What are the causes of empyema?

Causes of Empyema

Empyema is predominantly caused by bacterial pneumonia, with Streptococcus pneumoniae being the leading pathogen in developed countries, accounting for up to 75% of cases when molecular techniques are used. 1

Primary Etiologic Categories

Community-Acquired Pneumonia (Most Common)

• Bacterial pneumonia leads to parapneumonic effusions in approximately 50% of cases, with a significant proportion progressing to empyema 1
• Streptococcus pneumoniae is the predominant organism in both pediatric and adult populations in developed nations, with serotype 1 representing 53% of pediatric cases 1
• Anaerobic bacteria are isolated in 12–34% of positive cultures and can be the sole pathogen in approximately 14% of culture-positive empyema 1
• Streptococcus milleri and anaerobes are particularly associated with aspiration-related empyema, especially in patients with neurodevelopmental impairment, poor oral hygiene, or swallowing dysfunction 1

Geographic and Environmental Variations

• In developing countries, Staphylococcus aureus becomes the most common pathogen, particularly during hot and humid months when staphylococcal skin infections are more prevalent 1
• Gram-negative organisms (Klebsiella spp., Pseudomonas aeruginosa, other Enterobacteriaceae) are more frequently isolated in developing regions and are often linked to protein-energy malnutrition 1

Post-Surgical and Iatrogenic Causes

• 15–30% of empyemas occur after thoracic surgery, typically caused by nosocomial pathogens such as Staphylococcus aureus and aerobic gram-negative bacilli 2
• These post-surgical empyemas are more likely to be monomicrobial compared to the polymicrobial nature of community-acquired cases 2
• Empyema necessitans can rarely occur as an iatrogenic complication of chest drain insertion, where infection extends through the chest wall 3

Intra-Abdominal Sources

• Approximately 10% of empyemas occur in association with intra-abdominal infections, representing extension of infection through the diaphragm 2

Lung Abscess Complications

• Empyema can develop as a complication of lung abscess, with anaerobic bacteria being historically the most common cause 4

Pathophysiologic Progression

Empyema represents a continuum of pleural infection that progresses through three distinct stages: exudative (simple parapneumonic effusion with clear fluid and pH >7.2), fibropurulent (complicated parapneumonic effusion with fibrin deposition, septations, and pH <7.2), and organizational (chronic empyema with thick pleural peel formation that prevents lung re-expansion) 5

• During the fibropurulent phase, fibrin deposition creates septations and loculations that form separate compartments which do not communicate with one another 5
• In the organizational phase, fibroblasts infiltrate and transform thin fibrin strands into thick, non-elastic membranes ("pleural peel") that encase both visceral and parietal pleura, creating a "trapped lung" phenomenon 5

Critical Diagnostic Indicators

• Empyema should be suspected when a patient with pneumonia fails to improve after 48–72 hours of appropriate antibiotic therapy 1
• The simultaneous presence of pulmonary infiltrates and pleural fluid on chest radiography should immediately raise suspicion for a parapneumonic collection 1
• Anaerobic empyema often presents insidiously with lower fever, greater weight loss, and a history of aspiration or inadequate dental care 1

Common Pitfalls

• Culture positivity is only 17% in recent UK studies, mainly due to prior antibiotic use, so negative cultures do not exclude empyema 1
• Even with modern molecular techniques (16S PCR), an etiological agent is only detected in approximately 75% of culture-negative cases 1
• Historically, tuberculous infections were a leading cause of empyema necessitans, but this is now rare due to effective anti-tuberculous treatment 3