Arteriovenous malformations or AVM occur at the point where arteries and veins meet. In someone with a healthy circulatory system, blood passes from one type of small capillary to another without problem. In someone with an AVM, the blood vessel junctions have not formed correctly and this can cause excessive bleeding. This can lead to health problems, particularly if the AVM is in the brain.

What is an AVM and where does it form?

Blood flows in a circuit in a healthy human body. The heart pumps blood into the main artery, the aorta, which branches off into smaller arteries that take blood into every tissue and organ in the body. In the tissues, the blood vessels become very narrow and the flow of blood slows so that oxygen and nutrients can pass from the blood to the cells. Carbon dioxide and other waste then pass into the blood as it leaves the tissues and small and then larger veins take blood back to the heart.

Normally the capillaries where the arteries and veins meet are well-organised but in some people, a problem occurs. The arteries and veins meet via a knotted tangle of randomly interconnecting vessels, which are unstable and at high risk of allowing blood to escape. This is a vascular or arteriovenous malformation (AVM) and it can occur anywhere in the body. It is particularly dangerous when it affects blood vessels in the brain as a bleeding AVM can cause the same symptoms as a stroke.

What causes an AVM?

The exact cause of an AVM is not yet understood completely but most are thought to develop before or shortly after birth. An AVM is not a condition that develops later in life, although many AVMs can cause no symptoms for many years.

Some are only detected by accident during an investigation for an unrelated illness. Only in around 12% of people who have an AVM will their condition cause serious symptoms that mean the AVM must be treated.

Are AVMs rare?

They are quite rare. AVMs occur in about 18 people in every 100,000, and only 1-2 of these will show any symptoms. Comparing this with stroke in general, which affects 2,500 people in every 100,000, shows just how rare it is to become ill because of an AVM.

What symptoms does a brain AVM cause?

Common symptoms include:

  • Epileptic fits. These can occur as a result of tiny bleeds from a small AVM that disrupts the transmission of electrical signals within the brain.
  • A headache that is quite like a migraine. This also tends to occur when the AVM is small.
  • Back pain that has no other explanation.
  • Tinnitus.
  • Unexplained neurological symptoms: feeling weak, losing your balance, dizziness, poor memory, experiencing numbness or tingling.

If minor symptoms occur and all other causes are ruled out, an AVM is suspected and may then be detected using imaging technology. It then needs to be monitored very closely to follow its progress. If the AVM is in danger of bleeding significantly, it may need treatment to try to remove it.

If the AVM bleeds, this can lead to haemorrhagic stroke. Around 70% of people with an AVM have their condition diagnosed after they have a stroke.