Interstitial Lung Disease
The interstitial space is defined as a continuum of loose connective tissue throughout the lung composed of three subdivisions:
- the bronchovascular (axial), surrounding the bronchi, arteries, and veins from the lung root to the level of the respiratory bronchiole;
- the parenchymal (acinar), situated between the alveolar and capillary basement membranes; and
- the subpleural, situated beneath the pleura, as well as in the interlobular septa.
Interstitial lung disease may result in four patterns of abnormal opacity on chest radiographs and CT scans:
These patterns are more accurately and specifically defined on CT.
A linear pattern is seen when there is thickening of the interlobular septa, producing Kerley lines. These septal lines were first described by Kerley in patients with pulmonary edema (2). Kerley B lines are short, straight lines (1 to 2 cm) perpendicular to and abutting the lower lateral pleural edge. Kerley A lines are generally longer (2 to 6 cm), they radiate out from the hilum toward the pleura but are not contiguous with the pleura, and they are most obvious in the upper and middle lungs. The interlobular septa contain pulmonary veins and lymphatics. The most common cause of interlobular septal thickening, producing Kerley A and B lines, is pulmonary edema, as a result of pulmonary venous hypertension and distension of the lymphatics. Anything that causes thickening of the interlobular septa can produce Kerley lines, including edema, inflammation, tumor, or fibrosis. Septal thickening without architectural distortion is more likely to represent pulmonary edema.
A reticular pattern results from the summation or superimposition of irregular linear opacities. The term reticular is defined as meshed, or in the form of a network. Reticular opacities can be described as fine, medium, or coarse, as the width of the opacities increases. A classic reticular pattern is seen with pulmonary fibrosis, in which multiple curvilinear opacities form small cystic spaces along the pleural margins and lung bases (honeycomb lung).
A nodular pattern consists of multiple round opacities, generally ranging in diameter from 1 mm to 1 cm, which may be difficult to distinguish from one another as individual nodules on a chest radiograph. Nodular opacities may be described as miliary (1 to 2 mm, the size of millet seeds), small, medium, or large, as the diameter of the opacities increases. A nodular pattern, especially with an upper lung-predominant distribution, suggests a specific differential diagnosis.
A reticulonodular pattern results from a combination of reticular and nodular opacities, or it can appear when reticular opacities are seen end-on. This pattern is often difficult to distinguish from a purely reticular or nodular pattern, and in such a case a differential diagnosis should be developed based on the predominant pattern. If there is no predominant pattern, causes of both nodular and reticular patterns should be considered. An acute appearance suggests pulmonary edema or pneumonia . A lower lung-predominant distribution with decreased lung volumes suggests idiopathic pulmonary fibrosis, asbestosis, collagen vascular disease, or chronic aspiration.
A reticulonodular pattern and larger-than-normal lung volumes can be seen with lymphangioleiomyomatosis and Langerhan cell histiocytosis (LCH).
A middle or upper lung-predominant distribution suggests mycobacterial or fungal disease, silicosis, sarcoidosis, LCH, extrinsic allergic alveolitis (hypersensitivity pneumonitis), or, very rarely, ankylosing spondylitis. Kerley lines help limit the differential diagnosis.
Associated lymphadenopathy suggests sarcoidosis; neoplasm (lymphangitic carcinomatosis, lymphoma, metastases); infection (viral, mycobacterial, or fungal); and silicosis.
Associated pleural thickening and/or calcification suggest asbestosis.
Associated pleural effusion suggests pulmonary edema, lymphangitic carcinomatosis, lymphoma, collagen vascular disease, or lymphangioleiomyomatosis (especially if the effusion is chylous).
Associated pneumothorax suggests lymphangioleiomyomatosis or LCH.
Hydrostatic pulmonary edema is defined as abnormal water in the lungs secondary to elevated pulmonary venous pressure from a failing left ventricle, mitral stenosis, increased circulating blood volume (as with anemias), renal failure (causing fluid retention), or overhydration.
Interstitial edema is seen on chest radiographs and CT scans as blurring of the margins of the blood vessels and bronchial walls (peribronchial cuffing), thickening of the fissures (subpleural edema), and thickening of the interlobular septae (Kerley lines)