What is brain AVM?
Arterio-venous malformation (AVM) is a vascular disorder characterized by formation of abnormal vessel conglomerate in the brain.
How does the brain get its blood supply?
As any other organ of the body the brain requires constant blood circulation to maintain its functions. The blood is delivered to brain by arteries, that carry oxygen and nutrient rich blood from the heart. Arteries branch off getting progressively smaller and eventually form the capillary network in which oxygen and nutrients pass to the brain tissue. Also carbon dioxide and waste products are passing from brain to blood in capillaries. After exchange capillaries join each others to form veins which drain the oxygen-poor blood back to the heart. This artery-capillary-vein structure is a three-dimensional web and is essential for optimal brain functioning.
How does brain AVM develop?
AVM is a tangle of vessels in which blood from arteries delivered to veins by-passing capillary network. Usually, there are several arteries entering the AVM. They are called feeding arteries or feeders. The draining veins carry the blood out of AVM back to the venous system. In between these arteries and veins there is a highly complex, intermediate, convoluted 3D vessel complex lacking capillaries. This conglomerate is often called a nidus. Nidus is almost always buried in the brain tissue, yet some portion of it may reach the surface of brain. In a typical case the nidus resembles an inverted cone or a pyramid with the base at the surface and apex pointing deep to ventricular system. However, this classical description is not always the case, as AVM's come in various shapes and sizes. The hallmark of AVM is lack of capillary network in nidus. Therefore, the blood passing through doesn’t lose oxygen and thus veins draining AVM contain arterial blood (unlike regular veins containing venous blood).
Most AVMs are formed in fetus during early stages of brain development. However, in rare cases AVM may develop after birth (de novo AVM). The exact cause of brain AVM development is unknown. Since most of them are formed during pregnancy, in developing brain; genetic or enviromental factors impending normal vessel development are likely.
What are the symptoms of brain AVM?
The clinical presentation of AVM depends on whether it has bled or not. The vast majority of those that did not bleed are usually asymptomatic and discovered on brain imaging for unrelated reasons. In some cases, brain AVM’s can cause epileptic seizures due to irritation of the brain tissue. These seizures are usually hard to control with anti-epileptic medications — so called drug resistant epilepsy.
Brain bleeding is most fearsome complication and unfortunately quite frequent clinical presentation of AVM. It is almost always acute and the symptoms develop suddenly. Clinical presentation may vary from mild headache to deep coma depending on location and size of bleeding. Focal neurological symptoms such as paresis (weakness of muscles), hypesthesia (numbness), apahsia (speech impediment), visual impairment, gait abnormalities, etc., may also develop.
How is AVM diagnosis made?
Combination of clinical examination and imaging studies is used to make AVM diagnosis There are no specific clinical signs and symptoms indicating brain AVM and definitive diagnosis is made by imaging.
Brain CT scan, MRI and angiography are used to diagnose AVM. While CT and MRI are mostly used to evaluate AVM location and size, angiography is employed for demonstrating intricate network of vessels.
What is the treatment of AVM?
The decision to treat AVM or not is based on clinical presentation and the age of a patient. If bleeding is present the AVM has to be treated. Cases of drug resistant epilepsy also warrant treatment. However, asymptomatic AVM may not require treatment at all. For asymptomatic AVM the most serious complication is bleeding and the decision is made on the life-time risk of bleeding. If the risk is low the case might be followed without treatment.
There are three main treatment modalities available for brain AVM: surgery, endovascular treatment and radiation treatment
Surgical treatment is one of the most commonly used. During surgery the AVM nidus is exposed, feeding arteries and draining veins are closed and cut and the AVM is removed from the brain. If AVM is totally removed the chances of recurrence are extremely low. However, partial AVM removal is not an acceptable treatment option since it may actually increase the risk of bleeding. The advantage is total AVM removal in the vast majority of cases.
Endovascular treatment is based on angiographically guided closure of AVM by selective closure of its feeding and nidus. During the procedure the feeding vessels are catheterized and various embolizing materials are injected to close the blood flow. As with surgical treatment total occlusion leads to cure while partial closure may actually increase the risk of bleeding. Unfortunately, total closure can be achieved only in minor subset of brain AVMs. Embolization can also be used as adjunctive treatment prior to surgery to facilitate surgical removal.
Radiation treatment is also used to treat AVMs. Usually it is applied as stereotactic radiation therapy (SRT) which is highly focused form of RT.
Combination — in some cases a single modality fails to treat AVM and combination of 2 or even all 3 methods can be used. The decision of which method to use is up to treating team which should be highly experienced.