Doctors define edema as an increase in the volume of fluid in the spaces between the body's cells. Depending on its cause, edema-related swelling may occur in a specific area or it may be distributed throughout the body.

There are many forms of edema. One type, known as anasarca, may be characterized by puffiness in the face. Other common signs of anasarca include rings that fit more snugly, or shoes that become uncomfortably tight, particularly in the evening. Another form of edema, ascites, is characterized by an excess of fluid in the lining of the abdomen and is usually associated with liver disease. Hydrothorax is yet another type of edema in which fluid accumulates in the space surrounding the lungs. Finally, idiopathic edema is a syndrome that occurs almost exclusively in women. It involves the retention of water and the accumulation of “water weight” as a part of premenstrual syndrome.

The causes of edema are related to the way fluids are distributed and transported throughout the body. Much of the fluid in the body is exchanged between the capillaries and the spaces that surround the cells. In a healthy state, a balance of pressure is maintained between the fluid in the capillaries and the fluid outside the capillaries, which is referred to as interstitial fluid. The development of edema occurs when this delicate balance of pressure is disrupted and an excess of fluid flows out of the capillaries and into the interstitial space.

Edema can also be caused by trauma to the systems that maintain a healthy balance of fluids in the body. For instance, burns, cuts, bumps, and bruises can damage the sensitive capillary tissues that control the flow of pressure into and out of the spaces between cells. The swelling that occurs as a result of these wounds is a form of edema. Chemical and bacterial invaders can damage the capillaries and trigger edema as well.

Other forms of edema are often caused by a reduction in the total volume of blood in the body. In this situation, the body compensates for the lower blood volume by retaining more water and salt. If, however, this response is insufficient to restore a proper blood volume, the retention of water and salt increases and edema can develop. This cycle is often seen in patients with congestive heart failure. In this case, the heart is unable to pump enough blood out of the heart and fill the circulatory system. The body then responds to the reduction in blood volume by retaining more and more water. Edema often follows.

Still more causes of edema include kidney failure, protein deficiency, and pregnancy, as well as the use of certain estrogen therapies and vasodilators.