What is pleural effusion?

What is Pleural Effusion?

Pleural effusion is an abnormal accumulation of fluid in the pleural space exceeding the normal 0.1 to 0.2 mL/kg body weight that exists between the visceral and parietal pleura. 1

Definition and Normal Physiology

• Under normal circumstances, only 0.1 to 0.2 mL/kg body weight (approximately 15-20 mL total) of pleural fluid resides in the pleural space. 1, 2
• Any accumulation beyond this amount constitutes a pleural effusion and represents the most common clinical manifestation of pleural disease. 1

Pathophysiological Mechanisms

Pleural effusions develop through five primary mechanisms: 1

• Increased pulmonary capillary pressure (hydrostatic forces) - most commonly seen in heart failure 1
• Increased pleural membrane permeability - allowing proteins and fluid to leak into the pleural space 1
• Decreased oncotic pressure - reducing the ability to retain fluid in blood vessels 1
• Lymphatic obstruction - preventing normal drainage of pleural fluid 1
• Decreased negative intrapleural pressure - altering the pressure gradient that normally keeps the pleural space minimal 1

Classification System

Pleural effusions are fundamentally classified into two categories based on their underlying pathophysiology: 1

Transudative Effusions

• Occur when hydrostatic forces favoring fluid accumulation are altered while capillary permeability to proteins remains normal 1
• Usually reflect systemic disease processes rather than pleural pathology 1
• Most common causes: 3, 4
  • Congestive heart failure (accounts for >80% of transudative effusions and 29% of all pleural effusions) 3, 4
  • Liver cirrhosis (approximately 10% of transudates) 4
  • Renal failure (24.7% prevalence in end-stage renal disease) 4

Exudative Effusions

• Develop when the pleural surface and/or local capillary permeability are altered 1
• Result from processes localized to the pleura itself 1
• Most common causes: 1, 3
  • Infection (parapneumonic effusions, empyema, tuberculosis) 1
  • Malignancy (lung cancer is the primary cause; breast cancer is the second most common cause of malignant effusions) 3
  • Autoimmune disorders (rheumatoid arthritis, systemic lupus erythematosus) 1, 4

Clinical Significance

• Pleural effusions are encountered in 5-12% of patients referred for emergency medical treatment, making them a common finding on hospital admission. 2
• The condition prevents the lung from expanding fully during inspiration due to reduced lung volume, depression of the diaphragm, and reduced chest wall compliance. 5
• Typical presenting symptoms include: 6, 5
  • Dyspnea (initially on exertion) 6
  • Predominantly dry cough 6
  • Pleuritic chest pain 6, 5

Diagnostic Detection Thresholds

Different imaging modalities can detect varying amounts of pleural fluid: 1

• Chest radiographs: Can detect >75 mL on lateral view and >175 mL on frontal view 1
• Thoracic ultrasound: Can detect >20 mL of pleural fluid 1
• Chest CT: Can detect >10 mL of pleural fluid and is considered the reference standard for imaging 1

Diagnostic Approach

• The etiology of pleural effusion remains unclear in approximately 20% of cases despite thorough investigation. 6
• Physical examination findings alone have a lower positive likelihood ratio for detection, supporting the use of imaging to identify clinically significant effusions. 1
• Clinical assessment alone can often identify transudative effusions, particularly in settings strongly suggestive of heart failure with confirmatory chest radiograph. 1
• Thoracentesis should be performed for new and unexplained pleural effusions to determine whether the fluid is transudative or exudative using Light's criteria. 1, 6