Parapneumonic Effusions: Types and Classification
Parapneumonic effusions are classified into three main types based on their stage of evolution and complexity: uncomplicated (simple) parapneumonic effusions, complicated parapneumonic effusions, and empyema, with management decisions driven primarily by effusion size, pleural fluid characteristics, and degree of respiratory compromise. 1, 2
Classification System
Uncomplicated (Simple) Parapneumonic Effusions
- Small effusions (<10 mm rim on lateral decubitus or opacifying less than one-fourth of hemithorax) that are free-flowing and sterile 1
- These represent the exudative stage where pleural fluid accumulates but remains uninfected 2
- Characterized by negative bacterial culture and Gram stain, normal pH (>7.20), and absence of loculations 1, 3
- Resolve with antibiotic therapy alone without requiring drainage 1
Complicated Parapneumonic Effusions
- Moderate to large effusions (>10 mm rim but less than half hemithorax for moderate; more than half hemithorax for large) that show signs of bacterial invasion but are not yet frankly purulent 1
- Enter the fibropurulent stage with developing loculations and septations visible on ultrasound 1, 2
- Pleural fluid pH <7.20, glucose <60 mg/dl (3.4 mmol/l), or positive Gram stain/culture indicate complicated status requiring drainage 4, 3, 5
- Associated with pleural thickening on imaging and enhanced pleural surfaces on contrast CT 1
- Moderate effusions without respiratory compromise and without empyema characteristics may be observed, but those with respiratory distress require drainage 1
Empyema
- Frank pus in the pleural space, representing the organizational stage with thick fibrinous septations and potential for lung entrapment 1, 2
- Macroscopically purulent fluid or positive bacterial culture/Gram stain with multiloculated collections 1, 4
- Always requires drainage regardless of size 1, 3
- May progress to bronchopleural fistula, lung abscess, or empyema necessitatis if untreated 1
Key Diagnostic Features for Classification
Size-Based Assessment
- Small: <10 mm thickness on ultrasound or lateral decubitus radiograph 1
- Moderate: >10 mm but opacifies less than half of hemithorax 1
- Large: Opacifies more than half of hemithorax 1
Pleural Fluid Characteristics
- Ultrasound must be used to confirm presence and assess for loculations 1
- Echogenic effusions on ultrasound indicate exudates, with homogeneous echogenic patterns suggesting empyema or hemorrhage 1
- Pleural fluid must be sent for Gram stain and bacterial culture 1
- Biochemical analysis (pH, glucose, protein, LDH) is unnecessary in uncomplicated cases but pH <7.20 strongly indicates need for drainage 1, 4
Critical Management Decision Points
When Drainage is NOT Required
- Small effusions (<10 mm) can be observed and treated with antibiotics alone 1
- Moderate effusions without respiratory compromise, non-purulent appearance, and pH >7.20 1
When Drainage IS Required
- Any effusion with purulent appearance (empyema) 1, 3
- Large effusions (>50% hemithorax opacification) in most cases 1
- Moderate effusions with respiratory distress or empyema characteristics 1
- Pleural fluid pH <7.20, glucose <60 mg/dl, or positive microbiology 4, 3, 5
- Enlarging effusions compromising respiratory function 1
Common Pitfalls
Avoid relying solely on chest radiograph appearance without ultrasound confirmation, as lateral decubitus films and CT can miss loculations that ultrasound readily identifies 1
Do not perform repeated thoracentesis in significant pleural infections—insert a drain at the outset 1
Recognize that approximately 40% of pneumonia patients develop parapneumonic effusions, but only a minority require intervention 2, 5
Pleural fluid loculations on imaging do not always predict need for drainage, but they do indicate potential for more complicated hospital course and may require fibrinolytics or VATS if drainage is undertaken 1, 6
The absence of pleural thickening on CT suggests a simple parapneumonic effusion unlikely to require drainage 1