What is pleural effusion?

What is Pleural Effusion?

Pleural effusion is the abnormal accumulation of fluid in the pleural space beyond the normal 0.1 to 0.2 mL/kg body weight (or approximately 0.3 mL/kg in children), representing one of the most common clinical manifestations of pleural disease. 1

Pathophysiology and Mechanisms

Pleural effusions develop through several distinct mechanisms 1:

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

The normal pleural space contains only 15-20 mL of fluid, and the lymphatic system can typically handle several hundred milliliters of excess fluid per 24 hours before an effusion becomes clinically apparent. 1, 2

Classification: Transudates vs. Exudates

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

Transudative Effusions

• Result from altered hydrostatic forces favoring fluid accumulation while capillary permeability to proteins remains normal 1
• Reflect systemic disease processes rather than pleural pathology 1
• Most common causes include: 1
  • Congestive heart failure (most frequent)
  • Cirrhosis with ascites
  • Renal failure/nephrotic syndrome

Exudative Effusions

• Develop when the pleural surface and/or local capillary permeability are altered 1
• Result from processes localized to the pleura 1
• Most common causes include: 1, 3
  • Infection (pneumonia, empyema)
  • Malignancy (lung cancer, breast cancer, lymphoma)
  • Pulmonary embolism
  • Autoimmune disorders

Clinical Significance

Pleural effusions are extremely common in clinical practice 1, 2:

• Approximately 5-12% of patients presenting for emergency medical treatment are diagnosed with pleural effusion 2
• They indicate the presence of underlying disease that may be pulmonary, pleural, or extrapulmonary in origin 1
• The differential diagnosis is wide, necessitating a systematic diagnostic approach 1

Detection Thresholds by Imaging Modality

The sensitivity for detecting pleural fluid varies significantly by imaging technique 1:

• Chest radiograph (lateral view): Can detect >75 mL 1
• Chest radiograph (frontal view): Can detect >175 mL 1
• Thoracic ultrasound: Can detect >20 mL 1
• Chest CT: Can detect >10 mL and is considered the reference standard 1

Physical Examination Findings

The hallmark auscultatory finding is decreased or absent breath sounds over the effusion, with the degree of reduction proportional to effusion size. 4 Additional findings include 4:

• Dullness to percussion over the fluid-filled area
• Decreased or absent tactile fremitus
• Egophony (E-to-A change) at the upper border of the effusion
• Contralateral tracheal deviation with large effusions due to mediastinal shift

Important caveat: Physical examination findings have a lower positive likelihood ratio for detecting pleural effusions compared to imaging, and small effusions (<500 mL) may produce subtle or no auscultatory findings. 1, 4

Clinical Presentation

Patients most commonly present with 5:

• Dyspnea (initially on exertion, progressing to rest)
• Predominantly dry cough
• Pleuritic chest pain (sharp, worse with breathing)

The pathophysiology of dyspnea involves decreased chest wall compliance, mediastinal shift, decreased ipsilateral lung volume, and reflex stimulation from the lungs and chest wall. 4

Critical Diagnostic Principle

The initial diagnostic step is determining whether the effusion is a transudate or exudate, which begins with history and physical examination, followed by biochemical analysis of pleural fluid. 1 This distinction is essential because it fundamentally directs the subsequent diagnostic workup and treatment approach, as transudative effusions typically require treatment of the underlying systemic condition, while exudative effusions necessitate investigation of local pleural or pulmonary pathology. 1