Arteries normally carry oxygenated blood from the heart to the brain while the veins carry de-oxygenated blood away from the brain and back to the heart. An arteriovenous malformation (AVM) is a tangle of blood vessels in the brain which divert blood directly from the arteries to the veins, bypassing the normal brain tissue.
How common are brain AVMs?
Brain AVMs occur in less than one percent of the general population. It occurs in about one in 200–500 people, and is more common in males than females.
Why do brain AVMs occur?
We do not really know why brain AVMs occur. They are usually congenital, meaning someone is born with the condition. However, they are usually not hereditary. People probably do not inherit an AVM from their parents, and they probably will not pass an AVM on to their children.
Where do brain AVMs occur?
They can occur anywhere within the brain or on the covering of the brain. This includes the four major lobes of the front part of the brain (frontal, parietal, temporal, occipital), the back part of the brain (cerebellum), the brainstem, or the ventricles (deep spaces in the brain that produce the cerebrospinal fluid).
Do brain AVMs change or grow?
Most AVMs do not grow or change significantly, although the vessels involved may dilate. There are also reported cases of AVMs shrinking or enlarging. This can be explained by the clots in certain parts of an AVM, causing it to shrink, or the redistribution of blood flow into adjacent vessels, resulting in enlargement.
What are the symptoms of a brain AVM?
The symptoms of an AVM depend on where it occurs within the brain.
Over 50% of patients with an AVM present with intracranial haemorrhage.
20% - 25% of patients with an AVM have either focal or generalised seizures. Patients may experience localised pain in the head because of increased blood flow around an AVM.
15% of patients may have difficulty with movement, vision, or speech.
What causes brain AVMs to bleed?
A brain AVM contains abnormal and weakened blood vessels that direct blood away from the normal brain tissue. These weakened blood vessels dilate over time and may eventually burst due to the high pressure of blood flow from the arteries, causing bleeding into the brain.
What are the chances of a brain AVM bleeding?
Each AVM patient has a 1-3% chance of AVM bleeding per year.
Over a 15-year period, there is a 25% total chance of an AVM bleeding resulting in brain damage and stroke.
Does one bleed increase the chance of a second bleed?
The risk of a second bleeding is slightly elevated for a short period of time after the first bleed. In the first of two studies that were conducted, the risk during the first year after initial bleeding was 6% and then dropped to the baseline rate. In the second study, the risk of re-bleeding during the first year was 17.9%. The risk of recurrent bleeding may be even higher in the first year after the second bleed, and has been reported to be 25% during that year.
AVM patients between the ages of 11 and 35 are known to have a slightly higher risk of re-bleeding.
What can happen if a brain AVM bleeds?
There is a 10–15% risk of death related to each bleed and a 20–30% chance of permanent brain damage. Each time blood leaks into the brain, normal brain tissue is damaged. This results in the loss of normal function, which may be temporary or permanent. Some possible symptoms include arm or leg weakness/paralysis, or difficulty with speech, memory or vision. The extent of brain damage depends on the amount of blood leaking from the AVM.
How many different types of brain AVMs are there?
Although all blood vessel malformations involving the brain and its surrounding structures are commonly known as AVMs, there are actually 5 different classifications:
True arteriovenous malformation (AVM)
This is the most common brain vascular malformation and consists of a tangle of abnormal vessels connecting arteries and veins with no normal intervening brain tissue.
Venous malformation
This is an abnormality only of the veins. The veins are usually enlarged or appear in abnormal locations within the brain.
Occult AVM or cavernous malformations
This is a vascular malformation in the brain that does not actively divert large amounts of blood. It may bleed and often produces seizures.
Haemangiomas
Haemangiomas are abnormal blood vessels usually found at the surface of the brain and on the skin or facial structures. They are large and abnormal pockets of blood within normal tissue planes of the body.
Dural fistula
The covering of the brain is known as the "dura mater." When there is an abnormal connection between blood vessels in the dura, we call them dural fistula. There are three different types of dural fistulas:
- Dural carotid cavernous sinus fistula
These occur behind the eye, and usually cause symptoms as a result of having diverted too much blood towards the eye. Patients may experience eye swelling, decreased vision, redness and congestion of the eye. They often can hear a "swishing" noise.
