Medicine – Pleural Effusion

A pleural effusion is an abnormal accumulation of fluid within the pleural space. The key initial step is to determine whether the fluid is a transudate or an exudate, because this greatly narrows the differential diagnosis.

1. Classification of Pleural Fluid

The standard approach is to use Light’s criteria.

A pleural effusion is classified as an exudate if any one of the following is present:

• Pleural fluid protein : serum protein ratio > 0.5
• Pleural fluid LDH : serum LDH ratio > 0.6
• Pleural fluid LDH > two-thirds of the upper limit of normal serum LDH

If none of these criteria is met, the effusion is usually considered a transudate.

2. Transudative Pleural Effusion

A transudate is generally caused by a systemic disturbance in hydrostatic or oncotic pressure, rather than local pleural inflammation.

Cardiac Failure

Congestive cardiac failure is one of the commonest causes of transudative pleural effusion.

Raised pulmonary venous pressure increases hydrostatic pressure and drives fluid into the pleural space. Effusions are often bilateral, although one side may predominate.

Cirrhosis

Cirrhosis can cause pleural effusion, often through movement of ascitic fluid across small diaphragmatic defects.

This is sometimes referred to as hepatic hydrothorax and is commonly right-sided.

Hypoalbuminaemia

Marked hypoalbuminaemia reduces plasma oncotic pressure, favouring movement of fluid from the vascular compartment into tissues and body cavities, including the pleural space.

Nephrotic Syndrome

In nephrotic syndrome, heavy urinary protein loss causes hypoalbuminaemia and reduced oncotic pressure.

This can result in peripheral oedema, ascites, and pleural effusions.

Hypothyroidism

Hypothyroidism can occasionally cause pleural effusions.

These may be transudative or sometimes borderline/exudative depending on the mechanism, so it is not as classically transudative as heart failure or cirrhosis.

3. Exudative Pleural Effusion

An exudate results from local pleural disease, usually because of increased vascular permeability, inflammation, impaired lymphatic drainage, or direct pleural involvement.

Malignancy

Malignant pleural effusion is a common cause of an exudate.

It may occur with lung cancer, breast cancer, lymphoma, mesothelioma, and other metastatic malignancies. The fluid is often recurrent and may be haemorrhagic.

Parapneumonic Effusion

A parapneumonic effusion develops in association with bacterial pneumonia.

It may be uncomplicated or progress to a complicated parapneumonic effusion or empyema, especially when pleural fluid becomes infected.

Tuberculosis

Tuberculous pleuritis typically causes an exudative effusion.

The fluid is usually lymphocyte-predominant and may have a low glucose concentration.

Subphrenic Abscess

A subphrenic abscess can irritate the adjacent diaphragm and pleura, causing a reactive exudative pleural effusion.

Pulmonary Embolism

Pulmonary embolism can cause a small exudative effusion, especially when pulmonary infarction occurs.

The fluid may be blood-stained.

Pancreatitis

Pancreatitis can cause an exudative pleural effusion, usually through inflammatory spread or a pancreaticopleural fistula.

A clue is a markedly elevated pleural fluid amylase.

Rheumatoid Arthritis

Rheumatoid pleuritis can produce an exudative effusion, often with very low pleural fluid glucose.

The pleural fluid may also have a low pH and high LDH.

Systemic Lupus Erythematosus

SLE can cause pleuritis with an exudative pleural effusion.

This is often associated with other manifestations of active systemic lupus.

4. Low Pleural Fluid Glucose

A reduced pleural fluid glucose concentration suggests that glucose is being consumed by inflammatory cells, bacteria, or metabolically active tissue, or that transport across the pleura is impaired.

Important causes include rheumatoid pleuritis, tuberculosis, malignancy, and empyema.

Rheumatoid Arthritis

Rheumatoid effusions can produce very low glucose levels, sometimes approaching zero.

This is one of the classic laboratory clues to rheumatoid pleuritis.

Tuberculosis

Tuberculous pleural effusions may also have low pleural glucose, particularly in more intense or chronic pleural inflammation.

Malignancy

Malignant effusions can have reduced glucose because of high metabolic activity within the pleural space and impaired glucose transport.

Empyema

In empyema, bacteria and neutrophils consume glucose rapidly, so pleural fluid glucose may become very low.

Empyema is also typically associated with low pH and high LDH.

Key Clinical Pattern

Think of transudates as being caused mainly by systemic pressure or protein problems, especially heart failure, cirrhosis, hypoalbuminaemia, and nephrotic syndrome.

Think of exudates as being caused by local pleural inflammation or infiltration, especially malignancy, pneumonia, TB, pulmonary embolism, pancreatitis, RA, and SLE.

For classification, remember Light’s criteria, and for a particularly low pleural fluid glucose, remember RA, TB, malignancy, and empyema.