Arteriovenous Malformation (AVM)

Arteriovenous malformations, also known as AVMs, is a term used to describe a group of brain blood vessel abnormalities. The term AVM means different things to different kinds of physicians, and some explanation of the term is warranted. Classically, AVM includes four types of brain blood vessel anomalies: 1.) pial (“true”) arteriovenous malformations; 2.) cavernous malformations; 3.) venous angioma (developmental venous anomaly or DVA); and 4.) capillary hemangioma. While all of these entities can now be seen on brain imaging studies such as CT or MRI, most physicians are speaking of the true or pial type when they use the term AVM because of the greater risk of seizures, strokes, and hemorrhages associated with this type.

Around 12% of people with AVMs have symptoms and symptoms most commonly occur in patients aged 20 - 40 years of age.

Pial AVM’s are relatively rare, affecting about 0.01-0.50% of the population. Most AVMs are sporadic in that there is no known reason that any given person has developed an AVM. There are several hereditary conditions, such as Hereditary Hemorrhagic Telangiectesis (HHT), in which AVMs can run in families. Based on autopsy and population based data, incidence ranges from 1.1 – 2.1 per 100,000 population while prevalence ranges from 10.3 521 per 100,000. Around 12% of people with AVMs have symptoms and symptoms most commonly occur in patients aged 20 - 40 years of age. Although children less commonly develop symptoms if they have an AVM, it is recognized that AVMs are the next most common cause of brain bleeding in children after head trauma.

Traditionally, the risk of hemorrhage has been estimated to range from 2 – 4%. Lifetime risk can be estimated by subtracting the patient’s age from the number 105: Lifetime risk of AVM Hemorrhage = 105 Patient Age (years).

From this formula, it is clear that an elderly person with no AVM symptoms has a relatively low lifetime risk of complications. However, a young person has a very high life-time risk of developing complications from their AVM. It is this type of information that physicians use to guide patients when considering the risks, benefits, and alternatives of AVM treatment. In the only prospective determination of hemorrhage risk, the annual risk was 2% for AVMs that had not already hemorrhaged, but the risk was dramatically higher for patients with prior hemorrhage: 32.9% during the first year, and 11.3% thereafter.

Blood flows from the heart through arteries to supply all of the body’s organs, including the brain. There are four major arteries that supply the brain with blood – 2 carotid arteries in the front of the neck, and 2 vertebral arteries in the back of the neck.

In healthy people, the heart pumps blood into the large arteries at a high pressure and speed which decreases as the blood flow reaches progressively smaller arteries. The smallest vessels, known as capillaries, are tinier in diameter than a strand of human hair. As blood slowly flows through the capillaries, it relinquishes oxygen and nutrients into the brain tissue and receives carbon dioxide and other waste products.

The blood then enters tiny veins at a very low pressure, then flows to larger veins to eventually return to the heart and lungs. Blood flows in the veins at a speed and pressure much lower than the flow of blood in the arteries. The walls of the veins are much thinner and more delicate than those of the arteries, and are unable to handle blood flow at a high pressure.

In an AVM, large arteries within the brain become directly connected to veins. It is like a “short-circuit” in an electrical system: rapid, high pressure blood flow that normally occurs in the large arteries is delivered directly into the veins bypassing the brain. The thin, fragile blood vessels expand and push against nearby areas of the brain or possibly “steal” blood away from the surrounding brain. This may cause strokes or scarring of the brain that presents as symptoms such as weakness, numbness, loss of vision, or seizures. The malformed arteries supplying the blood—the AVM itself—or the enlarged veins may also rupture resulting in bleeding within or around the brain. Intracranial hemorrhage is a type of stroke. This is the most typical presentation of AVM – occurring in about 53% of all patients with AVM.1

AVM-Related Stroke
A stroke occurs when damage to the arteries or veins in the brain leads to interference with brain function. Strokes can be caused by hemorrhage when a vessel breaks or ruptures, allowing blood to escape. The blood may damage the surrounding areas of the brain. In addition, because of the tear in the vessel, the brain may not be receiving its normal blood flow.

Brain tissue requires a constant, steady flow of blood to survive. In fact, the brain ordinarily receives 25% of the blood pumped by the heart. When the flow of blood to the brain is hampered due to blockage of blood vessels, as in the presence of a blood clot, immediate interference with brain function can occur. Brain damage can be temporary or permanent; the extent and severity of the damage to the brain will depend on the length of time and the amount of interference with blood flow. In contrast to hemorrhagic strokes, which occur due to a ruptured vessel, ischemic stroke occurs when blood vessels to the brain are blocked. These types of strokes are much more common among Americans than hemorrhagic stroke, although the problems that result are similar in both cases. Ischemic stroke can lead to loss of vision, loss of speech, weakness, or numbness, depending on the area of the brain that is affected.

How do people with AVMs first come to medical attention?
There are no symptoms or signs that are completely specific to AVMs. Nevertheless, certain neurological symptoms or medicalsigns may cause a physican to request a brain imaging test such as Computed Tomography (CT) or Magnetic Resonance Imaging (MRI) that will help the radiologist to make the diagnosis. The most common indication of a possible AVM is the abrupt onset of stroke. Other symptoms and signs include headache, weakness, numbness, visual problems, or seizures. Radiological imaging studies are the cornerstone to accurate diagnosis of AVMs.

