It is excessive accumulation of fluids in tissue spaces and serous sacs.
A certain amount of fluid is distributed in the interstitial tissues of the body, which is regulated by:
1- The normal capillary permeability.
2- The normal hydrostatic pressure in the venous capillaries.
3- By the normal osmotic pressure of the plasma proteins. Causes of edema:
1- Increase in the permeability of the capillaries caused by toxins and anoxia.
2- Increase in the venous hydrostatic pressure of the capillaries as occurs in chronic venous congestion.
3- Decrease of the colloidal osmotic pressure of the plasma proteins below 5%. The decrease in albumin is more important because it has stronger osmotic pressure than globulin.
4- Lymphatic obstruction: The interstitial fluid is removed by lymphatics.
Types of edema:
It may be local or generalized.
I- Local edema which may be:
A- Inflammatory edema: This appears in acute inflammation and is due to increased capillary permeability and increase of the osmotic pressure in the interstitial tissues due to the high protein content in the inflammatory exudate.
B- Edema due to venous obstruction: As occurs in thrombosis of the femoral vein with edema of the lower limb. It is usually due to increased venous pressure due to stagnation of venous blood, helped by increased capillary permeability due to anoxia. It also occurs in a loop of intestine in volvulus and in strangulated hernia.
II- Generalized edema (anasarca) is of 3 main types:
a- Cardiac edema
b- Renal edema. 1 – Nephritic 2- Nephrotic c- Nutrional edema.
III- Lymphatic edema:
Due to lymphatic obstruction as in elephantiasis and edema of the skin of the breast and the arm in carcinoma of the breast where the lymphatics are obstructed by the malighant infiltration.
IV- Allergic edema:
Appears in some allergic diseases or due to hypersensitivity and is known as angioneurotic edema and is due to increased capillary permeability.
V- Other types of edema:
1- In normal pregnancy, edema of the ankles, which may
be accompanied by generalized edema may develop.
This is due to increased retention of fluids which occurs
in normal pregnancy. This may be helped in late months
of pregnancy by pressure of the enlarged uterus on the
veins and lymphatics of the lower limbs.
– In toxemia of pregnancy or pre-eclampsia, edema may appear due to hypertension and proteinurea which occurs in this disease.
2- Edema due to adrenal hormones:
In Cushing’s syndrome and during therapeutic administration of cortisone and ACTH, edema may develop and this is due to salt and water retention.
A- Cardiac edema: which occurs in right-sided heart failure with chronic generalized venous congestion. It is directly related to gravity and first appears in the ankles when the patient is walking and in sacral region when he is sleeping.
Pathogenesis or causes: It is usually due to the following factors:
1- Increase in the venous pressure of the capillaries.
2- Increased capillary permeability due to anoxia from congestion.
3- Accumulation of waste products in the tissues which by their osmotic action will tend to attract more water from blood.
4- Salt and water retention due to disturbance of the renal and adrenal functions resulting from decrease in the cardiac output. This is the main factor.
– The reduction in cardiac output, leads to renal vasoconstriction which helps the reabsorption of sodium with water retention.
– The generalized edema, leads to a decrease in the blood volume because increased fluid is retained in tissues. This decrease in blood volume stimulates the secretion of antidiuretic hormone and aldosterone, leading to salt and water retention.
B- Renal edema: which is of two forms.
1- Nephritic edema: which occurs in acute glomerul-
onephritis. It is a mild type of generalized edema which
affects mainly the face and eyelids and sometimes the
ankles and genital region. It is due to increase in the
capillary permeability based on allergic mechanism with
damage of the capillary walls. Another important factor
is the accompanying oligemea in acute glomerul-
onephritis, which leads to water retention.
2- Nephrotic edema: It is a severe type of generalized edema
which appears in nephrosis and in the nephrotic syndrome such as rapidly progressive glomerulonephritis and renal amyloidosis. It is mainly due to decrease in the osmotic pressure of the plasma proteins due to heavy albuminurea.
C- Nutritional edema: also known as "famine edema" which usually develops with prolonged malnutrition, chronic starvation and cachectic disease. It is usually due to decrease in the osmotic pressure of plasma proteins due to marked hypoproteinaemia.
Features of edema:
1- Generalized edema cannot be evident unless at least five liters of fluid had accumulated in tissues.
2- When we press with the finger on the edematous part, the fluid moves from one place to another and a pit is formed and this is known as pitting edema.
3- In severe generalized edema, fluid accumulates in the serous sacs leading to hydrothorax and hydroperitoneum. This fluid is known as transudate, which is quite different from the fluid which accumulates in the same sites as a result of inflammation and is called exudate.
Differences between transudate and exudates
Less than 1%
More than 3%
Does not clot
May clot due to presence of fibrinogen.
3- Specific gravity
Less than 1015
More than 1015
Few or absent
Present in large numbers and their type depends on the type of inflammation.