Arteriovenous malformations (AVM)
What is an Arteriovenous malformation (AVM)?
Normally, there is a fine network of capillaries running between the arteries and the veins throughout your brain. An arteriovenous malformation or AVM is the complex tangle of arteries and veins which develops when the capillaries are missing in one area. A more old-fashioned name for an AVM is an ‘angioma’.
Occasionally, only a single artery and vein are involved, forming a particular type of AVM called an arteriovenous fistula or AVF (a fistula is the medical name for an abnormal tunnel or connection).
Usually, capillaries slow down blood flow. In AVMs and AVFs where there are no capillaries, blood flows at high pressure from muscular arteries directly into thin-walled veins. This abnormal flow of blood is known as a shunt.
AVMs and AVFs range in size from just a few millimetres to several centimetres across. They can occur in any part of the brain and spinal cord. They can also develop within the brain’s outer covering, the dura mater or dura. An example of an AVF of the dura is a carotid-cavernous fistula (CCF). This is an abnormal connection between the main artery supplying the brain (the carotid artery) and one of the draining veins behind the eyes which can cause swelling and redness in one eye.
- Nidus: the knotted tangle of blood vessels in an AVM.
- Shunt: the direct flow of blood from an artery to a vein.
- Dura: the membrane or layer which forms the outer covering of the brain; the dura mater.
- Arteriovenous fistula (AVF): an abnormal connection between an artery and a vein. It allows blood to flow directly from the artery to the vein without passing through any capillaries.
We do not know the cause of AVMs in the brain but we do know that they are not cancerous (they are not linked to brain tumours) and they are not infectious.
We think AVMs are due to a problem with the normal growth of capillaries during early development in the womb. However, although an AVM might have been present before birth, it usually takes some time for it to produce any symptoms.
Very rarely, AVMs can run in the same family with an identifiable genetic cause. They are only inherited in a condition called hereditary haemorrhagic telangiectasia (HHT) in which AVMs can also occur in the lungs. People with this condition might experience nosebleeds, red spots on their skin and bleeding from their stomach lining and intestines. It is also known as Osler-Weber-Rendu disease.
Dural AVFs can be caused by head injuries, brain surgery, infections of the brain or blood clots in the large veins that drain the brain. Sometimes there is no obvious cause.