What causes it?

The process usually starts with damage to the inner lining of the vein due to inflammation, injury or infection. This is followed by stasis (reduced flow) of the blood with local increase in coagulability leading to thrombosis. Thrombosis often starts at the site of a vein valve and may itself cause inflammation of the vein.

Several factors may initiate the process by increasing pressure on and within a vein. These are:

• Increase intra-abdominal pressure can lead to partial obstruction of blood flow through the abdomen due to increased pressure on the main intra-abdominal veins. This increases the back pressure into the legs and if prolonged it can stretch and damage the walls of the veins with resulting inflammation and thrombosis. Conditions such as abdominal obesity, pregnancy and childbirth can increase venous back pressure. Associated inactivity further reduces the rate of flow and may cause stasis of the blood within the legs.
• Periods of inactivity such as in long-haul air travel or being confined to bed can also cause intravenous thrombosis from back pressure and stasis. Changes in levels of oxygen in the blood during prolonged flights can add to the effects of stasis.
• Varicose veins are caused by the absence of or damage to the valves within a vein which allows significant back pressure and stasis of blood within the legs. The back-pressure transmits to the smaller superficial veins and can result in superficial thrombophlebitis in the lower leg.
• Obstruction in the chest or neck from malignancy or other diseases can impede the flow of blood from the upper limbs and result in thrombophlebitis although it is much less common in the upper than the lower limbs.
• Damage to the vein wall from the injection of irritant substances, and injection using infected needles can cause thrombophlebitis as can prolonged insertion of cannulae for intravenous infusion.

In addition to mechanical factors conditions which increase the rate of coagulation of the blood can cause thrombophlebitis.

• Septic phlebitis can occur secondary to an infection in the vicinity of a vein