What are parapneumonia and empyema?

Definitions of Parapneumonic Effusion and Empyema

Parapneumonic effusion is a collection of pleural fluid that develops in association with underlying pneumonia, while empyema is the presence of frank pus in the pleural space. 1

Parapneumonic Effusion

A parapneumonic effusion is defined as any pleural fluid collection that occurs in the context of pneumonia. 1 This represents a spectrum of disease severity:

• Simple parapneumonic effusion: Clear fluid with low white cell count that accumulates due to the inflammatory process from the adjacent pneumonia 1
• Complicated parapneumonic effusion: Fluid that has thickened with increased white cells, often with fibrin deposition leading to septations and loculations 1
• Occurs in 2-12% of children with community-acquired pneumonia, though can be seen in up to 20% of Mycoplasma pneumoniae cases and 10% of viral pneumonias 1
• Most commonly associated with bacterial pneumonia, occurring in 50% of cases caused by S. pneumoniae, S. pyogenes, and S. aureus 1

Empyema

Empyema is specifically defined as the presence of pus in the pleural space associated with underlying pneumonia. 1 The definition of pus includes:

• Gross purulent appearance on visual inspection 1
• White blood cell count >50,000 WBCs/μL 1
• Positive bacterial culture from pleural fluid 1

Staging and Progression

Pleural infection exists on a continuum and classically progresses through three stages: 1

1. Exudative stage: Inflammatory process leads to accumulation of clear fluid with low white cell count (simple parapneumonic effusion) 1

2. Fibropurulent stage: Fibrin deposition creates septations and loculations, fluid thickens with increased white cells (complicated parapneumonic effusion), eventually becoming overt pus (empyema) 1

3. Organizational stage: Fibroblasts infiltrate the pleural cavity, thin membranes reorganize into thick, non-elastic "peel" that may prevent lung re-expansion ("trapped lung") and impair function 1

Clinical Presentation

Patients with parapneumonic effusions present with classic pneumonia symptoms but are typically more unwell than those with simple pneumonia: 1

• Fever, cough, dyspnea, exercise intolerance, decreased appetite, malaise 1, 2
• Pleuritic chest pain (patients may lie on affected side to splint the hemithorax) 1
• Abdominal pain (especially with lower lobe involvement) 1
• Physical examination reveals unilateral decreased chest expansion, dullness to percussion, reduced or absent breath sounds, and possible scoliosis 1, 2
• Cyanosis may occur due to ventilation-perfusion mismatch 1, 2

Key Clinical Pitfall

If a child remains febrile or unwell 48 hours after admission for pneumonia, parapneumonic effusion or empyema must be excluded with repeat clinical examination and chest radiography. 1, 2 This is a critical juncture where complications are often identified, and failure to recognize this can lead to delayed treatment and worse outcomes.