What is the diagnostic workup for empyema necessitans?

Diagnostic Workup for Empyema Necessitans

The diagnostic workup for empyema necessitans should begin with chest radiography followed by chest ultrasound, with contrast-enhanced CT chest being essential for definitive diagnosis and surgical planning. 1

Initial Imaging Approach

First-Line Imaging

• Chest radiography (posteroanterior and lateral views) should be performed as the initial imaging study to detect pleural effusion and assess for pulmonary infiltrates 1
• PA and lateral radiographs have significantly greater sensitivity (83.9%) compared to single-view AP radiographs (67.3%) for detecting pleural effusions 1
• Most missed parapneumonic effusions on radiographs occur in patients with coexistent lower lobe consolidation 1

Second-Line Imaging

• Chest ultrasound must be performed to confirm the presence of pleural fluid collection and guide further management 1
• Ultrasound can:
  • Differentiate free from loculated pleural fluid 1
  • Determine echogenicity of the fluid 1
  • Demonstrate pleural thickening 1
  • Guide thoracentesis or drain placement 1
  • Detect septations, increased echogenicity, and microbubbles which are associated with empyema 1

Advanced Imaging

Contrast-Enhanced CT Chest

• CT chest with IV contrast is recommended in cases of suspected empyema necessitans 1
• Key CT findings associated with empyema include:
  • Pleural enhancement (sensitivity 84%, specificity 83%) 1
  • Pleural thickening (sensitivity 68%, specificity 87%) 1
  • Loculation (sensitivity 52%, specificity 89%) 1
  • Extrapleural fat proliferation (sensitivity 53%, specificity 91%) 1
  • Increased attenuation of extrapleural fat (sensitivity 39%, specificity 97%) 1
• Technical considerations:
  • Acquire CT scan 60 seconds after IV contrast bolus to optimize visualization of the pleura 1
  • CTA is not recommended as it employs contrast timing earlier than 60 seconds, not allowing sufficient time for pleural enhancement 1

MRI Considerations

• MRI chest without and with IV contrast has been used as an adjunctive modality specifically for empyema necessitans in case reports 1
• Not recommended for routine use but may be considered in cases where radiation exposure is a concern 1

Laboratory Investigations

• Blood cultures (including anaerobic bottle) should be obtained 1
• Pleural fluid analysis when thoracentesis is performed:
  • Gram stain and culture (including for anaerobes) 1
  • Biochemical tests: pH, glucose, LDH 1
  • Acid-fast bacilli stain and culture (tuberculosis is a common cause of empyema necessitans) 2, 3
• Additional blood tests:
  • Complete blood count 1
  • C-reactive protein (useful marker of progress) 1
  • Serum albumin (often low) 1

Special Considerations

• Empyema necessitans is characterized by dissection of pus through the soft tissues of the chest wall and eventually through the skin 3, 4
• Point-of-care ultrasound can help distinguish between a superficial abscess and empyema necessitans by identifying a tract extending from a cutaneous nodule toward the pleural space 4
• Tuberculosis is a common cause of empyema necessitans, particularly in endemic areas and immunocompromised patients 2, 3
• Non-tuberculous causes include Actinomyces israelii and, rarely, gram-negative bacteria such as Proteus species 5

Pitfalls and Caveats

• Empyema necessitans may initially appear as a superficial abscess, leading to misdiagnosis and inadequate treatment 4
• CT scan is essential before surgical intervention (thoracotomy or thoracoscopy) to delineate anatomy and check for intrapulmonary abscess 1
• Parapneumonic effusions <2.5 cm in anteroposterior dimension can often be managed without thoracentesis 1
• The "split pleura" sign on contrast-enhanced CT (enhancing pleural tissue visible on both visceral and parietal pleural surfaces) is indicative of empyema 1