Brain Arteriovenous Malformation (AVM)

What is a Brain arteriovenous malformation (AVM)? 

A brain AVM is a tangle of abnormal and poorly developed blood vessels connecting arteries to veins. 

AVM’s are rare and occur in less than 1% of the population. 

In normal situations arteries take oxygen rich blood away from the heart to various parts of the body under high pressure and so have strong muscular walls. These arteries flow into small capillaries: capillaries are the tiny vessels that deliver oxygen-rich blood to tissues and carry away waste products. The high flow blood from arteries is slowed down through this capillary network. Capillaries then flow into veins to take the blood back to the heart. Veins have thinner walls because they take blood back to the heart under low pressure. 

In an AVM this process is disrupted by the tangle of abnormal and poorly formed blood vessels. This tangle is referred to as the ‘nidus’ of the AVM. 

 

What causes an AVM? 

The cause of AVM’s is not known. It is thought that AVM’s are acquired although some may be formed during development or shortly after birth although it usually takes some time before they cause problems or symptoms. 

AVM’s are not cancerous or infectious and don’t spread to other parts of the body. 

AVM’s affect men and women equally and occur in all races. 

Most are discovered between the ages of 20-40. 

Rarely, AVM’s are hereditary, and a genetic cause can be found. This inherited condition is called hereditary haemorrhagic telangiectasia (HHT). 

The size of AVM’s varies from person to person. Some are small, but others can cover large parts of the brain. Most AVM’s don’t grow or change much, although the blood vessels involved may dilate (widen) making them bigger. Less often they shrink if the blood clots in a part of the AVM. 

Types of AVM 

AVM’S can occur anywhere in the body but those in the brain are of particular concern because of the problems they cause if they bleed. 

The classic, more common type, is made up of the blood supply to the brain and called a pial AVM. 

A dural AVM is more commonly known as a dural arteriovenous fistula( dAVF) and involves one or more connections between an artery and vein. This is a less common type of AVM and may form as a result of head injuries, brain surgery, infections of the ear or brain, or blood clots in the large veins that drain the brain. Sometimes no obvious cause is found. 

Symptoms of an AVM 

In about 15% of cases AVM’s don’t cause symptoms and are found incidentally. 

Seizures: In approximately 20% of cases they are found because of seizures. Seizures occur because AVM’s can be irritants to the brain. 1% of people who have AVM’s are at risk of seizures. 

Haemorrhage: More than 50% of AVM’s are discovered following haemorrhage. Symptoms include a sudden onset of severe headache often with nausea, vomiting, neurological problems or a decreasing level of consciousness. The blood vessels in an AVM are weaker and so at a higher risk of bleeding than normal ones. 

Bleeding is the most serious complication of an AVM and is the main reason for recommending treatment. 

The risk of bleeding is approximately 3-4% each year. Following a bleed, the risk increases to about 6-7% for the next couple of years. 

Some bleeds cause mild symptoms but approximately 50% are significant. 

Headaches: Headaches may be caused by the high blood flow through the AVM. These headaches may be similar to a migraine or be actual migraines. They can be mild or disabling. 

Dural AVF’s can cause headaches because of the many pain fibres on the surface of the brain being irritated. 

Stroke: Brain AVMs may cause stroke-like symptoms by depriving the nearby brain tissue of oxygen and nutrients. The symptoms can include, weakness or paralysis on one side of the body; numbness and tingling; problems with vision, hearing, balance, memory and personality changes. 

Symptoms depend on the part of the brain the AVM is in. 

Some AVM’s can cause noises in the head as a result of the blood flowing through it. Dural AVFs may cause tinnitus which is a whooshing noise in time with the pulse. 

Diagnosing an AVM 

Computerised Tomography scan (CT scan): This can be done with or without dye to look for bleeding in the brain and to view the AVM. 

Angiogram: A thin tube is inserted into an artery in the groin or the wrist. This is passed through the blood vessels to those in the brain. Dye is then injected and X-ray pictures are taken. Doctors are able to see the exact location, shape and size of the AVM. It will also show if any other weaknesses are associated with the AVM such as blisters (aneurysms). Although there are risks associated with an angiogram, they are often essential to plan a management of the AVM. 

Magnetic Resonance Imaging (MRI scan): An MRI scan is used to make a detailed picture of the position of the AVM in the brain in relation to other important structures and is used to plan management. If the AVM is close to important (eloquent) parts of the brain for example vision or movement, then a functional fMRI scan will be taken to map the pathways of these structures in relation to the AVM. This will allow the doctors to know how safe or unsafe any treatment will be. 

Treatment for AVM 

AVM’s are complex and unique to each person. For this reason management is individual to each case. 

Once the required tests have been completed your medical team will sit together with other experts and make the safest and most appropriate management plan for your AVM. This is called a multidisciplinary team (MDT) meeting. 

Treatment depends on the size, shape, position and blood supply to and from the AVM. It also depends on how closely it is associated to important parts of the brain which may cause lifelong disability if damaged. 

Treatment also takes into account your health and wellbeing along with your personal wishes if appropriate. 

No treatment plan will be made without discussing the options with you. 

Treatment for AVM 

Treatment includes surgery, endovascular procedures, radiosurgery or a combination of treatments. The risks of surgery are considered to be high for AVMs that are located in deep parts of the brain or with very important functions nearby. 

Unfortunately some AVM’s cannot be treated as the risk is too high. If this is your plan, the medical team will discuss this with you and you will be monitored with scans. 

Surgery 

Whilst you are asleep under general anaesthesia in an operating theatre, your neurosurgeon removes part of your skull temporarily to gain access to the brain and AVM. With the help of a high-powered microscope, the surgeon seals off the AVM and carefully removes it from surrounding brain tissue. The skull bone is re-attached with small plates and the incision in your scalp is closed.