Bronchiolitis

Author: John Udeani, MD, FAAEM, Assistant Professor, Department of Emergency Medicine, Charles Drew University/UCLA School of Medicine

Introduction

Background

Bronchiolitis is an acute inflammatory injury of the bronchioles that is usually caused by a viral infection. Although it may occur in persons of any age, severe symptoms are usually only evident in young infants; the larger airways of older children and adults better accommodate mucosal edema. Bronchiolitis usually affects children younger than 2 years, with a peak in infants aged 3-6 months. Acute bronchiolitis is the most common cause of lower respiratory tract infection in the first year of life. It is generally a self-limiting condition and is most commonly associated with respiratory syncytial virus.

Bronchiolar injury and the consequent interplay between inflammatory and mesenchymal cells can lead to diverse pathological and clinical syndromes. Bronchioles are small airways, less than 2 mm in diameter, and lack cartilage and submucosal glands. The terminal bronchiole, a 16th generation airway, is the final conducting airway that terminates in the respiratory bronchioles. The acinus (ie, the gas exchange unit of the lung) consists of respiratory bronchioles, the alveolar duct, and alveoli. The bronchiolar lining consists of surfactant-secreting Clara cells and neuroendocrine cells, which are the source of bioactive products such as somatostatin, endothelin, and serotonin.

Wilhelm Lange first described obliterative bronchiolitis (OB) in 1901 by reporting 2 cases of interstitial bronchiolar disorder. In 1985,1 bronchiolitis obliterans-organizing pneumonia (BOOP) was described as a separate condition with different clinical, radiographic, and prognostic features than OB. BOOP is a histopathologic lesion, not a specific diagnosis. Its pathologic hallmark is proliferative bronchiolitis or bronchiolitis obliterans in association with organizing pneumonia. BOOP and OB are beyond the scope of this article and are not discussed further.

Pathophysiology

Bronchiolitis is very contagious. The virus that causes it is spread from person to person by direct contact with nasal secretions, airborne droplets, and fomites.

The effects of bronchiolar injury include the following:

* Increased mucus secretion
* Bronchial obstruction and constriction
* Alveolar cell death, mucus debris, viral invasion
* Air trapping
* Atelectasis
* Reduced ventilation that leads to ventilation/perfusion mismatch
* Labored breathing

Ninety percent of cases are caused by respiratory syncytial virus (RSV). Other causes of bronchiolitis are addressed in Causes. Complex immunologic mechanisms play a role in the pathogenesis of RSV bronchiolitis. Type 1 allergic reactions mediated by immunoglobulin E may account for some clinically significant bronchiolitis. Infants that are breastfed with colostrum rich in immunoglobulin A appear relatively protected from bronchiolitis.

Frequency

United States

Approximately 1 in 9 infants contracts bronchiolitis in the first year of life, usually during the fall and winter months.

In one study, an estimated 1.65 million hospitalizations for bronchiolitis occurred among children younger than 5 years from 1980-1996, accounting for 7 million inpatient days.2 Children younger than 6 months accounted for 57% of these hospital visits; those younger than 1 year accounted for 81%.

International

According to the World Health Organization bulletin,3 an estimated 150 million new cases occur annually; 11-20 million (7-13%) of these cases are severe enough to require hospital admission. Worldwide, 95% of all cases occur in developing countries.

Mortality/Morbidity

Acute respiratory tract infection in children younger than 5 years is still the leading cause of childhood mortality in the world. In 2000, acute respiratory tract infection accounted for an estimated 1.9 million deaths worldwide; 70% of these deaths occurred in Africa and Southeast Asia.

Race

Race and low socioeconomic status may adversely affect outcome in patients with acute bronchiolitis. In one study,4 RSV bronchiolitis seemed to be more severe in white children than in black children. The reason for this finding is unknown. A study by La Via et al5 demonstrated that although more minority children than white children were hospitalized with RSV infection, nothing indicated that the infections in minority children were more or less severe than those in white children.

Sex

The incidence of bronchiolitis is slightly higher in boys. The exact etiology is unclear.

Age

Age was found to be a significant factor in the severity of infection. The younger the person, the more severe the infection tended to be, as measured by the lowest oxygen saturation. Infants younger than 6 months are most severely affected, owing to smaller, more easily obstructed airways and a decreased ability to clear secretions.

Intrauterine cigarette-smoke exposure may impair in utero airway development or alter the elastic properties of the lung tissue. Second-hand cigarette smoke (eg, by a parent or family member) in the postnatal period compounds the severity of RSV bronchiolitis in infants.

Clinical

History

Profuse coryza, congestion, pharyngitis, nasal discharge, and fever usually characterize the clinical syndrome in children. Primary RSV infections are confined to the upper airways in more than 50% of patients. Symptoms reach a peak in 2-5 days, with involvement of the lower respiratory tract. Typical symptoms include the following:

* Cough
* Dyspnea
* Wheezing
* Poor feeding
* Hypothermia or hyperthermia

Physical

Physical findings for bronchiolitis are not restricted to the airway. They may include the following:

* Hypothermia or hyperthermia
* Otitis media
* Tachypnea
* Nasal flaring
* Intercostal retractions
* Irritability
* Fine rales
* Wheezing
* Hypoxia

Causes

RSV is the most commonly isolated agent. It is found in 75-90% of children younger than 2 years who are hospitalized for bronchiolitis. Abundant evidence suggests that complex immunologic mechanisms play a role in the pathogenesis of RSV bronchiolitis. Type I allergic reactions mediated by the immunoglobulin E antibody may account for clinically significant bronchiolitis. Babies breastfed with colostrum rich in immunoglobulin A appear relatively protected from bronchiolitis.

Other agents that cause bronchiolitis include the following:

* Parainfluenza virus types 1, 2, and 3
* Influenza B
* Echovirus
* Rhinovirus
* Adenovirus types 1, 2, and 5: These viruses cause bronchiolitis obliterans, a particularly destructive type of bronchiolitis.
* Mycoplasma: Bronchiolitis from this cause primarily occurs in school-aged children.

References

Contents
Overview: Bronchiolitis
Differential Diagnoses & Workup: Bronchiolitis
Treatment & Medication: Bronchiolitis
Follow-up: Bronchiolitis

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