Bronchiolitis

• Viral attachment and cellular invasion: Respiratory syncytial virus binds to fusion protein (F) and glycoprotein (G) on respiratory epithelial cells. The F protein mediates fusion between viral and host cell membranes, allowing viral entry into epithelial cells of the terminal and respiratory bronchioles. This initial infection triggers rapid epithelial cell lysis and denudation of the bronchiolar wall, exposing the underlying basement membrane and lamina propria to inflammatory mediators.

• Inflammatory cascade and mucus plugging: The viral infection and epithelial injury activate both innate and adaptive immune responses. Infected epithelial cells release cytokines (IL-6, IL-8, TNF-α) and chemokines (RANTES, MCP-1) that recruit neutrophils, macrophages, and lymphocytes to the bronchiolar wall. These inflammatory cells further damage epithelial cells and increase vascular permeability. Simultaneously, infected cells and inflammatory debris combined with excessive mucus production create mucus plugs that obstruct the small airways. Because the bronchioles are the smallest conducting airways, even small amounts of mucus and edema cause significant airflow obstruction, particularly in expiration when dynamic compression narrows these airways further.

• Airflow obstruction and gas trapping: The pathognomonic finding of bronchiolitis is air trapping (hyperinflation). Mucus plugging and bronchiolar edema create a ball-valve mechanism: air can enter the alveoli during inspiration (when airways are wider) but becomes trapped during expiration (when dynamic compression narrows airways further). This leads to increased functional residual capacity (FRC) and hyperinflation visible on chest imaging. The hyperinflation flattens the diaphragm, reducing its mechanical efficiency and increasing work of breathing. Additionally, obstruction of some bronchioles creates ventilation-perfusion (V/Q) mismatch, leading to hypoxemia that is often more severe than the degree of respiratory distress would suggest. Some bronchioles may be completely obstructed, causing atelectasis in dependent regions.

• Increased work of breathing and respiratory mechanics: The obstructed airways increase airway resistance, requiring infants to generate higher transpulmonary pressures to achieve adequate ventilation. Infants have compliant chest walls with horizontal ribs that collapse inward during increased negative intrathoracic pressure (paradoxical breathing), further increasing work of breathing. The increased metabolic cost of breathing combined with fever and potential feeding difficulty leads to caloric deficit and fatigue. Without adequate rest and support, respiratory muscle fatigue develops, potentially leading to hypoventilation and hypercapnic respiratory failure.

• Hypoxemia mechanisms and severity factors: Hypoxemia in bronchiolitis results from multiple mechanisms including V/Q mismatch (most significant), true intrapulmonary shunting from atelectasis, and diffusion impairment from airway edema. The hypoxemia is typically refractory to supplemental oxygen to a degree that seems disproportionate to clinical severity, particularly in infants less than 6 months old. This occurs because the primary obstruction prevents oxygen-enriched gas from reaching collapsed alveoli, and even high FiO2 cannot overcome pure ventilation-perfusion mismatch.

• Secondary bacterial superinfection: While bronchiolitis is viral, secondary bacterial infection can occur, particularly with Streptococcus pneumoniae and Haemophilus influenzae type b (now rare due to vaccination). However, true bacterial superinfection in uncomplicated bronchiolitis is uncommon, and the presence of fever or infiltrates should not automatically prompt antibiotic therapy without other clinical indicators.

• Respiratory Syncytial Virus (RSV): Accounts for 70-80% of bronchiolitis cases and is the definitive agent for classical disease presentation. RSV exhibits two major subgroups (A and B) with seasonal variation; RSV-A is associated with more severe disease and higher hospitalization rates. The virus demonstrates a remarkable capacity to evade immune responses through G protein antigenic variation and suppression of interferon production, explaining the high attack rates and potential for reinfection even in previously infected individuals.

• Parainfluenza viruses (Types 1, 2, 3): Responsible for 10-15% of bronchiolitis cases, particularly type 3, which causes year-round disease rather than strict seasonal patterns. Type 3 is particularly prevalent in immunocompromised hosts and can cause severe disease in transplant recipients.

• Influenza viruses: Account for 5-10% of cases, typically during influenza season (winter months). Influenza-associated bronchiolitis tends to present with higher fever and more prominent systemic symptoms than RSV disease.

