Abnormal accumulation of watery fluid in the spaces between connective-tissue cells. Usually a symptom of diseases of the kidneys, heart, veins, or lymphatic system, which affect water balance in the cells, tissues, and blood, edema can be pitting (retaining an imprint when compressed) or nonpitting. Edema may be local (e.g., hives from allergies) or generalized (also called dropsy), sometimes involving body cavities as well as tissues. Treatment must usually focus on the underlying cause.
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Oedema (or Edema in American English), formerly known as dropsy or hydropsy, is the increase of interstitial fluid in any organ — swelling. Generally, the amount of interstitial fluid is determined by the balance of fluid homeostasis, and increased secretion of fluid into the interstitium or impaired removal of this fluid may cause edema.
Abnormal removal of interstitial fluid is caused by obstruction of the lymphatic system. This may be due to, for example, pressure from a cancer or enlarged lymph nodes, destruction of lymph vessels by radiotherapy, or infiltration of the lymphatics by infection (such as elephantiasis).
Causes of edema which are generalized to the whole body can cause edema in multiple organs and peripherally. For example, severe heart failure can cause peripheral edema, pulmonary edema, pleural effusions and ascites.