Cause of phlebitis (superficial thrombophlebitis)
The common finding in virtually every case of phlebitis is a clot within the vein lumen. Normal veins have flowing blood in them. If the blood clots within the vein, the body tries to absorb the clot as part of the healing process and this is performed by inflammation. Therefore the clot within the vein wall causes the vein wall to become inflamed. This inflammatory process then spreads through the vein wall and starts causing further inflammation outside of the vein.
It is very important to note at this point that infection has not been mentioned. Inflammation which is part of the healing process, is virtually indistinguishable from some infections and this confuses many medics. Although many medics give antibiotics for phlebitis, it is exceptionally rare for phlebitis to be caused by an infection.
Of course blood normally flows within the veins without clotting and it is not normal for blood to clot under normal conditions. Therefore when the blood does clot in a superficial vein and cause phlebitis, there is usually an underlying reason for the thrombosis (blood clot).
The things that cause blood to clot within any blood vessel including veins were first described by a doctor called Rudolf Virchow. As there are three of them, they are known as “Virchow’s triad”. Virchow’s triad are:
- changes in the blood flow
- changes in the blood constituents
- changes in the vein wall (or blood vessel wall if other blood vessels are being considered such as arteries)
- Blood being pumped normally up the deep vein
- Blood refluxing from deep vein, out of the saphenofemoral junction into the Great Saphenous Vein (GSV) due to valve failure
- Blood refluxing down the Great Saphenous Vein (GSV) due to the valves not working
- Turbulence of the refluxing blood causes veins to dilate, which are called varicose veins
Probably the commonest cause of phlebitis in the legs is the presence of varicose veins, even if the varicose veins are not visible on the surface. Patients get varicose veins when the valves in the superficial veins fail and gravity causes blood to fall down the veins rather than being pumped up towards the heart (which is against the force of gravity). The falling of the blood down the veins often causes the wall to stretch, causing varicose veins (for a deeper understanding of venous reflux and the cause of varicose veins see The College of Phlebology’s textbook “Understanding Venous Incompetence: the cause of varicose veins and venous leg ulcers“).
- Reflux of blood in the superficial truncal veins causes inflammation of subcutaneous fat in the lower leg
- Valve failing at the Sapheno-Femoral junction allowing blood to reflux from the deep vein to the Great Saphenous Vein (GSV)
- Valves working normally in the deep vein
In this situation, both the normal blood flow has changed (blood is falling down the vein rather than being pumped up it) and the vessel wall has changed (the vein has dilated becoming “varicose”) meaning that two of the three elements of Virchow’s triad have been affected. To get a clot within the vein, or more accurately a “thrombosis”, it is only necessary to have one of the three elements of the triad changed. Hence it is not surprising that varicose veins, which affects two of the three elements, is the commonest cause of phlebitis (or superficial venous thrombophlebitis).
The one element of the triad that is not affected in normal varicose veins is “changes in the blood constituents”. However if a person becomes dehydrated so the constituents become more concentrated, or is a smoker so that the fibrin within the blood is increased making the blood more syrupy, or is on the oral contraceptive pill which increases some of the proteins within the blood, or has any other condition that changes the blood constituents, then they are much more likely to get phlebitis (or superficial venous thrombophlebitis). Some people are born with certain genes that cause blood to be thicker and more likely to clot. Such patients are said to have a “thrombophilia” which literally means “liking of clots”.
From Virchow’s triad, other causes of phlebitis (or superficial venous thrombophlebitis) can be identified including intravenous drug administration through needles or plastic tubes called “drips” (or “cannulae”) and occasionally people suffering from certain cancers can end up with multiple bouts of phlebitis in different veins in different areas of the body.
Next page: What are the symptoms of phlebitis?
This website was last updated on 11/10/16.