Chapter 7 Interstitial Lung Disease
Many chronic diseases can produce diffuse opacities in the lung. Some are primarily lung disorders, and some others are manifestations of diseases arising elsewhere. Although these disorders have frequently been referred to as interstitial lung diseases, many also involve the alveolar spaces. More than 150 such disorders have been described, and a comprehensive list is provided in Box 7-1. Despite the large number, approximately 15 to 20 constitute 90% of such disease states, and these are the entities that are discussed in this chapter. Pneumoconioses and vascular disorders are discussed in Chapters 8 and 9.
Box 7-1 Diffuse Interstitial (Parenchymal) Lung Diseases
IMMUNOLOGIC AND CONNECTIVE TISSUE DISORDERS
Progressive systemic sclerosis
Chronic eosinophilic pneumonia
Idiopathic pulmonary hemorrhage
IDIOPATHIC INTERSTITIAL PNEUMONIAS
Nonspecific interstitial pneumonia
Desquamative interstitial pneumonia
Lymphocytic interstitial pneumonia
Respiratory bronchiolitis interstitial lung disease
Pneumonia resulting from irradiation
PATTERN RECOGNITION
Classification
A graphic or morphometric classification is a better approach and is enumerated in Box 7-3. The patterns are described as nodular, irregular or linear, cystic, ground-glass, and parenchymal consolidation. Most patterns can be readily identified on standard radiographs, but ground-glass and cystic disease patterns are much more readily appreciated on HRCT. Many diseases demonstrate more than one pattern (see Box 7-3).
Box 7-3 Patterns of Opacities in Interstitial Lung Disease
LINEAR PATTERN (SMALL, IRREGULAR, RETICULAR OPACITIES)
Usual interstitial pneumonitis (idiopathic pulmonary fibrosis)*
Nonspecific interstitial pneumonitis
Lymphocytic interstitial pneumonitis
Fibrosis associated with collagen vascular disease
GROUND-GLASS ATTENUATION
Acute interstitial pneumonitis
Desquamative interstitial pneumonitis
Nonspecific interstitial pneumonitis
PARENCHYMAL CONSOLIDATION (AIRSPACE OR ALVEOLAR DISEASE)
Cryptogenic organizing pneumonia
Chronic eosinophilic pneumonia
Pattern Characteristics
The nodular pattern (Fig. 7-1) is composed of multiple, small nodules that range from 1 mm to 1 cm in diameter. Irregular linear opacities (Fig. 7-2) frequently form a reticular pattern that may be fine or coarse. There are two types of cystic patterns: thin-walled cysts (Fig. 7-3) and honeycombing. Honeycomb spaces usually are 1 cm or less in diameter with relatively thick walls (>2 mm), and they are a pathologic correlate of end-stage lung disease with fibrosis (Fig. 7-4). Ground-glass attenuation is a term used almost exclusively with CT. It consists of an amorphous opacification or increase in attenuation, which is mildly severe and is not sufficient to obliterate the pulmonary vessels. Parenchymal consolidation, which has been referred to as alveolar or airspace disease, is characterized by dense opacification often with air bronchograms (Fig. 7-5). This opacification obliterates the pulmonary vasculature. Septal lines are a common feature of many interstitial lung disorders but are particularly predominant in lymphangitic spread of carcinoma and in congestive heart failure.
Zonal Distribution
Diseases have zonal preferences in the lungs (Box 7-4), although severe diseases often become diffuse. For example, histiocytosis, sarcoidosis, silicosis, and coal worker’s pneumoconiosis typically favor the upper lobes, whereas idiopathic pulmonary fibrosis and fibrosis associated with collagen vascular disease tend to be a lower-zone phenomenon. Pleural disease may take one of several forms (Box 7-5). Pneumothorax may be seen as a complication of any cause of end-stage lung, but it may be identified early in the course of diseases such as histiocytosis X and lymphangioleiomyomatosis, in which there is a high prevalence of pneumothorax. Similarly, pleural effusions and diffuse thickening are often associated with collagen vascular disease and asbestos exposure. Pleural plaques, an uncommon feature, are produced almost exclusively by asbestos exposure. Adenopathy (Box 7-6), which is recognized on standard radiographs, is associated with silicosis and sarcoidosis, lymphangitic carcinomatosis, and lymphoma. CT is more sensitive in the identification of adenopathy and may demonstrate mildly enlarged lymph nodes in idiopathic pulmonary fibrosis, hypersensitivity pneumonitis, fibrosis associated with the collagen vascular diseases, and lymphangioleiomyomatosis.
Box 7-4 Zonal Preference
Box 7-6 Adenopathy
COMPUTED TOMOGRAPHY*
Nonspecific interstitial pneumonitis
Fibrosis associated with collagen vascular disease
HIGH-RESOLUTION COMPUTED TOMOGRAPHY FEATURES OF INTERSTITIAL LUNG DISEASE
Reticular or Linear Opacities
Box 7-8 High-Resolution Computed Tomography Findings for Linear Opacities
Thickening of bronchovascular bundles (axial)
Interlobular septal thickening (septal lines)
Intralobular interstitial thickening
Nodules and Nodular Opacities
Cystic Pattern
Cystic abnormalities include honeycombing, traction bronchiectasis, lung cysts, and cavitary nodules. Findings related to emphysema and small airways disease (e.g., bronchiolitis, which may cause decreased lung opacity) are discussed in Chapters 10 and 13.