Small airways diseases.

The histopathology of the small airways is frequently quite subtle, even in cases with clinically severe disease. In the present paper, we will demonstrate some recognizable straightforward pathological changes in the small airways, and also provide a list of clinico-pathological conditions that should be considered when each is encountered. In the second part of the paper, we will briefly overview some general histological patterns of lesions and specific diseases that may involve the small airways. The basic lesions will be subdivided into inflammatory (acute, chronic, granulomatous, with or without necrosis), proliferative (epithelial or mesenchymal), and remodeling reactions. Inflammatory and proliferative reactions may lead to a the third category of remodeling reactions, characterized by a variety of distortions of normal bronchiolar architecture, including occlusion, constriction, dilatation (with or without mucostasis), tortuosity and nodularity. In addition to this schematic distinction, it is important to recognize that these lesions are frequently combined together and evolve with one another. Each of the 3 reaction patterns may be exquisitely bronchiolar, or may extend in the surrounding parenchyma. In this case, it is important to distinguish between lesions that extend from the bronchiole to the parenchyma or vice versa (such as in organizing pneumonia patterns, where the main lesion is in the parenchyma). As most of these lesions are part of a dynamic process, it is important to recognize that a single causative agent may produce distinct pathologic features at different times in the natural history of the disease. In addition, the same clinical disease may result in a variety of pathologic lesions. Accordingly, there may be not always be an unequivocal relationship between the clinical disease/condition and specific histopathologic lesions in the small airways.