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Pathology and causes of Pleural effusion

Pleural cavity refers to the space between the two pleural layers between the lungs and ribs and normally contains 3~15 ml of fluid. This fluid is produced by capillaries in the pleural cavity, reabsorbed and broken down by the lymphatic system and transported to the blood system. In normal breathing process, an appropriate amount of fluid may serve as a lubricant for the pleural cavity and lungs. But certain medical conditions will speed up the production and slow down the absorption of this fluid. When excessive fluid is retained in the pleural cavity, the condition is known as pleural effusion or commonly as ‘water on the lungs’.

Pleural effusion is not a disease, but a complication caused by other diseases. There are two types of effusion: transudative and exudative pleural effusions.

1.      Transudative pleural effusion

It is formed when fluid leaks from blood vessels into the pleural space. Some of the conditions that can cause transudative pleural effusion are:

  • Heart failure
  • Liver failure or cirrhosis
  • Kidney failure

2.      Exudative pleural effusion

It is caused by blocked blood vessels or lymph vessels, lung diseases, inflammation, and tumours, including:

  • Lung cancer or breast cancer
  • Lymphoma
  • Pneumonia
  • Tuberculosis pleuritis
  • Tuberculosis empyema
  • Systemic lupus erythematosus
  • Rheumatoid arthritis
  • Kidney failure


Chronic pleural effusion may cause permanent damages to the lungs. If fluid is retained in the pleural cavity too long or pleural effusions are caused by infections, it may result in empyema. In this case, thoracentesis must be performed to remove the pus. One of the risks involved in this procedure is pneumothorax.


Difficulty in breathing

This is the most common symptom of pleural effusion. Excessive fluid takes up extra pleural cavity, leaving less space for the lungs to breathe and resulting in difficulty in breathing. Patients in early stages may only experience chest distress but no other obvious symptoms.

Chest pain

When the pleural surface is irritated, patients may experience sudden chest pain, stabbing pain as they breathe (pleurtitic chest pain). The pain becomes intense when coughing or taking a deep breath. As the condition develops, chest pain will become more severe. Patients may even experience shoulder and upper abdominal pain in case of diaphragm inflammation.  


Seek medical advice promptly if you have difficulty in breathing or chest pain. A doctor may judge whether a person has pleural effusions through auscultation and percussion. As patients with pleural effusions breathe in less air, dull sounds can be heard in the affected lung. In cases of pleurisy, rubbing sound may be heard. Other tests that may help confirm a diagnosis include:

  • X-ray examination 
    If only a small amount of fluid is retained, there is blunting of the costophrenic angles; if a moderate amount of fluid is retained, there is a shadow in the lungs. When a patient lies on his/her side, X-ray will show the fluid flowing.
  • Ultrasound examination
    It can identify pleural effusion quickly, and determine the location of effusion and the site for thoracentesis.
  • CT scan
  • Thoracentesis 
  • This is a procedure to aspirate pleural fluid for further testing. Usually a patient will receive an X-ray examination before having a thoracentesis to determine the site of insertion. This helps ensure that the procedure will not result in any traumatic pneumothorax. Areas covered in the analysis are:
    • The protein content of blood and pleural fluid are compared to determine whether it is a transudative or an exudative effusion. Exudates have a higher protein level than transudates.
    • LDH (lactate dehydrogenase) level is also used to differentiate these two types of effusion.
    • Check for any bacteria or cancer cells. This helps identify the causes of the condition.