Pleural effusion

Encyclopedia of Medicine, Apr 06, 2001 by David A. Cramer

Pleural effusion occurs when too much fluid collects in the pleural space (the space between the two layers of the pleura). It is commonly known as "water on the lungs." It is characterized by shortness of breath, chest pain, gastric discomfort (dyspepsia), and cough.

There are two thin membranes in the chest, one (the visceral pleura) lining the lungs, and the other (the parietal pleura) covering the inside of the chest wall. Normally, small blood vessels in the pleural linings produce a small amount of fluid that lubricates the opposed pleural membranes so that they can glide smoothly against one another during breathing movements. Any extra fluid is taken up by blood and lymph vessels, maintaining a balance. When either too much fluid forms or something prevents its removal, the result is an excess of pleural fluid -- an effusion. The most common causes are disease of the heart or lungs, and inflammation or infection of the pleura.

Pleural effusion itself is not a disease as much as a result of many different diseases. For this reason, there is no "typical" patient in terms of age, sex, or other characteristics. Instead, anyone who develops one of the many conditions that can produce an effusion may be affected.

There are two types of pleural effusion: the transudate and the exudate. This is a very important point because the two types of fluid are very different, and which type is present points to what sort of disease is likely to have produced the effusion. It also can suggest the best approach to treatment.

A transudate is a clear fluid, similar to blood serum, that forms not because the pleural surfaces themselves are diseased, but because the forces that normally produce and remove pleural fluid at the same rate are out of balance. When the heart fails, pressure in the small blood vessels that remove pleural fluid is increased and fluid "backs up" in the pleural space, forming an effusion. Or, if too little protein is present in the blood, the vessels are less able to hold the fluid part of blood within them and it leaks out into the pleural space. This can result from disease of the liver or kidneys, or from malnutrition.

An exudate -- which often is a cloudy fluid, containing cells and much protein -- results from disease of the pleura itself. The causes are many and varied. Among the most common are infections such as bacterial pneumonia and tuberculosis; blood clots in the lungs; and connective tissue diseases, such as rheumatoid arthritis. Cancer and disease in organs such as the pancreas also may give rise to an exudative pleural effusion.

Some of the pleural disorders that produce an exudate also cause bleeding into the pleural space. If the effusion contains half or more of the number of red blood cells present in the blood itself, it is called hemothorax. When a pleural effusion has a milky appearance and contains a large amount of fat, it is called chylothorax. Lymph fluid that drains from tissues throughout the body into small lymph vessels finally collects in a large duct (the thoracic duct) running through the chest to empty into a major vein. When this fluid, or chyle, leaks out of the duct into the pleural space, chylothorax is the result. Cancer in the chest is a common cause.

Among the most important specific causes of a transudative pleural effusion are:

• Congestive heart failure. This causes pleural effusions in about 40% of patients and is often present on both sides of the chest. Heart failure is the most common cause of bilateral (two-sided) effusion. When only one side is affected it usually is the right (because patients usually lie on their right side).
• Pericarditis. This is an inflammation of the pericardium, the membrane covering the heart.
• Too much fluid in the body tissues, which spills over into the pleural space. This is seen in some forms of kidney disease; when patients have bowel disease and absorb too little of what they eat; and when an excessive amount of fluid is given intravenously.
• Liver disease. About 5% of patients with a chronic scarring disease of the liver called cirrhosis develop pleural effusion.

A wide range of conditions may be the cause of an exudative pleural effusion:

• Pleural tumors account for up to 40% of one-sided pleural effusions. They may arise in the pleura itself (mesothelioma), or from other sites, notably the lung.
• Tuberculosis in the lungs may produce a long-lasting exudative pleural effusion.
• Pneumonia affects about 3 million persons each year, and four of every ten patients will develop pleural effusion. If effective treatment is not provided, an extensive effusion can form that is very difficult to treat.
• Patients with any of a wide range of infections by a virus, fungus, or parasite that involve the lungs may have pleural effusion.
• Up to half of all patients who develop blood clots in their lungs (pulmonary embolism) will have pleural effusion, and this sometimes is the only sign of embolism.
• Connective tissue diseases, including rheumatoid arthritis, lupus, and Sjögren's syndrome may be complicated by pleural effusion.
• Patients with disease of the liver or pancreas may have an exudative effusion, and the same is true for any patient who undergoes extensive abdominal surgery. About 30% of patients who undergo heart surgery will develop an effusion.
• Injury to the chest may produce pleural effusion in the form of either hemothorax or chylothorax.