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The idiopathic interstitial pneumonias (IIPs) are a group of interstitial lung diseases (ILDs) of unknown etiology. There are over 100 known causes of ILD, including certain inherited conditions, collagen vascular diseases, drugs, and dozens of occupational and environmental exposures. Historically, conditions that remained idiopathic have been collectively called by various names including “diffuse interstitial fibrosis,”“diffuse fibrosing alveolitis,”“Hamman-Rich syndrome,”“idiopathic pulmonary fibrosis,” and IIP. The currently preferred term for these conditions is IIP, and it will be used for the remainder of the chapter.

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The definition and classification of the IIPs have been a source of confusion for many years. It is now quite clear that classification of the IIPs based on the histopathological appearance of surgical lung biopsy specimens helps guide therapy and aids in prognostication. In 1969, Liebow and Carrington described five histopathological subgroups of IIP: undifferentiated or “usual” interstitial pneumonia (UIP), diffuse lesions similar to UIP with superimposed bronchiolitis obliterans termed bronchiolitis interstitial pneumonia (BIP), desquamative interstitial pneumonia (DIP), lymphoid interstitial pneumonia (LIP), and giant cell interstitial pneumonia (GIP). This classification was revised by Katzenstein and Myers in 1998, and updated by the American Thoracic Society/European Respiratory Society (ATS/ERS) in 2002.

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The current ATS/ERS classification schema for the IIPs includes seven distinct conditions based largely on histopathological appearance (Table 10–1): idiopathic pulmonary fibrosis (IPF) defined by the presence of a UIP pattern, nonspecific interstitial pneumonia (NSIP) defined by the presence of an NSIP pattern, cryptogenic organizing pneumonia (COP) defined by the presence of an organizing pneumonia pattern, acute interstitial pneumonia (AIP) defined by the presence of a diffuse alveolar damage (DAD) pattern, DIP defined by the presence of a DIP pattern, a closely related pattern termed respiratory bronchiolitis associated ILD (RBILD) defined by the presence of a respiratory bronchiolitis (RB) pattern, and lymphocytic interstitial pneumonia (LIP) defined by the presence of an LIP pattern. An integrated approach is critical to achieving an accurate diagnosis in patients with suspected IIP. Clinical, radiographic, and pathological data should be equally considered, with careful attention given to identifying known causes of ILD.

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Table 10–1. Idiopathic Interstitial Pneumonia: Clinical Diagnosis and Histopathologic Correlation.
American Thoracic Society/European Respiratory Society: International multidisciplinary consensus classification of the idiopathic interstitial pneumonias. Am J Respir Crit Care Med 2002;165(2):277.   [PubMed: 11790668] (A thorough review of the current classification schema for the idiopathic interstitial pneumonias and descriptions of the individual conditions.)
Katzenstein AA, Myers JL: Idiopathic pulmonary fibrosis: clinical relevance of pathologic classification. Am J Respir Crit Care Med 1998;157(4, ...

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