The Pleura

Chapter 18 The Pleura

Several imaging modalities are used to observe the pleural space. The most important is chest radiography, which remains the initial examination in the assessment of pleural disease. Other imaging techniques that may be used include computed tomography (CT) and ultrasound. Magnetic resonance imaging (MRI) plays a limited role in the assessment of pleural abnormalities. Positron emission tomography (PET) and integrated PET/CT scanning are increasingly used in the assessment of pleural malignancy.


The pleura consists of a visceral and parietal layer that is composed of a continuous surface epithelium of mesothelial cells and underlying connective tissue. The visceral pleura covers the lungs and interlobar fissures, whereas the parietal pleura lines the ribs, diaphragm, and mediastinum. A double fold of pleura extends from the hilum to the diaphragm to form the inferior pulmonary ligament. There is no communication between the two pleural cavities. The pleural space is a potential space that contains 2 to 10 mL of pleural fluid in the normal individual. The pleura can produce up to 100 mL of fluid in an hour, and the absorption capacity of the pleural surface is approximately 300 mL per hour.

The parietal pleura is supplied by systemic capillary vessels and drains into the right atrium by way of the azygos, hemiazygos, and internal mammary veins. The visceral pleura is supplied by pulmonary arterioles and capillaries and drains mainly into the pulmonary veins. Fluid is usually produced at the level of the parietal pleura and is drained by the visceral pleura. Lymphatics also play a role in the clearance of pleural fluid in health and disease. Lymphatic drainage occurs through the parietal pleural lymphatics and ultimately reaches the thoracic duct. The lymphatic drainage of the pleural space begins within lymphatic stomas located mainly in the mediastinal, intercostal, and diaphragmatic portions of the parietal pleura. They eventually drain into larger lymphatic channels. The visceral subpleural space is in continuity with the interlobular septa of the pulmonary interstitium. Unlike the parietal pleura, there is no communication between lymphatic channels of the visceral pleura and the pleural space. Lymph from the visceral pleura flows centripetally toward the hila.

The main manifestations of disease in the pleura include pleural effusion, pleural thickening (which may or may not be calcified), pleural air (i.e., pneumothorax), and pleural neoplasms. Primary disease of the pleura is rare. Most pleural abnormalities result from disease processes in other organs.


General Considerations and Clinical Features

Pleural effusions occur when the rates of entry and exit for pleural liquid and protein are no longer in equilibrium. Increased pleural fluid may result from one of six mechanisms (Box 18-1):

Pleural effusions may be transudates or exudates (Box 18-2). Transudates are usually caused by increased capillary hydrostatic pressure or decreased osmotic pressure, as in CHF, hypoalbuminemia, hepatic cirrhosis, and nephrotic syndrome. Management of transudates usually consists of treatment of the underlying cause, such as CHF. Exudates result from inflammatory and neoplastic processes involving the pleura. Other examples include pulmonary infarction and collagen vascular diseases.

The presence of an exudate requires a clinical investigation to determine the cause of the pleural effusion. Exudates are characterized by the following criteria: a pleural fluid protein concentration divided by the serum protein concentration that is greater than 0.5; a pleural fluid lactate dehydrogenase (LDH) level divided by the serum LDH level that is greater than 0.6; or a pleural fluid LDH level greater than two thirds of the upper limit of normal for the serum LDH level.

An exudative effusion with frank pus is referred to as an empyema. Hemothoraces may arise from traumatic laceration of vessels adjacent to the pleura. A pleural fluid hematocrit greater than 50% of the peripheral blood hematocrit establishes the diagnosis. Hemorrhagic effusions may also occur with neoplasms, tuberculosis, and infarction. Rupture or obstruction of major lymphatic channels, such as the thoracic duct, may result in a chylothorax, suggested by the presence of elevated triglyceride and cholesterol levels in the pleural fluid. Pleural effusions may also contain a high proportion of eosinophils. Causes include drug hypersensitivity, pneumonia, and pulmonary infarction. Intra-abdominal abnormalities may lead to pleural effusions such as ascites, benign ovarian fibroma (i.e., Meigs’ syndrome), hydronephrosis, and pancreatitis.

Predominantly left-sided effusions may be caused by pancreatitis, distal thoracic duct obstruction, Dressler’s syndrome, and postpericardiotomy syndrome. Predominantly right-sided effusions occur in proximal thoracic duct obstruction and ascites related to hepatic or ovarian disease and in endometriosis.

The leading causes of pleural effusion in the United States are CHF, pneumonia, malignancy, pulmonary embolism, viral infection, coronary artery bypass surgery, and cirrhosis. CHF and pneumonia account for more episodes of pleural effusion than the remaining five entities combined.

Radiologic Features

Standard Radiography

On an upright chest radiograph, a free pleural effusion demonstrates a meniscus sign, which is a concave, upward-sloping interface with the lung that causes sharp or indistinct blunting of the costophrenic angle (Figs. 18-1 and 18-2). Approximately 200 mL of fluid usually is necessary to blunt the lateral costophrenic angle, although smaller amounts (>75 mL) can produce a meniscus that blunts the posterior costophrenic angle on the lateral view (Fig. 18-3). A lateral decubitus view of the chest is much more sensitive than the upright view in the detection of pleural effusion, and as little as 5 mL of fluid can be demonstrated on decubitus views (Fig. 18-4).

