What is Pleural Effusion and What Causes It?
A pleural effusion is an abnormal accumulation of fluid in the pleural space exceeding 15-20 mL, resulting from imbalances in hydrostatic/oncotic pressures, increased pleural membrane permeability, decreased intrapleural pressure, or obstructed lymphatic drainage. 1
Definition and Pathophysiology
Pleural effusion represents fluid accumulation in the space between the visceral and parietal pleura. The underlying mechanisms include: 1
- Increased permeability of the pleural membrane
- Increased pulmonary capillary pressure (hydrostatic forces)
- Decreased negative intrapleural pressure
- Decreased oncotic pressure (hypoalbuminemia)
- Obstructed lymphatic flow preventing normal fluid clearance
Classification: Transudates vs Exudates
Transudative Effusions
Heart failure is the primary cause of transudative pleural effusions, accounting for more than 80% of cases. 2 Transudates occur when systemic factors influencing pleural fluid formation and reabsorption are altered. 1
Key causes of transudates include: 1, 2
- Congestive heart failure (most common)
- Cirrhosis with ascites (fluid moves directly from peritoneal cavity through diaphragmatic pores)
- Nephrotic syndrome (low oncotic pressure from proteinuria)
- Hypoalbuminemia
Exudative Effusions
Exudates occur when local factors affecting the pleura are altered, allowing fluid accumulation. 1
Leading causes of exudates include: 1, 3
- Pneumonia/parapneumonic effusions (approximately 40% of pneumonia patients develop effusions)
- Malignancy (lung cancer is primary cause; breast cancer is second most common for malignant effusions) 2
- Pulmonary embolism (75% present with pleuritic pain; effusions typically occupy <1/3 hemithorax) 1
- Tuberculosis (lymphocytic exudative effusion) 2
Comprehensive Causes by Category
Infectious Causes 1
- Parapneumonic effusions/empyema (purulent fluid collection)
- Tuberculosis pleuritis (highly complex with internal septations on ultrasound)
- HIV-related (Kaposi's sarcoma, parapneumonic, TB) 2
Malignant Causes 1
- Lung carcinoma
- Breast carcinoma
- Lymphomas and leukemias
- Malignant mesothelioma
Cardiac Causes 1, 2
- Left ventricular failure (transudation of pulmonary interstitial fluid overwhelms lymphatic clearance)
- Postcardiac surgery (usually left-sided with temporal relationship to surgery) 1
- Dressler syndrome 1
Inflammatory/Autoimmune Causes 1
- Rheumatoid arthritis
- Systemic lupus erythematosus
- Organizing pneumonia
End-Stage Renal Failure (ESRF) 1
Pleural effusions occur in approximately 24.7% of ESRF patients, with fluid overload being the leading cause (61.5%) rather than heart failure (9.6%). 1
ESRF-related mechanisms include: 1
- Fluid overload (most common)
- Hydrostatic and oncotic imbalances
- Hypoalbuminemia (nephrotic syndrome)
- Uraemic pleuritis (diagnosis of exclusion; exudative, often hemorrhagic)
- Urinothorax (urine diverted into pleural cavity; PF creatinine/serum creatinine >1)
- Vascular abnormalities from hemodialysis complications
- Peritoneal dialysis-associated pleuro-peritoneal leak
Other Causes 1
- Trauma (rib fractures, active bleeding on CT)
- Pulmonary embolism
- Benign asbestos-related pleural effusion (calcified pleural plaques may be present)
- Abdominopelvic pathology (cirrhosis, adnexal masses)
- Drug-induced (various medications can cause exudative effusions) 1
Clinical Pearls
Common diagnostic pitfall: 25-30% of cardiac and hepatic transudates are misclassified as exudates. 2
Bilateral effusions in a clinical setting strongly suggestive of transudate (e.g., heart failure) do not require aspiration unless atypical features are present or they fail to respond to therapy. 1
Malignancy can coexist with pleural infection in approximately 5% of cases, requiring follow-up imaging for up to 2 years if clinically concerning features persist. 1