What is the difference between a parapneumonic effusion and a sympneumonic effusion?

Parapneumonic vs. Synpneumonic Effusion

The term "synpneumonic" is not a recognized medical classification in pleural disease—the correct terminology is "parapneumonic effusion," which refers to any pleural fluid collection associated with underlying pneumonia. 1

Understanding Parapneumonic Effusions

Parapneumonic effusion is defined as a collection of fluid in the pleural space associated with an underlying pneumonia, occurring in approximately 40-50% of patients with bacterial pneumonia. 1, 2

Classification of Parapneumonic Effusions

The British Thoracic Society and other major guidelines classify parapneumonic effusions into three distinct categories based on fluid characteristics and clinical progression 1:

Simple Parapneumonic Effusion

• Clear fluid appearance 1
• pH >7.2 1
• LDH <1000 IU/L 1
• Glucose >2.2 mmol/L 1
• No organisms on culture or Gram stain 1
• Management: Usually resolves with antibiotics alone; chest tube drainage only for symptom relief if needed 1

Complicated Parapneumonic Effusion

• Clear or cloudy/turbid fluid 1
• pH <7.2 1
• LDH >1000 IU/L 1
• May have positive Gram stain/culture 1
• Management: Requires chest tube drainage 1

Empyema

• Frank pus on gross appearance 1
• May be defined by WBC count >50,000 WBCs/μL 1
• May have positive Gram stain/culture 1
• Management: Requires chest tube drainage; no additional biochemical tests necessary 1

Pathophysiological Progression

Parapneumonic effusions follow a predictable three-stage continuum 3:

1. Exudative Stage: Clear fluid with low white cell count, low LDH, physiological pH, and normal glucose; fluid remains sterile despite adjacent infection 3

2. Fibropurulent Stage: Fibrin deposition leads to septation and loculation, with dramatically increased white cell counts, rising LDH, protein >3 g/dL, pH falling below 7.20, and dropping glucose levels 3

3. Organizational Stage: Fibroblasts infiltrate the pleural cavity, converting fibrin into thick, non-elastic membranes that prevent lung re-expansion 3

Clinical Presentation

Parapneumonic effusions occur in 2-12% of children with community-acquired pneumonia and up to 50% of cases due to typical bacteria including S. pneumoniae, S. pyogenes, and S. aureus. 1

Key clinical features include 1:

• Prolonged fever despite antibiotic therapy
• Chest pain
• Abdominal pain
• Dullness to percussion
• Diminished breath sounds
• Change in quality of breath sounds over the effusion

Diagnostic Approach

Chest radiography with lateral decubitus views should confirm the presence of pleural fluid. 1 If uncertainty exists between pleural fluid versus parenchymal opacification, chest ultrasound is preferred over CT due to lack of ionizing radiation. 1

All non-purulent, possibly infected effusions require pleural fluid pH assessment. 1 Frankly purulent or turbid/cloudy pleural fluid on sampling mandates prompt pleural space chest tube drainage. 1

Common Pitfalls

• Do not assume all effusions with pneumonia are the same: Small effusions (<10 mm rim on lateral decubitus or less than one-fourth hemithorax opacified) typically resolve with antibiotics alone and rarely require drainage. 1

• Do not delay drainage when indicated: pH <7.2, presence of organisms on Gram stain/culture, or frank pus all require prompt chest tube drainage. 1

• Do not confuse terminology: There is no distinct entity called "synpneumonic effusion" in current medical literature or guidelines—all pleural effusions associated with pneumonia are classified as parapneumonic. 1