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The lung's interstitium consists of connective tissue, lymphatics, blood vessels, and bronchi. These are the structures that surround and support the airspaces. Interstitial lung disease is sometimes referred to as infiltrative lung disease. Interstitial lung disease produces what can be thought of as discrete 'particles' of disease that develop in the abundant interstitial network of the lung. These 'particles' of disease can be further characterized as having three patterns of presentation:
These 'particles' or 'packets' of interstitial disease tend to be inhomogeneous, separated from each other by visible areas of normally aerated lung. The margins of 'particles' of interstitial lung disease are sharper than the margins of airspace disease that tend to be indistinct. Interstitial lung disease can be focal (as in a solitary pulmonary nodule) or diffusely distributed in the lungs. Usually no air bronchograms are present, as there may be with airspace disease. Some Causes of Interstitial Lung Disease Just as with the airspace pattern, there are many diseases that produce an interstitial pattern in the lung. Several will be discussed briefly here. They are roughly divided into those diseases that are predominantly reticular and those that are predominantly nodular. Keep in mind that many diseases have patterns that overlap and many interstitial lung diseases have mixtures of both reticular and nodular changes (reticulonodular disease). Predominantly Reticular Interstitial Lung Diseases
Pulmonary interstitial edema can occur because of increased capillary pressure (congestive heart failure), increased capillary permeability (allergic reactions), or decreased fluid absorption (lymphangitic blockade from metastatic disease). Considered the precursor of alveolar edema, pulmonary interstitial edema classically manifests four key radiologic findings: fluid in the fissures (major and minor), peribronchial cuffing (from fluid in the walls of bronchioles), pleural effusions, and Kerley B lines. Classically, the patient may have few physical findings in the lungs (rales) even though their chest radiograph demonstrates considerable pulmonary interstitial edema, because almost all of the fluid is in the interstitium of the lung rather than in the airspaces. With appropriate therapy, pulmonary interstitial edema usually clears rapidly (<48>
A disease of unknown etiology, usually occurring in older men who develop cough and shortness of breath. The early stage is a milder form known as desquamative interstitial pneumonia (DIP) and its findings are usually seen best on high-resolution CT scans of the chest. Later in the disease, it is called usual interstitial pneumonia (UIP), and there is marked thickening of the interstitium, bronchiectasis, and a pattern of cystic changes in the lung called honeycombing. UIP is also best demonstrated on high-resolution CT scans of the chest. Conventional radiographs of the chest may show a fine or, later in the disease, a coarse reticular pattern that is bilaterally symmetrical, most prominent at the bases, subpleural in location and frequently associated with volume loss. Idiopathic pulmonary fibrosis is considered the end-stage disease along the spectrum of these interstitial pneumonias.
Rheumatoid lung disease is found in some patients with rheumatoid arthritis. The three most common manifestations of rheumatoid lung disease are (in order of decreasing frequency) pleural effusions, interstitial lung disease, and nodules in the lung called necrobiotic nodules. Pleural effusions are usually unilateral and characteristically remain unchanged in appearance for long periods of time. Rheumatoid interstitial lung disease is usually reticular, can be seen diffusely throughout the lung, but is usually most prominent at the lung bases. Necrobiotic nodules are identical to subcutaneous rheumatoid nodules and occur mostly at the lung bases near the periphery of the lung; cavitation is frequent. Unlike the joint findings of rheumatoid arthritis, which are more common in women, the thoracic manifestations of rheumatoid arthritis are more common in men. Predominantly Nodular Interstitial Diseases
Bronchogenic carcinoma has four major cell types:adenocarcinoma, squamous cell carcinoma, small cell carcinoma, and large cell carcinoma. Adenocarcinomas, in particular, can present as a solitary peripheral pulmonary nodule. As a rule, on conventional chest radiographs, nodules or masses in the lung are more sharply marginated than airspace disease, producing a relatively clear demarcation between the nodule and the surrounding normal lung tissue. CT scans may demonstrate spiculation or irregularity of the lung nodule that may not be apparent on conventional radiographs.
Metastases to the lung can be divided into three categories depending on the pattern of disease demonstrated in the lung: hematogenous, lymphangitic and direct extension. Hematogenous metastases arrive via the bloodstream and usually produce two or more nodules in the lungs, sometimes called cannonball metastases because of their large, round appearance. Primary tumor sites that classically produce nodular metastases to the lung include breast, colorectal, renal cell, bladder and testicular, head and neck carcinomas, soft tissue sarcomas, and malignant melanoma. The second form of tumor dissemination is lymphangitic spread. The pathogenesis of lymphangitic spread to the lungs is somewhat controversial but most likely involves blood-borne spread to the pulmonary capillaries and then invasion of adjacent lymphatics. An alternative means of lymphangitic spread is obstruction of central lymphatics usually in the hila with retrograde dissemination through the lymphatics in the lung. Regardless of the mode of transmission, lymphangitic spread to the lung tends to resemble pulmonary interstitial edema from congestive heart failure, except, unlike congestive heart failure, it tends to be localized to a segment or involve only one lung. Primary tumor sites that classically produce the lymphangitic pattern of metastases to the lung include breast, lung, stomach, pancreatic, and, infrequently, prostate carcinoma. Findings include: Kerley lines, fluid in the fissures, and pleural effusions Direct extension is the least common form of tumor spread to the lungs because the pleura is surprisingly resistant to the spread of malignancy through direct violation of its layers. Direct extension would most likely produce a localized subpleural mass in the lung, frequently with adjacent rib destruction. Mixed Reticular and Nodular Interstitial Disease (Reticulonodular Disease)
In addition to the bilateral hilar and right paratracheal adenopathy chracteristic of this disease, about half of patients with thoracic sarcoid also demonstrate interstitial lung disease. The interstitial lung disease is frequently a mixture of both reticular and nodular components. There is a progression of disease in sarcoid that tends to start with adenopathy (Stage I), proceed to a combination of both interstitial lung disease and adenopathy (Stage II), and then progress to a stage in which the adenopathy regresses while the interstitial lung disease remains (Stage III). Most patients with parenchymal lung disease will undergo complete resolution of the disease. From: Learning Radiology Recognizing The Basics |
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