Empyema

Most empyemas evolve from parapneumonic effusions. Inadequate treatment of the underlying infection results in bacterial invasion of the pleural space with accumulation of large volumes of pleural fluid which has a high white cell count, low pH and glucose concentration and a high LDH concentration, contains abundant cellular debris and from which bacteria can be cultured. Fibrin is deposited over the visceral and parietal pleura, reducing the rate of pleural fluid resorption and resulting in loculation of pleural fluid. There is progressive thickening of the pleura and adjacent soft tissues with the development of a pleural peel, an inelastic fibrous membrane which encases the lung and restricts function. If the patient is inadequately treated, the empyema may drain spontaneously through the chest wall (empyema necessitatis) or bronchial tree (bronchopleural fistula).

On chest radiography empyemas may appear similar to uncomplicated pleural effusions but, more usually, are loculated with an oval or circular shape on either the frontal or lateral projection (see loculated pleural effusion).

CT is particularly good for demonstrating an empyema (Fig.1). The features are of a loculated collection, with uniformly thickened pleura which enhances following intravenous contrast administration. The collection usually has obtuse angles with the chest wall and displaces adjacent pulmonary vessels and bronchi. The enhanced pleura may be seen to "split" at the margins of the collection. These features usually enable ready distinction from lung abscess. Chest ultrasonography typically reveals an anechoic space within which there are loculations; less commonly, the fluid is diffusely echogenic.

The treatment of empyema is immediate drainage. CT or ultrasound are used to enable accurate placement of the drainage tube within the fluid collection. Traditionally, large-bore drains have been used but empyemas may be managed successfully using smaller gauge (1214F) catheters (see percutaneous drainage thoracic). Drains should be regularly irrigated to prevent blockage and kept on constant suction. Fibrinolytic agents (streptokinase, urokinase) improve drainage of empyemas and are best administered via the drainage tube early in management. Empyemas which present at a late stage with a significantly thickened pleura require surgical decortication as do those collections which do not respond adequately to tube drainage.

CF - HM