Bronchiolitis. Pathologic considerations.

Bronchiolitis represents a cellular and mesenchymal reaction involving bronchioles. The interplay between the cellular infiltrate and the mesenchymal reaction affects the lumen size, lamina propria, muscular layer, and bronchiolar adventitia. The result is a variety of clinical, radiologic, and functional patterns of bronchiolar disease. The anatomy of the small airways is discussed, and a pathologic classification applicable to the surgical pathology of bronchiolitis is presented. The classification is practical and includes asthma, chronic obstructive pulmonary disease, cellular bronchiolitis, respiratory bronchiolitis, respiratory bronchiolitis-associated interstitial lung disease, bronchiolitis obliterans (including bronchiolitis obliterans with intraluminal polyps and constrictive bronchiolitis), dust-related small airway fibrosis, and (postinflammatory) bronchiolar scarring and peribronchiolar fibrosis.