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This is a clinician-written, evidence-based summary aligned to the USMLE Step 2 CK Content Outline. It is intended for medical students preparing for USMLE Step 2 CK. Management reflects current ACC/AHA, USPSTF, and APA guidelines. Always cross-reference with UpToDate, institutional protocols, and clinical judgment.
The Bottom Line
- Bronchiolitis affects infants and young children, usually from RSV, with URI prodrome followed by cough, wheeze, crackles, tachypnea, and feeding difficulty
- Diagnosis is clinical; routine chest X-ray, viral testing, bronchodilators, corticosteroids, and antibiotics are not recommended in typical cases
- Treatment is supportive: nasal suction, hydration, oxygen if hypoxemic, and respiratory support when needed
- Highest-risk infants include age <12 weeks, prematurity, chronic lung disease, hemodynamically significant congenital heart disease, and immunocompromise
- Apnea may be the presenting feature in very young infants
Overview
Bronchiolitis is inflammation and obstruction of small airways, most commonly caused by RSV. It is a clinical diagnosis based on age, season, URI prodrome, and lower respiratory signs. The AAP guideline emphasizes avoiding low-value interventions and focusing on hydration, oxygenation, and work of breathing. It is distinct from asthma: first-time wheeze in an infant during viral season is bronchiolitis until proven otherwise.
Epidemiology
Bronchiolitis is a leading cause of infant hospitalization in the United States, with seasonal peaks in fall and winter. Nearly all children are infected with RSV by early childhood, but severe disease is concentrated in young infants, preterm infants, and those with cardiopulmonary or immune risk factors.
Clinical Features
Symptoms
Rhinorrhea, congestion, and cough preceding lower respiratory symptoms
Wheeze, tachypnea, increased work of breathing
Poor feeding, vomiting after cough, or decreased wet diapers
Apnea, especially in young or premature infants
Fever may be present but high fever or toxic appearance suggests alternative diagnosis
Signs
Diffuse wheeze and crackles
Nasal flaring, grunting, retractions, tachypnea
Hypoxemia or cyanosis
Dehydration signs: dry mucosa, poor tears, decreased urine output
Focal lung findings or severe toxicity suggesting pneumonia/sepsis
Investigations
First-line
Clinical diagnosisTypical age, season, URI prodrome, wheeze/crackles, and work of breathing are sufficient
Pulse oximetryAssess oxygenation in moderate/severe disease; continuous monitoring is not required for all improving infants
Hydration assessmentFeeding volume, urine output, weight, mucous membranes, and ability to maintain intake
Second-line
Chest X-rayNot routine; consider if severe disease, focal findings, ICU-level illness, or diagnostic uncertainty
Viral testingNot routinely needed; may help cohorting or infection control in hospitalized patients
Sepsis/UTI evaluationConsider in febrile young infants, ill appearance, or age/risk factors requiring fever workup
Specialist
Hospital admission/PICUHypoxemia, apnea, dehydration, severe work of breathing, exhaustion, or high-risk infant with worsening course
Respiratory support teamHigh-flow nasal cannula, CPAP, or ventilation for escalating respiratory distress
Management
AAP Clinical Practice Guideline for the Diagnosis, Management, and Prevention of Bronchiolitis1
Supportive care
- Nasal saline and gentle suctioning, especially before feeds
- Maintain hydration with oral, nasogastric, or IV fluids depending respiratory effort and feeding safety
- Supplemental oxygen when persistent oxygen saturation is below accepted threshold or clinical hypoxemia is present
2
Avoid routine low-value treatments
- Do not routinely use albuterol, epinephrine, corticosteroids, antibiotics, chest physiotherapy, or hypertonic saline in typical ED/outpatient bronchiolitis
- Antibiotics only when bacterial infection is suspected or confirmed
- Chest X-ray can increase unnecessary antibiotic use and should be reserved for atypical/severe cases
3
Escalation
- Admit for apnea, persistent hypoxemia, moderate/severe retractions, inability to maintain hydration, or social concern
- High-flow nasal cannula may reduce work of breathing in hospitalized infants who fail low-flow oxygen
- PICU for impending respiratory failure, recurrent apnea, exhaustion, or need for CPAP/intubation
4
Prevention
- Hand hygiene, smoke avoidance, breastfeeding support, and limiting exposure during RSV season
- RSV immunoprophylaxis/monoclonal prevention is risk- and season-dependent per current CDC/AAP recommendations
Complications
- Apnea: Especially in premature or very young infants
- Dehydration: Poor feeding and tachypnea may require NG or IV fluids
- Respiratory failure: Escalating work of breathing can require high-flow oxygen, CPAP, or ventilation
- Secondary bacterial infection: Uncommon but consider with focal findings, toxic appearance, or persistent high fever
- Recurrent wheeze: Severe RSV bronchiolitis is associated with later wheezing, but bronchiolitis itself is managed supportively
USMLE Step 2 CK Exam Tips
- 1Typical bronchiolitis = supportive care, not albuterol or steroids
- 2First episode of wheezing in an infant during RSV season is bronchiolitis, not asthma
- 3Routine chest X-ray is not indicated and may lead to unnecessary antibiotics
- 4Apnea can be the presenting sign in young infants
- 5Admission triggers: hypoxemia, apnea, dehydration, severe work of breathing, young age/prematurity
- 6Diffuse crackles and wheeze after URI prodrome = bronchiolitis
- 7Antibiotics are only for suspected bacterial coinfection, not RSV itself
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