Viral lower respiratory tract infection of infants causing wheeze, hypoxia, and dehydration.
Also known as: bronchiolitis, RSV, respiratory syncytial virus, viral bronchiolitis
Overview
Acute viral lower respiratory tract infection most common in children under 2 years, characterized by inflammation of the bronchioles producing cough, wheeze, tachypnea, and increased work of breathing. RSV causes the majority of cases.
Epidemiology
Most common lower respiratory infection in infants worldwide. Peak incidence in winter months. >2 million US outpatient visits and ~58,000-80,000 hospitalizations annually among children <5 years. Leading cause of infant hospitalization in the US.
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Question 1PulmonaryMedium
A 7-month-old presents in January with 3 days of rhinorrhea and cough, now with audible wheezing and poor feeding. Temp 38.1 C, RR 58, SpO2 94% on room air. Exam shows nasal flaring, subcostal retractions, and diffuse expiratory wheezes with scattered crackles. The infant attends daycare and has no prior respiratory episodes. Which of the following is the most likely diagnosis?
APertussis infection
BAcute bronchiolitis
CFirst episode of viral-triggered asthma
DCommunity-acquired pneumonia
EAcute laryngotracheobronchitis
Reveal answer & full explanation
Correct answer: B — Acute bronchiolitis
APertussis infection
BAcute bronchiolitis✓
CFirst episode of viral-triggered asthma
DCommunity-acquired pneumonia
EAcute laryngotracheobronchitis
Why acute bronchiolitis is correct
Classic acute bronchiolitis presents in an infant under 24 months in winter with a viral prodrome followed by lower respiratory signs
RSV infects bronchiolar epithelium, producing edema, sloughed cells, and mucus plugging of small airways, which manifests as wheezing, crackles, tachypnea, and increased work of breathing
The age (7 months), winter seasonality (January), daycare exposure, preceding URI, and combination of diffuse expiratory wheezes plus crackles make bronchiolitis the best fit
Diagnosis is clinical without need for routine imaging or viral testing
Why the others are wrong
Community-acquired pneumonia — Community-acquired pneumonia typically presents with focal crackles, higher fever, and a lobar infiltrate rather than diffuse wheezing in an infant with a URI prodrome
First episode of viral-triggered asthma — Viral-triggered asthma is not diagnosed on a first wheezing episode in an infant this young; recurrent wheezing episodes outside the bronchiolitis age window would be required
Acute laryngotracheobronchitis — Croup (laryngotracheobronchitis) causes inspiratory stridor and a barky cough from subglottic edema, not diffuse expiratory wheezing and crackles
Pertussis infection — a coryzal prodrome and daycare exposure fit pertussis too, but it causes paroxysmal coughing spells with post-tussive emesis or apnea, usually with little or no fever and a clear chest between paroxysms, not diffuse expiratory wheezes and crackles with retractions.
Question 2PulmonaryEasy
A 4-month-old infant in January has cough, rhinorrhea, diffuse wheezing, and mild retractions. Oxygen saturation is 96% and the infant is feeding adequately. Which of the following is the most appropriate initial management?
AOral prednisolone for five days
BNebulized racemic epinephrine
CScheduled albuterol nebulizer therapy
DSupportive care with nasal suctioning
EOral amoxicillin for bacterial coverage
Reveal answer & full explanation
Correct answer: D — Supportive care with nasal suctioning
AOral prednisolone for five days
BNebulized racemic epinephrine
CScheduled albuterol nebulizer therapy
DSupportive care with nasal suctioning✓
EOral amoxicillin for bacterial coverage
Why Supportive care with nasal suctioning is correct
Mild bronchiolitis is managed with supportive care: hydration, nasal suctioning, and monitoring.
A well-feeding infant with 96% saturation needs no pharmacologic therapy and can be managed at home.
Bronchodilators, antibiotics, and corticosteroids do not improve outcomes in routine bronchiolitis.
Why the others are wrong
Scheduled albuterol nebulizer therapy — Asthma-transfer trap: bronchodilators are not recommended in bronchiolitis because the wheeze comes from edema, sloughed epithelium, and mucus plugging rather than reversible bronchospasm, and trials show no benefit.
Oral amoxicillin for bacterial coverage — Reflex-antibiotic trap: bronchiolitis is viral, and antibiotics are reserved for a documented bacterial coinfection.
Nebulized racemic epinephrine — Wrong-disease trap: racemic epinephrine treats croup with stridor, not bronchiolitic wheezing with adequate saturation.
Oral prednisolone for five days — anti-inflammatory trap: diffuse wheezing invites an asthma-style steroid burst, but AAP guidance advises against systemic corticosteroids in bronchiolitis in any setting because they have not been shown to improve outcomes.
