Edema

Edema is observable swelling from fluid accumulation in body tissues. Edema most commonly occurs in the feet and legs, where it is referred to as peripheral edema. 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 of the cells are normally stored in two spaces; the blood vessels (as the "liquid" or serum portion of your blood) and the interstitial spaces (not within the cells). 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 edema. 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 of the tissues and cavities of the body at the same time.

Pitting edema 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 edema is referred to as pitting edema. Any form of pressure, such as from the elastic in socks, can induce pitting with this type of edema.

In non-pitting edema, which usually affects the legs or arms, pressure that is applied to the skin does not result in a persistent indentation. Non-pitting edema can occur in certain disorders of the lymphatic system such as lymphedema, which is a disturbance of the lymphatic circulation that may occur after a mastectomy, lymph node surgery, or congenitally. Another cause of non-pitting edema of the legs is called pretibial myxedema, which is a swelling over the shin that occurs in some patients with hyperthyroidism. Non-pitting edema of the legs is difficult to treat. Diuretic medications are generally not 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 edema, as it is by far the most common form of edema.

Source: MedicineNet.com
http://www.medicinenet.com/edema/article.htm