• Human metapneumovirus: An emerging pathogen causing 5-10% of viral lower respiratory infections in infants; clinically indistinguishable from RSV bronchiolitis. This virus was only discovered in 2001 and is increasingly recognized as a significant pathogen in pediatric populations.

• Rhinovirus and enterovirus: Increasingly recognized causes of bronchiolitis, particularly with expanding molecular diagnostic capabilities. Some rhinovirus species (particularly RV-C) are associated with more severe disease and may account for cases previously attributed to RSV.

• Other viral etiologies: Adenovirus, bocavirus, coronavirus (including SARS-CoV-2), and other viruses less commonly cause bronchiolitis, typically in immunocompromised or severely debilitated infants.

• Age and developmental factors: Infants under 12 months are at highest risk, with peak incidence at 2-12 months. The immature airway anatomy (smaller diameter, more compliant walls), underdeveloped collateral ventilation, and relative immunologic immaturity of infants place them at particular risk. Infants born prematurely (especially <32 weeks) remain at higher risk beyond chronologic age 12 months.

• Prematurity and chronic lung disease: Infants with bronchopulmonary dysplasia (BPD) face markedly increased risk of severe bronchiolitis and hospitalization, with potential for rapid deterioration. These patients may require prolonged mechanical ventilation and are candidates for palivizumab prophylaxis.

• Congenital heart disease: Particularly cyanotic heart disease and conditions with pulmonary hypertension or left-to-right shunts (ASD, VSD) predispose to more severe disease due to increased pulmonary blood flow and baseline respiratory compromise.

• Immunocompromised states: Primary immunodeficiency (particularly severe combined immunodeficiency [SCID], DiGeorge syndrome), HIV infection, and iatrogenic immunosuppression (from chemotherapy, solid organ/bone marrow transplantation) significantly increase severity risk.

• Environmental and socioeconomic factors: Passive smoke exposure, air pollution, crowded living conditions, lack of breastfeeding, and low socioeconomic status increase risk of both infection and severity. Daycare attendance increases exposure but also provides earlier immunity in some cases.

• Season and climate: Peak winter incidence in temperate climates; year-round circulation in tropical regions. Cold, dry weather correlates with increased RSV transmission.

• Prodromal upper respiratory symptoms: Bronchiolitis typically begins with a 1-3 day prodrome of rhinorrhea, nasal congestion, and low-grade fever (often 38-39°C, though RSV may present with higher fever). Many infants exhibit poor feeding related to nasal congestion and increased work of breathing. Older siblings or daycare contacts often have concurrent upper respiratory infection, establishing epidemiologic context.

• Progressive lower respiratory involvement and cough: By days 3-5 of illness, persistent cough develops that is typically non-productive and often described as tight or barky. The cough worsens with agitation and is triggered by feeding. Progressive airway obstruction leads to wheezing and crackles on auscultation. The character of respiratory sounds often evolves: initial harsh upper airway sounds progress to lower airway wheezing and fine crackles as disease progresses.

• Tachypnea and increased work of breathing: Respiratory rate typically increases to 40-80 breaths per minute (well above the normal 30-40 for infants). Retractions (subcostal, intercostal, and suprasternal) develop as infants generate higher negative intrathoracic pressures to overcome airway obstruction. The use of accessory muscles becomes apparent, particularly the scalene muscles in the neck. Nasal flaring indicates significant respiratory distress and is a sign of more severe disease requiring closer monitoring. The rapid, labored breathing appears disproportionate to the degree of fever, differentiating bronchiolitis from simple upper respiratory infection.

• Hypoxemia and cyanosis: Oxygen desaturation is common in bronchiolitis, often more severe than clinical appearance would suggest—a critical distinction. Infants may maintain adequate work of breathing but have saturation in the 88-92% range at rest, dropping further with exertion or agitation. Perioral or peripheral cyanosis appears in more severe cases and indicates the need for supplemental oxygen. The hypoxemia is often refractory to low-flow oxygen, requiring higher concentrations.

