Bronchiolitis – Definition, Epidemiology, Clinical Presentation, Natural Course, Management, and Post‑Bronchiolitis Syndrome

Definition and Pathophysiology

Bronchiolitis is an acute viral infection of the lower respiratory tract in children < 2 years, clinically characterized by tachypnea, wheeze and/or diffuse crackles following an upper‑respiratory prodrome. 1

It is the most common acute lower‑respiratory infection in infants and the leading cause of hospitalization for children < 1 year worldwide. 1
Respiratory syncytial virus (RSV) accounts for > 80 % of bronchiolitis cases during epidemic seasons, with ~ 90 % of children infected by age 2 years. 1
Other viral agents include human metapneumovirus, influenza, adenovirus, parainfluenza, rhinovirus, and coronavirus. 1
Seasonal peaks occur between December and March in North America, with regional variations. 1

Key signs:
- Tachypnea (rapid breathing). 1
- Diffuse bilateral crackles/crepitations on auscultation. 1
Diagnosis is primarily clinical, based on history and physical examination, without routine need for chest radiography or laboratory testing.

Bronchiolitis is self‑limited in the majority of infants; 90 % are cough‑free by day 21, with the median cough duration of 8–15 days. 1
Hospitalization is required in approximately 1–2 % of infants < 1 year of age. 1
Mortality in the United States declined from ~ 4,500 deaths annually in 1985 to ~ 390 deaths in 1999. 1

Recommended supportive measures:
- Gentle nasal suction of secretions.
- Supplemental oxygen when pulse oximetry < 90 %.
- Adequate hydration.
- Prone positioning for hospitalized infants.
- Escalation to respiratory support (e.g., CPAP or mechanical ventilation) if clinically indicated.
Routine use of bronchodilators, systemic or inhaled corticosteroids, and antibiotics does not improve outcomes and is not recommended. 1

A subset of children develop persistent respiratory symptoms beyond 4 weeks, termed post‑bronchiolitis syndrome. 1
Recurrent episodic wheeze with subsequent viral infections may occur during the following 6 months. 1

The American Academy of Pediatrics defines bronchiolitis as a viral lower respiratory tract infection characterized by acute inflammation, edema and necrosis of epithelial cells lining small airways, increased mucus production, and bronchospasm, typically presenting with viral upper respiratory prodrome followed by lower respiratory tract involvement 2, 3
Infants under 2 years of age, with the highest risk under 12 weeks, are most commonly affected by bronchiolitis, as seen in this 2-month-old infant 4
The clinical presentation of bronchiolitis includes expiratory wheeze, increased work of breathing with nasal flaring, and viral upper respiratory prodrome such as rhinorrhea and cough 2, 4
The absence of fever does not exclude bronchiolitis, as it can occur but is not a necessary symptom for diagnosis 4

The American Academy of Pediatrics states that children with recurrent wheezing episodes after the first year of life likely have asthma, but a first episode in an infant under 2 years with viral prodrome is more likely bronchiolitis 5
Pneumonia typically presents with focal findings on examination rather than diffuse bilateral wheezing, distinguishing it from bronchiolitis 4
Viral croup presents with inspiratory stridor, barky cough, and upper airway obstruction, unlike bronchiolitis which presents with expiratory wheezing and lower airway disease 2, 4

The American Academy of Pediatrics recommends careful monitoring for progression in infants under 12 weeks due to significant risk factor for severe disease 4
Additional risk factors to assess include history of prematurity, underlying cardiopulmonary disease, and immunodeficiency 4, 5
The diagnosis of bronchiolitis should be made clinically based on history and physical examination alone, without routine chest x-rays and laboratory studies 2, 4
Treatment for bronchiolitis is supportive only, including nasal suctioning, oxygen if saturations are less than 90%, and ensuring adequate hydration, as bronchodilators, corticosteroids, and antibiotics do not improve outcomes 4, 5