Background: Empyema is inflammatory fluid and debris within the pleural space. It results from an untreated pleural-space infection which progresses from free-flowing pleural fluid to a complex collection in the pleural space.

Empyema most commonly occurs in the setting of bacterial pneumonia. A parapneumonic effusion is associated with 20-60% of pneumonia cases. With appropriate antibiotic therapy a parapneumonic effusion most often resolves without complication and is of little clinical significance. However, in some cases, the parapneumonic effusion does not resolve; this is called a complicated effusion. The resulting infection and inflammatory response can proceed until adhesive bands form and the infected fluid becomes loculated pus within the pleural space.

Empyema may also result from causes other than a complication of bacterial pneumonia. Any process that introduces pathogens into the pleural space can lead to an empyema. Some other causes are the following:

• Thoracic trauma (About 1-5% of cases lead to an empyema.)

   

• Rupture of lung abscess into the pleural space

   

• Extension of a non–pleural-based infection (eg, mediastinitis, abdominal infection)

   

• Esophageal tear

   

• Iatrogenic introduction at the time of thoracic surgery

   

• Indwelling catheter that is a nidus for infection

Empyema has a high associated mortality rate related to respiratory failure and systemic sepsis, which occurs when the immune response and antibiotics are inadequate to control the infection. Drainage procedures are used to remove the collection and improve outcomes. The proper intervention depends on the severity of the disease and ranges from minimally invasive catheter drainage to open surgical decortication. Early intervention decreases the mortality rate from empyema. Prompt diagnosis, treatment, and proper management of empyema are crucial.

Pathophysiology: Normally, the secretion of fluid into the pleural space is in equilibrium with drainage via the subpleural lymphatics. The pleural lymphatic system is capable of draining almost 500 mL/d. When the ability of the lymphatics to drain the pleural space is exceeded, an effusion begins to form.

Parapneumonic effusions are the most common cause of empyema. Pneumonia triggers an inflammatory response. Inflammation near the pleura results in increased permeability of the mesothelial cells, the outer most monolayer of cells on the pleura. The mesothelial cells demonstrate an increased permeability to albumin and other proteins, which is why pleural effusions due to infection are rich in protein.

Chemical mediators of the inflammatory process stimulate the mesothelial cells to release chemokines, which recruit other inflammatory cells to the area. The mesothelial cells play an important role in attracting neutrophils into the pleural space. Under normal conditions, neutrophils are not found in pleural fluid. They are found only in the pleural space when they are recruited as part of the inflammatory process. Neutrophils, mononuclear phagocytes, and lymphocytes amplify the inflammatory response and release mediators to attract more inflammatory cells to the pleural space.

The formation of an empyema has 3 stages:

1. Exudative stage: Protein-rich pleural fluid remains free-flowing. The number of neutrophils is rapidly increasing. Glucose and pH levels are normal. Drainage of the effusion and appropriate antimicrobial therapy are normally sufficient for treatment.

    

2. Fibrinolytic stage: Viscosity of the pleural fluid is increasing. Coagulation factors are activated, and fibroblast activity begins coating the pleural membrane with an adhesive meshwork. Glucose and pH levels are lower than normal.

    

3. Organizing stage: Loculations are forming. Fibroblast activity causes adherence to the visceral and parietal pleura. This activity may progress with the formation of pleural peels in which the pleural layers are indistinguishable. Pus, which is a protein-rich fluid with inflammatory cells and debris, is present in the pleural space. Surgical intervention is often required at this stage.

Simple parapneumonic effusions resolve with treatment of the underlying disease. The lymphatics drain the effusion, and the mesothelial layer returns to normal. A parapneumonic effusion is considered complicated if the effusion does not resolve with antibiotic therapy. Complicated effusions progress to empyema if they are not drained.

In rare cases, an infected pleural collection can extend through the pleural space into the chest wall, called empyema necessitatis. A fluctuant mass can be palpable and if the empyema is left untreated, the infection can extend to the surface and drain spontaneously. This is considered a pleurocutaneous fistula.

Frequency:

• In the US: One million pleural-space infections occur each year in the United States.

  In a review of several published case series, Strange and Sahn (1999) analyzed the causes of pleural infections. A parapneumonic effusion was identified as the origin of pleural infection in 70% of the cases.

  A simple parapneumonic effusion proceeds to complicated effusion in 5-10% of cases.

  As many as 5% of patients with thoracic trauma develop an empyema.

• Internationally: Overall, the international incidence of pleural-space infection or empyema is not known; however, 4000 pleural-space infections occur each year in the United Kingdom.

Mortality/Morbidity:

• In a series of patients with clinical pneumonia, radiographic predictors of mortality and rates were as follows (Hasley, 1996):
  
  
  • No effusion: 7-day mortality rate, 1.3%; 30-day mortality rate, 4%
    
    
  • Unilateral effusion: 7-day mortality rate, 2.5%; 30-day mortality rate, 5.9%
    
    
  • Bilateral effusion: 7-day mortality rate, 10%; 30-day mortality rate, 28% (Bilateral effusions secondary to congestive heart failure were not excluded from this analysis.)

• In the United Kingdom, mortality rates due to frank empyema are 20%.

• Comorbid conditions increase the mortality due to pleural-space infection. In elderly and chronically debilitated patients, reported mortality rates are 25-75%.

