AVMs and the Circulation of the Brain
Arteries are thick-walled blood vessels designed to carry oxygen, bound to the red blood cells, to parts of the body where it will be used to fuel the activities of the organ's cells. Once the oxygen has been unloaded from the red cells the blood is carried back to the lungs in thinner blood vessels called veins so that it may collect more blood and begin the return jouney. An arteriovenous malformation (AVM) forms in the brain when an abnormal connection develops between its arteries and the veins. Instead the blood passes through a tangle of abnormal vessels called a nidus (from the latin for "nest"). The arteries often enlarge in response to increased amounts of blood being pushed through them and the veins, unused to the higher pressure, oxygen-laden blood, tend to become larger and more tortuous.
The high blood-flow through an AVM places stresses on the vessels which may cause bleeding. Aneurysms may develop on the arteries, veins or within the nidus which represent particularly weak area prone to haemorrhage.
An AVM may develop in many parts of the body but in the nervous system are of particular concern because of the damage that bleeding within the brain or spinal cord may cause. Such AVMs are sometimes termed pial AVMs. Brain AVMs are thought to occur in about 0.15% of the population although it is difficult to be certain given that people may harbor one which does not cause any symtoms.
It was believed that AVMs formed in-utero or in the period immediately after birth. In recent years cases have emerged where the AVM clearly developed later in childhood. This tells us that the development of an AVM can be a dynamic process. Most AVMs encountered in clinical practice are isolated lesions but there are uncommon conditions where an aberrent gene programs AVMs and other vascular malformations to develop throughout life. An example of such a condition is Hereditary Haemorrhagic Telangectasia (HHT). This is a collective term for a group of conditions characterised by the development of multiple AVMs in various locations including the brain, skin, eyes, mucus membranes and lungs. Telangectasia are small bright red vascular malformations in the skin. It is thought to affect between 0.1-0.2% of the population. Increasingly it is recognised that abnomalities of genes play a greater role in AVMs formerly believed to be sporadic.
An AVM will often be first seen on either a computed tomography (CT) or magnetic resonance (MR) scans. These are imaging tools which allow the brain to be viewed in cross-section and they provide importance information about where the malformation is located within the brain when planning treatment. They are also very sensitivie ways of detecting bleeding from the AVM. To fully evaluate the angioarchitecture of the malformation a Digital Subtraction Angiography (DSA) is recommended. After a local anaesthetic injection a small tube is passed into a large artery at the top of the leg. Occasionally an artery at the elbow is used. Guided by x-rays the tube is directed into the blood vessls of the brain where dye is injected to produces very detailed pictures of the circulation. DSA is carried out by a specialist neuroradiologist who will discuss the potential risks of the procedure with the patient and/or their carers. It will identify characteristics which are assosciated with bleeding such as aneurysms and narrowing in the veins taking blood away from the nidus (stenoses).
Treatment of a brain AVM requires that all shunting of arterial blood through the venous circulation must be abolished completely. Some are not safely treatable but many are by a number of means and you can learn more by exploring the menu on the right of this page.