What is the most common cause of empyema thoracis?

Common Cause of Empyema Thoracis

The most common cause of empyema thoracis is bacterial pneumonia, with Streptococcus pneumoniae being the predominant pathogen in both children and adults in developed countries. 1, 2

Primary Etiology

Bacterial pneumonia accounts for the vast majority of empyema cases, with parapneumonic effusions occurring in approximately 50% of bacterial pneumonias and progressing to empyema in a subset of these patients. 1

Most Common Pathogens in Developed Countries

• Streptococcus pneumoniae is the leading causative organism, identified in 75% of culture-negative pleural fluid samples when PCR techniques are used, with serotype 1 representing 53% of these cases in pediatric populations. 2
• In adults with community-acquired pneumonia complications, S. pneumoniae remains the most frequently isolated pathogen. 2
• Staphylococcus aureus is the second most common pathogen, particularly in posttraumatic empyema where it accounts for 60% of cases. 3
• Anaerobic bacteria are increasingly recognized, present in 12-34% of positive pleural fluid cultures, and may occur as sole pathogens in 14% of culture-positive cases. 1

Geographic Variations

In developing countries, the bacteriological profile differs significantly:

• S. aureus becomes the predominant pathogen, especially during hot and humid months when staphylococcal skin infections are more prevalent. 1, 2
• Gram-negative organisms (Klebsiella spp, Pseudomonas aeruginosa, other Enterobacteriaceae) are more common than in developed nations and may be associated with protein-energy malnutrition. 1, 2

Secondary Causes

Beyond pneumonia, empyema develops from:

• Thoracic surgery complications - a significant predisposing factor, particularly with Gram-negative enteric bacilli as the predominant pathogens in postoperative cases. 4
• Gastrointestinal surgery complications - another important surgical cause. 4
• Chest trauma with tube thoracostomy - where S. aureus contamination occurs during tube insertion or from the injury itself, rather than from parapneumonic processes (only 21% correlation between respiratory and pleural cultures). 3
• Aspiration pneumonia - associated with anaerobes and Streptococcus milleri, particularly in children with delayed neurodevelopment or poor dental hygiene. 1

Diagnostic Challenges

Culture positivity is notably low:

• Only 17% of cases yield positive cultures in recent UK studies, primarily due to prior antibiotic use before hospital admission. 2
• Even with modern molecular techniques (16S PCR), an etiological agent is detected in only approximately 75% of culture-negative cases. 2
• Despite 63% of patients receiving appropriate antibiotics before admission, organisms were still isolated in 63% of these cases. 4

Clinical Pitfalls

Key considerations to avoid missed diagnoses:

• Empyema should be suspected in any patient failing to respond to appropriate antibiotic therapy within 48-72 hours. 1
• The presence of pulmonary infiltrates with pleural fluid on chest radiograph should immediately raise suspicion for parapneumonic collection. 1
• Anaerobic infections present more insidiously with less fever, greater weight loss, and are more common with aspiration history or poor dental hygiene. 1