Oedema

Oedema means that too much fluid, mainly water, has accumulated in the body. The term comes from a Greek word meaning a swollen condition.

The accumulation of fluid may be in a particular location or it may be in several places in the body. In generalised oedema, fluid accumulates in any of the tissues, but especially in the air spaces of the lungs and in the spaces in the abdomen surrounding the bowels and other organs (the peritoneal cavity). Localised oedema is usually temporary and often corrects itself. Generalised oedema, however, is almost always abnormal and requires specific treatment.

The body is made of millions of cells, most of them linked together to form tissues. The cells themselves are largely filled with (and surrounded by) fluid. So a considerable proportion of the body weight - about 60 per cent - consists of water.

The processes that result in the movement of water into and out of cells are important in human body function. Also important are the factors that control the total amount of water in the body at any particular time. For example, in a healthy person without oedema, the kidneys deal with surplus fluid in the body by disposing of it in the urine.

The amount of water normally remaining in the body is provided by the difference between fluid taken in and fluid discharged. Fluid is taken into the body by drinking, food, and by water produced by bodily processes. It is discharged from the body via urine, faeces, non-visible perspiration (for example, when breathing out), and sweat.

Factors affecting the quantity of water in the body include high surrounding temperatures and taking strenuous exercise. In healthy people, such factors usually result in a significant increase in the intake of fluid, which is almost immediately compensated for by an increase in the urinary output.

CausesBlood is carried from the heart in arteries and returns to it in veins. Between the arteries and the veins are networks of tiny blood vessels called capillaries. The capillary walls have microscopic gaps through which water from the blood can pass. Normally the red blood cells and the large protein molecules in the blood can't get through these gaps.

If a membrane, such as the capillary wall, through which water can pass, separates two liquids - in this case blood and tissue fluid - water will flow from the liquid with the lesser amount of substance dissolved in it into the stronger solution.

When the balance of strength is equal on the two sides, no water flows. This is called an equal osmotic pressure. Dissolved protein is one of the most important substances that increases the osmotic pressure in the blood. In healthy people there is plenty of protein in the blood.

If blood protein levels drop significantly, there will be little or nothing to draw the water back into the blood from the tissue spaces through the capillary walls. The result is an accumulation of fluid in the tissue - also called oedema.

Generalised oedema may result from:

- Kidney disease, such as nephrotic syndrome where protein loss in the urine is considerable.
- Acute glomerulonephritis which also causes protein loss.
- Liver disease, such as cirrhosis, in which the production of protein is reduced.
- Starvation, in which the intake of protein is inadequate.

Problems with osmotic pressure are not the only causes of oedema. In women who have had breast cancer surgery involving the removals of the lymph nodes in the armpit, excess tissue fluid can no longer return via the lymph system. The result can be a condition of arm oedema called lymphoedema.

Anything that increases the leakiness (permeability) of capillaries can cause oedema. This can result from physical or chemical injury, from burns or from hormonal causes, as in premenstrual syndrome. The exact mechanism in this last case is uncertain, but some experts believe it is due to increased leakage of protein into the tissue fluid.

Oedema also occurs in heart failure, when the heart can't pump blood round fast enough to clear fluid from the tissues. There is a rise in back-pressure in the veins and an accumulation of fluid.

Oedema of the ankles is quite common, especially in hot weather, at high altitude, and if movement is restricted. This swelling is usually dependent, meaning that the pressure of fluid in the legs, from prolonged standing or sitting, pushes it into the surrounding tissues. Rotating the ankles regularly is recommended, as is keeping the feet up where possible. The use of support stockings will also help.

Generalised oedema causes weight increase. In mild cases there may be no more effect than this, but more severe oedema can cause difficulty in breathing. If the lung oedema becomes severe, the situation may become life-threatening

Oedema may be visible as an obvious puffiness of the face, the ankles or the small of the back. Oedema can often be demonstrated by pressing firmly on the skin with a finger. If oedema is present, a hollow may be left that will take some time to disappear. This is called pitting oedema.

This is based on the physical signs, such as appearance, the weight gain, and the presence of pitting oedema. Oedema in the lungs produces typical crackling sounds on breathing that can be heard while listening to the chest with a stethoscope.

Oedema is treated by correcting the cause, where possible, and by the use of diuretic drugs that increase the urinary water output.

When large accumulations of fluid occur, as in the abdomen, it may be necessary to drain fluid out through a tube. This can be done painlessly with a local anaesthetic.

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