What is a brain arteriovenous malformation (AVM)?

A brain arteriovenous malformation (AVM) is an abnormality of the blood vessels of the brain that looks like a tangle of blood vessels. Normally, arteries (blood vessels that carry blood to the brain) drain out though veins after passing though smaller blood vessels called capillaries and then into veins.  In an AVM this normal connection is not present and commonly there is a direct connection between the arteries and the veins. This means that the high pressure in the arteries is transmitted into the veins which are not normally used to this higher pressure

How common are brain AVMs and why do they happen?

In general, brain AVMs are rare and only 1 in 200 of the general population has an AVM.  The reason why brain AVMs occur is not known. It is thought that most brain AVMs are present from birth although some can form in adult life.

How do brain AVMs present?

Most AVM’s cause no symptoms and are picked up on routine scans often done for other reasons. 

When an AVM is symptomatic:

  • About half of patients present with a brain haemorrhage
  • About a quarter of patients present with a seizure or epilepsy
  • About 15% may have problems with weakness or speech disturbances

What is the risk of a haemorrhage from a brain AVM

The risk of a haemorrhage from an AVM depends on whether the AVM has bled before or not. In general the risk of an AVM bleeding is about 1-2% per year. If there has been a previous haemorrhage then the risk of having a repeat haemorrhage is higher (6-20%) in the first two years, but this risk then gradually falls to a baseline rate of about 4% per year. Predicting which AVMs are going to bleed, and when, is difficult.

How are brain AVMs diagnosed?

Most brain AVMs can be identified on a CT scan or an MRI scan. Confirmation of the diagnosis of an AVM almost always requires a CT or MR angiogram. Most patients would also routinely have a cerebral angiogram which helps in understanding the detail of the blood vessel abnormality a bit better. This is almost certainly required if there are plans to treat an AVM.

How is an AVM treated?

In general, an AVM may be considered for treatment if it has bled, if it’s in an area of the brain that can be easily treated and if it’s not too large. Recent clinical trial evidence suggests that it is best not to treat brain AVMs if they have not ruptured, but these decisions are made on an individual patient basis. 

All clinical scenarios and imaging is normally discussed with a panel of doctors that have experience in treating a brain AVM (multidisciplinary team (MDT) discussion).

The various methods of treating brain AVMs are outlined below. 

  1. Conservative management.  If an AVM has been found incidentally on a brain scan and there are no symptoms associated with it, conservative management may be recommended.
  2. Surgery. If an AVM has bled and/or is in an area that can be easily operated upon, then surgical removal may be recommended. The patient is put to sleep with anaesthesia, a portion of the skull is removed, and the AVM is surgically removed. When the AVM is completely taken out, the possibility of any further bleeding should be eliminated.~

  3. Stereotactic radiosurgery. An AVM can also be treated by focused radiotherapy treatment. Often this means a trip to the national centre for radiosurgery in Sheffield to meet with experts that perform this treatment.   In this procedure, a cerebral angiogram is done to localize the AVM. Focused-beam high energy sources are then concentrated on the brain AVM to produce direct damage to the vessels that will cause a scar and allow the AVM to “clot off.”  The effects of this treatment are not immediate. It normally takes about 3 years to be sure that the AVM has been treated. 

  4. Interventional neuroradiology/endovascular neurosurgery. It can sometimes be possible to treat part or all of the AVM by placing glue into the AVM using catheters that have been passed into the blood vessels supplying the AVM.  This approach may be sufficient to treat the AVM on its own, but sometimes this procedure is used to help make surgery safer.

Are there different types of brain AVMs?

All blood vessel malformations involving the brain and its surrounding structures are commonly referred to as AVMs. We do, however, recognise several types:

  • True arteriovenous malformation (AVM). This is the most common brain vascular malformation. It consists of a tangle of abnormal vessels connecting arteries and veins with no normal intervening brain tissue.

  • Occult or cryptic AVM or cavernous malformations. This is a vascular malformation in the brain that doesn’t actively divert large amounts of blood. It may bleed and often produce seizures.

  • Venous malformation. This is an abnormality only of the veins. The veins are either enlarged or appear in abnormal locations within the brain.

  • Haemangioma. These are abnormal blood vessel structures usually found at the surface of the brain and on the skin or facial structures. These represent large and abnormal pockets of blood within normal tissue planes of the body.

  • Dural fistula. The covering of the brain is called the “dura mater.” An abnormal connection between blood vessels that involve only this covering is called a dural fistula. Dural fistulas can occur in any part of the brain covering. Three kinds of dural fistulas are: 

Dural carotid cavernous sinus fistula. These occur behind the eye and usually cause symptoms because they divert too much blood toward the eye. Patients have eye swelling, decreased vision, redness and congestion of the eye. They often can hear a “swishing” noise.

Transverse/sigmoid sinus dural fistula. These occur behind the ear. Patients usually complain of hearing a continuous noise (bruit) that occurs with each heartbeat, local pain behind the ear, headaches and neck pain.

Sagittal sinus and scalp dural fistula. These occur toward the top of the head. Patients complain of noise (bruit), headaches, and pain near the top of the head. They may have prominent blood vessels on the scalp and above the ear.