Oedema 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. Oedema can accumulate in almost any location in the body, but the most common sites are the feet and ankles.

Oedema 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 oedema.

Generation of interstitial fluid is regulated by the Starling equation of tissue fluid which states that it depends on the balance of osmotic pressure and of hydrostatic pressure which act in opposite directions across the semipermeable 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 oedema. Increased hydrostatic pressure inside the blood vessel (for example in heart failure) will have the same effect. If the permeability of the capillary walls increases, more fluid will tend to escape out of the capillary, as can happen when there is inflammation.

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.

Peripheral oedema

Oedema without a modifier usually refers to peripheral or dependent oedema, 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 oedema. If severe enough, peripheral oedema may progress to involve the abdominal or even thoracic wall (this may be referred to as generalized oedema or anasarca). In particular oedema states (e.g. nephrotic syndrome, see below), periorbital oedema (around the eyes) may be present.

Some phenomena may distinguish different causes of peripheral oedema. Most peripheral oedema is pitting oedema - pressing down will lead to a shift in the interstitial fluid and the formation of a small pit that resolves over seconds. Non-pitting oedema may reflect lymphedema, a form of oedema that develops when the lymph vessels are obstructed.

Causes of peripheral oedema 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 cirrhosis
      o malnutrition
      o nephrotic syndrome (renal protein loss)
      o epidemic dropsy

      • obstruction of lymph drainage
      • o infection
        o cancer
        o fibrosis after surgery
        o filariasis

        • 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 (tumour - or swelling - is one of the main characteristics of inflammation)

          Organ-specific oedema

          Oedema of specific organs (cerebral oedema, pulmonary oedema, macular oedema) may also occur, each with different specific causes to peripheral oedema, but all based on the same principles. Ascites is effectively oedema within the peritoneal cavity, as pleural effusions are effectively oedema in the pleural cavity. Causes of oedema which are generalised to the whole body can cause oedema in multiple organs and peripherally. For example, severe heart failure can cause peripheral oedema, pulmonary oedema, pleural effusions and ascites.

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