Parapneumonic effusion

Parapneumonic effusion refers to an exudative pleural effusion associated with pneumonia.


Approximately 40% of patients who are hospitalized for pneumonia develop a parapneumonic effusion .


Fluid leaks into the pleural space due to increased permeability of the visceral pleura adjacent to the infected lung. Bacterial pneumonia is a more common cause of parapneumonic effusions than viral pneumonia .

Radiographic features

Plain radiograph / CT

Similar findings to a simple pleural effusion :

  • usually small volume
  • normal meniscus sign
  • dependent
  • no loculation
  • "split pleura sign" is not typical and more specific for empyema

Treatment and prognosis

Parapneumonic effusions often resolve with the treatment of underlying pneumonia. However, ~7.5% of patients with parapneumonic pneumonia progress to a fibrinopurulent stage such as empyema .