ETIOLOGY

Bronchiolitis is the term used for first-time wheezing with a viral respiratory infection. The distinctive element of acute bronchiolitis is respiratory tract inflammation with airway obstruction resulting from swelling of small bronchioles leading to inadequate expiratory airflow. Most severe cases of bronchiolitis occur among infants, probably as a consequence of smaller airways and an immature immune system. Bronchiolitis is potentially life-threatening.

Respiratory syncytial virus (RSV) is the primary cause of bronchiolitis, followed in frequency by human metapneumovirus, parainfluenza viruses, influenza viruses, adenoviruses, rhinoviruses, and, infrequently, Mycoplasma pneumoniae. Viral bronchiolitis is extremely contagious and is spread by contact with infected respiratory secretions. Although coughing produces aerosols, hand carriage of contaminated secretions is the most frequent mode of transmission.

EPIDEMIOLOGY

Bronchiolitis is the leading cause of hospitalization of infants. Approximately 50% of children experience bronchiolitis during the first 2 years of life, with a peak age at 2 to 6 months. The incidence falls rapidly between the ages of 1 and 5 years, after which bronchiolitis is uncommon. It is estimated that only 10% of healthy children with bronchiolitis and wheezing require hospitalization. Children with chronic lung disease such as bronchopulmonary dysplasia, hemodynamically significant congenital heart disease, neuromuscular weakness, or immunodeficiency are at increased risk of severe, potentially fatal disease. Children acquire infection after exposure to infected family members, who typically have symptoms of an upper respiratory tract infection, or from infected children in day care. In the United States, annual peaks are usually in the late winter months from December through March. Boys are affected more commonly than girls in a ratio of 1.5:1.

CLINICAL MANIFESTATIONS

Bronchiolitis caused by RSV has an incubation period of 4 to 6 days. Bronchiolitis classically presents as a progressive respiratory illness that is similar to the common cold in its early phase with cough, coryza, and rhinorrhea. It progresses over 3 to 7 days to noisy, raspy breathing and audible wheezing. There is usually a low-grade fever accompanied by irritability, which may reflect the increased work of breathing. In contrast to the classic progression of disease, young infants infected with RSV may not have a prodrome and may have apnea as the first sign of infection.

Physical signs of bronchiolar obstruction include prolongation of the expiratory phase of breathing, intercostal retractions of the lower ribs, suprasternal retractions, and air trapping with hyperexpansion of the lungs. During the wheezing phase of the illness, percussion of the chest usually reveals only hyperresonance, but auscultation usually reveals diffuse wheezes and crackles throughout the breathing cycle. With more severe disease, grunting and cyanosis may be present.

LABORATORY AND IMAGING STUDIES

Routine laboratory tests lack specificity for diagnosing bronchiolitis and are not required to confirm the diagnosis. A mild leukocytosis of 12,000 to 16,000 cells/μL is encountered frequently but is not specific. It is important to assess oxygenation in severe cases of bronchiolitis. Pulse oximetry is generally adequate for monitoring oxygen saturation. Frequent, regular assessments and cardiorespiratory monitoring of infants are necessary because respiratory failure may develop precipitously in very tired infants even though blood gas values taken before rapid decompensation are not alarming.

Antigen tests (usually by immunofluorescence or ELISA [enzyme-linked immunosorbent assay]) of nasopharyngeal secretions for RSV, parainfluenza viruses, influenza viruses, and adenoviruses are the most sensitive tests to confirm the infection. Rapid viral diagnosis also is performed by polymerase chain reaction (PCR). Identifying the viral agent is helpful for cohorting children with the same infection.

The chest radiograph frequently shows signs of hyperexpansion of the lungs, including increased lung radiolucency and flattened or depressed diaphragms. Areas of increased density may represent either viral pneumonia or localized atelectasis.

DIFFERENTIAL DIAGNOSIS

The major difficulty in the diagnosis of bronchiolitis is to differentiate other diseases associated with wheezing. It may be impossible to differentiate asthma from bronchiolitis by physical examination, but age of presentation, presence of fever, and no history (personal or family) of asthma are the major differential factors. Bronchiolitis occurs primarily in the first year of life and is accompanied by fever, whereas asthma usually presents in older children with previous wheezing episodes that usually are not accompanied by fever unless a respiratory tract infection is the trigger for the asthma attack.

Wheezing also may be due to a foreign body in the airway, congenital airway obstructive lesion, cystic fibrosis, exacerbation of bronchopulmonary dysplasia, viral or bacterial pneumonia, and other lower respiratory tract diseases (see Chapter 78). Cardiogenic asthma, which can be confused with bronchiolitis in infants, is wheezing associated with pulmonary congestion secondary to left-sided heart failure.

Wheezing associated with gastroesophageal reflux is likely to be chronic or recurrent, and the patient may have a history of frequent emesis. Cystic fibrosis is associated with poor growth, chronic diarrhea, and a positive family history. A focal area on radiography that does not inflate or deflate suggests a foreign body.

TREATMENT

Treatment of bronchiolitis consists of supportive therapy, including respiratory monitoring, control of fever, good hydration, upper airway suctioning, and oxygen administration. Supplemental oxygen by nasal cannula is often necessary, with intubation and ventilatory assistance for respiratory failure or apnea.

Indications for hospitalization include young age (<6 months old), moderate to marked respiratory distress, hypoxemia (PO₂ < 60 mm Hg or oxygen saturation <92% on room air), apnea, inability to tolerate oral feeding, and lack of appropriate care available at home. Hospitalization of high-risk children with bronchiolitis should be considered. Bronchodilators and corticosteroids have not been shown to be effective treatment of bronchiolitis.

COMPLICATIONS AND PROGNOSIS

Most hospitalized children show marked improvement in 2 to 5 days with supportive treatment alone. The course of the wheezing phase varies, however. There may be tachypnea and hypoxia after admission, progressing to respiratory failure requiring assisted ventilation. Apnea is a major concern for very young infants with bronchiolitis.

Most cases of bronchiolitis resolve completely, although minor abnormalities of pulmonary function and bronchial hyperreactivity may persist for several years. Recurrence is common but tends to be mild and should be assessed and treated similarly to the first episode. The incidence of asthma seems to be higher for children hospitalized for bronchiolitis as infants, but it is unclear whether this is causal or if children prone to asthma are more likely to be hospitalized with bronchiolitis. There is a 1% to 2% mortality rate, highest among infants with preexisting cardiopulmonary or immunologic impairment.

PREVENTION

Monthly injections of palivizumab, an RSV-specific monoclonal antibody, initiated just before the onset of the RSV season confers some protection from severe RSV disease. Palivizumab is indicated for some infants under 2 years old with chronic lung disease (bronchopulmonary dysplasia), very low birth weight infants, and infants with hemodynamically significant cyanotic and acyanotic congenital heart disease. Immunization with influenza vaccine in children above 6 month of age may prevent influenza-associated disease.