- Sagittal sinus and scalp dural fistula
These occur near the top of the head. Patients usually complain of noise, headaches, and pain near the top of the head and they may present with prominent blood vessels on the scalp and above the ear.
- Transverse-Sigmoid sinus dural fistula
These occur behind the ear and patients may complain of hearing a continuous noise that occurs with each heartbeat, local pain behind the ear, headaches, and neck pain.
What is the best available treatment for Dural Fistula?
The best treatment is usually endovascular surgical blocking of the abnormal connections that produce the fistula. This is done by guiding small tubes (catheters) inside the blood vessel and blocking off the abnormal connections under X-ray guidance. Depending upon the location and size, many of these can be successfully treated and cured using this technique.
How are AVMs diagnosed?
Most AVMs are detected either on a Computed Tomography (CT) brain scan or with a Magnetic Resonance Imaging (MRI) brain scan. These tests are very good at detecting brain AVMs. They also tell us about the location and size of the AVM and information on bleeding. A radiologist may also perform a cerebral angiogram, which involves the insertion of a catheter through an artery in the leg and guiding it into each of the vessels in the neck going to the brain, injecting contrast material (dye) and taking an X-ray of all the blood vessels in the brain. For treatments involving an AVM, an angiogram is usually done so that the doctor can identify the type of AVM present and plan the appropriate treatment.
What factors influence whether an AVM should be treated?
In general, an AVM may be considered for treatment if it has bled, if it is in an area of the brain that can be easily treated, and if it is not too large.
What is the best treatment for an AVM?
Treatment usually depends on the type of AVM (location and size) and the symptoms that it may be causing.
What are the different types of treatment available?
Medical Therapy
If the patient has no symptoms, or if the AVM occurs in a region of the brain that cannot be easily treated, conservative medical management may be necessary. The AVM patient will be advised to avoid blood thinners (Warfarin), and any activities that may excessively raise blood pressure (e.g. carrying heavy things or strenuous exercise).
They are encouraged to have regular checkups with a neurologist.
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. With the AVM completely removed, there will be no possibility of further bleeding.
Stereotactic Radiosurgery (Gamma Knife)
An AVM that is not too large, but is in an area that is difficult to reach by regular surgery may be treated by performing Gamma Knife Radiosurgery.
A stereotactic frame is fixed onto the patient’s head by the neurosurgeon on the morning of the treatment.
Wearing this frame, the patient will have to undergo an MRI scan followed by a Cerebral Angiogram (X-ray to show blood vessels in the brain).The images of both studies are collected and fed into a special planning computer which allows the neurosurgeon to localise the AVM target, and create a good treatment plan.
A good treatment plan is one that enables us to deliver a high enough dose of radiation to the AVM for obliteration, while at the same time, sparing the surrounding normal brain tissue from radiation damage.
Once the plan is done and a dosage is prescribed by the radiation oncologist, a team of radiotherapists will carry out the treatment, which involves positioning the patient within the Gamma Knife machine so that focused beams of radiation are concentrated onto the AVM. This produces direct damage to the blood vessels and causes scarring of the vessel walls, allowing the AVM to “clot off”.
Interventional neuroradiology/endovascular neurosurgery
It may be possible to treat part – or all – of the AVM by placing a catheter (small tube) inside the blood vessels that supply the AVM, and blocking off the abnormal blood vessels with a variety of different materials. These include liquid tissue adhesives (glues), micro-coils, particles and other materials used to stop blood flowing to the AVM. The best course of treatment usually depends upon the type of symptoms, type of AVM present and the AVM’s size and location.
Who are the specialists involved in treating brain AVMs?
Vascular neurosurgeons
Specialise in surgical removal of brain AVMs.
Radiation therapists/neurosurgeons
Specialise in the stereotactic radiosurgery treatment of brain AVMs.
Interventional neuroradiologists/endovascular neurosurgeons
Specialise in the endovascular therapy of brain AVMs.
Stroke neurologists
Specialise in the medical management of brain AVMs.
Neuroradiologists
Specialise in the diagnosis and imaging of the head, neck, brain, and spinal cord. They perform and interpret the CT, MRI, and cerebral angiograms necessary for evaluation, management, and treatment. Each of these specialists has gone through extensive training and is highly skilled at treating complex brain AVMs.