[The small airways: normal histology and the main histopathological lesions]

Lesions of the small airway are observed in a wide variety of pulmonary conditions, most of which are due to infection, tobacco and connective tissue diseases. They are sometimes isolated or, more often, associated with involvement of other pulmonary structures such as the bronchi, the lung parenchyma and the pleura. The pathological spectrum of the bronchiolar response to injury is relatively limited. Thus, the same lesion is observed in various clinical settings. There is no correlation between the severity of the small airway involvement seen by the pathologist and the clinical and functional manifestations of bronchiolitis. The causes of bronchiolitis may be classified on a clinical basis, on aetiology or on histological appearance, yet no single classification appears to be suitable. An integrated clinical, radiological, functional and histological approach is needed. As they are seen by the pathologist microscopically, small airway lesions may be subdivided into three categories: (1) simple nonspecific lesions (bronchiolitis - cellular, follicular, granulomatous, obliterative, constrictive) that are never exclusively related to one clinical picture, (2) or displaying a more specific pattern like the respiratory bronchiolitis of the smoker or the histolgical changes of asthma, (3) bronchiolar lesions in conditions described as "interstitial", predominantly centrilobular, involving the small airways and the lung parenchyma, and visible radiologically. After recalling the normal histological appearances of the bronchioles, this review describes the diversity of the histopathological lesions of the small airways.