Introduction
Background
Many benign and malignant diseases can cause pleural effusion. The characteristics of the fluid depend on the underlying pathophysiologic mechanism. The fluid can be transudate, nonpurulent exudate, pus, blood, or chyle. Imaging studies are valuable in detecting and managing pleural effusions but not in accurately characterizing the biochemical nature of the fluid.
Pathophysiology
A small amount of fluid is normally present in the pleural space. The parietal pleura continuously produce the fluid, which is absorbed by the visceral pleura and by the lymphatics of the parietal pleura. The hydrostatic, colloid osmotic, and tissue pressures affect circulation of the fluid. Alteration of 1 or more of these factors causes abnormal accumulation of fluid in the pleural space and is the primary mechanism of transudative effusions. For instance, increased hydrostatic pressure and decreased osmotic pressure cause effusions in congestive heart failure (CHF) and nephrotic syndrome, respectively.
In addition to alteration in the circulation of pleural fluid, injury to the pleura or subpleural lung parenchyma can cause increased vascular permeability and a shift of fluid from the pulmonary interstitium. This mechanism is primarily seen in exudative effusions, such as effusion associated with pneumonia and infarction. In certain cases, such as traumatic hemothorax or postsurgical chylothorax, the fluid accumulates from leaking damaged vessels or lymphatic ducts. Because peritoneal and pleural spaces communicate through defects in the diaphragm in some patients, peritoneal fluid crosses the diaphragm due to the negative pressure in the pleural cavity. Hepatic hydrothorax and pleural effusion secondary to gynecologic malignancies with ascites are 2 examples of this mechanism.
Frequency
United States
Because pleural effusion is a manifestation of a wide spectrum of diseases rather than a primary entity, its exact incidence is not known. In addition, pleural effusion can be an intermittent phenomenon in some case, such as in CHF.
An estimated 1 million patients develop pleural effusion each year. Most pleural effusions are secondary to CHF, malignancy, pneumonia, or pulmonary emboli. Approximately 72% of patients with CHF were found to have effusion at autopsy. Parapneumonic effusions develop in 36-66% of hospitalized patients with bacterial pneumonia. The incidence of effusion is lower in fungal and viral lung infections than in bacterial infection (2-9% in Coccidioides immitis infection and 7-19% in viral pneumonia). About 10-50% of patients with pulmonary embolism develop pleural effusion.
The incidence of the effusion varies from approximately 90-100% after coronary bypass or heart and/or lung transplantation to 16-37% in systemic lupus erythematosus, 4-20% in acute pancreatitis, and 5% in rheumatoid arthritis.
International
In 1 international epidemiologic study of pleural effusion in a well-defined region of central Bohemia, the incidence was 0.32%. Extrapolated to the entire population of the former Czechoslovakia, this rate represents 48,000 cases annually.
Mortality/Morbidity
Morbidity and mortality are primarily related to the underlying disease.
- Dyspnea is the main clinical symptom directly related to pleural effusion. The degree of respiratory function compromise is a function of the size of the effusion and the presence of associated lung parenchymal abnormalities.
- In some cases, the nature of the pleural effusion may affect the course of the underlying disease. For instance, uninfected parapneumonic effusions spontaneously clear without altering the outcome of the pneumonia, whereas effusions that require drainage increase the risk of morbidity and mortality. Effusions can resolve spontaneously, or they can resolve when the underlying disease is medically treated.
- In general, effusions that resolve spontaneously in less than 2 months are caused by CHF; pneumonia; acute pancreatitis; lung, heart, or liver transplant; pulmonary embolism; systemic lupus erythematosus; traumatic chylothorax; or uremia. Effusions after coronary artery bypass or cardiac injury and those related to sarcoid can resolve in less than 2 months; however, they can persist up to 6 months. Tuberculous pleurisy and chronic pancreatitis effusions resolve in 2-6 months. Rheumatoid pleurisy and benign asbestos pleural effusions resolve spontaneously in 2 -6 months; in some cases, these resolve in 6 months to 1 year. Effusion, especially malignant effusion, can rapidly recur after thoracentesis.
Presentation
The pleuritic chest pain associated with pleural irritation is localized, sharp, and severe. It is exacerbated by deep inspiration or coughing. The development of effusion may relieve the pleuritic pain. The mass effect associated with large pleural effusions can cause dyspnea. In cases of associated lung pathology, small effusions can cause dyspnea. The clinical history may help in limiting the differential diagnosis of the underlying etiology.
Small effusions might not be detectable on physical examination. Large effusions produce dependent, diminished breath sounds and dullness to percussion. Signs of an underlying process, such as pneumonia or CHF, can be detected on physical examination.
Preferred Examination
Different imaging modalities can be used to diagnose and manage pleural disease. Findings on chest radiographs frequently confirm the presence of pleural effusion. Lateral decubitus projections enhance the sensitivity of conventional radiography.
Depending on the clinical context, ultrasonography or CT can be used to confirm a pleural effusion, especially in cases of loculated pleural effusion, complete opacification of hemithorax, or associated lung parenchymal abnormalities. Ultrasonography and CT are more accurate than chest radiography in identifying the underlying etiology. Both modalities can depict small effusions not visualized radiographically. Ultrasonography and CT are also used to guide interventional procedures to manage pleural effusions.
MRI is sometimes used to evaluate questionable CT findings. MRI has been reported to be more sensitive than CT in differentiating benign from malignant causes of effusion.
Limitations of Techniques
Radiographic studies may not help in differentiating parenchymal processes from pleural processes. In addition, chest radiography is limited in evaluating the underlying etiology, as in differentiating benign disease from malignant pleural disease.
Differential Diagnoses
Other Problems to Be Considered
Elevated hemidiaphragm and/or herniation on a chest radiograph
Pleural thickening and/or fibrothorax on a chest radiograph
Subpleural fat on a chest radiograph
Ascites on a CT scan
Subphrenic abscess on a CT scan
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Further Reading
Keywords
increased pleural fluid, hydrothorax, hemothorax, pyothorax, chylothorax
