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What is pleural effusion?
Pleural effusion is when excess fluid builds up in the pleural space of the lungs.
The lungs are surrounded by a thin membrane, the inner layer of pleura. A second, outer membrane lines the inside of the chest cavity attaching to the ribcage. The narrow pleural space between the two membranes contains about four teaspoons (10-20mls) of fluid.  This fluid allows the two layers to glide smoothly across one another as the lungs expand and contract during breathing.
The fluid enters the pleural space from blood capillaries, and drains through lymph ducts. If excess fluid builds up in the pleural space, this is known as pleural effusion.
Pleural effusion can be the result of many medical conditions, and an analysis of the effusion can be useful for diagnosing these conditions.
Signs and symptoms
General signs and symptoms of pleural effusion include:
- Chest pain, and;
- Shortness of breath and rapid breathing.
Additional signs and symptoms can appear, depending on the type and cause of the effusion.
Methods for diagnosis
Your doctor will diagnose pleural effusion by:
- Your symptoms and medical history;
- A physical examination of the chest;
- Chest X-rays, chest ultrasound, or computerized tomography (CT) scan, and;
- Bronchoscopy or thoracoscopy, in some cases. 
Your doctor will determine the cause of pleural effusion by aspirating the pleural fluid - taking a sample of fluid from your pleura using a needle and syringe. The fluid sample is then analyzed for the presence of blood, bacteria, immune cells, a tumor, and levels of glucose and certain proteins.
Types of treatment
The treatment of pleural effusion depends on the cause, and on whether the person has symptoms as a result of the pleural fluid.
For pleural effusions due to underlying medical conditions, such as heart failure and liver disease, optimizing the management of these conditions is necessary. This can be combined with drainage of the pleural fluid, using a procedure called thoracentesis (see below). Without treating the underlying condition, the pleural effusion will re-accumulate. Infections caused by bacteria are treated with antibiotics. Pleural effusions due to cancer need specialist treatment, as therapies depend on the type of cancer.
To treat pleural effusion, a doctor can remove the excess fluid using a needle and syringe, or by inserting a tube into your pleural space. This is known as thoracentesis, or 'tapping', and can be done periodically if fluid keeps building up. In some cases, draining tubes may be left in the chest cavity.
In particular cases of pleural effusion, your doctor may suggest pleurodesis, a procedure in which the two layers of the pleura are fused together. This is performed for recurrent pleural effusions that do not respond to other therapies.
In addition to being a sign of another condition, if left untreated, pleural effusions can lead to complications such as:
For others, pleural effusions will recur if the underlying cause is not treated successfully. In these cases, the fluid will be periodically drained whenever it accumulates.