What are the differential diagnoses for an infant (≤12 months, up to 24 months) presenting with cough, wheeze, tachypnea and crackles suggestive of bronchiolitis?

Differential Diagnoses for Bronchiolitis

When an infant presents with cough, wheeze, tachypnea, and crackles suggestive of bronchiolitis, the primary differential diagnoses include asthma (recurrent wheezing), bacterial pneumonia, viral croup, and congenital heart disease—though the first episode of wheezing in a child under 24 months with viral prodrome is bronchiolitis until proven otherwise. 1, 2

Primary Differential: Asthma vs. Bronchiolitis

Age and episode history are the critical distinguishing factors:

• Children under 24 months with their first wheezing episode should be diagnosed with bronchiolitis, regardless of the specific virus causing the illness 2
• Recurrent wheezing episodes after the first year of life likely represent asthma, not bronchiolitis 1
• Asthma is extremely rare as a first presentation in infants under 12 months, making bronchiolitis the correct diagnosis in this age group with acute febrile illness and viral prodrome 3

Bacterial Pneumonia

Key distinguishing features from bronchiolitis:

• Focal findings on examination (localized crackles, decreased breath sounds in one area) rather than diffuse bilateral wheezing and crackles 1
• Higher fever and more toxic appearance than typical bronchiolitis 4
• Chest radiography, if obtained, shows focal consolidation rather than hyperinflation with diffuse infiltrates 3

Critical pitfall: Do not routinely order chest X-rays to differentiate these conditions, as radiographic studies should not be obtained routinely when bronchiolitis is diagnosed clinically 4, 2

Viral Croup (Laryngotracheobronchitis)

Distinct clinical presentation:

• Inspiratory stridor (not expiratory wheezing) as the hallmark finding 1
• Barky, seal-like cough rather than the productive cough of bronchiolitis 1
• Upper airway obstruction symptoms (stridor, hoarseness) versus lower airway disease (wheezing, crackles) 1
• Typically affects older children (6 months to 3 years) compared to bronchiolitis peak in infants under 6 months 4

Congenital Heart Disease

Consider in infants with:

• Hemodynamically significant cardiac lesions presenting with respiratory distress that mimics bronchiolitis 4, 1
• Poor feeding, failure to thrive, or hepatomegaly in addition to respiratory symptoms 4
• Disproportionate tachycardia or signs of congestive heart failure 4

This is a high-risk condition: Infants with underlying congenital heart disease who develop bronchiolitis have increased risk of progression to severe disease and mortality 4, 3

Other Viral Etiologies Within Bronchiolitis Spectrum

While all are clinically managed as bronchiolitis, viral etiology may predict clinical course:

• RSV bronchiolitis (60-75% of hospitalized cases) typically presents with longer clinical course and higher inflammatory markers 2, 5
• Rhinovirus bronchiolitis is associated with shorter clinical courses, atopic predisposition, and higher risk of subsequent asthma development 2, 6
• Co-infections (RSV + rhinovirus most common) may mount lower inflammatory response with less fever and lower C-reactive protein levels 5

Important caveat: Routine viral testing is not recommended as it does not change acute management, except for infants receiving palivizumab prophylaxis 2, 3

Risk Stratification Regardless of Diagnosis

Assess these high-risk features in all infants with respiratory distress:

• Age under 12 weeks (significant risk factor for severe disease and apnea) 4, 2, 3
• Prematurity (especially born before 32 weeks gestation) 4, 3
• Chronic lung disease (bronchopulmonary dysplasia) 4, 3
• Hemodynamically significant congenital heart disease 4, 3
• Immunodeficiency or immunocompromised state 4, 2
• Neuromuscular disease 2, 3

Clinical Pitfalls to Avoid

• Do not diagnose asthma based on a single wheezing episode in an infant under 24 months—this is bronchiolitis until proven otherwise 2
• Do not routinely order chest radiographs, as they do not correlate well with disease severity and may lead to unnecessary antibiotic use 2, 3
• Do not interpret rhinovirus PCR results without caution, as the assay may detect prolonged viral shedding from previous unrelated illness rather than active disease 2
• Antibiotics are not indicated for uncomplicated bronchiolitis and should only be considered if there is concern for secondary bacterial pneumonia with focal findings 1, 7