Edema, or dropsy, is an excess of a clear watery fluid within the tissues between the cells. This fluid differs from the blood-plasma in that it has less albumin, is of a lower specific gravity, is rich in salts, but does not coagulate spontaneously, as it contains very little fibrin. This is called a transudate to distinguish it from the fluid present in inflammations, the latter being called an exudate.

It may be caused by:

1. Differences of pressure, a filtration process.

2. An increased secretion by the endothelial cells of the vessels.

3. Osmosis, resulting from variations in the relative concentration of salts, particularly sodium chlorid, on either side of the osmotic membranes, the cell walls.

4. The most satisfactory explanation of the occurrence of edema can probably be based upon the properties of colloidal (non-crystalloid) bodies such as gelatin and, presumably, other proteins. If dried gelatin is placed in water it will absorb a definite quantity of that water and will swell up to a certain point. If the water be slightly acidified the amount absorbed becomes very much greater. It has been shown that an inadequate supply of oxygen results in the production in the tissues of acids, particularly lactic acid and carbon dioxid, which, in turn, increase the tendency of colloids to take up water; consequently, edema is brought about. There is probably little of it due to increased capillary pressure or to secretory activity on the part of the endothelium of the blood-vessels.

5. Neuropathic edema, as herpes zoster and angioneurotic edema, may be due to something more than simple uncomplicated vasomotor disturbances.

6. Hydrops, or edema ex vacuo, is that which occurs when an organ, as a result of atrophy, does not completely fill its cavity, the remaining space becoming filled with fluid. It usually occurs in the cranial cavity and in the spinal canal.

The Types Of Edema

The types of edema are as follows:

1. Congestive edema, the commonest form. Present in cases of obstruction to the venous outflow. Possibly the interference with the nutrition of the tissues and the changes thus resulting play a part.

2. Edema from lymphatic obstruction does not occur in healthy tissues, but does take place if there is some disturbance of nutrition in the areas involved in the lymphatic obstruction.

3. Inflammatory Edema - Probably due to degenerative changes occurring in the endothelial cells of the capillaries and of the tissue cells as well.

4. Toxic Edema - Various toxic substances circulating in the blood have possibly different effects on the capillary walls.

According to the seat of the edema, special terms are employed.

When the subcutaneous tissues are generally involved, it is known as anasarca. Ascites refers to a collection of fluid within the abdominal cavity.

Hydrothorax, a collection within the pleural cavities.

Hydro pericardium, when within the pericardium.

Hydrocephalus, fluid within the ventricles of the brain.

Hydrocele, when within the tunica vaginalis testis.

The common clinical causes are: (1) Cardiac insufficiency, the edema usually first noticed about the ankles. (2) Kidney disease, first seen about the eyes. (3) Cirrhosis of the liver, accompanied by ascites. (4) Anemia and cachexia. (5) Pressure upon the veins or lymphatics.

Under the microscope the cells of the involved tissues will appear more or less widely separated and in some instances may be vacuolated.

Interstitial emphysema is an infiltration of the tissues by gas, usually the result of some injury involving the respiratory tract. It may be due to the presence of some gas-producing bacteria, such as the bacillus of malignant edema or the Bacillus aerogenes capsulatus, within various organs, particularly the uterus and liver. It is a comparatively rare condition.