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.
- Edema is a swelling, usually of the legs, due to the accumulation of excessive fluid in the tissues.
- The edema that occurs in diseases of the heart, liver, and kidneys is mainly caused by salt retention, which holds the excess fluid in the body.
- In certain liver and kidney diseases, low levels of albumin in the blood can contribute to fluid retention.
Heart failure, cirrhosis of the liver, and a kidney disease called nephrotic syndrome are the most common systemic diseases that cause edema.
- Excess fluid that accumulates in the lungs is called pulmonary edema.
- Excess fluid that accumulates in the abdominal cavity is called ascites.
- Edema of unknown cause occurs primarily in women.
- Varicose veins or thrombophlebitis (a blood clot in an inflamed vein) of the deep veins in the legs causes edema that is localized to the legs.
- Therapy for edema consists of treating the underlying conditions, restricting salt intake, and often using diuretics (medicines to induce urination).
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.
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.
Edema can become a problem in systemic diseases of the heart, liver or kidneys. Diuretic therapy then can be initiated, which often alleviates the edema. The most potent diuretics are the loop diuretics, so-called because they work in that portion of the kidney tubules referred to as the loop of Henle. The kidney tubules are small ducts that regulate salt and water balance, while transporting the forming urine. These loop diuretics are clinically available as furosemide (Lasix), torsemide (Demedex), and butethamide (Bumex). The doses of these diuretics vary depending upon the clinical circumstances. These agents can be given orally, although seriously ill patients in the hospital may receive them intravenously for more prompt or effective response. If one of the loop diuretics is not effective alone, it may be combined with an agent that works further down (more distally) in the tubule. These agents include the thiazide type diuretics, such as hydrochlorthiazide (Hydrodiuril), or a similar but more potent type of diuretic called metolazone (Zaroxolyn). When diuretics that work at different sites in the kidney are used together, the response often is greater than the combined responses to the individual diuretics (synergistic response).
Some diuretics frequently cause an excessive loss of potassium in the urine, leading to the depletion of body potassium. These drugs include the loop diuretics, the thiazide diuretics, and metolazone. Patients on these diuretics, therefore, are commonly advised to take potassium supplements and/or to eat foods that contain a lot of potassium. High potassium foods include certain fruits, orange juice, tomatoes, and potatoes. Patients with impaired kidney function, however, often do not require potassium supplements with their diuretics because their damaged kidneys tend to retain potassium. In certain instances, the volume of urine that is induced by the diuretic can be improved by adding a potassium-sparing diuretic, that is, one that does not cause depletion of potassium. These diuretics include spironolactone (Aldactone), triamterene (Dyrenium, which is a component of Dyazide), and amiloride (Midamor). Adding one of these diuretics to the patient's diuretic regimen may preclude the need for potassium supplements. Another diuretic that can be used is acetazolamide (Diamox), which counteracts the development of a high concentration of bicarbonate (too much alkali) in the blood. A high bicarbonate sometimes occurs in patients receiving other diuretics.