Respiratory syncytial virus (RSV) is one of those “simple” Step 1 bugs that keeps showing up—because it’s extremely testable. It’s the classic cause of bronchiolitis in infants, it has a distinctive diagnostic clue (syncytia), and it forces you to think through airway obstruction physiology (wheezing + hyperinflation) in a way that connects micro, path, peds, and pharm.
Quick ID: What RSV Is (and Why You Care)
RSV is an enveloped, negative-sense, single-stranded RNA virus in the Paramyxoviridae family (same broader family vibe as parainfluenza and measles).
High-yield identifiers
- Enveloped → relatively fragile in the environment; spread mainly via respiratory droplets and direct contact (fomites are still important in daycare/hospital settings).
- (-)ssRNA → must carry an RNA-dependent RNA polymerase in the virion.
- Causes bronchiolitis and pneumonia in infants/young children
- Famous for syncytia formation via F (fusion) protein
First Aid cross-reference: Microbiology → Viruses → Respiratory viruses (RSV); Pediatrics → Bronchiolitis; Immunology/Pharm sections for palivizumab.
Virology Deep Dive (Step-Relevant)
Structure & Key Proteins
RSV has two “Step 1 favorite” surface proteins:
| Protein | Function | Board-style consequence |
|---|---|---|
| F (fusion) protein | Fuses viral envelope with host cell membrane; also fuses infected cells together | Syncytia (multinucleated giant cells), enhanced cell-to-cell spread |
| G (attachment) protein | Mediates attachment to respiratory epithelium | Tropism for airway epithelium |
Buzzword: Syncytia = multinucleated giant cells formed by fused infected cells (a cytopathic effect).
Mechanism: F protein promotes membrane fusion → infected cells merge.
Pathophysiology: Why RSV Causes Wheeze + Increased Work of Breathing
RSV infects ciliated respiratory epithelial cells, especially in the small airways (bronchioles).
Key downstream effects:
- Inflammation + edema of bronchiolar walls
- Increased mucus production
- Sloughing of necrotic epithelium → debris plugs
- Air trapping (ball-valve effect) → hyperinflation
- Patchy atelectasis can occur from obstruction
How this shows up clinically (connect the dots)
- Wheezing: narrowed bronchioles + mucus plugging
- Crackles: small airway + alveolar involvement, secretions
- Hypoxemia: V/Q mismatch from obstructed/atelectatic regions
- Increased work of breathing: nasal flaring, retractions, tachypnea
First Aid cross-reference: Pathology/Physiology of obstructive disease patterns; Pediatrics bronchiolitis.
Epidemiology & Risk Factors (What NBME Loves)
Who gets RSV?
- Peak: infants < 2 years, especially 2–6 months
- Seasonality: winter (often tested alongside influenza season, but RSV is a pediatric staple)
Risk factors for severe disease
- Prematurity (immature lungs + fewer maternal antibodies)
- Bronchopulmonary dysplasia
- Congenital heart disease
- Immunodeficiency
- Young age (small airway caliber → big effect from mild swelling)
Classic test stem setting: daycare exposure + winter + infant with wheeze.
Clinical Presentation: Bronchiolitis “Picture”
Typical bronchiolitis
Symptoms
- Starts like a URI: rhinorrhea, congestion, low-grade fever
- Progresses to lower respiratory signs: cough, wheezing, tachypnea
Signs
- Intercostal/subcostal retractions
- Nasal flaring
- Diffuse wheezes and/or crackles
- Poor feeding, dehydration in infants
- Apnea can be an early sign in very young infants (especially premature)
Pneumonia (RSV can do both)
- More prominent crackles, hypoxemia
- Can overlap with bronchiolitis findings
Diagnosis: Mostly Clinical (But Know the Tools)
In real life and on Step 1
- Clinical diagnosis is common for typical bronchiolitis.
- Testing is often used for infection control/cohorting in hospitals.
