Bronchiolar disorders.

Bronchiolar abnormalities are relatively common and occur in a variety of clinical settings. Various histopathologic patterns of bronchiolar injury have been described and have led to confusing nomenclature with redundant and overlapping terms. Some histopathologic patterns of bronchiolar disease may be relatively unique to a specific clinical context but others are nonspecific with respect to either etiology or pathogenesis. Herein, we present a scheme separating (1) those disorders in which the bronchiolar disease is the predominant abnormality (primary bronchiolar disorders) from (2) parenchymal disorders with prominent bronchiolar involvement and (3) bronchiolar involvement in large airway diseases. Primary bronchiolar disorders include constrictive bronchiolitis (obliterative bronchiolitis, bronchiolitis obliterans), acute bronchiolitis, diffuse panbronchiolitis, respiratory bronchiolitis, mineral dust airway disease, follicular bronchiolitis, and a few other rare variants. Prominent bronchiolar involvement may be seen in several interstitial lung diseases, including hypersensitivity pneumonitis, respiratory bronchiolitis-associated interstitial lung disease, cryptogenic organizing pneumonia (idiopathic bronchiolitis obliterans organizing pneumonia), and pulmonary Langerhans' cell histiocytosis. Large airway diseases that commonly involve bronchioles include bronchiectasis, asthma, and chronic obstructive pulmonary disease. The clinical relevance of a bronchiolar lesion is best determined by identifying the underlying histopathologic pattern and assessing the correlative clinico-physiologic-radiologic context.