What is a pleural effusion?

What is a Pleural Effusion?

A pleural effusion is the accumulation of fluid in the pleural space—the potential space between the visceral and parietal pleura surrounding the lungs—representing a common medical problem that indicates underlying pulmonary, pleural, or systemic disease. 1

Definition and Normal Physiology

• Under normal circumstances, only 0.1 to 0.2 mL/kg of body weight of pleural fluid exists in the pleural space. 1
• When fluid accumulates in excess of 15 to 20 mL, it constitutes a pleural effusion. 2
• This abnormal accumulation prevents the lung from expanding fully during inspiration, causing dyspnea, reduced lung volume, depression of the diaphragm, and reduced chest wall compliance. 3

Pathophysiologic Mechanisms

Pleural effusions develop through several distinct mechanisms: 1

• Increased pulmonary capillary pressure (hydrostatic forces)
• Increased permeability of the pleural membrane (capillary leak)
• Decreased negative intrapleural pressure
• Decreased oncotic pressure (hypoalbuminemia)
• Obstructed lymphatic flow 1

Classification: Transudates vs. Exudates

The fundamental classification distinguishes transudative from exudative effusions, which have different underlying pathophysiology and causes: 1

Transudative Effusions

• Occur when hydrostatic forces favoring fluid accumulation are altered, but capillary permeability to proteins remains normal. 1
• Typically reflect systemic disorders rather than pleural disease. 1
• Most common causes include: 1, 4
  • Congestive heart failure (accounts for ~80% of transudates and 29% of all pleural effusions)
  • Liver cirrhosis (~10% of transudates)
  • End-stage renal failure (24.7% prevalence in ESRD patients)

Exudative Effusions

• Develop when the pleural surface and/or local capillary permeability are altered. 1
• Usually result from processes localized to the pleura. 1
• Most common causes include: 1, 5
  • Pneumonia and parapneumonic effusions
  • Malignancy (lung cancer, breast cancer, lymphoma, mesothelioma)
  • Pulmonary embolism
  • Autoimmune disorders (rheumatoid arthritis, lupus)
  • Tuberculosis

Clinical Significance

• Pleural effusions are encountered in 5% to 12% of patients referred for emergency medical treatment, making them a common finding on hospital admission. 2
• The differential diagnosis is wide, requiring a systematic approach to investigation. 1
• The etiology remains unclear in nearly 20% of cases despite thorough evaluation. 6
• Patients most commonly present with: 6
  • Dyspnea, initially on exertion
  • Predominantly dry cough
  • Pleuritic chest pain

Detection Thresholds by Imaging Modality

Different imaging modalities have varying sensitivities for detecting pleural fluid: 1

• Chest radiography: Can detect >75 mL on lateral view and >175 mL on frontal view
• Thoracic ultrasound: Can detect >20 mL of pleural fluid
• Chest CT: Can detect >10 mL and is considered the reference standard for imaging

Important Clinical Pitfall

• Physical examination findings alone have a lower positive likelihood ratio for detecting pleural effusions, supporting the use of imaging for identification of clinically significant effusions. 1
• Clinical assessment alone can often identify transudative effusions in appropriate settings (such as left ventricular failure), and these may not require sampling unless atypical features are present or they fail to respond to treatment. 1