Pleural Effusion

Definition

Pathophysiology & Classification

Transudates

• Heart failure (most common cause overall): elevated pulmonary venous pressure overwhelms parietal pleural lymphatic drainage
• Nephrotic syndrome
• Cirrhosis / hepatic hydrothorax (5–10% of cirrhotic patients): small diaphragmatic defects allow ascitic fluid into the pleural space; 85% are right-sided
• Myxoedema: borderline transudate/exudate

Exudates

• Parapneumonic effusion — most common exudative cause in the US; seen in up to 50% of community-acquired pneumonia
• Malignancy (2nd most common exudate) — lung, breast, lymphoma account for ~75% of malignant effusions
• Tuberculosis, pulmonary embolism, pancreatitis, rheumatoid disease, SLE, oesophageal rupture, post-cardiac injury syndrome, aortic dissection

Laterality clues

Clinical Features

• May be clinically silent or present with dyspnoea, cough, pleuritic chest pain
• Percussion dullness and decreased breath sounds at the lung base
• Absence of mediastinal shift despite a large effusion: suggests ipsilateral lung collapse or extensive pleural malignancy (e.g., mesothelioma)

Imaging

Chest X-ray (PA erect)

CXR (A) and CT (C) demonstrating pleural effusion with the meniscus sign and dependent layering — Grainger & Allison's Diagnostic Radiology

• Small effusion (<200–500 mL): initially subpulmonary/invisible; first sign is blunting of the posterior then lateral costophrenic angle
• Moderate effusion: homogeneous opacity with obliteration of the costophrenic angle and hemidiaphragm; meniscus sign (concave upper border, higher laterally)
• ~1000 mL: reaches the level of the 4th anterior rib
• Massive effusion: dense opacification with contralateral mediastinal shift
• Lateral decubitus view: fluid layers and the lung floats — increases sensitivity for small effusions

CT

• Distinguishes free vs. loculated fluid; identifies underlying lung/pleural pathology
• Split-pleura sign (parietal + visceral pleural enhancement): highly indicative of exudate (infection or malignancy)
• Cannot reliably distinguish transudate from exudate by CT number alone

Ultrasound

• Most sensitive bedside tool for small effusions
• Simple effusion: anechoic; complex/exudative: echogenic with mobile strands or septations
• Guides safe thoracentesis

A: CT — simple free-flowing left effusion (red arrow). B: CT — loculated effusion with two pockets. C: Ultrasound — simple effusion. D: Ultrasound — complex effusion with septations — Harrison's Principles of Internal Medicine

Pleural Fluid Analysis (Thoracentesis)

Light's Criteria (sensitivity 98–99%, specificity 65–86%)

Diuresis caveat: Diuretics concentrate pleural protein, potentially misclassifying a transudate as exudate. Use the serum-to-pleural albumin gradient >1.2 g/dL in this setting (though sensitivity drops >10%).

Additional Tests

Parapneumonic Effusion — Indications for Evacuation

1. Loculated pleural fluid
2. Pleural fluid pH <7.20
3. Pleural fluid glucose <3.3 mmol/L (<60 mg/dL)
4. Positive Gram stain or culture
5. Grossly purulent fluid (empyema)
6. Elevated LDH >900 IU/L

Management

General

• Therapeutic thoracentesis: indicated for dyspnoea at rest; drain 1.0–1.5 L; avoid >1.5 L acutely to prevent re-expansion pulmonary oedema
• Treat the underlying cause

Heart Failure

• Diuresis and optimisation of cardiac therapy resolves >80% within 2 weeks
• Diagnostic thoracentesis warranted if: unilateral, patient is febrile, or pleuritic pain present

Empyema

• Chest tube drainage mandatory
• Fibrinopurulent phase: consider intrapleural tPA (10 mg) + DNase (5 mg) if incomplete drainage
• Thoracoscopy with adhesion breakdown, or surgical decortication if above fails

Malignant Effusion

• Primarily symptomatic (disease is usually disseminated, not curable)
• Options: repeated thoracentesis, indwelling pleural catheter, pleurodesis

Hepatic Hydrothorax (Cirrhosis)

• Sodium restriction + diuretics (first-line)
• TIPS: initial response rate 70–80%, limited by long-term complications
• Video-assisted thoracoscopic surgery (VATS) for diaphragmatic repair + pleurodesis in refractory cases
• Indwelling pleural catheter as bridge to liver transplant or for palliation

Trapped Lung

• Fibrous visceral peel prevents re-expansion
• Suspected when: thick pleural rind on imaging, pneumothorax ex vacuo after thoracentesis, rapid recurrence
• Definitive treatment: surgical decortication