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.