Ensayo Bronchiolitis

INTRODUCTION — Bronchiolitis, a lower respiratory tract infection (LRTI) that primarily affects the small airways (bronchioles), is a common cause of illness and hospitalization in infants and young children.

The microbiology, epidemiology, clinical features, evaluation, and diagnosis of bronchiolitis will be presented here. The treatment, outcome, and prevention of bronchiolitis are discussed separately, as is the emergent evaluation of children with acute respiratory distress. (See "Bronchiolitis in infants and children: Treatment; outcome; and prevention" and "Emergent evaluation of acute respiratory distress in children".)

DEFINITION — Bronchiolitis is defined as follows:

The definition for most clinical studies is the first episode of wheezing in a child younger than 12 to 24 months who has physical findings of a viral respiratory infection and has no other explanation for the wheezing, such as pneumonia or atopy [1,2].The broader definition is an illness in children <2 years of age characterized by wheezing and airway obstruction due to primary infection or reinfection with a viral or bacterial pathogen, resulting in inflammation of the small airways/bronchioles.

In young children, the clinical diagnosis of bronchiolitis may overlap with virus-induced wheezing and an acute viral-triggered asthma event.

MICROBIOLOGY — Bronchiolitis is typically caused by a viral infection. The proportion of disease caused by specific viruses varies depending upon the season and the year [3]. Respiratory syncytial virus (RSV) is the most common cause, followed by rhinovirus. Less common causes include parainfluenza virus, human metapneumovirus, influenza virus, adenovirus, coronavirus, and human bocavirus (discovered in 2005) [4-10]. With molecular diagnostics, coviral infection may occur in approximately one-third of young children hospitalized with bronchiolitis [6,8,10]. In addition, LRTI and wheezing episodes in infants occasionally are associated with Mycoplasma pneumoniae. (See "Mycoplasma pneumoniae infection in children".)

Respiratory syncytial virus — RSV is the most common cause of bronchiolitis and the virus most often detected as the sole pathogen. RSV is ubiquitous throughout the world and causes seasonal outbreaks. In temperate climates, late fall and wintertime epidemics of bronchiolitis usually are linked to RSV. In the northern hemisphere, outbreaks usually occur from November to April, with a peak in January or February. In the southern hemisphere, wintertime epidemics occur from May to September, with a peak in May, June, or July. In tropical and semitropical climates, the seasonal outbreaks usually are associated with the rainy season. (See "Respiratory syncytial virus infection: Clinical features and diagnosis".)Rhinovirus — Human rhinoviruses consist of more than 100 serotypes and are the main cause of the common cold. Rhinovirus is associated with lower respiratory tract infection in young children and in individuals with chronic lung disease [11]. Dual viral infections are often detected. Rhinovirus is associated often with bronchiolitis in the spring and fall [12]. (See "Epidemiology and clinical manifestations of rhinovirus infections in children".)Parainfluenza virus — Parainfluenza virus type 3, which is associated with epidemics in early spring and fall, is another cause of bronchiolitis. Parainfluenza virus types 1 and 2 also can cause bronchiolitis [13]. (See "Parainfluenza viruses in children".)Human metapneumovirus — Human metapneumovirus (HMPV) is a paramyxovirus first recognized in 2001. It can occur in conjunction with other viral infections [14] and has been identified as an etiology of bronchiolitis and pneumonia in children. Human metapneumoviruses are discussed separately. (See "Human metapneumovirus infections".)Influenza virus — The lower respiratory tract manifestations of influenza are clinically indistinguishable from those due to respiratory syncytial virus (RSV) or parainfluenza viral infections. Influenza virus infections in children are discussed separately. (See "Clinical features and diagnosis of influenza in children".)Coronavirus — Human coronaviruses are the second-most common agents that cause the common cold. Since the identification of severe acute respiratory syndrome-associated coronavirus (SARS-CoV), two new human coronaviruses (CoV-NL63 and CoVHKU1) have been identified. Non-SARS coronaviruses can cause lower respiratory tract infection, including bronchiolitis [15]. (See "Coronaviruses".)Human bocavirus — Human bocavirus is a pathogen discovered in 2005 [4]. It causes upper and lower respiratory infections during the fall and winter months [16-18]. Bronchiolitis and pertussis-like illness can occur.Human polyomaviruses — Human polyomaviruses were discovered in 2007 in children with respiratory tract disease including bronchiolitis. These viruses are frequently detected in the presence of other viruses and in asymptomatic children. Because polyomaviruses cause persistent asymptomatic infections in immunocompetent individuals, the medical significance of respiratory polyomaviruses remains unclear [19].

EPIDEMIOLOGY — The majority of cases of bronchiolitis are caused by infection with RSV. Thus, the epidemiology of bronchiolitis is similar to that of RSV infection. (See "Respiratory syncytial virus infection: Clinical features and diagnosis", section on 'Epidemiology'.)

Bronchiolitis typically affects infants younger than two years, principally during the winter months [20]. In one prospective study of 1179 healthy infants, bronchiolitis accounted for 60 percent of the cases of LRTI during the first year of life [21]. Bronchiolitis has a peak incidence between two and six months of age and remains a significant cause of respiratory disease during the first five years of life [13,22].

Bronchiolitis is a leading cause of hospitalization in infants and young children [13,22,23]. Data from the United States National Hospital Discharge Survey from 1980 to 1996 indicate that [22]:

An estimated 1.65 million hospitalizations for bronchiolitis occurred among children younger than five years during this 17-year period.Infants younger than one year and those younger than six months accounted for 81 and 57 percent of the hospitalizations, respectively.The median length of hospital stay was three days.The hospitalization rates more than doubled during the study period (from 13 to 31 per 1000 children younger than one year of age and from 1.3 to 2.3 per 1000 children one to four years of age).The proportion of hospitalizations for bronchiolitis tripled during the study period

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