What are the common causes of bilateral pleural effusions?

Bilateral Pleural Effusion Causes

Bilateral pleural effusions are most commonly caused by heart failure, which accounts for more than 80% of transudative effusions, followed by hepatic cirrhosis, nephrotic syndrome, and end-stage renal disease. 1, 2

Transudative Causes (Most Common for Bilateral Effusions)

Bilateral effusions in a clinical setting strongly suggestive of transudate should not be aspirated unless atypical features are present or they fail to respond to therapy. 3

Primary Transudative Causes:

• Congestive heart failure: The dominant cause, responsible for >80% of transudative effusions, where elevated pulmonary venous pressure drives fluid accumulation 1, 2, 4

  • NT-proBNP >1500 μg/mL in serum or pleural fluid can accurately diagnose heart failure as the cause 1, 2

• Hepatic cirrhosis with ascites: Fluid crosses the diaphragm into the pleural space, typically right-sided but can be bilateral 5, 4

• Nephrotic syndrome: Reduced plasma oncotic pressure combined with salt retention increases hydrostatic pressure 1

  • May appear exudative despite being transudative, requiring clinical correlation rather than relying solely on Light's criteria 1

• End-stage renal failure: Multiple mechanisms including fluid overload, decreased lymphatic clearance, and uraemic pleuritis 1

  • Prevalence of 24.7% in dialysis patients 6
  • Associated with significantly worse prognosis: 31% mortality at 6 months, 46% at 1 year 1

• Peritoneal dialysis: Dialysate leaks into pleural cavity, producing extreme transudates with very low protein and markedly elevated glucose 1

Exudative Causes (Less Common for Bilateral Presentation)

Key Exudative Causes That Can Present Bilaterally:

• Malignancy: Accounts for 26% of all pleural effusions 6

  • Breast cancer commonly causes bilateral effusions through chest wall lymphatics or hepatic metastases 3
  • Lymphoma and metastatic disease from unknown primary 3

• Drug-induced effusions: Multiple medications can cause exudative effusions; accurate drug history is essential 3

• Collagen vascular diseases: Systemic lupus erythematosus and rheumatoid arthritis can present bilaterally 5

• Tuberculosis: Produces lymphocyte-predominant exudates (6% of cases), though typically unilateral 6

Critical Diagnostic Pitfalls

Misclassification occurs in 25-30% of cardiac and hepatic transudates as exudates when using Light's criteria alone. 6, 2

• Use serum-effusion albumin gradient >1.2 g/dL to reclassify as transudate when heart failure is suspected 1, 6
• NT-proBNP >1500 μg/mL corrects misclassification errors 1, 2

Systematic Approach to Bilateral Effusions

1. Clinical assessment first: If bilateral effusions with clear signs of heart failure, cirrhosis, or nephrotic syndrome are present, treat the underlying cause without aspiration 3

2. When to aspirate bilateral effusions: 3

   • Atypical features present
   • Failure to respond to therapy
   • Diagnostic uncertainty

3. If aspiration performed: Analyze for protein, LDH, pH, cell count with differential, Gram stain, culture, and cytology 3

4. Apply Light's criteria to distinguish transudate from exudate, but use albumin gradient or NT-proBNP to correct misclassifications 1, 6