Idiopathic Lung Diseases Other than Pulmonary Fibrosis

Abstract

Idiopathic lung diseases are diffuse parenchymal lung diseases that are grouped under the term interstitial lung disease (ILD). The term interstitial (or interstitium) is, however, misleading as the term interstitium refers to the microscopic anatomic space between the basement membranes of the endothelial and epithelial cells. The pathologic processes involved in these diseases, however, are not limited to the interstitium and can affect other elements of the gas exchange units as well as bronchiolar lumen, terminal bronchioles, pulmonary parenchyma, and pleural and vascular spaces. Since there are potentially hundreds of agents and clinical situations that are associated with ILD, a simplified grouping scheme includes seven main entities: ILD associated with (1) occupational and environmental factors (inhalation cause), (2) collagen vascular diseases, (3) granulomatous lung disease of known and unknown causes (eg, hypersensitivity pneumonitis [HP], sarcoidosis), (4) inherited diseases, (5) iatrogenic/drug induced, (6) certain specific entities (eg, pulmonary Langerhans cell histiocytosis [PLCH], lymphangioleimyomatosis, and (7) idiopathic interstitial pneumonia (IIP). As idiopathic pulmonary fibrosis (IPF) is a subgroup of IIP, this review focuses on the clinical features and management of the major IIPs, and IPF is discussed more in the review, “Idiopathic Pulmonary Fibrosis,” found elsewhere in this publication. This review also focuses on HP as it is a key disease in the differential diagnosis of the IIPs. Figures depict chest radiographs, high resolution computed topography (HRCT) scans, and histopathologic features of various ILDs. Tables list the American Thoracic Society (ATS) and the European Respiratory Society (ERS) classification of IIPs, and common antigens associated with HP and their potential sources. This review contains 22 highly rendered figures, 2 tables, and 156 references.