• Slowed blood flow, e.g. through bed rest, plaster cast or splints, marked fluid deficiency or also through a pre-existing venous disorder (chronic venous insufficiency)
• Damage to the vessel wall, e.g. during an operation, injury or inflammation, but also in age-related changes to the leg veins (e.g. varicose veins)
• Increased tendency for the blood to clot, e.g. if the clotting factors are increased or the normal equilibrium between clotting and clot dissolution is disturbed by certain drugs.

As a general rule, the risk of thrombosis increases with advancing age, with excess body weight, in pregnancy and after childbirth and also with smoking.

What happens if a thrombosis is left untreated:

The venous system in the leg consists of superficial veins in the connective tissue directly under the skin and deep leg veins in the leg muscles. If a clot forms in a deep leg vein, the path of the blood back to the heart is far more seriously impaired than with a blood clot in the superficial veins. The blood dams up in the leg and the vein becomes inflamed. A dull pain develops, similar to muscle stiffness after exercise. Typically, the pain decreases if the legs are raised and the swelling reduces. Other typical features are a hypersensitive, overheated and sometimes bluish discolouration of the skin.

In addition to the pain of venous inflammation, it is the complications of deep leg vein thrombosis that are most feared: if parts of the blood clot become detached, an embolism can occur in the lungs and this can be fatal. The greatest risk from an embolism is in the first three to five days of the thrombosis.

The post-thrombotic syndrome is a common complication in the late phase of venous occlusion: this syndrome is caused by thrombosis-induced damage to the valves that can lead to permanent damage to the vein with all its consequences. These can include venous leg ulcers.