There are two main systems of veins in the leg. There are the deep veins inside of the muscles, which are the main pumping veins of the leg, and there are the superficial veins which lie in the fat just under the skin.

Superficial thrombophlebitis (often called phlebitis)

If a thrombosis (clot) occurs in the superficial veins just under the skin, there is often a painful lump that can be red. It is usually localised to just one area although it can be long or branched just as the vein is.

Unfortunately, many general practitioners or nurses see a painful red lump on the leg and think this is an infection and often give antibiotics. This is obviously the wrong treatment as there are no bugs to kill - the redness and pain are coming from inflammation of the clot. The correct treatment would be aspirin or other nonsteroidal anti-inflammatory drugs.

Superficial thrombophlebitis is painful and can leave brown stains on the skin. However it is not usually dangerous and only rarely does the clot propagate and spread into the deep veins.

Deep vein thrombosis (DVT)

A deep vein thrombosis is a clot in the deep system of veins in the leg. The severity of the symptoms and the risks really depend on where the clot occurs.

A DVT in the calf (or below knee) is usually tender but poses very little risk. Most doctors will treat this with a short course of heparin and will keep the patient mobile and will scan again in a few days to check the clot has not propagated (got bigger).

An above knee deep vein thrombosis in the thigh, often causes more pain and swelling in the leg. As such there is more risk that this will damage the deep veins of the leg in a way that made cause more problems in the future. In addition, there is a small chance that this may give rise to a clot that flies off to the lungs -a pulmonary embolism.

A deep vein thrombosis that goes into the pelvis is the riskiest of all. Not only will it cause the whole leg swell if it blocks off the vein, but if it does not attach firmly to the wall it will fly off as a pulmonary embolism. A pulmonary embolism can make a patient very unwell or even kill them.