What are the causes of a unilateral right pleural effusion?

Causes of Unilateral Right Pleural Effusion

A unilateral right pleural effusion requires systematic evaluation to distinguish between transudative and exudative causes, with the most common etiologies being malignancy, parapneumonic effusion/empyema, pulmonary embolism, and tuberculosis for exudates, while heart failure (though typically bilateral) can occasionally present unilaterally on the right side. 1, 2

Classification Framework

The differential diagnosis is organized by whether the effusion is a transudate or exudate, which fundamentally guides the diagnostic approach:

Transudative Causes (Less Common for Unilateral Right-Sided)

• Heart failure: Accounts for >80% of all transudates but typically presents bilaterally; when unilateral, right-sided predominance is more common than left 2, 3
• Hepatic hydrothorax (cirrhosis): Represents ~10% of transudates and characteristically appears on the right side due to diaphragmatic defects 2, 4
• End-stage renal failure: Prevalence of 24.7% among ESRF patients, usually from fluid overload or uremic pleuritis 2, 5
• Hypoalbuminemia/nephrotic syndrome: Causes transudative effusions through decreased oncotic pressure 2, 5

Exudative Causes (Most Common for Unilateral Presentation)

Malignant effusions:

• Lung cancer: Leading cause of malignant pleural effusions, frequently unilateral and ipsilateral to the primary tumor 2, 5
• Breast cancer: Second most common malignant cause 2, 5
• Lymphoma: Accounts for ~10% of malignant effusions 2
• Mesothelioma: Important consideration given that 40% of needle incisions become invaded by tumor 1

Infectious causes:

• Parapneumonic effusion/empyema: Represents 16% of all pleural effusions; CT features include lentiform configuration, split pleura sign, and adjacent consolidation 1, 2
• Tuberculosis: Accounts for 6% of effusions; may mimic malignancy with circumferential pleural thickening but lacks chest wall invasion 1, 2
• HIV-related: Kaposi's sarcoma, parapneumonic effusions, and TB are leading causes 2

Vascular causes:

• Pulmonary embolism: Associated with effusions in up to 40% of cases; typically small, unilateral, and ipsilateral to the embolus 5, 6
• Approximately 75% present with pleuritic pain; effusions occupy <1/3 of hemithorax with dyspnea out of proportion to size 1
• 80% are exudates and 80% are bloodstained 5

Inflammatory/autoimmune causes:

• Rheumatoid arthritis: Affects pleura in ~5% of patients, more common in men; pleural fluid glucose <30 mg/dL strongly suggests this etiology 1, 2, 5
• Systemic lupus erythematosus: Causes pleural disease in up to 50% during disease course; LE cells in pleural fluid are diagnostic 2, 5
• Dressler syndrome: Post-cardiac surgery effusions, though more commonly left-sided 1

Trauma-related:

• Rib fractures, hemothorax (high density on acute CT), active bleeding 1

Occupational:

• Benign asbestos pleural effusion: Occurs within first two decades after exposure; calcified pleural plaques may be present 1, 5

Abdominopelvic pathology:

• Signs of cirrhosis, adnexal masses, pancreatitis 1

Drug-induced:

• Multiple medications can cause exudative effusions; accurate drug history is essential 1

Critical Diagnostic Considerations

Multiple etiologies are common: 30% of unilateral pleural effusions have more than one identifiable cause, which significantly impacts treatment decisions 7

NT-proBNP utility: Serum or pleural fluid NT-proBNP ≥1,500 pg/mL accurately diagnoses heart failure as primary or contributory cause (sensitivity 79%, specificity 88%) and predicts multiple etiologies 2, 4, 7

Imaging features on CT help differentiate infection from malignancy, though sensitivity is poor (0.20-0.48), necessitating diagnostic thoracentesis in unexplained cases 1

Common Pitfalls

• Misclassification: 25-30% of cardiac and hepatic transudates are misclassified as exudates by Light's criteria; use serum-effusion albumin gradient >1.2 g/dL to reclassify 2, 5, 4
• Coexistent disease: Malignancy and infection coexist in ~5% of cases; follow-up imaging for up to 2 years may be needed 1
• Persistent undiagnosed effusions: Always reconsider pulmonary embolism and tuberculosis as they are amenable to specific treatment 2, 5, 4
• Approximately 15% remain undiagnosed despite repeated testing; many ultimately prove malignant with continued observation 5