preterm birth | [224]A1b[421] |
| Severe disease | Prematurity, chronic lung disease, congenital heart disease, Down syndrome, ICU/PICU admission, death, hospitalisation ≥7 days | [396][57]B2b |
| Viral surveillance | RSV prevalence, HMPV detection, adenovirus clusters, genomic variation | [51]B2a[52]B2a[318] |
| Service outcomes | Oxygen threshold, respiratory support, length of stay, family and longer-term outcomes | [364][284]B2b[323] |
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