Effusion (Fluid Accumulation) Types, Symptoms, Causes, Treatment

What is an effusion?

Effusion refers to an abnormal accumulation of fluid in a body cavity. This condition is also known as water retention because a major proportion of the accumulated fluid is usually water. Sometimes, however, other components such as blood and pus may also constitute a part of the accumulated fluid. Effusion should not be confused with edema, which is another type of abnormal fluid accumulation in the body.

Effusion refers to abnormal fluid accumulation within a body cavity, whereas edema refers to abnormal fluid accumulation within tissue spaces. For this reason, edema is also sometimes referred to as tissue swelling. The most frequently observed sites of effusion include the lungs, cranium, middle ear, heart, abdomen and joints.

Types of Fluid Accumulation

Some of these regions have fluid-filled cavities between double membranes that surround these organs (such as pleura, pericardium, and peritoneum). The normal quantity of fluid within these cavities helps in lubrication and protection of the organs from physical shocks. The excess fluid that accumulates within the body cavities during effusion can be broadly classified into two types.


Transudate refers to fluid that seeps out from the blood vessels or tissue spaces into the body cavities due to a difference in osmotic colloid pressure or hydrostatic pressure between the two compartments. Inflammatory conditions are not the cause of transudate fluid accumulation.


Exudate refers to fluid that seeps out of the blood vessels into the body cavity due to inflammation. During inflammation, the walls of the blood vessels become more porous, resulting in exudation of fluid and blood proteins into tissue spaces and cavities. Sometimes, whole blood may also exude out when capillaries are damaged.
Effusion can also be classified based on the composition of the fluid that accumulates in the body cavities. The following are some of the types of effusion based on fluid composition:

Types of Fluids

  • Serous effusion: In serous effusion, the accumulated fluid has very little protein, and is similar in composition to the tissue fluids or serum component of the blood.
  • Fibrinous effusion: Fibrinous effusion contains a large amount of fibrin and fibrinogen proteins. These proteins are critical for the formation of blood clots. The presence of these proteins in body cavities may lead to scar tissue formation.
  • Purulent effusion: Also known as suppurative effusion, purulent effusion refers to the presence of pus in the accumulated fluid. The amount of pus in the accumulated fluid may vary.
  • Hemorrhagic effusion: Hemorrhagic effusion refers to the presence of blood in the accumulated fluid. Sometimes, blood may be the only component that accumulates in a body cavity (for example, accumulation of blood in pleural space). At other times, blood may be mixed with previous effusion.

Signs and Symptoms

In the initial stages, effusion does not elicit any signs and symptoms. Only when excess fluid accumulation causes swelling, signs and symptoms may appear. The following are some of the key signs and symptoms of effusion:
Swelling: The main sign of effusion is localized swelling in the affected region. However, swelling may not be visible in all cases, even when present. For example, swelling within the cranium or thoracic cavity cannot be seen with the naked eye.


Swelling may be accompanied by pain, which is usually a result of tissue compression or stretching of the membranes that surround the cavity. The intensity of pain may differ based on the amount of accumulated fluid and the extent of the swelling. In some cases, pain may be felt only as a dull discomfort or a sensation of fullness. In other cases, the pain may be excruciating.

Loss of function

Accumulation of fluid in the body cavities may lead to compression of nearby tissues. This could lead to constriction of nearby blood vessels, resulting in a reduction or total loss of blood supply to the tissues. A lack of blood supply leads to the death of the tissue, which can compromise the function of the affected organs. For example, breathing may become difficult when lung tissue is affected by effusion.

Causes and Locations

The causes of effusion depend on the site of effusion. Inflammation of the membranous linings of the body cavities is one of the main causes of effusion. Infections, trauma, medications, chemotherapy, radiotherapy, hepatic disease, renal failure, and heart failure can all cause effusion. However, the reasons for effusion at different sites vary.

Effusion of the lung

The lung is enclosed within the pleura, which is a sac made of two layers of membrane. The space between the two pleural membranes (known as the pleural cavity) contains about 1-5 mL of serous fluid (also known as pleural fluid). This fluid acts as a lubricant. Abnormal accumulation of fluid within the pleural cavity is termed as pleural effusion or effusion of the lung. Pleural effusion can be caused by a variety of conditions such as pneumonia, pleuritis, asbestosis, pulmonary tuberculosis, cancer, pulmonary embolism, kidney disease, liver disease, heart failure and trauma to the chest.

Read more on fluid in the lungs.

Effusion of the heart

Like the lung, the heart is also enclosed within a double layered membranous sac (known as the pericardium). A small amount of pericardial fluid is present between the two layers of the pericardium (known as the pericardial space). Excessive collection of fluid within the pericardial space is known as pericardial effusion. Effusion around the heart can be caused by a variety of conditions such as pericarditis, infections, trauma to the chest wall, hypothyroidism, kidney failure, AIDS, and systemic lupus erythematosus.

Effusion of the brain

Effusion around the brain refers to abnormal increase in the intracranial pressure due to increase in the volume of cerebrospinal fluid (abbreviated as CSF). The main causes of effusion of the brain are hydrocephalus and subdural effusion.

Effusion of the joints

The space between two joints contains a small space, which contains synovial fluid. The synovial fluid is secreted by the synovial membrane that lines the joint surfaces. Joint effusion occurs when excessive fluid accumulates in the space between the joints. The excess fluid in most cases is synovial fluid. However, blood and pus may also be present in some cases. Arthritis (especially rheumatoid, septic, and psoriatic), infections, gout and trauma are common causes of effusion in the joint space.

Effusion of the middle ear

Middle ear normally contains very little fluid. Effusion of fluid in the middle ear can occur during inflammation of the middle ear (known as otitis media), which is mostly caused by infections. This is especially common in young children.

Read more on otitis media.

Effusion of the abdomen

Many abdominal organs are enveloped by a double layered membranous sac known as the peritoneum. The space between the two layers of the peritoneum (known as the peritoneal space) contains a small amount of lubricating fluid known as the peritoneal fluid. Peritoneal effusion (also known as ascites) refers to abnormal accumulation of fluid within the peritoneal space. The main causes of ascites include cirrhosis, liver cancer, biliary disease, alcoholic hepatitis, and non-alcoholic steatohepatitis.

Treatment of Effusion

The exact treatment of effusion depends on the underlying cause of effusion. Medications may be given. Sometimes, drainage of fluid may also be required. Surgery is also one of the treatment options that might be considered based on the degree of tissue damage and the cause of the effusion.

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