What are pleural effusions?

Pleural Effusions

Pleural effusions are abnormal accumulations of fluid in the pleural space that exceed the normal amount of 0.1-0.2 mL/kg body weight, resulting from an imbalance between pleural fluid formation and drainage. 1

Definition and Normal Physiology

• The pleural space normally contains approximately 0.3 mL/kg body weight of pleural fluid
• Normal pleural fluid circulates continuously with lymphatic vessels capable of handling several hundred milliliters of extra fluid per 24 hours
• When formation exceeds drainage, pleural effusion develops 2
• Normal pleural fluid contains primarily mesothelial cells, macrophages, and lymphocytes with low protein concentration 2

Pathophysiologic Mechanisms

Pleural effusions develop through several mechanisms:

• Increased pulmonary capillary pressure
• Increased pleural membrane permeability
• Decreased negative intrapleural pressure
• Decreased oncotic pressure
• Obstructed lymphatic flow 1

Classification

1. Transudative Effusions

• Result from systemic conditions altering hydrostatic forces
• Characterized by:
  • Clear fluid
  • Protein <25 g/L
  • LDH <1000 IU/L
  • Glucose >2.2 mmol/L 1
• Common causes:
  • Congestive heart failure
  • Cirrhosis
  • Renal failure 2

2. Exudative Effusions

• Result from altered pleural surface and/or increased capillary permeability
• Further classified as:
  • Simple parapneumonic
  • Complicated parapneumonic
  • Empyema 1
• Common causes:
  • Infection (pneumonia, tuberculosis)
  • Malignancy
  • Autoimmune disorders 2
  • Post-surgical
  • Trauma 2

Clinical Presentation

• Dyspnea: Most common symptom, often out of proportion to effusion size in conditions like pulmonary embolism 1
• Pleuritic chest pain: More common in inflammatory conditions
• Cough: Often dry and non-productive
• Physical findings:
  • Decreased breath sounds on affected side
  • Dullness to percussion
  • Decreased tactile fremitus 1

Diagnostic Approach

Imaging

• Chest radiography: Initial imaging modality
  • Can detect >175 mL on frontal view
  • Can detect >75 mL on lateral view 2, 1
• Thoracic ultrasound: Essential for initial evaluation
  • Can detect >20 mL of pleural fluid
  • Guides thoracentesis 2, 1
• Chest CT: Reference standard for imaging
  • Can detect >10 mL of pleural fluid
  • Helps differentiate empyema from lung abscess
  • Shows "split pleura" sign in empyema 2, 1

Pleural Fluid Analysis

Diagnostic thoracentesis should include:

• Appearance and odor
• Nucleated cell count and differential
• Total protein and LDH (to distinguish transudate vs. exudate)
• Glucose
• pH
• Amylase (if pancreatic disease or esophageal rupture suspected)
• Cytology (for malignancy)
• Microbiology studies when infection is suspected 1

Management

Management depends on the underlying cause:

Transudative Effusions

• Treat the underlying condition (heart failure, cirrhosis, renal failure)
• Therapeutic thoracentesis for symptomatic relief 1

Exudative Effusions

• Infectious (parapneumonic/empyema):

  • Appropriate antibiotics
  • Drainage for complicated parapneumonic effusions and empyema
  • Chest tube placement for adequate drainage 2, 1

• Malignant effusions:

  • Drainage for symptomatic relief
  • Consider pleurodesis to prevent recurrence
  • Options include thoracentesis, chest tube, or tunneled pleural catheter 2, 1

Special Considerations

• Bilateral effusions: May indicate systemic disease but can also occur in tuberculosis or parasitic infections 2
• Loculated effusions: Result from fibrin deposition in the pleural space, often requiring ultrasound-guided drainage 2, 1
• Recurrent effusions: May require repeated thoracentesis or placement of indwelling pleural catheters 1

Potential Pitfalls

• Assuming bilateral effusions are always transudative
• Inadequate pleural fluid sampling leading to false-negative cytology
• Overlooking pulmonary embolism as a cause
• Failing to send sufficient volume (at least 50mL) for cytology when malignancy is suspected 1

Follow-up

• Patients should be followed until complete recovery
• Chest radiograph should return to near normal
• Consider underlying diagnoses (immunodeficiency, connective tissue disease) in recurrent cases 2