Pleural Effusion

Pleural effusion is an excess amount of fluid that accumulates in the pleural cavity, which is the fluid filled space that surrounds the lungs.  Two categories exist; one is  “transudative pleural effusions” where there are systemic factors that cause fluid accumulation; most commonly due to left ventricular failure and cirrhosis.  The second is “exudative pleural effusions” where local factors affect fluid accumulation; most commonly due to bacterial pneumonia, malignancy, viral infection, and pulmonary embolism.

The incidence in the United States is estimated to be about 1.5 million cases annually, with congestive heart failure, bacterial pneumonia, malignancy, and pulmonary embolus responsible for most cases.  The estimated international occurrence is 320 cases per 100,000 people in industrialized countries.  The incidence is equal between the sexes.

The pleural space, which lies between the lung and chest wall, normally contains a thin layer of fluid, which helps with lung inflation.  Due to various pathological processes, excess fluid may accumulate in this space and cause a pleural effusion.

Pleural fluid normally enters the space from capillaries in the parietal pleura, and is removed by lymphatic drainage.  Pleural fluid accumulation occurs when there is either excess fluid formation or decreased fluid removal by lymphatics.

The most common cause of effusions (either transudative or exudative) is left ventricular failure (a transudative effusion).  This occurs because of the increased amounts of fluid in the lung interstitial spaces that exit across the visceral pleura; this overwhelms the capacity of the lymphatics to remove fluid.

The other main cause of transudative effusion is in patients with cirrhosis and ascites (AKA hepatic hydrothorax), in which the main mechanism is direct movement of peritoneal fluid through small openings in the diaphragm into the pleural space; these are frequently right sided effusions.  

Exudative effusions include parapneumonic effusion, which are effusions that form in the pleural space adjacent to a bacterial pneumonia (also associated with lung abscess and bronchiectasis).  Pneumonia causes an increase in lung interstitial fluid, which then moves across into the pleural space. Empyema refers to a grossly purulent effusion.   The next most common exudative effusion is malignant effusion due to metastatic disease (commonly lung cancer, breast cancer, and lymphoma).  Chylothorax (accumulation of chyle in pleural space) occurs due to trauma, in which the thoracic duct is disrupted and chyle accumulates. Hemothorax (blood in pleural space) occurs as a result of trauma and injury to the chest.

A detailed history should be obtained, with a complete evaluation of past medical and occupational history.  

The most common symptom is dyspnea, and is related more to distortion of the diaphragm and chest wall during breathing than to hypoxia.  Cough is present, and is often mild and nonproductive. Severe cough or purulent/bloody sputum suggests an underlying pneumonia.  

The presence of chest pain may result from pleural irritation, and raises the likelihood of exudative effusion such as pleural infection, mesothelioma, or pulmonary infarction.  Pain is described as sharp or stabbing, and is exacerbated by deep inspiration.

On physical exam, findings are variable, and depend on volume of the effusion.  Effusions smaller than 300 mL will generally have no physical findings; however over 300mL, one will expect to find diminished or inaudible breath sounds, dullness to percussion, decreased tactile fremitus, asymmetrical chest expansion, egophony at superior aspect of effusion, and pleural friction rub.  Effusions greater than 1000 mL may show mediastinal shift away from effusion with displacement of the trachea.

Once suspected, the patient should undergo chest imaging to diagnose the extent.  Chest X-Ray is the easiest initial modality, and if suspected on chest x-ray, should undergo chest CT scanning to detect thickened pleura or invasion of adjacent structures.

Once the effusion is confirmed, the next step is to determine the cause.  This is best accomplished by taking a sample of fluid via thoracentesis to determine its etiology.  

Transudative and exudative pleural effusions are distinguished by a set of criteria named Light’s criteria.  If any of the following criteria are met, then the effusion is exudative:

1) Pleural fluid protein/serum protein > 0.5

2) Pleural fluid LDH/serum LDH > 0.6

3) Pleural fluid LDH more than 2/3 the normal upper limit for serum

If none of these criteria met, the effusion is transudative.  If exudative, further workup is required to evaluate for malignancy, bacterial infections, pulmonary embolus, and tuberculosis.

Treatment will depend on the type of effusion.  Transudative effusions are treated by managing the underlying disorder (congestive heart failure, liver disease, renal disease, etc.). Exudative effusions due to infectious etiology should be drained via thoracentesis for source control.  Regardless of the type, if a large enough effusion, therapeutic thoracentesis should be performed to alleviate dyspnea, and prevent ongoing inflammation and fibrosis in parapneumonic effusions.