What is empyema, a condition characterized by pus accumulation in the pleural space, often in elderly patients or those with underlying conditions like pneumonia or lung abscess?

What is Empyema?

Empyema is the presence of frank pus in the pleural space, representing an advanced stage of pleural infection that requires chest tube drainage and antibiotic therapy. 1

Definition and Core Characteristics

Empyema is defined by the accumulation of purulent material in the pleural cavity, which can be identified by:

• Gross appearance of frank pus on visual inspection of pleural fluid 1
• White blood cell count exceeding 50,000 WBCs/μL in the pleural fluid 1, 2
• Positive bacterial culture or Gram stain (though not always present) 1

When empyema is identified, no additional biochemical tests (such as pH measurement) are necessary on the pleural fluid since the presence of pus itself mandates drainage. 1

The Pathophysiological Continuum

Empyema exists on a spectrum of pleural infection that progresses through three distinct stages: 1, 3

Stage 1: Exudative (Simple Parapneumonic Effusion)

• Clear fluid with low white cell count accumulates 1, 2
• pH >7.2, LDH <1000 IU/L, glucose >2.2 mmol/L 1, 2
• No organisms on culture or Gram stain 1, 2
• Usually resolves with antibiotics alone without requiring drainage 2, 3

Stage 2: Fibropurulent (Complicated Parapneumonic Effusion → Empyema)

• Fibrin deposition creates septations and loculations in the pleural space 1, 2, 3
• White cell counts increase dramatically, fluid thickens and eventually becomes overt pus 1, 3
• pH <7.2, LDH >1000 IU/L, glucose may drop 1, 2, 3
• Requires chest tube drainage 1, 2

Stage 3: Organizational

• Fibroblasts infiltrate and create thick, non-elastic fibrous "peel" on pleural surfaces 1, 3
• Prevents lung re-expansion ("trapped lung") and impairs lung function 1, 3
• May require surgical decortication if symptomatic 3

Clinical Context and Epidemiology

Empyema most commonly develops as a complication of bacterial pneumonia:

• Approximately 50% of patients with acute bacterial pneumonia develop a parapneumonic effusion, though most do not progress to empyema 3
• In children with community-acquired pneumonia, parapneumonic effusions occur in 2-12% of cases, with up to 50% of bacterial pneumonias developing effusions 1, 2
• Mortality rates range from 7-33% in typical cases but may exceed 50% in elderly patients or those with comorbidities such as malignancy, diabetes, or alcohol abuse 4

Microbiology

The causative organisms vary by setting: 1, 3

• Community-acquired: Streptococcus pneumoniae, Staphylococcus aureus, β-hemolytic streptococci, Haemophilus influenzae 1, 3
• Anaerobes are present in 12-34% of positive cultures and may occur alone in 14% of cases 1
• Anaerobic infections present more insidiously with less fever, greater weight loss, and association with aspiration or poor dental hygiene 1
• Microbiological yields are only around 56% due to prior antibiotic use 3

Diagnostic Approach

Clinical Presentation

Suspect empyema in patients with: 1, 2

• Prolonged fever despite appropriate antibiotic therapy for pneumonia 1, 2
• Chest pain and abdominal pain 1, 2
• Physical examination findings: dullness to percussion, diminished breath sounds, change in breath sound quality over the effusion 1, 2

Imaging Strategy

The British Thoracic Society recommends: 1

• Chest radiography (posteroanterior or anteroposterior views) to identify pleural fluid; lateral views may confirm fluid not seen on PA films 1
• Ultrasound is the preferred investigation to confirm presence of fluid, identify septations and loculations, and guide thoracentesis or drain placement 1
• Contrast-enhanced CT scanning is reserved for diagnostic difficulty (differentiating empyema from lung abscess) or when drainage fails to assess for undrained locules 1

Key imaging distinction: Empyemas appear lenticular in shape and compress lung parenchyma, while lung abscesses have indistinct boundaries with lung tissue. The "split pleura sign" (enhancing pleural tissue on both visceral and parietal surfaces) is characteristic of empyema on CT. 1

Management Principles

All patients with empyema require chest tube drainage in addition to antibiotics: 1, 2

Drainage Indications

Absolute indications for chest tube drainage include: 1, 2, 5

• Frank pus or turbid/cloudy pleural fluid 1, 2, 5
• Positive Gram stain or culture from pleural fluid 1, 2, 5
• pH <7.2 in non-purulent effusions 1, 2, 5
• Presence of loculations 5

Antibiotic Selection

The British Thoracic Society guidelines recommend: 1

• Second-generation cephalosporin (e.g., cefuroxime) or aminopenicillin (e.g., amoxicillin) for community-acquired infection 1
• Add beta-lactamase inhibitor or metronidazole to cover penicillin-resistant aerobes and anaerobes 1
• Clindamycin can provide appropriate single-agent coverage 1
• Avoid aminoglycosides due to poor pleural space penetration and inactivity in acidic pleural fluid 1
• Antibiotics penetrate the pleural space well; intrapleural administration is unnecessary 1

Critical Pitfalls to Avoid

• Delaying drainage when indicated by fluid characteristics leads to increased morbidity and progression to organized stage requiring surgery 5, 3
• Failing to suspect empyema in pneumonia patients who remain febrile or unwell 48-72 hours after admission 1
• Not using ultrasound guidance for drain placement, which improves success rates 1
• Assuming clear fluid rules out significant infection—complicated parapneumonic effusions may appear clear initially but still require drainage based on pH and biochemistry 1, 2

Rare Complications

If untreated or inadequately managed, empyema can lead to: 1, 6

• Bronchopleural fistula 1
• Lung abscess 1
• Empyema necessitans: extension through the chest wall with external drainage 1, 6
• Septicemia and disseminated abscesses 4