In the upright position in the normal individual, a small amount of pleural fluid accumulates in a subpulmonic position. In certain individuals, a large amount of free-flowing pleural fluid may accumulate in this position before spilling into the costophrenic angles. On the frontal view, this produces a characteristic appearance with elevation of the apparent ipsilateral hemidiaphragm, flattening of the medial aspect, and displacement of the peak of the apparent diaphragm laterally. On the left side, this is easy to recognize because of separation of the stomach bubble from the apparent left hemidiaphragm (Fig. 18-5). A massive effusion produces a complete or nearly complete opacification of a hemithorax, with displacement of the mediastinum to the opposite side (Fig. 18-6). This appearance contrasts with complete atelectasis of the lung, in which the shift of the mediastinum is toward the side of the opaque hemithorax. Moderate to large amounts of pleural effusion may be missed on supine radiographs. These effusions layer posteriorly and produce a generalized increase in opacity of the hemithorax, through which the pulmonary vessels can be visualized (Fig. 18-7). There may be blunting of the costophrenic angle, and the fluid occasionally tracks over the apex of the lung, producing an apical cap.

Fluid may occasionally accumulate within fissures, and these accumulations may produce the appearance of a mass or pseudotumor (Fig. 18-8). Differentiation from a mass can be easily made because the fluid is free and shifts on decubitus views.

Pleural effusions frequently loculate (Fig. 18-9); i.e., they do not shift freely in the pleural space because of adhesions between the visceral and parietal pleura. Loculation of fluid occurs in exudative effusions, particularly in patients with empyema or hemothorax.

Computed Tomography

Free-flowing pleural fluid produces a sickle-shaped opacity in the most dependent part of the thorax posteriorly on CT scanning (Fig. 18-12). CT allows very small amounts of pleural fluid to be detected. Loculated fluid collections are seen as lenticular opacities in fixed positions (see Fig. 18-9). CT is of limited value in differentiating transudates from exudates or in the diagnosis of chylous pleural effusions. Although most chylous effusions are indistinguishable from other causes of pleural effusion on CT, low attenuation consistent with fat or a fat-fluid level in the pleural collection is rarely seen. Acute pleural hemorrhage, however, can be identified by the presence of a fluid-fluid level or by increased attenuation of the pleural fluid collection (Fig. 18-13).

Pleural fluid can be distinguished from ascites by several CT features (Box 18-3), including the displaced crus sign, the interface sign, the diaphragm sign, and the bare area sign. If the diaphragmatic crus is displaced away from the spine by an abnormal fluid collection, the fluid is located in the pleural space (Fig. 18-14). The location of ascites is lateral and anterior to the crus. The interface sign describes a sharp interface that can be identified between fluid and the liver or spleen when ascites is present. In cases of ascites, the interface is sharp, whereas in cases of pleural effusion, the interface is ill defined (Fig. 18-15). If the diaphragm is identifiable adjacent to an abnormal fluid collection in the right upper quadrant, the diaphragm sign is probably the most reliable means of differentiating fluid from ascites (Fig. 18-16). The location of the diaphragm is readily visible in patients with ascites but may not be identified in patients with pleural effusions. Pleural effusion is visualized outside the hemidiaphragm, whereas ascites is seen within the hemidiaphragmatic contour. The bare area is the portion of the right lobe of the liver that lacks peritoneal covering. Restriction of peritoneal fluid by the coronary ligaments from that area is another useful distinguishing sign. To differentiate pleural effusions from intra-abdominal fluid collections, all four signs should be assessed in each case. The attempt to differentiate ascites from pleural effusion may have pitfalls. A large pleural effusion, particularly on the left side, may cause inversion of the diaphragm, resulting in the pleural fluid being located centrally rather than peripherally.

CT is helpful in the assessment and management of loculated pleural effusions (see Fig. 18-9). Accurate localization of loculated collections is useful before drainage. Loculated effusions have a lenticular configuration with smooth margins, and they displace the adjacent parenchyma. This typical appearance for any pleural process is a distinguishing feature that can help differentiate a pleural from parenchymal process.

Magnetic Resonance Imaging

The role of MRI in the evaluation of the pleura is somewhat limited. MRI does provide certain advantages because of its ability to image the thorax directly in the axial, sagittal, and coronal planes. MRI may be slightly superior to CT in the characterization of pleural fluid (Fig. 18-17). Typically, fluid collections in the pleural cavity have a low signal intensity on T1-weighted images and relatively high signal intensity on T2-weighted images because of the water content. It may be possible to differentiate transudates, simple exudates, and exudates with the use of a triple-echo pulse sequence. Complex exudates have greater signal intensity than simple exudates, which are brighter than transudates. Preliminary results also suggest chylothorax may have distinctive findings on MRI, with signal intensity characteristics similar to those of subcutaneous fat. Subacute or chronic hematomas demonstrate typical signal intensity on MRI with a concentric ring sign, consisting of an outer dark rim composed of hemosiderin and bright signal intensity in the center because of the T1 shortening effects of methemoglobin.


Radiologic Features

The radiologic features of empyema are described in Box 18-5.

Feb 28, 2016 | Posted by in RESPIRATORY IMAGING | Comments Off on The Pleura
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