What is a Parapneumonic Effusion?
A parapneumonic effusion is a collection of pleural fluid that develops as a complication of bacterial pneumonia, occurring in 2-12% of children with community-acquired pneumonia and up to 50% of cases caused by typical bacteria such as S. pneumoniae, S. pyogenes, and S. aureus. 1
Definition and Pathophysiology
A parapneumonic effusion represents fluid accumulation in the pleural space secondary to an adjacent pneumonic process. 1 In previously healthy children, these effusions are typically secondary to acute bacterial pneumonia, with S. pneumoniae being the predominant pathogen. 1
The condition follows a predictable three-stage continuum: 2
- Exudative stage: Clear fluid with low white cell count and normal glucose
- Fibropurulent stage: Fibrin deposition with septation and loculation
- Organizational stage: Progressive organization requiring more aggressive intervention
Classification System
The British Thoracic Society provides a clinically useful classification based on fluid characteristics: 2
Simple parapneumonic effusion:
- Clear fluid appearance
- pH >7.2
- LDH <1000 IU/L
- Glucose >2.2 mmol/L
- Negative culture and Gram stain
- Usually resolves with antibiotics alone 2
Complicated parapneumonic effusion:
- Clear or cloudy/turbid fluid
- pH <7.2
- LDH >1000 IU/L
- May have positive Gram stain/culture
- Requires chest tube drainage 2
Empyema:
- Frank pus on gross appearance
- May be defined by WBC count >50,000 WBCs/μL
- May have positive Gram stain/culture
- Requires chest tube drainage 2
Clinical Presentation
Key clinical features that strongly suggest parapneumonic effusion include: 1
- Prolonged fever despite antibiotic therapy (most important clue)
- Chest pain
- Abdominal pain
- Respiratory distress (present in 60% of cases) 3
Physical examination findings include: 1
- Dullness to percussion over the affected area
- Diminished breath sounds
- Altered quality of breath sounds or transmitted speech over the effusion
Diagnostic Approach
Imaging strategy: 1
- Chest radiography with lateral decubitus views should confirm the presence of pleural fluid
- If uncertainty exists, chest ultrasound is preferred over CT due to lack of ionizing radiation
- Ultrasound can identify loculations, which are associated with longer hospital stay and more frequent need for surgery 3
Pleural fluid analysis: 1
- Gram stain and bacterial culture should be performed whenever pleural fluid is obtained
- pH assessment is required for all non-purulent, possibly infected effusions 2
- Frankly purulent or turbid/cloudy pleural fluid mandates prompt chest tube drainage 2
Common Pitfalls
Loculations on admission are a critical prognostic factor—they are associated with longer hospital stay, longer antibiotic courses, and more frequent need for surgery. 3 Failure to identify loculations early can lead to inadequate drainage and treatment failure.
Underestimating effusion size on plain radiography is common, as chest X-rays may not fully reveal the extent and complexity of the effusion. 4 This is why ultrasound is preferred when uncertainty exists.