Pleural Empyema

Pleural Empyema

Surgery – Clear fluid with extensive fibrinous and exudate adherent to the parietal and visceral pleura.

CR – Large non-layering left pleural fluid with lower lobe infiltrate and contralateral shift of the mediastinum.

CT – Biconvex pleural fluid with enhancing and thickened pleura; note the “split pleura” sign indicating complex pleural process.

Inflammation related to pneumonia increases permeability of the mesothelial cells of the pleura and consequently leads to accumulation of fluid and protein. Cytokines and other inflammatory mediators recruit neutrophils, lymphocites and phagocytes to the pleural space and amplify the inflammatory response. About half of empyemas are caused by Gram-positive bacteria (Staphylococcus aureus, Streptococcus pneumoniae); the remainder are Gram-negative organisms or anaerobes.

Pneumonia related pleural disease has 3 stages: 1) Protein-rich pleural fluid remains free flowing. Glucose and pH levels are normal. Drainage of the effusion and appropriate antimicrobial therapy are normally sufficient for treatment. 2) Viscosity of the pleural fluid increases. Coagulation factors are activated, and fibroblastic activity begins coating the pleural membrane. Glucose and pH levels are lower than normal. 3) Loculations form. Fibroblastic activity causes adherence to the visceral and parietal pleura. Protein-rich fluid with inflammatory cells and debris, is present in the pleural space. Surgical intervention is often required at this stage.

On CT, pleural separation (“split pleura”) is commonly seen with empyema and is associated with pleural thickening and enhancement. Empyema can be accompanied by swelling of the extrapleural subcostal tissue and infiltration of the subpleural fat. Pleural changes similar to those of empyema can be seen with malignant effusions and hemothorax.

Kraus GJ. The split pleura sign. Radiology (2007); 243, 297-298.