• Edema refers to an excess of fluid volume in the interstitial space. This excess in fluid will always be apparent histologically but in certain cases it will occur on such a scale that it will also manifest clinically. When the word "Edema" is used in a clinical setting it refers to clinically apparent edema while histological references to edema may or may not be clinically apparent.
  • Overview
    • Edema is ultimately caused by a derangement in proper interstitial fluid handling. The total amount of interstitial fluid is largely determined by the total amount of extracellular fluid in the body. The amount of interstitial fluid in any particular tissue is determined by the balance between the rate of fluid leakage from local capillary beds and the rate of local interstitial fluid siphoning by the lymphatic system. Derangement of any of these factors can lead to edema.
  • Excess Extracellular Fluid
    • The total amount of body extracellular fluid is ultimately regulated by the kidneys and is described further in ECF volume regulation. Briefly, ECF volume is largely dependent on the total amount of sodium in the body. Consequently, pathophysiological mechanims which induce sodium retention can cause edema.
  • Excess Capillary Fluid Leakage
    • Interstitial fluid is derived from leakage of fluid from the capillaries into the interstitial space. The rate of capillary fluid leakage is determined by the Starling Forces, as described in microcirculatory physiology. Consequently, any derangement in the capillary Starling Forces which induce excess fluid leakage can result in edema.
  • Reduced Lymphatic Return
    • Interstitial fluid leaked from the capillaries is constantly recycled back to the blood stream by lymphatic vessels, as described in lymphatic physiology. Reduced lymphatic return due to from tissues due to physical damage to lymphatics can thus cause edema.
Clinical Consequences
  • Edema generally does not disturb the function of most tissues and so is not dangerous in and of itself. Therefore, edema is usually important only as a sign of a potentially more ominous disease process. The major exceptions to this pattern are edema in the lung and brain which can disturb the functions of these delicate organs. Consequently, pulmonary edema and cerebral edema are always of proximate clinical concern.
  • Overview
    • Edema can occur in a generalized form over the entire body or can occur locally. Furthermore, the composition of the edematous fluid can vary. Each of these properties can provide some clues as to the etiology of the edematous process.
  • Generalized Edema
    • Generalized Edema typically results from disturbances of total ECF volume regulation or body-wide derangements of Starling Forces. Consequently, primary or secondary derangements of renal sodium handling along with hypoalbuminemia which disturbs Starling Forces throughout the body are typically to blame. In severe cases, generalized edema is referred to as anasarca or is termed hydrops fetalis when it occurs in a neonate.
  • Localized Edema
    • Localized Edema generally results from local derangements in Starling Forces or lymphatic fluid return. Various monickers are in usage to describe localized edema of particular organs.
    • Peripheral Edema: Edema of the extremities
    • Cerebral Edema: Edema of the brain
    • Pulmonary Edema: Edema of the lung interstitium
    • Hydrothorax: Edema in the pleura
    • Ascites: Edema in the peritoneum
    • Hydropericardium: Edema in the pericardium
  • Edema Fluid Composition
    • Edematous fluid can have a higher or lower density depending on the amount of protein within the fluid. Because the capillary barrier is responsible for preventing leakage of plasma proteins, excessive protein concentration in the edema fluid suggests defects in the capillary barrier which occur during capillary injury or inflammation
    • Transudate: Protein poor (density
< 1.012) and suggests a normal vascular permeability **'''Exudate:''' Protein rich (density > 1.020) and occurs when vascular permeability is increased