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Causes of Phlebitis


Phlebitis is the natural reaction for a vein that has been damaged and can occur in any vein in the body. Either the vein wall becomes inflamed, leading to a clot in the vein, or a clot forms in the vein itself, leading to inflammation of the vein wall.

Most phlebitis occurs in the legs and is caused by varicose veins or hidden varicose veins. There are other veins that get phlebitis and other causes, and so we will look at some of the more well-known ones here.

Varicose veins and hidden varicose veins

The commonest cause of phlebitis is the clotting of blood in varicose veins or hidden varicose veins.

Blood will clot when one or more of 3 factors are affected (collectively known as Virchow’s Triad). These are:

  • Changes in the blood
  • Changes in the blood vessels wall
  • Changes in the blood flow

In varicose veins, the valves do not work and so blood flows backwards down the veins. This is a change in the blood flow. In addition, the flow is turbulent which stretches the vein wall to become varicose. This is the second change ie. change in the blood vessel wall. Either one of these changes by themselves could cause a clot and phlebitis. It is little wonder when both occur together that phlebitis occurs so frequently with varicose veins and hidden varicose veins.

Infusion or ‘drip’ phlebitis

Phlebitis can occur in any veins that have medication or fluid injected into them. This is often a reaction of the vein wall to the chemical that is being infused into the blood stream or the cannula (tube) that is inside the vein delivering an infusion. For example, it is very common to find hard, lumpy and tender veins in the arms of hospitalised patients who have had (or still have), a drip going into the vein in question.

These hard lumpy veins are usually blocked by the clot (thrombus) that accumulates inside the vein. With time, the clot usually resolves, opening the vein up again. However the vein wall may have become irreparably scarred and may either fibrose and disappear away as a scar, or may keep getting new bouts of phlebitis in it.

Malignancy or blood disorders

There are some rarer causes of phlebitis. Certain medical conditions such as some cancers or blood disorders (where the blood becomes thicker and more likely to clot) can cause different veins to suddenly clot unexpectedly around the body. This is a specialised area and we will not discuss it in more detail here. However, if a person suddenly starts getting phlebitis in more than one area for no apparent reason, they should see a doctor as soon as possible to try to find out why.