• Poor feeding, dehydration, and metabolic complications: Increased work of breathing and nasal congestion impair feeding, leading to potential dehydration, hypoglycemia, and failure to thrive. Feeding difficulty is particularly concerning in young infants and those with marginal respiratory status. The metabolic cost of increased respiratory work combined with reduced caloric intake creates a negative energy balance.

• Apneic episodes in severe disease: Apnea occurs in 10-20% of hospitalized cases and is particularly common in premature infants and those <6 months of age. Apneic episodes may be central (from respiratory center depression due to hypoxemia, hypercapnia, or metabolic derangement) or obstructive (from secretion-related airway obstruction). Episodes typically last 15-20 seconds but can be prolonged and life-threatening.

• Systemic manifestations: Fever is present in 60-70% of cases and may be moderate to high. Irritability and lethargy reflect both the systemic inflammatory response and, in severe cases, hypoxemia or hypercapnia. Some infants develop hepatomegaly (20-30% of cases), typically mild and related to increased intrathoracic pressure rather than true hepatitis.

• Clinical severity spectrum: Mild disease presents with cough and minimal distress; moderate disease includes wheezing, tachypnea, and clear respiratory effort but normal oxygen saturation; severe disease includes marked tachypnea (>60), retractions, altered mental status, poor feeding, and hypoxemia (saturation <90% on room air).

• Important clinical variants: Atypical presentations can occur, particularly in specific populations. Immunocompromised infants may present with insidious onset of hypoxemia without prominent cough or wheezing. Recurrent wheezing during the acute illness suggests underlying reactive airway disease or atopy and predicts post-viral wheezing. Fulminant presentations with rapid deterioration to respiratory failure suggest more aggressive viral strains (particularly RSV-A) or immunologic factors.

• Clinical diagnosis based on presentation: Bronchiolitis is fundamentally a clinical diagnosis in infants 2-24 months with a viral prodrome followed by lower respiratory tract findings (cough, tachypnea, wheezing, and/or crackles). No laboratory test is required for diagnosis in typical cases. The presence of wheezing during a winter viral illness in an infant is pathognomonic for bronchiolitis. Diagnostic criteria from the American Academy of Pediatrics require: (1) first episode of wheezing and/or crackles; (2) signs of viral respiratory infection; (3) age typically <24 months. Peak diagnostic certainty occurs during winter months when RSV is prevalent.

• Viral identification and rapid testing: Nasopharyngeal aspirate or nasal swab for rapid viral antigen detection (RSV, parainfluenza, influenza) has sensitivity 80-95% and specificity >95% depending on the assay and pathogen. Rapid molecular testing (PCR) provides faster results (within 1-2 hours) and higher sensitivity than antigen detection. Viral identification is not required for diagnosis but helps confirm etiology and guide infection control measures (cohorting RSV-positive patients, for instance). Real-world practice note: overuse of viral testing can lead to unnecessary testing in clear cases and false reassurance in negative tests, as clinical diagnosis is more reliable.

• Chest radiography findings: Hyperinflation is the hallmark radiographic finding, evidenced by increased anteroposterior diameter, flattened diaphragms, and increased retrosternal air space. Bronchial wall thickening (bronchial markings extended to lung periphery) and peribronchial infiltrates are common. Atelectasis, typically subsegmental or segmental, occurs in dependent lung regions and may create areas of opacification that raise concern for bacterial pneumonia. Hyperinflation is so characteristic that its absence should raise doubt about the diagnosis. However, chest X-rays are not routinely recommended for uncomplicated bronchiolitis as they change management infrequently but may lead to unnecessary antibiotic treatment if infiltrates are misinterpreted as bacterial pneumonia.

• Laboratory studies: Complete blood count may show lymphocytosis or leukocytosis (nonspecific) but is not routinely necessary. Blood culture should be obtained only if bacteremia is suspected (high fever, toxic appearance, significant leukocytosis >15,000); it is not routinely indicated. Electrolyte assessment may be warranted in infants with poor intake or signs of dehydration. Blood glucose should be checked in severely ill infants due to risk of hypoglycemia from increased metabolic demands. Arterial or capillary blood gas is useful in severe disease to assess hypercapnia (PCO2 >45 mmHg), which indicates respiratory muscle fatigue and potential need for mechanical support. Pulse oximetry is the most useful continuous assessment tool, though values should be interpreted carefully: infants can have significant hypoxemia despite appearing relatively well.

