What is Acute Bronchiolitis?
Acute bronchiolitis is a viral lower respiratory tract infection affecting children under 2 years of age, characterized by acute inflammation, edema, and necrosis of epithelial cells lining the small airways (bronchioles ≤2 mm in diameter), accompanied by increased mucus production and bronchospasm. 1
Clinical Definition and Diagnosis
Bronchiolitis is a clinical diagnosis made in children aged 1 month to 2 years who present with tachypnea, wheeze and/or crepitations/crackles following an upper respiratory illness. 1
Typical Clinical Presentation
The disease typically begins with:
- Rhinitis and cough as initial symptoms 1
- Progressive respiratory symptoms including tachypnea, wheezing, rales/crackles 1, 2
- Signs of increased respiratory effort such as use of accessory muscles and/or nasal flaring 1, 2
- Respiratory distress in the majority of cases (approximately 93% of patients) 3
Epidemiology and Causative Agents
Respiratory syncytial virus (RSV) is the most common causative agent, with peak incidence occurring between December and March in North America, though regional variations exist. 1, 4
Key Epidemiological Facts
- 90% of children are infected with RSV within the first 2 years of life, and up to 40% will experience lower respiratory tract infection during their initial infection 1, 4
- RSV leads to more than 90,000 hospitalizations annually in the United States 1, 4
- Mortality has significantly decreased from approximately 4,500 deaths annually in 1985 to around 390 deaths in 1999 1, 4
- Reinfection is common throughout life as RSV infection does not confer permanent immunity 1, 4
Other Viral Causes
Beyond RSV, bronchiolitis can be caused by:
- Human metapneumovirus 1, 4
- Influenza virus 1, 4
- Adenovirus 1, 4
- Parainfluenza viruses 1, 4
- Human rhinovirus 1
- Coronavirus 1
Pathophysiology
The disease process involves extensive inflammation of the airways with several key pathological features:
- Acute inflammation and edema of epithelial cells lining small airways 1
- Necrosis of airway epithelial cells 1
- Increased mucus production 1
- Bronchospasm contributing to airway narrowing 1, 2
- Wheezing results from inflammation and obstruction of bronchioles that are ≤2 mm in diameter 2
Natural History and Prognosis
Bronchiolitis is a self-limiting condition in most children, with the majority recovering without complications. 1
Timeline of Resolution
- 90% of children are cough-free by day 21 (mean time of cough resolution is 8-15 days) 1
- Chronic symptoms persisting beyond 4 weeks may represent a different clinical problem, sometimes termed post-bronchiolitis syndrome 1
Risk Factors for Severe Disease
Children at higher risk for severe disease include those with:
- Age less than 12 weeks 2
- History of prematurity 2
- Underlying cardiopulmonary disease 2
- Immunodeficiency 2
- Birth weight under 1500 grams 3
- Non-exclusive breastfeeding and early bottle feeding (statistically significant risk factors) 3
Important Clinical Distinctions
The guideline specifically applies to children aged 1 month through 23 months and excludes those with immunodeficiencies, chronic neonatal lung disease, neuromuscular disease, cystic fibrosis, or hemodynamically significant congenital heart disease from management recommendations. 1
Persistent wheezing following bronchiolitis requires careful evaluation as it may represent post-bronchiolitis syndrome or the development of asthma, and should not automatically trigger asthma medication use unless other evidence of asthma is present. 2