What are the causes of a transudative pleural effusion?

Transudative Pleural Effusion: Differential Diagnosis

The most common causes of transudative pleural effusions are congestive heart failure (by far the leading cause), cirrhosis with ascites, and nephrotic syndrome, with less common causes including pulmonary embolism, peritoneal dialysis, and urinothorax. 1

Pathophysiology

Transudative effusions develop when systemic factors alter the balance of hydrostatic and oncotic pressures across the pleura, favoring fluid accumulation while the pleural membranes themselves remain intact and have normal capillary permeability. 1 This distinguishes them from exudates, where the pleural surface or local capillary permeability is directly altered. 1

Common Causes (in order of frequency)

Congestive Heart Failure (Most Common)

• Accounts for the vast majority of transudative effusions in clinical practice. 2, 3, 4
• Most patients have left ventricular failure specifically. 4
• The mechanism involves transudation of pulmonary interstitial fluid across the visceral pleura that overwhelms lymphatic drainage capacity. 4
• Clinical pitfall: Do not aspirate bilateral effusions in a clinical setting strongly suggestive of heart failure unless atypical features are present or the effusions fail to respond to therapy. 1

Cirrhosis with Ascites

• Second most common cause of transudative effusions. 2, 4
• Most patients with cirrhotic pleural effusion also have ascites. 4
• Fluid moves directly from the peritoneal cavity into the pleural space through diaphragmatic pores (hepatic hydrothorax). 3, 4
• Right-sided effusions are more common due to anatomical positioning. 5

Nephrotic Syndrome

• Effusions result from decreased oncotic pressure due to hypoalbuminemia. 1, 2
• Many patients with nephrotic syndrome develop transudative effusions. 3

Pulmonary Embolism

• Can present as a transudative effusion (though often exudative). 2, 4
• Key clinical features: Approximately 75% have pleuritic chest pain, effusions typically occupy less than one-third of hemithorax, and dyspnea is often disproportionate to effusion size. 1
• Critical pitfall: Pleural fluid tests are unhelpful in diagnosing pulmonary embolism—maintain high clinical suspicion to avoid missing this diagnosis. 1

Less Common Causes

Peritoneal Dialysis

• Fluid can accumulate in the pleural space during peritoneal dialysis through transdiaphragmatic communication. 1, 3

Urinothorax

• Occurs in the setting of renal failure with fluid overload. 3, 6

Hypoalbuminemia (from any cause)

• Low plasma oncotic pressure promotes fluid accumulation. 1

Important Clinical Considerations

When to Suspect Concomitant Conditions

• Malignant effusions can occasionally be transudative (approximately 5% of cases) when a concomitant transudative state exists. 6
• Look for: fluid overload from renal failure, congestive heart failure, deep venous thrombosis/pulmonary embolism, or superior vena cava obstruction. 6

Diagnostic Approach

• Clinical assessment alone can often identify transudative effusions, particularly in left ventricular failure with confirmatory chest radiograph. 1
• When pleural fluid protein is between 25-35 g/L, use Light's criteria to accurately differentiate transudates from exudates. 1
• Light's criteria for transudate: pleural fluid/serum protein ratio <0.5, pleural fluid/serum LDH ratio <0.6, and pleural fluid LDH <2/3 upper limit of normal for serum. 2, 4, 7

Management Principles

• Treatment should be directed at the underlying systemic condition causing the transudate. 2, 3
• If the underlying problem is corrected, the effusion will be reabsorbed since the pleural membranes are intact. 3
• For refractory massive transudative effusions, tetracycline pleurodesis can be considered. 2