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What is oedema?

Oedema (formerly known as dropsy or hydropsy) is observable swelling from fluid accumulation in body tissues. Oedema most commonly occurs in the feet and legs, where it is referred to as peripheral oedema. The swelling is the result of the accumulation of excess fluid under the skin in the spaces within the tissues. All tissues of the body are made up of cells and connective tissues that hold the cells together. This connective tissue around the cells and blood vessels is known as the interstitium. Most of the body's fluids that are found outside the cells are normally stored in two spaces; the blood vessels (as the ‘liquid’ or serum portion of your blood) and the interstitial spaces. In various diseases, excess fluid can accumulate in either one or both of these compartments.

The body's organs have interstitial spaces where fluid can accumulate. An accumulation of fluid in the interstitial air spaces ( alveoli) in the lungs occurs in a disorder called pulmonary oedema. In addition, excess fluid sometimes collects in what is called the third space, which includes cavities in the abdomen ( abdominal or peritoneal cavity - called ‘ascites’) or in the chest (lung or pleural cavity - called ‘pleural effusion’). Anasarca refers to the severe, widespread accumulation of fluid in the all the tissues and cavities of the body at the same time.

What is pitting oedema and how does it differ from non-pitting oedema?

Pitting oedema can be demonstrated by applying pressure to the swollen area by depressing the skin with a finger. If the pressing causes an indentation that persists for some time after the release of the pressure, the oedema is referred to as pitting oedema. Any form of pressure, such as from the elastic in socks, can induce pitting with this type of oedema.

In non-pitting oedema, which usually affects the legs or arms, pressure that is applied to the skin does not result in a persistent indentation. Non-pitting oedema can occur in certain disorders of the lymphatic system such as lymphoedema, which is a disturbance of the lymphatic circulation that may occur after a mastectomy, lymph node surgery, or congenitally. Another cause of non-pitting oedema of the legs is called pretibial myxoedema, which is a swelling over the shin that occurs in some patients with hyperthyroidism. Non-pitting oedema of the legs is difficult to treat. Diuretic drugs are not usually effective, although elevation of the legs periodically during the day and compressive devices may reduce the swelling.

The focus of the rest of this article is on pitting oedema, as it is by far the most common form of oedema.

What causes pitting oedema?

Oedema is caused by either systemic diseases, that is, diseases that affect the various organ systems of the body, or by local conditions involving just the affected extremities. The most common systemic diseases associated with oedema involve the heart, liver, and kidneys. In these diseases, oedema occurs primarily because of the body's retention of too much salt ( sodium chloride). The excess salt causes the body to retain water. This water then leaks into the interstitial tissue spaces, where it appears as oedema.

The most common local conditions that cause oedema are varicose veins and thrombophlebitis (inflammation of the veins of the legs). These conditions can cause inadequate pumping of the blood through the veins (venous insufficiency). The resulting increased back-pressure in the veins forces fluid to accumulate in the extremities (especially the ankles and feet). The excess fluid then leaks into the interstitial tissue spaces, causing oedema.

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