Common diagnostic tests
- RT-PCR of nasopharyngeal swab (sensitive)
- Rapid antigen tests exist but can be less sensitive depending on setting.
Imaging (board relevance)
- CXR may show:
- Hyperinflation
- Peribronchial cuffing
- Patchy atelectasis
- Not required for straightforward cases; used if severe/atypical.
High-yield pitfall: bronchiolitis is viral—antibiotics don’t help unless there’s concern for bacterial superinfection.
Treatment: Supportive First, Targeted Prevention for High-Risk
Main treatment (most important)
Supportive care
- Hydration
- Oxygen if hypoxemic
- Suctioning of nasal secretions (big impact in infants)
- Ventilatory support if severe (HFNC/CPAP/intubation depending on course)
Bronchodilators and steroids: generally not routinely recommended for classic RSV bronchiolitis (Step 2 nuance). Step 1 takeaway: supportive care is the core.
Antivirals (know but don’t overuse)
- Ribavirin: historically used in severe RSV, especially in high-risk/immunocompromised patients; limited routine use due to variable benefit and toxicity/administration issues.
Prevention (very high yield)
Palivizumab
- Monoclonal antibody against RSV F protein
- Used for prophylaxis in high-risk infants (e.g., prematurity, chronic lung disease of prematurity, significant congenital heart disease)
Testable logic chain:
RSV F protein → syncytia + fusion → palivizumab targets F to prevent infection/severe disease.
First Aid cross-reference: Pharm/immunology: monoclonal antibodies; Micro RSV section often mentions palivizumab.
High-Yield Associations & Classic USMLE Clues
Rapid-fire HY facts
- RSV = Paramyxovirus, enveloped, (-)ssRNA
- F protein → syncytia
- Most common cause of bronchiolitis in infants
- Wheezing + hyperinflation pattern
- Treatment: supportive
- Prevention for high risk: palivizumab (anti-F)
Common stem patterns (translate symptoms → diagnosis)
- Infant + winter + URI prodrome → wheeze/retractions → RSV bronchiolitis
- Premature infant with apnea episodes + mild URI signs → consider RSV early
- Question mentions multinucleated giant cells in respiratory epithelium → think RSV (via F protein)
Differential to keep straight
| Condition | Typical age | Hallmark | Common confusion point |
|---|---|---|---|
| RSV bronchiolitis | <2 years | Wheeze, retractions, hyperinflation | Often first wheezing episode |
| Croup (parainfluenza) | 6 mo–3 yr | Barking cough, inspiratory stridor | Upper airway (laryngotracheal) |
| Influenza | any | Abrupt fever, myalgias | Systemic symptoms prominent |
| Asthma | older kids/teens | Recurrent episodic wheeze | Not usually first-time infant bronchiolitis picture |
Mini “First Aid-Style” Memory Hooks (Without the Fluff)
- RSV = Retract (retractions in infants)
- Syncytia = Stuck-together cells (F protein fusion)
- Viral bronchiolitis = Ventilatory support + hydration (supportive care)
Step-Style Questions: What They’re Really Testing
-
Mechanism question: “Which viral protein causes multinucleated giant cells?”
→ F (fusion) protein -
Management question: “Best next step in an infant with RSV bronchiolitis and mild hypoxemia?”
→ Supportive care + oxygen as needed -
Prevention question: “Premature infant entering RSV season—what prophylaxis?”
→ Palivizumab -
Pathophys question: “Why hyperinflation?”
→ Small airway obstruction from edema/mucus/debris → air trapping
Takeaway Summary (What to Have Memorized)
- RSV is an enveloped (-)ssRNA paramyxovirus causing bronchiolitis in infants.
- F protein is the star: fusion + syncytia.
- Clinical picture: URI → wheeze, tachypnea, retractions, hyperinflation.
- Diagnosis is usually clinical; PCR can confirm.
- Treatment is supportive; severe cases may need respiratory support.
- Palivizumab (anti-F) prevents severe RSV in high-risk infants.