ATS/ERS Classification of the Idiopathic Interstitial Pneumonias
Diffuse Alveolar Damage/ARDS
Bronchiolitis Obliterans Organizing Pneumonia
Usual Interstitial Pneumonia
Nonspecific Interstitial Pneumonia
Respiratory Bronchiolitis With Interstitial Lung Disease and Desquamative Interstitial Pneumonia
Langerhans Cell Histiocytosis
Pulmonary Alveolar Proteinosis
Define the pathologic features of ARDS (pathologic diffuse alveolar damage).
Define the pathologic features of bronchiolitis obliterans organizing pneumonia.
Define the pathologic features of usual interstitial pneumonia.
Define the pathologic features of nonspecific interstitial pneumonia.
Define the pathologic features of Langerhans cell histiocytosis.
Define the pathologic features of hypersensitivity pneumonitis.
Define the pathologic features of sarcoid.
Define the pathologic features of lymphangioleiomyomatosis.
Define the pathologic features of pulmonary alveolar proteinosis.
Define the pathologic features of acute and chronic eosinophilic pneumonia.
Pathologists use the term acute lung injury to refer to both diffuse alveolar damage, the pathologic finding in ARDS, and to bronchiolitis obliterans organizing pneumonia. This should be discouraged because in almost all cases the combination of morphology, imaging, and clinical features will separate these entities. Usual interstitial pneumonia is characterized by peripheral macroscopic honeycombing and fibrosis, microscopic honeycombing, patchy fibrosis, and fibroblast foci. In contrast, nonspecific interstitial pneumonia shows uniform involvement of the alveolar walls and can be cellular, fibrotic, or mixed. Respiratory bronchiolitis with interstitial lung disease consists of smoker's macrophages in the lumens of respiratory bronchioles with or without fibrosis of the walls of the bronchioles. Desquamative interstitial pneumonia looks like respiratory bronchiolitis with interstitial lung disease writ large, with filling of airspaces by smoker's macrophages over a large area. Langerhans cell histiocytosis shows a combination of cellular centrilobular nodules containing, early on, Langherns cells, often with cysts, and stellate fibrosis in the healed stage. In hypersensitivity pneumonitis, there is a centrilobular interstitial inflammatory infiltrate, usually but not always associated with small noncaseating granulomas. Sarcoid has numerous noncaseating well-defined granulomas that follow the bronchovascular bundles and interlobular septa. In lymphangioleiomyomatosis, the lung shows cysts with a partial lining of abnormal smooth muscle cells that are positive for HMB-45 on immunostaining. In pulmonary alveolar proteinosis the alveoli are filled by densely eosinophilic granular material. In chronic eosinophilic pneumonia, the alveolar are filled by eosinophils, but acute eosinophilic pneumonia looks like diffuse alveolar damage with small numbers of eosinophils.