Interstitial Lung Disease

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.



Pattern recognition in diffuse interstitial lung disease has been the subject of controversy for many years. Traditional interpretation of chest radiographs separates these processes into two groups: diseases that radiographically appear to involve the terminal airspaces or alveoli and those that appear to involve the interstitium. However, several problems limit this approach to differential diagnosis. Many pulmonary diseases produce pathologic changes in both compartments, and disease processes that are pathologically classified as interstitial may produce an alveolar pattern on the radiograph.

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).

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.

Other features should be considered in the differential diagnosis. They include distribution of disease, pleural abnormalities, the size of the lungs, the presence of pulmonary arterial hypertension, and mediastinal and hilar adenopathy.

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.

The size of the lung (i.e., lung volumes) may be a clue to the differential diagnosis (Box 7-7). The fibrotic disorders are characterized by marked restriction, and small lungs invariably are seen in idiopathic pulmonary fibrosis and related disorders. However, histiocytosis X and sarcoidosis in the early stages are usually associated with normal lung volumes, but lymphangioleiomyomatosis produces air trapping with large lung volumes. Pulmonary arterial hypertension usually indicates end-stage disease with pronounced obliteration of the pulmonary vasculature. Except for pulmonary vascular diseases, signs of pulmonary arterial hypertension are rarely identified.


The five classifications of patterns of diffuse parenchymal lung disease on HRCT are linear or reticular opacities, nodular opacities, cystic lesions, ground-glass opacification, and parenchymal consolidation (i.e., alveolar or airspace disease). Webb and colleagues describe such HRCT findings in interstitial lung disease further in their work (see Suggested Readings).

Reticular or Linear Opacities

Reticular opacities usually are caused by interstitial thickening by cells, fluid, or fibrous tissue (Box 7-8).

Axial Interstitial Thickening

Thickening of the axial interstitium (i.e., interstitium in a peribronchovascular location) (Fig. 7-6) occurs in many diseases, such as lymphangitic spread of carcinoma, pulmonary fibrosis, and sarcoidosis. It is manifested by bronchial wall thickening and apparent enlargement of central pulmonary vessels. The thickening may be smooth or nodular. This appearance must be differentiated from a primary airway problem, bronchiectasis. In bronchiectasis, the bronchi show evidence of bronchial wall thickening, but they also are dilated and larger than adjacent pulmonary artery branches. This results in the appearance of large ring shadows. In patients with isolated bronchiectasis, there are no other signs of lung disease.

Interlobular Septal Thickening

Thickening of the interlobular septa (Fig. 7-7) is common in many interstitial lung diseases. In the peripheral lung, it appears as 1- to 2-cm lines that extend perpendicularly from the pleural surface into the substance of the lung. In the more central portion of the lung, the thickened septa can outline the secondary pulmonary lobules, producing polygonal structures that are 1 to 2.5 cm in diameter. These structures typically have a central dot that represents the pulmonary artery. Occasionally, lines that are 2.5 cm long and that outline more than one lobule can be identified, particularly in the periphery of the lung. They have been called parenchymal bands and long lines.

Centrilobular Abnormalities

Prominence of the central dot (Fig. 7-8) within the secondary pulmonary lobule (i.e., centrilobular vessel) may occur in a number of interstitial lung diseases. The intralobular bronchiole often becomes visible when there is centrilobular thickening. Centrilobular abnormalities can also be seen in patients with diseases of the peripheral airways (i.e., bronchioles).

Nodules and Nodular Opacities

Some investigators have attempted to differentiate interstitial from airspace or acinar nodules on HRCT. Because the anatomy of the secondary pulmonary lobule can be readily observed on HRCT, this distinction often may be possible, even though overlap in the appearance of interstitial and alveolar nodules occurs and many disease processes involve both compartments.

Interstitial Nodules

Interstitial nodules (Fig. 7-10) tend to be well defined and can be seen in numerous interstitial lung diseases. They may be located in the axial interstitium along the peribronchovascular bundles, in the interlobular septa in a subpleural location adjacent to fissures, and in the central portion of the secondary pulmonary lobule.

Masses of Fibrosis or Conglomerate Masses

Large masses of fibrous tissue may occur, usually in the central or axial interstitium (Fig. 7-12). They are usually associated with architectural distortion and volume loss. They typically produce traction bronchiectasis centrally in the bronchi that they encompass. This appearance is typical for silicosis and for coal worker’s pneumoconiosis, but it may also occur in end-stage sarcoidosis.

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.

Traction Bronchiectasis

Traction bronchiectasis (Fig. 7-14) is a phenomenon that occurs in the presence of severe lung fibrosis and distortion of lung architecture, in which the fibrous tissue produces traction on the bronchial walls, resulting in irregular bronchial dilation. It usually involves the more central bronchi.

Feb 28, 2016 | Posted by in RESPIRATORY IMAGING | Comments Off on Interstitial Lung Disease

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