• Scoring systems for severity assessment: The Respiratory Distress Assessment Instrument (RDAI) and Bronchiolitis Severity Score quantify severity using respiratory rate, retractions, nasal flaring, and oxygen requirements. These are most useful for research standardization and prognostication but are less commonly used in routine clinical practice. The Woods Clinical Asthma Score (designed for asthma) is sometimes applied to bronchiolitis and ranges from 0-3, with scores >2 suggesting inadequate response to therapy.

• Differential diagnosis considerations:
• Acute asthma exacerbation: Can be difficult to distinguish in older infants (>18 months) but typically has a history of prior wheezing episodes. Rapid response to bronchodilators suggests asthma over bronchiolitis.
• Foreign body aspiration: Should be considered if unilateral wheezing, sudden onset without prodrome, or history of foreign body exposure. Lateral neck radiography or fluoroscopy may show air trapping on affected side.
• Bacterial pneumonia: Focal consolidation on examination or imaging, high fever, elevated inflammatory markers, or toxic appearance. Streptococcus pneumoniae and Haemophilus influenzae are most common. However, crackles alone without consolidation are not indicative of bacterial infection.
• Viral croup: Presents with inspiratory stridor and barky cough rather than wheeze; different anatomic location (subglottic vs bronchioles).
• Epiglottitis: Presents with drooling, tripod positioning, and respiratory distress with minimal cough; requires urgent airway intervention.
• Congestive heart failure: Presents with crackles, wheezing, and hyperinflation but typically has hepatomegaly, gallop rhythm, and cardiac history. Chest X-ray shows pulmonary edema with Kerley B lines and

Immediate stabilisation (severe disease)

• Airway and oxygenation: nasal suctioning of secretions first — infants are obligate nasal breathers, and clearing the nares alone often improves work of breathing and feeding. The AAP 2014 Clinical Practice Guideline supports supplemental oxygen when saturation falls persistently below ~90%; oxygen may be withheld above that threshold in an otherwise stable infant.
• Apnea, exhaustion, or hypercapnia mandate escalation to a monitored setting.

First-line therapy — supportive care only

• Hydration: nasogastric or IV fluids when tachypnea (typically >60-70/min) makes oral feeding unsafe because of aspiration risk. Use isotonic maintenance fluid (e.g., 0.9% NaCl with dextrose); the AAP 2018 maintenance IV fluid guideline warns that hypotonic fluid plus illness-related ADH release causes hyponatremia.
• Suctioning and positioning: superficial nasal suction; deep suctioning is discouraged.

Escalation

• Heated humidified high-flow nasal cannula (HFNC): washes out nasopharyngeal dead space and provides modest distending pressure, reducing work of breathing.
• CPAP for impending failure; intubation and mechanical ventilation for apnea, refractory hypoxemia, or hypercapnic failure with exhaustion. Ventilate with long expiratory times to avoid worsening gas trapping.
• Nebulized hypertonic (3%) saline: the AAP states it may be used in hospitalized infants (rehydrates airway surface liquid, thins mucus plugs) but not in the emergency department for a patient going home.

What is NOT recommended (AAP 2014)

• Beta-2 agonists (albuterol): infant bronchioles are obstructed by plugs and edema, not smooth-muscle spasm; no change in course, and tachycardia and worsened V/Q matching may follow.
• Nebulized epinephrine: not for routine inpatient use.
• Corticosteroids (systemic or inhaled): no benefit, alone or with epinephrine.
• Antibiotics: reserve for a documented bacterial coinfection — radiographic infiltrates in bronchiolitis usually represent atelectasis.
• Chest physiotherapy, routine chest radiography, routine viral testing.

Prevention

• Nirsevimab (long-acting anti-RSV F monoclonal): single IM dose for infants entering their first RSV season, per CDC/ACIP and AAP.
• Maternal RSV prefusion F vaccine at 32-36 weeks (ACIP/ACOG).
• Palivizumab: monthly IM during season for eligible high-risk infants (extreme prematurity, chronic lung disease of prematurity, hemodynamically significant congenital heart disease).

