Edema

What is edema?

Edema is an observable swelling from fluid accumulation in certain body tissues. Edema most commonly occurs in the feet and legs, where it also 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 that are outside of the blood vessels. These spaces are known as interstitial spaces or compartments. Most of the body's fluids that are found outside of the cells are normally stored in two spaces; the blood vessels (where the fluids are called the blood volume) and the interstitial spaces (where the fluids are called the interstitial fluid). In various diseases, excess fluid can accumulate in either one or both of these compartments.

The body's organs also have areas between cells (interstitial spaces) where fluid can accumulate. For example, an accumulation of fluid in the interstitial air spaces (alveoli) in the lungs occurs in heart failure and is called pulmonary edema. In addition, excess fluid sometimes collects in what has been called the third space, which includes cavities in the abdomen (abdominal or peritoneal cavity) or in the chest (lung or pleural cavity). The term anasarca refers to the severe, widespread accumulation of fluid in the various tissues and cavities of the body.

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

Pitting edema can be demonstrated by applying pressure to, for example, the skin of a swollen leg, by depressing the skin with a finger. If the pressing causes an indentation in the skin that persists for some time after the release of the pressure, the edema is referred to as pitting edema. Actually, any form of pressure, such as from the elastic part of socks, can induce the pitting of this 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 radical mastectomy, or congenital lymphedema. Another cause of non-pitting edema of the legs is called pretibial myxedema, which is a swelling over the shins 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.

What causes pitting edema?

Edema 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 that are associated with edema involve the heart, liver, and kidneys. In these diseases, edema occurs primarily because of the body's retention of too much salt (which is the chemical compound sodium chloride). The excess salt holds excess water in the interstitial tissue spaces, where the retained surplus of fluid is recognized as edema. Idiopathic (of unknown cause) edema, also sometimes called cyclical edema, occurs most often in women and just prior to each menstrual period. The most common local conditions that cause edema are varicose veins and thrombophlebitis (a blood clot with inflammation of the veins) of the deep veins of the legs. These conditions can cause inadequate pumping of the blood by the veins (venous insufficiency). The resulting increased back-pressure in the veins forces fluid to leak into the interstitial tissue spaces, where the retained excess fluid is recognized as edema.

How does salt intake affect edema?

The body's balance of salt is usually well-regulated. Under normal circumstances intake of small or large quantities of salt in the diet without concern for developing salt depletion or retention. The intake of salt is determined by dietary patterns and the removal of salt from the body is accomplished by the kidneys. The kidneys have a great capacity to control the amount of salt in the body by changing the amount of salt eliminated (excreted) in the urine. The amount of salt excreted by the kidneys is regulated by hormonal and physical factors that signal whether retention or removal of salt by the kidneys is necessary. One of the most important hormonal factors is the renin-angiotensin-aldosterone hormonal system. An important physical factor is the flow of blood to the kidneys.

If the flow of blood to the kidneys is decreased by an underlying condition such as heart failure, the kidneys react by retaining salt. This salt retention occurs because the kidneys perceive that the body needs more fluid to compensate for the decreased blood flow. If, on the other hand, the patient has a kidney disease that impairs the function of the kidneys, the ability to excrete salt in the urine is limited. In both of these conditions, the amount of salt in the body increases, which causes the patient to retain water and develop edema.

Horses experiencing a disturbance in their ability to normally excrete salt may need to either be placed on a diet limited in salt and/or given diuretic medications. In the past, patients with diseases associated with edema were placed on diets that were very restricted in salt intake. With the development of new and very potent diuretic agents (water pills), this marked restriction in dietary salt intake is generally no longer necessary. These diuretics work by blocking the reabsorption and retention of salt by the kidneys, thereby increasing the amount of salt and water that is eliminated in the urine.