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by John T. McBride, MD

Bronchiolitis is an acute viral infection of the lower respiratory tract affecting infants < 24 mo and is characterized by respiratory distress, wheezing, and crackles. Diagnosis is suspected by history, including presentation during a known epidemic; the primary cause, respiratory syncytial virus, can be identified with a rapid assay. Treatment is supportive with O 2 and hydration. Prognosis is generally excellent, but some patients develop apnea or respiratory failure.

Bronchiolitis often occurs in epidemics and mostly in children < 24 mo, with a peak incidence in infants < 6 mo. The annual incidence in the first year of life is about 11 cases/100 children. In the temperate northern hemisphere, most cases occur between November and April, with a peak incidence during January and February.


Most cases are caused by

Less frequent causes are influenza viruses A and B, parainfluenza viruses 1 and 2, metapneumovirus, and adenoviruses. Rhinoviruses, enteroviruses, measles virus, and Mycoplasma pneumoniae are uncommon causes.


The virus spreads from the upper respiratory tract to the medium and small bronchi and bronchioles, causing epithelial necrosis and initiating an inflammatory response. The developing edema and exudate result in partial obstruction, which is most pronounced on expiration and leads to alveolar air trapping. Complete obstruction and absorption of the trapped air may lead to multiple areas of atelectasis, which can be exacerbated by breathing high inspired O 2 concentrations.

Symptoms and Signs

Typically, an affected infant has URI symptoms with progressively increasing respiratory distress characterized by tachypnea, retractions, and a wheezy or hacking cough. Young infants may present with recurrent apneic spells followed by more typical symptoms and signs over 24 to 48 h. Signs of distress may include circumoral cyanosis, deepening retractions, and audible wheezing. Fever is usually but not always present. Infants initially appear nontoxic and in no distress, despite tachypnea and retractions, but may become increasingly lethargic as the infection progresses. Hypoxemia is the rule in more severely affected infants. Dehydration may develop from vomiting and decreased oral intake. With fatigue, respirations may become more shallow and ineffective, leading to respiratory acidosis. Auscultation reveals wheezing, prolonged expiration, and, often, fine moist crackles. Many children have accompanying acute otitis media.


  • Clinical presentation

  • Pulse oximetry

  • Chest x-ray as needed

  • RSV antigen test on a nasal swab or washing for seriously ill children

Diagnosis is suspected by history, examination, and occurrence of the illness as part of an epidemic. Symptoms similar to bronchiolitis can result from an asthma exacerbation, which is often precipitated by a respiratory viral infection and is more likely in a child > 18 mo of age, especially if previous episodes of wheezing and a family history of asthma have been documented. Gastric reflux with aspiration of gastric contents also may cause the clinical picture of bronchiolitis; multiple episodes in an infant may be clues to this diagnosis. Foreign body aspiration occasionally causes wheezing and should be considered if the onset is sudden and not associated with manifestations of URI. Heart failure associated with a left-to-right shunt manifesting at age 2 to 3 mo also can be confused with bronchiolitis.

Patients suspected of having bronchiolitis should undergo pulse oximetry to evaluate oxygenation. No further testing is required for mild cases with normal O 2 levels, but in cases of hypoxemia and severe respiratory distress, a chest x-ray supports the diagnosis and typically shows hyperinflated lungs, depressed diaphragm, and prominent hilar markings. Infiltrates may be present resulting from atelectasis and/or RSV pneumonia; RSV pneumonia is relatively common among infants with RSV bronchiolitis. RSV rapid antigen testing done on a nasal swab or washing is diagnostic but not generally necessary; it may be reserved for patients with illness severe enough to require hospitalization. Other laboratory testing is nonspecific; about two thirds of the children have WBC counts of 10,000 to 15,000/μL. Most have 50 to 75% lymphocytes.


Prognosis is excellent. Most children recover in 3 to 5 days without sequelae, although wheezing and cough may continue for 2 to 4 wk. Mortality is < 1% when medical care is adequate. An increased incidence of asthma is suspected in children who have had bronchiolitis in early childhood, but the association is controversial and the incidence seems to decrease as children age.


  • Supportive therapy

  • O 2 supplementation as needed

  • IV hydration as needed

Treatment is supportive, and most children can be managed at home with hydration and comfort measures.

Indications for hospitalization include accelerating respiratory distress, ill appearance (eg, cyanosis, lethargy, fatigue), apnea by history, hypoxemia, and inadequate oral intake. Children with underlying disorders such as cardiac disease, immunodeficiency, or bronchopulmonary dysplasia, which put them at high risk of severe or complicated disease, also should be considered candidates for hospitalization.

In hospitalized children, 30 to 40% O 2 delivered by tent or face mask is usually sufficient to maintain O 2 saturation > 90%. Endotracheal intubation is indicated for severe recurrent apnea, hypoxemia unresponsive to O 2 therapy, or CO 2 retention or if the child cannot clear bronchial secretions.

Hydration may be maintained with frequent small feedings of clear liquids. For sicker children, fluids should be given IV initially, and the level of hydration should be monitored by urine output and specific gravity and by serum electrolyte determinations.

There is some evidence that systemic corticosteroids are beneficial when given very early in the course of the illness or in children with underlying corticosteroid-responsive conditions (eg, bronchopulmonary dysplasia, asthma), but benefit in previously well infants is unproved.

Antibiotics should be withheld unless a secondary bacterial infection (a rare sequela) occurs. Bronchodilators are not uniformly effective, but a substantial subset of children may respond with short-term improvement. This is particularly true of infants who have wheezed previously. Hospital stays probably are not shortened.

Ribavirin, an antiviral drug active in vitro against RSV, influenza, and measles, is probably not effective clinically and is no longer recommended except for children who are severely immunosuppressed; it also is potentially toxic to hospital staff. RSV immune globulin has been tried but is probably ineffective.

Prevention of RSV infection by passive immunoprophylaxis with monoclonal antibody to RSV (palivizumab) decreases the frequency of hospitalization but is costly and is indicated primarily in high-risk infants (see Prevention for indications and dosage).

Key Points

  • Bronchiolitis is an acute, viral, lower respiratory tract infection affecting infants < 24 mo and is typically caused by RSV or parainfluenza 3 virus.

  • Edema and exudate in medium and small bronchi and bronchioles cause partial obstruction and air trapping; atelectasis and/or pneumonia cause hypoxemia in more severe cases.

  • Typical manifestations include fever, tachypnea, retractions, wheezing, and cough.

  • Clinical evaluation is usually adequate for diagnosis, but more severely ill children should have pulse oximetry, chest x-ray, and rapid antigen testing for RSV.

  • Indications for hospitalization include accelerating respiratory distress, ill appearance (eg, cyanosis, lethargy, fatigue), apnea by history, hypoxemia, and inadequate oral intake.

  • Treatment is supportive; bronchodilators sometimes relieve symptoms but probably do not shorten hospitalization, and systemic corticosteroids are not indicated in previously well infants with bronchiolitis.

  • There is no vaccine; monoclonal antibody to RSV (palivizumab) may be given to certain high-risk infants to decrease the frequency of hospitalization.

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