Edema

Edema

Write a short note on edema.
Answer:

Edema is defined as abnormal and excessive accumulation of free fluid in the interstitial tissue spaces and serous cavities.

Edema Classification

1. Based on distribution and extent of involvement

• Localized: It is limited to an organ or limb, For Example. lymphatic edema, inflammatory edema, allergic edema, and pulmonary edema.
• Generalized: It is noticeable in the subcutaneous tissue, For Example. renal edema, cardiac edema and nutritional edema.

Edema types and causes

2. Based on fluid composition

• Transudate: It is more often the case, such as in edema of cardiac and renal disease.
• Exudate: Such as in inflammatory edema.

Edema causes and types

Pathogenesis of Edema

1. Decreased plasma oncotic pressure: It is exerted by the total amount of plasma proteins that tend to draw fluid into the vessels normally. A fall in the total plasma protein level (hypoproteinemia of less than 5 g/dl, mainly hypoalbuminemia), results in a lowering of plasma oncotic pressure in a way that it can no longer counteract the effect of the hydrostatic pressure of blood. This results in the increased outward movement of fluid from the capillary wall and decreased inward movement of fluid from the interstitial space causing edema. Hypoproteinemia usually produces generalized edema. Out of the various plasma proteins, albumin has four times higher plasma oncotic pressure than globulin; thus it is mainly hypoalbuminemia (albumin below 2.5 g/dl) that generally results in edema.

2. Increased capillary hydrostatic pressure: The hydrostatic pressure of the capillary is the force that normally tends to drive fluid through the capillary wall into the interstitial space by counteracting the force of plasma oncotic pressure. A rise in the hydrostatic pressure at the venular end of the capillary which is normally low to a level more than the plasma oncotic pressure results in minimal or no reabsorption of fluid at the venular end, consequently leading to edema.

What is edema in pathology

3. Lymphatic obstruction: Normally, the interstitial flid in the tissue spaces escapes by way of lymphatics. Obstruction to the outflow of these channels causes localized edema, known as lymphedema.

4. Tissue factors: The two forces acting in the interstitial space—oncotic pressure of the interstitial space and tissue tension, are normally quite small and insignificant to counteract the effects of plasma oncotic pressure and capillary hydrostatic pressure respectively. However, in some situations, the tissue factors in combination with other mechanisms play a role in the causation of edema.

5. Increased capillary permeability: An intact capillary endothelium is a semipermeable membrane that permits the free flow of water and crystalloids but allows minimal passage of plasma proteins normally. However, when the capillary endothelium is injured by various ‘capillary poisons’ such as toxins and their products (For Example. histamine, anoxia, venoms, certain drugs, and chemicals), the capillary permeability to plasma proteins is enhanced due to the development of gaps between the endothelial cells causing leakage of plasma proteins into the interstitial fluid. This in turn causes reduced plasma oncotic pressure and elevated oncotic pressure of the interstitial fluid, consequently producing edema.

6. Sodium and water retention: The mechanism of edema by sodium and water retention in the extravascular compartment is best described in relation to derangement in the normal regulatory mechanism of sodium and water balance. Normally about 80% of sodium is reabsorbed by the proximal convoluted tubule under the influence of either intrinsic renal mechanism or extrarenal mechanism while retention of water is affected by the release of antidiuretic hormone. The possible factors responsible for causing edema by excessive retention of sodium and water in the extravascular compartment via stimulation of intrinsic renal and extrarenal mechanism as well as via release of ADH are:

• Reduced glomerular filtration rate in response to hypovolemia.
• Enhanced tubular reabsorption of sodium and consequently its decreased renal excretion.
• Increased filtration factor, i.e. increased filtration of plasma from the glomerulus.
• Decreased capillary hydrostatic pressure is associated with increased renal vascular resistance.

Pitting vs non-pitting edema

Generalized edema

Consequences of Edema

Edema can compromise the cellular function in the following ways, i.e.

• Due to the expansion of interstitial space, there is an increase in diffusion distance for oxygen and other nutrients, which hampers cellular metabolism. An example is impaired gas exchange because of pulmonary edema.
• Expansion of interstitial space interferes with the removal of toxic byproducts of cellular metabolism.