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High-Yield Facts

  • Bronchiolitis is a self-limited, virally mediated, acute inflammatory disease of the lower respiratory tract, resulting in obstruction of the small airways that occurs almost exclusively in infants.

  • It is a clinical diagnosis characterized by rapid respiration, chest retractions and wheezing, and, frequently, hypoxia.

  • Respiratory failure may occur secondary to respiratory muscle fatigue or apnea, especially in very young and premature infants.

  • Treatment is largely supportive. Routine treatment with bronchodilators or corticosteroids has not been shown to be of benefit.

  • Indications for hospital admission include need for supportive care (oxygen or IV fluids), persistent respiratory distress or respiratory failure, adjusted age <6 weeks, or significant underlying disease.

Bronchiolitis is an acute inflammatory disease of the lower respiratory tract that is characterized by acute inflammation, edema, and necrosis of epithelial cells lining small airways, increased mucous production, and bronchospasm.1 The term describes a clinical syndrome that occurs in infancy and is characterized by rapid respiration, chest retractions, wheezing, and, frequently, hypoxia. It is a disease that occurs almost exclusively in children younger than 2 years. It is the leading cause of hospitalization in infancy in the United States, accounting for 3% of all hospitalizations. This results in nearly 150,000 hospital admissions per year with an associated annual cost over $500 million.2 There is evidence that hospitalization rates are increasing as well. It has a seasonal pattern, being most common in the winter and spring.3


The most common etiologic agent in bronchiolitis is respiratory syncytial virus (RSV) which is responsible for 70% of all bronchiolitis cases and even higher in winter months.4 Nearly all children are RSV seropositive by the age of 2. Infection with RSV does not grant permanent or long-term immunity; reinfections are common throughout life.5 Numerous other viruses have been implicated in bronchiolitis. This includes rhinovirus, human bocavirus, metapneumovirus, enterovirus, coronavirus, parainfluenza, adenovirus, influenza, mumps, picornavirus, and echovirus. Adenovirus causes a severe form of the disease. These viruses may also be associated with varying severity of disease and seasonality from the more typical RSV bronchiolitis.69 Mycoplasma pneumoniae and Chlamydia trachomatis also have been associated with bronchiolitis. The principal agent in school-aged children with bronchiolitis is Mycoplasma.


Infection produces inflammation of the bronchiolar epithelium, causing necrosis, sloughing, and luminal obstruction. Ciliated epithelium that has sloughed is replaced by cuboidal cells without cilia. The absence of ciliated epithelium prevents adequate mobilization of secretions and debris. The bronchioles and small bronchi are obstructed by submucosal edema, peribronchiolar cellular infiltrate, mucous plugging, and intraluminal debris. The obstruction is not uniform throughout the lungs, leading to ventilation/perfusion mismatching, resultant hypoxia, and compensatory hyperventilation. If the obstruction is severe, hypercapnia may occur. Distal to the obstructed bronchiole, air trapping or atelectasis may occur. The epithelium usually regenerates from the basal layer within 3 to 4 days. ...

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