What is the role of Cerebral Angiography in the Diagnosis and Treatment of AVMs?
When an AVM is diagnosed, another type of imaging study called an angiogram or arteriogram may be performed. This type of imaging study is used to study in detail the brain’s blood vessel anatomy involved in the AVM, and to assist the doctors in developing a plan for treatment.

Angiograms are an invasive type of x-ray procedure that are performed by specialists trained in performing and interpreting blood vessel examinations of the brain. There are doctors, nurses, and technologists who specialize in this procedure. An angiogram involves inserting a small tube or catheter into an artery in the groin and maneuvering the catheter, via x-ray technology, into the vessels in the neck that supply blood to the brain. A dye, or contrast, is injected into the vessel to make them visible. As the dye flows through the AVM, still images are taken so that the physician can examine the vessels and the flow of blood in greater detail.

Angiograms can be performed while the patient is awake. A local anesthetic or numbing medication is placed on the site of the groin prior to the insertion of the catheter, and other medications may be made available if the patient feels pain. Although there may be some mild discomfort, the procedure is generally not painful due to the administration of the anesthetic.

The dye that is used to make the vessels visible is not associated with any risk. Some people describe feeling a warm sensation when the dye is injected. The process of moving the catheter through the vessel is a painless procedure. There are no nerves in these arteries and one will not be able to feel the catheter traveling to the neck.

Most people are able to undergo an angiogram as an outpatient. Once the procedure is completed, the catheter will be removed and pressure will be applied to the leg to prevent bleeding. It is necessary to keep the leg straight for several hours. Although most people are discharged the same day, strenuous activity must be avoided following an angiogram.

Treatment of AVM’s
When AVM’s are present, treatment is generally warranted due to the severe complications that they can cause. It is estimated that once a person experiences problems due to AVM’s, the risk of neurological difficulties due to further rebleeding can be more than 30%.

There are three major approaches to treating an AVM which can be used either alone or in combination. The type of treatment recommended will depend on the patient’s history and symptoms, as well as the features of the AVM including its size, location within the brain, and the arteries and veins involved.

One method of treatment for AVM involves is radiosurgery. Despite its name, radiosurgery does not actually involve surgery. Instead, beams of radiation are used to cause scarring within the blood vessels of the AVM, thereby eliminating it. This procedure is recommended for patients with AVM’s that are small and located in specific areas of the brain. Radiosurgery is generally successful in completely eliminating the AVM in over 80% of cases, provided that the AVM is sufficiently small. However, a period of 2 to 3 years must pass before the full effect of radiosurgery can be determined. The obvious advantage of this procedure is that treatment can be provided without incisions in the skin or opening in the skill; however, it is not suitable for everyone with AVMs.

Another type of treatment for AVM involves open surgical treatment. This entails removing a portion of the skull so that surgical instruments can be inserted to remove the AVM. This procedure is often done in combination with the third method for treating AVM, embolization. Embolization prior to surgery is considered to be a lower-risk option than surgery alone.

Embolization
Embolization is an endovascular technique that is used to block the vessels of the AVM. The term endovascular refers to the procedure being performed from within the blood vessels. Some of the procedures involved in embolization are similar to those used in angiography, described above.
A tiny catheter or tube is inserted into the groin and guided via x-ray technology to the affected vessels in the brain that are causing the AVM. Material is then injected into the catheter to permanently block and close off the vessels of the AVM. These materials might include glue-like substances or small platinum coils .
There is a large advantage to performing an embolization prior to undergoing other methods of treating AVM. Embolization can often decrease the size of the AVM, rendering radiosurgery or open surgery much safer. In addition, embolization may totally block the AVM’s blood flow, causing other types of treatment unnecessary.

The facts about embolization
The night preceding the procedure, the patient will not be permitted to ingest any food or drink after midnight. Food in the stomach can make the patient nauseous during the embolization procedure and can lead to vomiting and severe consequences. Patients are generally advised to take their normal morning medications with a small amount of water.
A doctor will recommend either sedation or general anesthesia. An anesthesiologist will administer the medications so that the patient can be sedated or completely asleep during the procedure. Blood pressure and heart rate will be monitored through the use of equipment, and a catheter will be placed in the bladder to allow urine to drain.

The entire procedure can take about 4-6 hours. Once embolization is complete, it will be necessary to remain still and avoid bending at the area of the insertion for at least 8 hours. The patient, therefore, must lie flat and allow the puncture hole to heal.
A hospital stay of several days is typical following embolization. The first night following the procedure is usually spent in the neurological intensive care unit where vigilant monitoring will occur. Transfer to a regular hospital room usually takes place for 24 hours until the patient is discharged. When surgery is also recommended following embolization, it is likely to occur the day after the embolization.
In some cases, multiple embolization procedures are recommended to obtain maximum closure of the AVM. These may be schedule several weeks to several months apart.

Although embolization can be very successful in preventing further damage to the brain due to additional bleeding, there is no method that exists to repair damage to the brain already done by the AVM. Therefore, those neurological problems that may have resulted from the AVM hemorrhage are likely to continue to be present after treatment of the AVM. Patients may experience improvement, however, following treatment, particularly if they undergo rehabilitation.