Race: Race statistics for pleural-space infection or empyema are not known.

Sex: The male-to-female ratio is 1.8:1. No definite cause for the increased rate in men has been described. Men may seek treatment at a later stage of infection when antibiotics are insufficient for treatment.

Age: Age statistics for pleural-space infection are not known.

Anatomy: Within the thoracic cavity are 2 pleural cavities and the mediastinum. The mediastinum contains the heart, the esophagus, the trachea, the great vessels, and other structures. The left and right pleural cavities are lateral to the mediastinum and contain the lungs and their associated structures. Each lung is surrounded by a pleural sac. The pleura is a continuous layer of mesothelial cells and submesothelial matrix, which covers the chest wall and lung.

The parietal pleura lines the wall of the pleural cavity and is attached by connective tissue to the internal thoracic wall and superior surface of the diaphragm. During respiration, the parietal pleura moves with the contraction of the diaphragm and expansion of the chest wall. The parietal pleura is continuous with the visceral pleura at the hilum where structures enter and leave the lung.

The visceral pleura envelops the lung and is attached to the lung by connective tissue. Reflections of the visceral pleura line the lobes of the lungs and are visualized as the lung fissures on chest radiographs. Normally, the visceral pleural is in close proximity to the parietal pleura as the lung expands to fill the potential pleural space.

The pleural space contains a minimal amount of fluid (approximately 5 mL in a typical 70-kg individual). The pleural fluid decreases friction between the surfaces. Subpleural lymphatics drain excessive pleural fluid.

Clinical Details: As many as 70% of all cases of empyema are due to complications of pneumonia. In the setting of pneumonia, empyema may be associated with several symptoms. Patients may report chills, high-grade fever, sweating, poor appetite, malaise, and cough. Pleurisy and dyspnea may be symptoms in some patients. Pleurisy and dyspnea are not dependent on the size of the effusion. If an effusion of sufficient size is present, the physical findings may include dullness to percussion and absent breath sounds.

Before the modern era of antibiotics, most effusions and empyemas were related to Streptococcus pneumoniae pneumonia. Pneumococcal infections normally respond to antibiotic therapy, and these are less frequently seen in association with pleural-space infections today. Staphylococcal species and anaerobic pathogens are now the most common microorganisms associated with empyema. Because half of staphylococcal effusions progress to empyema, early drainage may be indicated if this organism is isolated. The re-emergence of tuberculosis may result in an increased association of Mycobacterium species with empyema.

Most parapneumonic effusions resolve with appropriate and timely antibiotic therapy. However, other effusions can progress to an empyema if prompt drainage is not performed. Interventions are uncomfortable and potential complications are risks.

See Intervention for more information about treatments.

Preferred Examination: A standard 2-view chest examination remains the first study for evaluating effusion or empyema. If an effusion is present, bilateral decubitus views are indicated for further characterization. These examinations remain informative and cost-effective.

Ultrasonography may show smaller volumes of pleural fluid and provide information on viscosity. Ultrasonography also may quickly demonstrate the presence or absence of septa within the pleural fluid collection.

CT of the chest is the imaging study that provides the most information. CT imaging depicts fluid, loculation, and thickening of the pleural membranes. CT and ultrasonography are also used in the placement of drainage catheters.

Limitations of Techniques: Two-view chest radiographs and decubitus views are not always possible in the intensive care unit. Radiographs are often limited to the bedside supine or semierect anteroposterior view in the very ill. A small fluid collection in the subpulmonic recess may be present and not detected on radiographs. Ultrasonography may demonstrate septa within the pleural fluid, but it poorly demonstrates the thickness of the pleura.

DIFFERENTIALS

Effusion, Pleural
Mesothelioma, Malignant

X-RAY

Findings: Free-flowing pleural fluid collects in the dependent portion of the pleural space. On 2-view chest radiographs, pleural fluid obscures the costophrenic angle (see Image 1). Approximately 75 mL of fluid is required to blunt the posterior costophrenic angle on a lateral chest radiograph. Almost 200 mL of fluid is required to blunt the lateral costophrenic angles on frontal radiographs. If loculations have formed, fluid opacity may be seen in a nondependent area. The D configuration of loculated fluid bulging out from the chest wall is a classically described but infrequently observed finding.

Although supine or semierect radiographs do not show pleural effusion as well as upright 2-view chest radiographs, an ill patient is often unable to stand. A unilateral free-flowing effusion shows increased hazy opacity on the side of the affected hemithorax.

If a pleural effusion is suspected, bilateral decubitus views are recommended. When an effusion is identified, the width of the layering fluid may be measured. If the fluid is less than 10 mm, the effusion may be managed medically and followed up with serial radiographs. However, if the effusion is more than 10 mm, thoracentesis or catheter drainage is should be performed, if clinically indicated. CT or ultrasonographic guidance is best for placement of pleural catheter for drainage.

Degree of Confidence: When 2-view chest radiographs are used for detecting pleural fluid, the sensitivity is 67% and the specificity is 70%. Decubitus views increase the degree of confidence. However, decubitus views are often skipped, and instead, the patient undergoes a CT examination.

False Positives/Negatives: The diagnosis of empyema is not made strictly on the basis of traditional radiographic findings. Further imaging with CT and confirmation of pleural infection with thoracentesis are usually required to diagnose empyema.