Arteriovenous malformation (AVM)

What is an arteriovenous malformation?

Arteries transport blood to your brain from your heart, whereas veins take deoxygenated blood back from your brain to your heart. An AVM takes place when the blood vessels within the brain become muddled and cause blood to be diverted directly to your veins from your arteries.

AVMs can occur in different parts of the brain, including the four frontal lobes, the cerebellum, the brainstem and the ventricles.

AVMs are very rare; it is estimated that they affect less than one per cent of the population. Males are more commonly affected by females.

What causes an AVM?

In most cases, the cause of an AVM is unknown. Most cases are congenital, which means that a baby has been born with the AVM. However, this is not an inherited condition and having an AVM does not mean that you will have a child with an AVM.

Types of AVM

In addition to the true AVM, which is characterised by the tangle of blood vessels, there are many different types of malformations, which affect the blood vessels in the brain and are categorised as AVMs; examples include:

• Occult AVM (also known as cavernous or cryptic malformations): occult AVM is a form of malformation, which doesn’t actually divert blood supply. It often causes fitting and there is a risk of bleeding.
• Hemangioma: hemangioma are large abnormal collections of blood, which are usually found on the skin surface or the brain surface.
• Venous malformation: this occurs when an abnormality affects only the veins. It causes the veins to be abnormally large or placed in abnormal locations in the brain.
• Dural fistula: This is an abnormality that affects the brain surface area known as dura mater. There are three main types of dural fistula.

The best course of action for dural fistula is usually surgery to block off the ends of the blood vessels to resolve the abnormal tangling or connection.

Diagnosing an AVM

AVMs are usually diagnosed using CT or MRI scan images. As well as confirming the AVM’s presence, these scans can also provide more details about the AVM’s exact size and location, which can help with treatment planning. A diagnostic test, known as cerebral angiogram, may also be carried out. This involves placing a small tube (a catheter) into the groin and guiding it into the vessels, which carries blood into the brain. Coloured dye is inserted into the tube and images of the brain are taken. This procedure helps doctors to see the blood vessels and follow blood flow in more detail.

AVM and bleeding risks

Once an AVM develops, there is a risk of it bleeding, which occurs as a result of weakness in the blood vessels, causing gradual dilation. Increased blood pressure may eventually cause the weakened vessels to burst, resulting in bleeding n the brain. The possibility of bleeding increases with time and is estimated at around 1-3 per cent a year. This means that by the time you’ve had an AVM for 15 years, there is an approximate risk of suffering a stroke of 25 per cent.

Once you’ve had an intracranial bleed, the risk of having another is a little higher, but only for a small period of time. Research studies differ in their findings: two studies conducted in the USA showed that the risk in the 1st year increased to 6 per cent in one study and 17 per cent in the second study. Research suggests that younger people aged between 11 and 35 years old are slightly more likely to suffer an intracranial bleed.

What are the implications of a bleed?

Whenever blood gets leaked into the brain, there is a risk of damage and the more frequent the bleeds, the higher the threat of extensive or serious damage. Bleeding can result in impaired function and this may affect sensations, cognitive and motor skills, memory loss and problems with vision. It is estimated that each bleed carries a 10-15% risk of death.

Different parts of the brain are responsible for different actions and functions and the effects of a bleed will usually be dependent on the area affected. Here are the different parts of the brain and their main roles:

• the frontal lobe: this controls mood and traits
• parietal lobe: is in control of motion and actions in the upper and lower limbs
• occital lobe: controls sight and vision
• temporal lobe: controls understanding, memory and speech and language
• cerebellum: controls balance and coordination
• brainstem: the brainstem is responsible for controlling the signals to the body from all parts of the brain
• ventricles: control the release of fluid in the spinal cord and brain (known as cerebrospinal fluid)

When are AVMs treated?

Generally speaking, an AVM will be treated if there is a high risk of bleeding or bleeding has already occurred in cases where the brain area is easily accessible, or the AVM is large and there is a high chance of successful treatment.

AVM treatment options

There are various treatments that may be used for an AVM. The type of treatment used will usually depend on the AVM’s size and location, the type of AVM and the nature of the symptoms caused by it. Treatment options include:

• Medical management: in cases where there aren’t any symptoms or the AVM is in an area that is hard to reach, medical administration may be employed. This will involve conservative monitoring and patients will be advised to avoid any kind of activity that may increase blood pressure. Patients will also usually be advised not to take medication to thin the blood.
• Surgery: surgery might be suggested if the AVM is easily accessible or there has already been a bleed. Surgery is carried out under anaesthetic and involves removing part of the skull and then taking out the AVM.
• Stereotactic radiosurgery: when an AVM is relatively small or it is in a difficult to access area, stereotactic radiosurgery may be recommended. This technique involves using high-energy beams to damage and break down the AVM.
• Neuroradiology and endovascular surgery: in some cases it might be achievable to treat an AVM by blocking off the flow of blood to the AVM using materials such as liquid glue. These materials are placed inside a catheter, which is placed within the blood vessels that are supplying blood to reach the AVM.

Treatment for an AVM is provided by specialists including neurosurgeons, radiotherapists and neurologists with specialist expertise in treating stroke patients.