Causes of Bilateral Pleural Effusion
Heart failure is the leading cause of bilateral pleural effusions, accounting for more than 80% of transudative effusions and representing the most common etiology of bilateral presentations. 1, 2
Primary Causes by Mechanism
Transudative Causes (Most Common for Bilateral Effusions)
- Heart failure dominates as the cause of bilateral pleural effusions, responsible for 29% of all pleural effusions overall and over 80% of transudates 1, 2
- Liver cirrhosis (hepatic hydrothorax) accounts for approximately 10% of transudative effusions and 3% of all pleural effusions 1, 2
- End-stage renal failure causes pleural effusions in 24.7% of ESRF patients, typically from fluid overload, heart failure, or uremic pleuritis 1, 3
- Hypoalbuminemia and nephrotic syndrome can produce bilateral transudates through decreased oncotic pressure 1
Exudative Causes (Less Common for Bilateral Presentations)
- Malignancy accounts for 26% of all pleural effusions, with lung cancer being the most common neoplasm, followed by breast cancer 1, 2
- Lymphoma causes approximately 10% of malignant effusions and may present bilaterally, particularly in Hodgkin's disease where obstruction of lymphatic drainage by enlarged mediastinal nodes occurs 4, 1
- Pneumonia (parapneumonic effusions) represents 16% of all pleural effusions but typically presents unilaterally unless bilateral pneumonia is present 1
- Tuberculosis causes approximately 6% of pleural effusions and should always be reconsidered in undiagnosed cases 1, 3
Autoimmune and Rheumatologic Causes
- Systemic lupus erythematosus causes pleural disease in up to 50% of patients during their disease course and can present bilaterally 1, 3
- Rheumatoid arthritis affects the pleura in approximately 5% of patients, though bilateral presentation is less common 1, 3
Critical Diagnostic Distinction
The first step is determining whether the effusion is a transudate or exudate using Light's criteria, which has 98% sensitivity but only 72% specificity for identifying exudates. 2
When Heart Failure is Suspected but Light's Criteria Suggest Exudate:
- Apply the serum-effusion albumin gradient >1.2 g/dL to reclassify as transudate, as misclassification of cardiac transudates as exudates occurs in 25-30% of cases 1, 3
- NT-BNP levels >1500 μg/mL in serum or pleural fluid accurately diagnose heart failure as the cause 1, 2
Important Clinical Pitfall: Contarini's Syndrome
Bilateral pleural effusions can rarely have different causes on each side (Contarini's syndrome), though this is uncommon. 5, 6
- A frequent combination is parapneumonic effusion triggering heart failure, which produces a contralateral transudate 5
- While bilateral effusions nearly always share the same etiology, consider sampling both sides if clinical features are asymmetric or response to treatment is unilateral 6
Approach to Persistent Undiagnosed Bilateral Effusions
- Reconsider pulmonary embolism and tuberculosis as they are amenable to specific treatment 2, 3
- Pulmonary embolism associates with pleural effusions in up to 40% of cases, with 80% being exudates 3
- Many "undiagnosed" effusions ultimately prove malignant with continued observation, and approximately 15% remain undiagnosed despite repeated testing 3
- Consider thoracoscopy if malignancy is suspected after routine tests fail 1