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Prematurity (<29 weeks especially) — among the strongest host risk factors for severe disease, along with age <12 weeks, BPD, and hemodynamically significant congenital heart disease
Congenital heart disease with heart failure — Failure to thrive, murmur, hepatomegaly, cardiomegaly on CXR
Anaphylaxis — Acute onset after exposure, urticaria, hypotension
Pertussis — Paroxysmal cough with whoop, post-tussive emesis, age extremes; PCR positive
Cystic fibrosis — Failure to thrive, recurrent infections, salty skin, family history; sweat chloride
Croup (laryngotracheobronchitis) — 6 months-3 years, barky cough and inspiratory stridor from subglottic edema, steeple sign on neck film; treated with dexamethasone ± racemic epinephrine, unlike bronchiolitis
Diagnostic workup
Labs
Clinical diagnosis — routine viral testing NOT recommended in typical cases (does not change management)
RSV testing may help with cohorting or public health surveillance
Pulse oximetry to assess hypoxia
Consider CBC, BMP, and blood cultures only if febrile neonate (<28 days), toxic appearance, or sepsis concern (very rare bacterial coinfection)
Imaging
Chest radiograph NOT routinely recommended (AAP 2014) — risk of overdiagnosis of pneumonia and unnecessary antibiotics
Obtain CXR if focal exam findings, severe disease, fails to improve, or alternative diagnosis suspected — findings: hyperinflation, peribronchial thickening, atelectasis
Diagnostic algorithm
Intervention
AAP Recommendation
Hydration and nasal suctioning
Supportive — recommended
Supplemental O2 for SpO2 <90%
Recommended
Routine bronchodilator (albuterol)
Do not use
Systemic or inhaled corticosteroids
Do not use
Antibiotics
Do not use unless bacterial coinfection
Chest physiotherapy
Do not use
Routine viral testing or CXR
Do not use in typical cases
Nirsevimab or clesrovimab (prevention)
Recommended for infants <8 months entering first RSV season unless mother received RSVpreF vaccine ≥14 days before delivery
AAP 2014 management recommendations for bronchiolitis; prevention row per AAP RSV prevention guidance (2025 policy statement; FAQ updated October 16, 2026).
Treatment
First-line
Supportive care — cornerstone:
Suction nasal secretions (gentle bulb suction before feeds; deep suction NOT recommended)
Hydration: oral/NG feeds if tolerated; IV fluids if severe respiratory distress or unable to feed
Bronchodilators (albuterol, racemic epinephrine) — no proven benefit; AAP recommends against routine use
Systemic or inhaled corticosteroids — no benefit in viral bronchiolitis
Antibiotics — only if bacterial coinfection (very rare) or alternative diagnosis
Chest physiotherapy
Hypertonic saline (mixed data, modest if any benefit; not routine)
Second-line / adjunct
High-flow nasal cannula (HFNC) for moderate-severe hypoxia or respiratory distress
Nasal CPAP or BiPAP for refractory respiratory distress
Intubation/mechanical ventilation for impending respiratory failure
Admission criteria: SpO2 <90% on room air, apnea, severe respiratory distress, dehydration, inability to feed, age <3 months with significant illness, social concerns
Prevention:
Maternal RSVpreF vaccine (Abrysvo) at 32-36 weeks gestation (September-January); most infants need either the maternal vaccine or an infant monoclonal antibody, not both
Nirsevimab (Beyfortus) — single-dose long-acting monoclonal antibody for infants <8 months entering their first RSV season if the mother did not receive RSVpreF vaccine, her status is unknown, or the infant was born <14 days after maternal vaccination; also for high-risk children 8-19 months entering their second season
Clesrovimab (Enflonsia) — single 105 mg IM dose regardless of weight; AAP recommends it or nirsevimab for eligible infants <8 months in their first RSV season (same maternal-vaccination criteria as nirsevimab), and high-risk children entering a second season receive nirsevimab, not clesrovimab; palivizumab was discontinued in the US as of December 31, 2025
Hand hygiene, breastfeeding, avoiding tobacco smoke and crowded settings
Post-bronchiolitis wheeze and increased asthma risk later in childhood
Severe RSV in immunocompromised, congenital heart disease, BPD
PANCE pearls
Bronchiolitis is a clinical diagnosis — routine RSV PCR, CXR, and labs are NOT recommended in typical cases (AAP 2014).
Bronchodilators, steroids, and antibiotics have no proven benefit in viral bronchiolitis and should be AVOIDED.
Long-acting monoclonal antibodies (nirsevimab, clesrovimab) have dramatically changed prevention — a single intramuscular dose covers an entire RSV season for any eligible infant <8 months (not just high-risk) unless the mother received RSVpreF vaccine at least 14 days before delivery; AAP recommends either nirsevimab or clesrovimab.
Apnea is a common and worrying complication in young infants (<2 months) and preterm infants; low threshold for admission.
Wheeze without prior episodes in a child <2 during RSV season = bronchiolitis; recurrent episodes after age 2 suggest asthma.
References
AAP 2014 — Clinical Practice Guideline: The Diagnosis, Management, and Prevention of Bronchiolitis (Ralston et al., Pediatrics 2014)
CDC/ACIP 2023 — Use of Nirsevimab for the Prevention of RSV Disease Among Infants and Young Children: Recommendations of ACIP — United States, 2023 (Jones et al., MMWR 2023)
MELODY Trial — Nirsevimab for Prevention of RSV in Healthy Late-Preterm and Term Infants (Hammitt et al., NEJM 2022)
MATISSE Trial — Bivalent Prefusion F Vaccine in Pregnancy to Prevent RSV Illness in Infants (Kampmann et al., NEJM 2023)
CDC/ACIP 2025 — Use of Clesrovimab for Prevention of Severe Respiratory Syncytial Virus-Associated Lower Respiratory Tract Infections in Infants: Recommendations of the Advisory Committee on Immunization Practices — United States, 2025 (Moulia et al., MMWR 2025;74:508-514; the June 2025 vote's legal status is uncertain after the March 16, 2026 preliminary court stay)
AAP 2025 — Recommendations for the Prevention of RSV Disease in Infants and Children: Policy Statement (AAP Committee on Infectious Diseases, Pediatrics 2025;156(5):e2025073923)
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