Pleural effusion (summary)

Last revised by Mohammad Osama Hussein Yonso on 17 Jan 2024
This is a basic article for medical students and other non-radiologists

Pleural effusions are collections of fluid within the pleural space. The term is usually reserved for collections of serous fluid and therefore excludes hemothorax, chylothorax, and pyothorax (empyema). Effusions may cause mass effect on the adjacent lung causing collapse, breathlessness, and respiratory compromise.

Both treatment and prognosis are completely dependent on the underlying cause:

  • a small, simple parapneumonic effusion can be observed and will resolve with appropriate treatment of the underlying infection
  • an infected effusion should be drained with an intercostal drain
  • a complex effusion with septa may require video assisted thoracoscopic drainage
  • a large effusion secondary to heart failure may respond to diuretics
  • an effusion in a patient with cirrhosis may respond to diuretics
  • an effusion secondary to malignancy may require pleurodesis following drainage

Prognosis will depend on the underlying cause of the effusion and the timing of any treatment.

Assuming that an effusion excludes non-serous pleural collections, they can be categorized as a transudate or an exudate based on the specific gravity and protein concentration of the fluid.

Transudates are caused by fluid shifts because of increased in-vessel pressures (left heart failure) or decreased oncotic pressures (hypoalbuminemia, cirrhosis, nephrotic syndrome).

Exudates are caused by disruption of the pleural membrane which results in a reduction in the ability to absorb pleural fluid. This may be secondary to infection (pneumonia), inflammation (e.g. rheumatoid arthritis), or neoplasia (e.g. lung cancer).

Fluid in the pleural space initially accumulates in the posterior pleural recess when erect and is not seen on a frontal radiograph until there is at least 250 mL. When a film is semi-recumbent or supine, larger volumes of fluid may be masked.

Fluid on a plain film will appear relatively dense compared to the air in the lung and form a meniscus at the costophrenic angle.

Small effusions are therefore seen as a meniscus of increased density at the costophrenic angle.

Large volume pleural effusions cause pressure on the adjacent lung resulting in collapse. However, if the volume of the effusion is greater than the degree of collapse, there will be accompanying mediastinal shift.

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