- Author: Jeffrey Rubins, MD; Chief Editor: Ryland P Byrd, Jr, MD more...
A pleural effusion is an abnormal collection of fluid in the pleural space resulting from excess fluid production or decreased absorption or both. It is the most common manifestation of pleural disease, with etiologies ranging from cardiopulmonary disorders to symptomatic inflammatory or malignant diseases requiring urgent evaluation and treatment. Approximately 1.5 million pleural effusions are diagnosed in the United States each year (see the images below).
The pleural space is bordered by the parietal and visceral pleurae. The parietal pleura covers the inner surface of the thoracic cavity, including the mediastinum, diaphragm, and ribs. The visceral pleura envelops all lung surfaces, including the interlobar fissures. The right and left pleural spaces are separated by the mediastinum.
The pleural space plays an important role in respiration by coupling the movement of the chest wall with that of the lungs in 2 ways. First, a relative vacuum in the space keeps the visceral and parietal pleurae in close proximity. Second, the small volume of pleural fluid, which has been calculated at 0.13 mL/kg of body weight under normal circumstances, serves as a lubricant to facilitate movement of the pleural surfaces against each other in the course of respirations. This small volume of fluid is maintained through the balance of hydrostatic and oncotic pressure and lymphatic drainage, a disturbance of which may lead to pathology.
The normal pleural space contains approximately 10 mL of fluid, representing the balance between (1) hydrostatic and oncotic forces in the visceral and parietal pleural vessels and (2) extensive lymphatic drainage. Pleural effusions result from disruption of this balance.
Pleural effusion is an indicator of an underlying disease process that may be pulmonary or nonpulmonary in origin and may be acute or chronic.[4, 5] Although the etiologic spectrum of pleural effusion is extensive, most pleural effusions are caused by congestive heart failure, pneumonia, malignancy, or pulmonary embolism. The following mechanisms play a role in the formation of pleural effusion:
Altered permeability of the pleural membranes (eg, inflammation, malignancy, pulmonary embolus)
Increased capillary permeability or vascular disruption (eg, trauma, malignancy, inflammation, infection, pulmonary infarction, drug hypersensitivity, uremia, pancreatitis)
Increased capillary hydrostatic pressure in the systemic and/or pulmonary circulation (eg, congestive heart failure, superior vena cava syndrome)
Reduction of pressure in the pleural space, preventing full lung expansion or "trapped lung" (eg, extensive atelectasis, mesothelioma)
Decreased lymphatic drainage or complete blockage, including thoracic duct obstruction or rupture (eg, malignancy, trauma)
Increased peritoneal fluid, with migration across the diaphragm via the lymphatics or structural defect (eg, cirrhosis, peritoneal dialysis)
Movement of fluid from pulmonary edema across the visceral pleura
Persistent increase in pleural fluid oncotic pressure from an existing pleural effusion, causing further fluid accumulation
The net result of effusion formation is a flattening or inversion of the diaphragm, mechanical dissociation of the visceral and parietal pleura, and a restrictive ventilatory defect as measured by pulmonary function testing.
Pleural effusions are generally classified as transudates or exudates, based on the mechanism of fluid formation and pleural fluid chemistry. Transudates result from an imbalance in oncotic and hydrostatic pressures, whereas exudates are the result of inflammation of the pleura or decreased lymphatic drainage. In some cases, the pleural fluid may have a combination of transudative and exudative characteristics.
Transudates are usually ultrafiltrates of plasma in the pleura due to imbalance in hydrostatic and oncotic forces in the chest. However, they can also be caused by the movement of fluid from peritoneal spaces or by iatrogenic infusion into the pleural space from misplaced or migrated central venous catheters or nasogastric feeding tubes. Transudates are caused by a small, defined group of etiologies, including the following:
Congestive heart failure
Cirrhosis (hepatic hydrothorax)
Atelectasis - Which may be due to malignancy or pulmonary embolism
Urinothorax - Usually due to obstructive uropathy
Cerebrospinal fluid (CSF) leaks to the pleura - Generally in the setting of ventriculopleural shunting or of trauma or surgery to the thoracic spine
Duropleural fistula - Rare, but may be a complication of spinal cord surgery
Extravascular migration of central venous catheter 
Glycinothorax - A rare complication of bladder irrigation with 1.5% glycine solution following urologic surgery
Exudates are produced by a variety of inflammatory conditions and often require more extensive evaluation and treatment than transudates. Exudates arise from pleural or lung inflammation, impaired lymphatic drainage of the pleural space, transdiaphragmatic movement of inflammatory fluid from the peritoneal space, altered permeability of pleural membranes, and increased capillary wall permeability or vascular disruption. Pleural membranes are involved in the pathogenesis of the fluid formation. The permeability of pleural capillaries to proteins is increased in disease states resulting in an elevated protein content.
