Bronchiolitis

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.

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.