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Acute bronchiolitis is the most frequent cause of infant hospitalization in the United States. Thus, a significant portion of a pediatric hospitalist’s time is spent caring for infants with bronchiolitis. Both bronchiolitis and laryngotracheobronchitis (croup) are common and lead to significant morbidity. Epiglottitis, although rare since introduction of the Haemophilus influenzae type B (Hib) vaccine, warrants inclusion because of its life-threatening nature.


Bronchiolitis, a common communicable respiratory illness, is characterized by signs and symptoms of both upper and lower respiratory tract infection. Most episodes occur in otherwise healthy children younger than 2 years. Children younger than 3 months are at high risk for severe disease. Other groups at risk for serious illness secondary to bronchiolitis include young infants less than 37 weeks gestation with and without chronic lung disease, children with hemodynamically significant congenital heart disease, and those with severe immune deficiency such as hematopoietic stem cell transplantation, solid organ transplantation, and cellular immune deficiencies (e.g. 22q11.2 chromosome deletions).

Respiratory syncytial virus (RSV), a single-stranded RNA paramyxovirus, accounts for 50% to 70% of cases of bronchiolitis. Less common causes of bronchiolitis include parainfluenza virus types 1, 2, and 3, influenza viruses A and B, and adenovirus. More recently, human metapneumovirus (hMPV) has been recognized as a cause of bronchiolitis.1,2 hMPV has been detected in 20% to 25% of children with bronchiolitis and negative direct fluorescent antibody testing of nasal aspirates for RSV, parainfluenza types 1 to 3, influenza A and B, and adenovirus.1,3 Rhinovirus appears to be increasing as an etiology of bronchiolitis in older infants.4 Rare causes of bronchiolitis include Mycoplasma pneumoniae and enteroviruses.

Overall, bronchiolitis is most prevalent between October and May, although sporadic cases occur throughout the year. Epidemic RSV bronchiolitis displays remarkable seasonality, with a peak occurrence between December and April and virtually no occurrence between June and October. Peak activity of hMPV occurs in March and April, just as RSV activity begins to wane. The seasonal prevalence of specific pathogens is shown in Figure 101-1. In the United States, 1% to 3% of affected infants require hospital care; most hospital admissions for bronchiolitis occur between December and March.

FIGURE 101-1.

The seasonal prevalence of bronchiolitis caused by specific pathogens. The number of cases represents the mean number of cases per year from 2000 to 2004. RSV, respiratory syncytial virus. (Used with permission of Dr. Richard L. Hodinka, University of South Carolina, School of Medicine, Greenville.)

Although the clinical manifestations of bronchiolitis and asthma overlap to some extent, the pathogenesis of the two conditions differs, a fact that has important therapeutic implications. In bronchiolitis, progressive infection of the respiratory mucosa induces desquamation of ciliated respiratory epithelial cells and lymphocytic infiltration of peribronchial epithelial cells. These changes lead to ...

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