More than 1 million cases of pleural effusion occur in the United States each year. Pleural effusions are a common presentation of many pulmonary and systemic diseases, including congestive heart failure (CHF), malignancy, sarcoidosis, and infection. About 25% to 30% of patients seen by a pulmonary consultant have evidence of pleural disease.
In this article, we will describe the steps to follow when evaluating a patient who has suspected pleural effusion. Specifically, we will discuss the clinical presentation, chest film findings, and the role of thoracentesis and medical thoracoscopy.
CAUSES OF PLEURAL EFFUSION
The pleural space is 10 to 20 µm in width and normally contains about 0.1 mL/kg of fluid. A volume greater than 7 to 14 mL is abnormal. Many mechanisms can result in abnormal amounts of pleural fluid, including:
- Increased hydrostatic pressures in the microvascular circulation.
- Decreased oncotic pressures in the microvascular circulation.
- Decreased pleural space pressure (resulting from lung collapse).
- Increased permeability of the microvascular circulation.
- Obstruction of lymphatic drainage (Figure 1).
Generally, transudative effusions are formed in response to increased hydrostatic pressure, while exudative effusions form when pleural inflammation or disrupted lymphatic drainage results in increased protein leak or decreased protein removal from the pleural space. In CHF, pleural effusions are secondary to pulmonary venous hypertension. Neoplasms can cause pleural effusions by direct involvement of the pleura, by lymphatic obstruction, or in association with a post-obstructive pneumonia. Pleural effusions associated with pulmonary embolism are secondary to increased capillary permeability, pleuropulmonary hemorrhage, and increased hydrostatic pressure.
Many patients with pleural effusions are asymptomatic. However, when symptoms arise, they do so because of pleural inflammation or the effusion's effects on mechanics. The most common symptoms of pleural effusion are dyspnea, nonproductive cough, and pleuritic chest pain.
The mechanisms by which dyspnea occurs are not well understood, but they do not appear to correlate with blood oxygen levels or the size of the pleural effusion. Dyspnea is probably related to increased thoracic cage size, which affects respiratory muscle function. Nonproductive cough may occur secondary to lung compression and resultant bronchial irritation.
Pleuritic chest pain is associated with inflammation of the parietal pleura. Pain is occasionally referred to the abdomen. If the central portion of diaphragmatic pleura is involved, patients experience pain in the lower chest and ipsilateral shoulder simultaneously. Historical features, including underlying disease processes, drug use, and radiation therapy, can alert you to the possibility of pleural effusion in a patient with less common symptoms.
Several physical findings suggest the presence of pleural effusion. Tactile fremitus is lost over the area of effusion because voice-induced vibrations are attenuated by the fluid adjacent to aerated lung. This finding is more sensitive than the use of percussion for detecting pleural fluid collections.
Absent or diminished breath sounds over the area of effusion are characteristic. Pleural friction rubs are occasionally noted in the initial stages or as the effusion resolves and are caused by roughened pleural surfaces moving across one another.
Radiography. Once you suspect a pleural effusion, obtain chest films to confirm its presence and to look for other abnormalities that can help identify the cause (Table 1). Small pleural effusions appear on plain films as blunting of the normally sharp costophrenic angles. These effusions usually represent more than 100 mL of fluid. If the effusion volume is greater than about 500 mL, chest films are 100% sensitive (Figure 2).1
Lateral decubitus films are essential to confirm that the effusion is not loculated. Loculated effusions may be misinterpreted as parenchymal infiltrates.
Cardiomegaly with bilateral pleural effusions is most consistent with a diagnosis of CHF. The differential diagnosis of a normal heart size with bilateral effusions includes malignancy (most common), rheumatoid pleurisy, systemic lupus erythematosus (SLE), esophageal rupture, nephrotic syndrome, and cirrhosis with ascites. Massive effusions are most commonly caused by malignancy.
Ultrasonography. An ultrasound scan can detect as little as 5 mL of pleural fluid. Ultrasonography is extremely sensitive in identifying septations within a pleural fluid collection. Small or loculated effusions are best tapped under ultrasonographic guidance.
CT. A CT scan of the chest is not appropriate for the initial confirmation of pleural effusion. It is most helpful after thoracentesis for detecting suspected parenchymal or pleural abnormalities. A CT scan is ideal if pleural abnormalities are seen on the chest radiograph, because it allows imaging of the entire pleural space and differentiates pleural fluid from pleural thickening or pleural-based masses.2
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