The more common causes of exudates include the following:
Parapneumonic causes 
Malignancy (most commonly lung or breast cancer, lymphoma, and leukemia; less commonly ovarian carcinoma, stomach cancer, sarcomas, melanoma) 
Collagen-vascular conditions (rheumatoid arthritis, systemic lupus erythematosus  )
Postcardiac injury syndrome
Status-post coronary artery bypass graft surgery
Meigs syndrome (benign pelvic neoplasm with associated ascites and pleural effusion)
Ovarian hyperstimulation syndrome
Drug-induced pleural disease (see Pneumotox On Line for an extensive list of drugs that can cause pleural effusion)
Asbestos-related pleural disease
Yellow nail syndrome (yellow nails, lymphedema, pleural effusions)
Trapped lung (localized pleural scarring with the formation of a fibrin peel prevents incomplete lung expansion, at times leading to pleural effusion)
Chylothorax (acute illness with elevated triglycerides in pleural fluid)
Pseudochylothorax (chronic condition with elevated cholesterol in pleural fluid)
Fistula (ventriculopleural, biliopleural, gastropleural)
Occurrence in the United States
Because pleural effusion is a manifestation of underlying disease, its precise incidence is difficult to determine. However, the incidence in the United States is estimated to be at least 1.5 million cases annually. Congestive heart failure, bacterial pneumonia, malignancy, and pulmonary embolus are responsible for most of these cases.
The estimated prevalence of pleural effusion is 320 cases per 100,000 people in industrialized countries, with a distribution of etiologies related to the prevalence of underlying diseases.
In general, the incidence of pleural effusion is equal between the sexes. However, certain causes have a gender predilection. About two thirds of malignant pleural effusions occur in women, in whom they are associated with breast and gynecologic malignancies.
Pleural effusion associated with systemic lupus erythematosus is also more common in women than in men. In the United States, the incidence of pleural effusion in the setting of malignant mesothelioma is higher in males, probably because of their higher occupational exposure to asbestos.
Pleural effusions associated with chronic pancreatitis are more common in males, with the majority of male cases having alcoholism as the etiology. Rheumatoid effusions also occur more commonly in males than in females.
Race- and age-related demographics
Because pleural effusion is a manifestation of underlying disease, racial differences will most likely reflect racial variation in incidence of the causative disorder.
Pleural effusions usually occur in adults. However, they appear to be increasing in children, often in the setting of underlying pneumonia. Fetal pleural effusions have also been reported and under certain circumstances may be treated prior to delivery.
The prognosis in pleural effusion varies in accordance with the condition’s underlying etiology. However, patients who seek medical care earlier in the course of their disease and those who obtain prompt diagnosis and treatment have a substantially lower rate of complications than do patients who do not.
Morbidity and mortality
Morbidity and mortality of pleural effusions are directly related to cause and stage of the underlying disease at the time of presentation, and biochemical findings in the pleural fluid.
Morbidity and mortality rates in patients with pneumonia and pleural effusions are higher than those in patients with pneumonia alone. Parapneumonic effusions, when recognized and treated promptly, typically resolve without significant sequelae. However, untreated or inappropriately treated parapneumonic effusions may lead to empyema, constrictive fibrosis, and sepsis.
Development of a malignant pleural effusion is associated with a very poor prognosis, with median survival of four months and mean survival of less than one year. . The most common associated malignancy in men is lung cancer. The most common associated malignancy in women is breast cancer. Median survival ranges from 3-12 months, depending on the malignancy. Effusions from cancers that are more responsive to chemotherapy, such as lymphoma or breast cancer, are more likely to be associated with prolonged survival, compared with those from lung cancer or mesothelioma.[15, 16]
Cellular and biochemical findings in the fluid may also be indicators of prognosis. For example, a lower pleural fluid pH is often associated with a higher tumor burden and a worse prognosis.
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