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Vascular malformations of the brain

A guide for patients and carers

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Arteriovenous malformations (AVMs)

Normally, there is a fine network of capillaries running between the arteries and the veins throughout your brain. An AVM is the complex tangle of arteries and veins which develops when the capillaries are missing in one area. Occasionally, doctors will use the more old-fashioned names, angioma or arteriovenous anomaly, to describe an AVM.

The knotted tangle of blood vessels in an AVM is known as a nidus (the Latin word for ‘nest’).

Occasionally, only a single artery and vein are involved, forming a particular type of AVM called an arteriovenous fistula (AVF). In cases of AVMs and AVFs, blood flows at high pressure from muscular arteries directly into thin-walled veins via the shortcut created by the absence of capillaries that would usually slow it down. 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 fibrous outer covering, the dura mater or dura. An example of a dural AVF is the carotid-cavernous fistula (CCF).

Nidus: the knotted tangle of blood vessels in an AVM.

Shunt: the direct flow of blood from an artery to a vein.

Dura: the tough, fibrous membrane which forms the outer covering of the brain; the dura mater.

Carotid-cavernous fistula (CCF): 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.

What causes AVMs?

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.

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