What are the causes of unilateral transudative pleural effusion?

Causes of Unilateral Transudative Pleural Effusion

The most common causes of unilateral transudative pleural effusion include heart failure with atypical presentation, pulmonary embolism, hepatic hydrothorax, nephrotic syndrome, and peritoneal dialysis-associated effusions. While transudates are typically bilateral, several conditions can present with unilateral fluid accumulation.

Common Causes of Unilateral Transudative Effusions

1. Heart Failure

• Heart failure is the most common cause of transudative effusions overall 1
• Usually presents bilaterally, but can be unilateral in:
  • Early stages of heart failure
  • Patients with pre-existing pleural adhesions
  • Right-sided heart failure (right-sided effusion predominance)
• Diagnosis supported by:
  • NT-proBNP levels >1500 μg/mL in serum or pleural fluid 1
  • Response to diuretic therapy

2. Pulmonary Embolism

• Approximately 75% of patients with pulmonary embolism and pleural effusion have pleuritic pain 2
• Effusions typically occupy less than one-third of the hemithorax
• Dyspnea often disproportionate to effusion size
• Pleural fluid tests are generally unhelpful for diagnosis
• High index of suspicion required to avoid missing diagnosis 2

3. Hepatic Hydrothorax

• Occurs in patients with cirrhosis and ascites
• Usually right-sided (63-85% of cases)
• Results from direct passage of ascitic fluid through diaphragmatic defects
• Characterized by low protein and LDH levels consistent with transudate

4. Nephrotic Syndrome

• Causes transudative effusions due to low oncotic pressure and increased hydrostatic pressure 1
• Can present unilaterally, though bilateral effusions are more common
• Associated with hypoalbuminemia and significant proteinuria

5. Peritoneal Dialysis-Associated Effusions

• Occurs due to increased intra-abdominal pressures and diaphragmatic porosities 1
• Presents as extreme transudate with low protein and high glucose values
• Usually right-sided due to anatomical factors

6. Urinothorax

• Rare cause of unilateral transudative effusion
• Results from obstructive uropathy with retroperitoneal urine leakage
• Characterized by pleural fluid with low pH and pleural fluid-to-serum creatinine ratio >1

Diagnostic Approach

1. Clinical Assessment

   • Often sufficient for identifying transudative effusions 2
   • Consider underlying medical conditions (heart, liver, kidney disease)
   • Note any pleuritic pain (suggests pulmonary embolism)
   • Review medication history 2

2. Pleural Fluid Analysis

   • Diagnostic thoracentesis with 21G needle 2
   • Analyze for protein, LDH, pH, cytology, and cultures
   • Apply Light's criteria to differentiate transudate from exudate:
     • Pleural fluid/serum protein ratio ≤0.5 (transudate)
     • Pleural fluid/serum LDH ratio ≤0.6 (transudate)
     • Pleural fluid LDH ≤2/3 upper limit of normal serum LDH (transudate) 1

3. Additional Testing

   • Serum-effusion albumin gradient >1.2 g/dL suggests transudate even when Light's criteria indicate exudate 1
   • NT-proBNP testing for suspected heart failure
   • Consider imaging (chest CT) for undiagnosed effusions 2

Important Considerations

• Multiple etiologies may coexist in up to 30% of patients with unilateral pleural effusions 3
• Elevated NT-pro BNP (≥1,500 pg/ml) is predictive of multiple etiologies 3
• Transudative effusions indicate intact pleural membranes, suggesting good prognosis if underlying cause is treated 4
• Atypical presentations may require more extensive evaluation, including contrast-enhanced CT imaging 2

Management Principles

• Treatment should target the underlying cause 1
• Drainage generally not required unless symptoms are severe
• Avoid removing >1.5L in a single thoracentesis to prevent complications 1
• For refractory effusions, consider:
  • More aggressive treatment of underlying condition
  • Alternative modes of renal replacement therapy (for dialysis-associated effusions)
  • Pleurodesis for persistent symptomatic effusions 5