Emergencies

• Apnea: most common in infants <2 months and former premature infants; results from immature central respiratory drive perturbed by hypoxemia and viral inflammation, plus obstructive events from secretions. Signalled by witnessed pauses, bradycardia, or desaturation before significant wheeze — a mandatory indication for admission and continuous monitoring.
• Hypercapnic respiratory failure: gas trapping plus respiratory muscle fatigue. The ominous sign is a quiet chest with diminishing retractions, rising PCO2, and lethargy — apparent "improvement" that actually means the infant can no longer move air. Requires immediate CPAP/intubation.
• Severe dehydration and hypoglycemia: from poor feeding plus insensible losses through tachypnea; signalled by sunken fontanelle, dry mucosa, decreased urine output.

Other disease complications

• Atelectasis: complete bronchiolar plugging with distal gas absorption; focal opacity on radiograph, often misread as bacterial pneumonia and treated with unnecessary antibiotics.
• Acute otitis media: the most frequent bacterial complication; eustachian tube dysfunction from viral inflammation. Suspect with new fever and a bulging tympanic membrane.
• Secondary bacterial pneumonia and bacteremia: uncommon in uncomplicated disease, but risk rises in ventilated infants.
• Hyponatremia from SIADH: illness-driven ADH release; worsened by hypotonic IV fluids — the reason the AAP specifies isotonic maintenance fluid. Signalled by seizure or altered mental status with low serum sodium.
• Post-bronchiolitis recurrent wheezing: airway injury and immune priming after severe RSV infection; associated with later asthma, though causality is debated.
• Bronchiolitis obliterans: rare fibrotic obliteration of small airways, classically after adenovirus; suggested by persistent hypoxemia and mosaic attenuation on CT.

Treatment-related complications

• Beta-agonists/nebulized epinephrine: tachycardia, tremor, and transient worsened hypoxemia from pulmonary vasodilation in poorly ventilated units.
• Positive-pressure support: air leak (pneumothorax, pneumomediastinum) from over-distension of already hyperinflated lungs — sudden desaturation with asymmetric breath sounds is a tension pneumothorax until proven otherwise; HFNC also causes gastric distension and nasal trauma.
• Hypotonic IV fluid: iatrogenic hyponatremia, as above.

• The stem: an infant under 12 months, in winter, with 2-3 days of rhinorrhea followed by cough, tachypnea, retractions, and diffuse wheezes and fine crackles — first episode of wheezing. That is bronchiolitis, and RSV is the answer for etiology.
• Single best next step is almost always supportive: nasal suctioning, hydration, and oxygen for saturation persistently <90%. If an answer choice says "nebulized albuterol," "systemic corticosteroids," "chest physiotherapy," or "chest radiograph," it is the distractor — the AAP 2014 guideline recommends against all of these routinely.
• Antibiotics are the classic trap. A patchy infiltrate on film in bronchiolitis is usually atelectasis from mucus plugging, not pneumonia. Treat bacterially only with a genuinely documented coinfection; concurrent acute otitis media is the one common exception worth treating.
• Apnea is the tested complication, disproportionately in infants <2 months and ex-premature infants, and it can precede significant wheezing. Age <2 months, prematurity, hypoxemia, dehydration, or apnea are the admission triggers.
• The paradox to recognize: hypoxemia is often worse than the infant looks (V/Q mismatch), while a quiet chest with falling respiratory rate and rising PCO2 means impending failure, not recovery.
• Prevention is the association examiners now test: nirsevimab (anti-RSV F monoclonal) for infants entering their first RSV season, or maternal RSV prefusion F vaccination at 32-36 weeks, per CDC/ACIP; palivizumab monthly during season is reserved for extreme prematurity, chronic lung disease of prematurity, and hemodynamically significant congenital heart disease. There is no live RSV vaccine for infants.
• Distinguishing distractors: sudden onset with unilateral wheeze and no prodrome = foreign body; inspiratory stridor with barky cough = croup; hepatomegaly with gallop and cardiomegaly = heart failure; repeated wheezing episodes in a toddler responsive to albuterol = asthma.
• Mechanism pearl: bronchodilators fail because obstruction is mucus plugging and mucosal edema in airways with little smooth muscle — not bronchospasm.