Pleural effusion

Dr Jeremy Jones et al.

Pleural effusion tends to be used as a catch-all term denoting a collection of fluid within the pleural space. This can be further divided into exudates and transudates depending on the biochemical analysis of aspirated pleural fluid (see below). Essentially it represents any pathological process which overwhelms the pleura's ability to reabsorb fluid.

Although sometimes the term pleural effusion is used to include all kinds of fluid that may accumulate in the pleural cavity. In clinical practice, it usually excludes non-transudate types of fluid, which can have distinctly different aetiologies, despite appearing identical on radiography, and as such, the following are discussed separately:

As the accumulation of fluid in the pleural space occurs in a large and disparate clinical scenarios, no single demographic is affected; rather the epidemiology will match that of the underlying condition. However, it is probably safe to say that as congestive cardiac failure and malignancy are some of the most common causes, older patients would be over-represented.

A small amount of fluid is completely asymptomatic. In fact, depending on the respiratory reserve of the patient, even large amounts of fluid can accumulate within the pleural space before any symptoms are recognised.

Eventually as the amount of fluid increases, with resulting elastic/compressive atelectasis of the adjacent lung, the patient will experience reduced exercise tolerance and breathlessness.

Normally the pleural cavities contain approximately 15 ml of fluid 6. Any process which results in more fluid forming than can be absorbed will produce a pleural effusion.

There are many causes of pleural effusion that are broadly split into transudates and exudates. This categorisation relies upon the biochemical analysis of aspirated pleural fluid 5:

  • transudate
    • protein concentration
      • <30g/L absolute
      • total protein fluid: serum <0.5
    • lactic acid dehydrogenase (LDH)
      • <20 IU/L
      • LDH fluid: serum <0.6
    • specific gravity <1.016
  • exudate
    • protein concentration
      • >30g/l
      • total protein fluid: serum >0.5
    • lactic acid dehydrogenase (LDH)
      • >20 IU/L
      • LDH fluid: serum >0.6
    • specific gravity >1.016

It occurs due to the increase in permeability in microcirculation or alteration in the pleural space drainage to lymph nodes. As examples:


It occurs when there is an increase in hydrostatic pressure or a decrease of capillary oncotic pressure. As examples:

Plain radiograph

Chest radiographs are the most commonly used examination to assess for the presence of a pleural effusion; however, it should be noted that on a routine erect chest x-ray as much as 250-600 ml of fluid is required before it becomes evident 6. A lateral decubitus film is most sensitive, able to identify even a small amount of fluid. At the other extreme, supine films can mask large quantities of fluid.

Chest radiograph (lateral decubitus)

A lateral decubitus film (obtained with the patient lying on their side, effusion side down, with a cross table shoot through technique) can visualise small amounts of fluid layering against the dependent parietal pleura.

Chest radiograph (erect)

Both PA and AP erect films are insensitive to small amounts of fluid. Features include:

  • blunting of the costophrenic angle
  • blunting of the cardiophrenic angle
  • fluid within the horizontal or oblique fissures
  • eventually, a meniscus will be seen, on frontal films seen laterally and gently sloping medially (note: if a hydropneumothorax is present, no such meniscus will be visible)
  • with large volume effusions, mediastinal shift occurs away from the effusion (note: if coexistent collapse dominates then mediastinal shift may occur towards the effusion)

Lateral films are able to identify a smaller amount of fluid as the costophrenic angles are deepest posteriorly.

A subpulmonic effusion (aka infrapulmonary effusion) may be seen when there is previously established pulmonary disease, but can also be encountered in normal lungs. It can be difficult to identify on frontal radiographs. They are more common on the right, and usually unilateral. The following features are helpful 6:

  • right: peak of the hemidiaphragm is shifted laterally
  • left: increased distance between lower lobe air and gastric air bubble

A lateral decubitus film is again ideal.

Chest radiograph (supine)

Large amounts of fluid can be present on supine films with minimal imaging changes, as the fluid is dependant and collects posteriorly. There is no meniscus, and only a veil-like increased density of the hemithorax may be visible. It is therefore especially difficult to identify similar sized bilateral effusions as the density of the lungs will be similar.


Ultrasound allows the detection of small amounts of pleural locular fluid, with positive identification of amounts as small as 3 to 5 ml, that cannot be identified by x-rays, which is only capable of detecting volumes above 50 ml of liquid. Contrary to the radiological method, ultrasound allows an easy differentiation of loculated pleural fluid and thickened pleura. Moreover, it is effective in guiding thoracentesis (thoracocentesis), even in small fluid collections 4.

The ultrasound image of pleural effusion is characterised by an echo-free space between the visceral and parietal pleura. Septations may be seen in the pleural fluid, and may indicate underlying infection but can be seen in chylothorax or haemothorax 8.


CT scanning is excellent at detecting small amounts of fluid and is also often able to identify the underlying intrathoracic causes (e.g. malignant pleural deposits or primary lung neoplasms) as well as subdiaphragmatic diseases (e.g. subdiaphragmatic abscess).

In addition, CT can also help distinguish between a pleural effusion and a pleural empyema (see pleural effusion vs pleural empyema).

The treatment of pleural effusions is usually targeted to the underlying condition (e.g. congestive cardiac failure or malignancy). In some instances patients are symptomatic from large effusions (especially if they have an underlying cardiovascular disease) and therapeutic aspiration (thoracentesis) can be carried out.

When effusions are very large, this can safely be done 'blind' although increasingly ultrasound is used to at least mark an appropriate site. Ultrasound guided aspiration is reliable and fast and enables loculated effusions to be drained. A catheter can be left in situ, although care must be taken to ensure that it is connected either to an underwater drain or to a sealed system such that air cannot enter the pleural cavity.

If effusions re-accumulate despite repeated aspirations and systemic therapy (where appropriate), a tunnelled semi-permanent pleural drain or video-assisted thoracic surgery (VATS) pleurodesis can be considered.

Imaging differential considerations include:

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Article information

rID: 6159
System: Chest
Section: Gamuts
Tag: pleura
Synonyms or Alternate Spellings:
  • Differential of a pleural effusion
  • Differential diagnosis of a pleural effusion
  • Differential diagnosis for a pleural effusion
  • Pleural effusions

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Cases and figures

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    Figure 1
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    Fluid overload
    Case 1: from fluid overload
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    Right sided pleur...
    Case 2: on ultrasound
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    Case 3: pleural metastases
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    Case 4: subpulmonic effusion
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    Atelectasis of le...
    Case 5
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    Case 6: meniscus well illustrated
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    Pleural effusion
    Case 7: with right lobe lobe atelectasis
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    Case 8: on right
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    Case 9
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    Case 10
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    Case 11: loculated
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    Case 12: large
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    Case 13
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    Case 14: from cirrhosis (note the TIPS)
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