Article: Edema

Edema (BE: oedema, formerly known as dropsy or hydropsy) is swelling of any organ or tissue due to accumulation of excess fluid, without an increase of the number of cells in the affected tissue.

Principles

Edema is the increase of [intracellular fluid] (ICF) in any organ. Generally, the amount of ICF is in balance. Increased secretion of fluid into the interstitium or impaired removal of this fluid may be associated with edema. Mechanisms causing increased secretion are inflammation, increased hydrostatic pressure and abnormalities in osmolality; abnormal removal can be found in low oncotic pressure and abnormal pressure on the lymphatic system.

Peripheral edema

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
  • venous obstruction, e.g. deep vein thrombosis (typically one-sided)
  • congestive heart failure
  • varicose veins
• low oncotic pressure
  • cirrhosis
  • malnutrition
  • nephrotic syndrome (renal protein loss)
  • epidemic dropsy
• obstruction of lymph drainage
  • infection
  • cancer
  • fibrosis after surgery
  • filariasis
• inflammation (active secretion of fluid into the interstitial space due to increased membrane permeability by inflammatory mediators):
  • allergic conditions (e.g. angioedema)
  • any other form of inflammation (tumor - or swelling - is one of the main characteristics of inflammation)

Organ-specific edema

Edema of specific organs (cerebral edema, pulmonary edema, macular edema) may impair function. It has different causes from peripheral edema, although pulmonary edema is seen together with peripheral edema in severe fluid overload.

Common and usually harmless appearances of cuteneous edema are observed with mosquito bites and skin contact with certain plants (urticaria).

Edema in plants

Edema in plants is the extended swelling in plant organs caused primarily by an excessive accumulation of water.

Reference

• Cho S, Atwood JE. Peripheral edema. Am J Med 2002;113:580-586. PMID 12459405.