Arterio-venous malformation (AVM) is a vascular disorder characterized by the formation of abnormal vessel conglomerate in the brain.
As with any other organ of the body, the brain requires constant blood circulation to maintain its functions. The blood is delivered to the 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 passed from the brain to the blood in capillaries. After exchange, capillaries join each other 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.
AVM is a tangle of vessels in which blood from arteries is delivered to veins, bypassing the 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 the brain. In a typical case, the nidus resembles an inverted cone or a pyramid with the base at the surface and the apex pointing deep to the ventricular system. However, this classical description is not always the case, as AVMs come in various shapes and sizes. The hallmark of AVM is the lack of a capillary network in the 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 the fetus during the 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 the developing brain, genetic or environmental factors impending normal vessel development are likely.
The clinical presentation of AVM depends on whether it has bled or not. The vast majority of those who did not bleed are usually asymptomatic and discovered on brain imaging for unrelated reasons. In some cases, brain AVMs 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 the 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 the location and size of the bleeding. Focal neurological symptoms such as paresis (weakness of muscles), hypesthesia (numbness), aphasia (speech impediment), visual impairment, gait abnormalities, etc., may also develop.
A combination of clinical examination and imaging studies is used to make an AVM diagnosis. There are no specific clinical signs and symptoms indicating brain AVM and a 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 an intricate network of vessels.
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 lifetime 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 a cure, while partial closure may actually increase the risk of bleeding. Unfortunately, total closure can be achieved only in a 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 a highly focused form of RT.
Combination — in some cases, a single modality fails to treat AVM, and a combination of two or even all three methods can be used. The decision of which method to use is up to the treating team, which should be highly experienced.
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