Pleural effusion

Pleural effusions are abnormal accumulations of fluid within the pleural space.  They may result from a variety of pathological processes which overwhelm the pleura's ability to reabsorb fluid.

"Pleural effusion" is commonly used as a catch-all term to describe any abnormal pleural fluid. The lack of specificity is mainly due to the limitations of the imaging modality. Given that most effusions are detected by x-ray, which generally cannot distinguish between fluids, the fluid in question may by simple (transudative) fluid, blood, pus, chylous fluid, etc.

If additional corroborative evidence is available, certain (mostly non-transudative) effusions are preferentially designed using more specific terminology. This is important because these effusions may be managed distinctly. These 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 volume 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
      • <30 g/L absolute
      • total protein fluid:serum ratio <0.5
    • lactic acid dehydrogenase (LDH)
      • <20 IU/L
      • LDH fluid:serum ratio <0.6
    • specific gravity <1.016
  • exudate
    • protein concentration
      • >30 g/L
      • total protein fluid:serum ratio >0.5
    • lactic acid dehydrogenase (LDH)
      • >20 IU/L
      • LDH fluid:serum ratio >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:

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.

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

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 bubble

A lateral decubitus film is again ideal.

Large amounts of fluid can be present on supine films with minimal imaging changes, as the fluid is dependent 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 3-5 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 characterized 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 hemothorax 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).

CT is not able to differentiate between a transudative or exudative pleural effusion with similar fluid densities and non-differentiating rates of loculation and pleural thickening 9,10. However, CT can 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). Symptomatic patients with large effusions may be treated by therapeutic aspiration (thoracentesis).

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:

  • strange or atypical configurations of pleural fluid can be due to either adhesions (i.e. loculated effusion) or underlying atelectasis. The latter are more likely to change with patient positioning 12
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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

  • 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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  • 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: massive with mediastinal shift
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  • Case 14: from cirrhosis (note the TIPS)
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  • Case 15: supine pleural effusion
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