Aetiology

The most common cause is bacterial pneumonia: (1)

• the largest single cause is pneumococcal pneumonia
• the incidence of empyema formation is highest amongst patients with klebsiella pneumonia
• staphylococcal and Haemophilus influenzae pneumonias are important causes of empyemata in children
• tuberculosis is now a rare cause of empyema in the UK

Other causes include:

• lung abscess
• bronchiectasis
• pulmonary infarction
• complication of thoracic surgery
• complication of mediastinitis of oesophageal rupture (usually left-sided)
• hepatic infection may pierce the diaphragm:
  • amoebic liver abscess
  • actinomycosis
  • hydatid disease
• penetrating chest injuries, including the insertion of a chest drain
• haematogenous spread of infection

Note:

• Klebsiella is associated with a higher mortality than other micro-organisms and is more common among patients who have underlying chronic disease, such as diabetes or alcoholic liver disease
• mixed aerobic and anaerobic bacteria are commonly isolated from empyemic pleural fluid, though sometimes purely anaerobic bacteria are found. The most common anaerobes are the Bacteroides fragilis group, pigmented Prevotella species and Fusobacterium species
• hospital-acquired empyema
  • commonly isolated bacteria in hospital-acquired empyema (either iatrogenic or secondary to hospital-acquired pneumonia) include staphylococci including meticillin-resistant strains, enterobacteria, enterococci and Pseudomonas aeruginosa

Reference:

1. Shen KR et al. The American Association for Thoracic Surgery consensus guidelines for the management of empyema. J Thorac Cardiovasc Surg. 2017 Jun;153(6):e129-46.