Edema (American English) or oedema (British English), formerly known as dropsy or hydropsy, is swelling of any organ or tissue due to accumulation of excess lymph fluid, without an increase of the number of cells in the affected tissue. Edema can accumulate in almost any location in the body, but the most common sites are the feet and ankles.
Edema is the increase of interstitial fluid in any organ. Generally, the amount of interstitial fluid is in the balance of homeostasis. Increased secretion of fluid into the interstitium or impaired removal of this fluid may cause edema.
Generation of interstitial fluid is regulated by Starling equation of Tissue Fluid which states that it depends on the balance of oncotic pressure and of hydrostatic pressure across the capillary walls. Consequently, anything that increases oncotic pressure outside blood vessels (for example inflammation), or reduces oncotic pressure in the blood (states of low plasma osmolality, for example cirrhosis) will cause edema. Increased hydrostatic pressure inside the blood vessel (for example in heart failure) will have the same effect.
Abnormal removal of interstitial fluid is caused by obstruction of the lymphatic system, for example due to pressure from a cancer or enlarged lymph nodes, destruction of lymph vessels by radiotherapy, or infiltration of the lymphatics by infection such as elephantiasis.
Edema without a modifier usually refers to peripheral or dependent edema, the accumulation of fluid in the parts of the body that are most affected by gravity. In ambulatory people these are the legs, although in those who are bedbound the first manifestation may be sacral edema. If severe enough, peripheral edema may progress to involve the abdominal or even thoracic wall (this may be referred to as generalized edema or anasarca). In particular edema states (e.g. nephrotic syndrome, see below), periorbital edema (around the eyes) may be present.
Some phenomena may distinguish different causes of peripheral edema. Most peripheral edema is pitting edema - pressing down will lead to a shift in the interstitial fluid and the formation of a small pit that resolves over seconds. Non-pitting edema may reflect lymphedema, a form of edema that develops when the lymph vessels are obstructed.
Causes of peripheral edema are:
* high hydrostatic pressure of the veins, leading to poor reabsorption of fluid
o venous obstruction, e.g. deep vein thrombosis (typically one-sided)
o congestive heart failure
o varicose veins
o asymmetric compression of thigh and leg (e.g., knee pads, tight jeans)
* low oncotic pressure
o nephrotic syndrome (renal protein loss)
o epidemic dropsy
* obstruction of lymph drainage
o fibrosis after surgery
* inflammation (active secretion of fluid into the interstitial space due to increased membrane permeability by inflammatory mediators):
o allergic conditions (e.g. angioedema)
o any other form of inflammation (tumor - or swelling - is one of the main characteristics of inflammation)
Edema of specific organs (cerebral edema, pulmonary edema, macular edema) may also occur, each with different specific causes to peripheral edema, but all based on the same principles. Ascites is effectively edema within the peritoneal cavity, as pleural effusions are effectively edema in the pleural cavity. Causes of edema which are generalised 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.
Common and usually harmless appearances of cuteneous edema are observed with mosquito bites and skin contact with certain plants (urticaria).