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Normally, there is a fine network of capillaries running between the arteries and veins throughout your brain. When the capillaries in one area are missing, a complex tangle of arteries and veins develops, resembling a knotted rope, called an arteriovenous malformation (see Figures 1 and 2 in the free downloadable pdf or the printed bound copy of the booklet).

The knotted tangle of blood vessels is known as a nidus. Occasionally, only a single artery and vein are involved, forming a particular type of arteriovenous malformation, called an arteriovenous "fistula". In both cases, blood flows at high pressure from muscular arteries into thin-walled veins because of the absence of capillaries to slow it down. This is like an electrical short circuit, and is known as a shunt.

An arteriovenous malformation is also known as an AVM, and an arteriovenous fistula is known as an AVF. Some doctors may use the more old-fashioned term "angioma" or "arteriovenous anomaly".

AVMs range in size from only a few millimetres to several centimetres across. They may occur in any part of your brain, or they may develop within its fibrous outer covering, called the dura mater, or dura for short. AVMs are rare, affecting less than 1% of the population, and they do not appear to be more common in certain countries or races. Men and women are affected equally.

What causes arteriovenous malformations of the brain and dura?
We do not know the cause of brain AVMs, but we do know that they are not cancerous, nor are they infectious. AVMs are thought to be due to a problem with the normal growth of capillaries during early development in the womb. Even though the AVM may havebeen present before birth, it usually takes some time for it to produce symptoms. Although AVMs of the brain very occasionally run in families, a gene for them is not known.They are only inherited in a condition known as hereditary haemorrhagic telangiectasia, or HHT for short, in which AVMs also occur in the lungs, and people with this condition may experience bleeding from their nose and gut.

Dural AVMs can be caused by knocks to the head, brain surgery, infections of the brain, or thrombosis of blood in the large veins that drain the brain. Sometimes, there is no obvious cause.