Diseases And Conditions

Cerebral arteriovenous malformations

AVM’s are complex abnormalities of the brain and spine that are present at birth. AVM’s occur in less than 1% of the population. An AVM results from the “short circuiting” of normal brain and spine circulation. AN understanding of normal brain and spine circulation is critical to understanding an AVM. Normally, arteries divide up into smaller arteries called arterioles. Arterioles then divide into capillaries. At the level of the capillaries, nutrients and oxygen are delivered to brain and spine cells. The capillaries then coalesce into venules, then veins then bigger veins. Blood pressure is normally high in arteries. Because arterioles and capillaries are so much smaller, this blood pressure is dissipated by the high resistance of these small caliber vessels. As a result, blood pressure is pretty low by the time it gets to the capillaries and veins. Because arteries function at high pressure, they are thick and surrounded by a muscle wall. Because veins function at low pressure they are thin and lack muscle.

In an AVM “short circuit,” there is a direct connection between the arteries and veins of the brain. Because there are no arterioles and capillaries in between, high pressure blood flows from the arteries to the veins. Over many years, these abnormal connections between the arteries and veins result in higher and higher blood flow. The abnormal connections grow and become entangled, forming a “nidus.” Problems arise because veins are not designed to handle high pressure blood. Aneurysms, or blisters of an artery or vein, may form within the nidus or in the veins due to the insufficiency of these vessels to handle high pressure. These aneurysms can cause bleeding. Aneurysms may even form in the arteries as a response to the very high blood flow that has evolved over time. Sometimes, the high pressure seen in the veins of an AVM can cause a back up of pressure in normal brain and spine veins, causing swelling. Another problem arises when blood flows so quickly from the arteries into the veins of an AVM that normal surrounding brain does not receive its share of blood. In essence, the AVM steals blood away from the normal brain. This can cause a dysfunction in the normal surrounding brain. This is referred to as “steal” phenomena.

Patients with AVM’s may present with a myriad of symptoms. The most common symptom is hemorrhage. Patients with a brain AVM will have up to a 4% per year risk of brain hemorrhage. If a hemorrhage has already occurred, then there is an 18% per year rate of re-hemorrhaging. Overall, 50% of patients with an AVM will present with hemorrhage. Hemorrhage will be fatal in 10% of cases and result in loss of function or disability in 30%-50% of cases. Other presentations include stroke due to steal, seizure and headache.

There are several ways to treat AVM’s. AVM’s can sometimes be removed surgically, depending on the size and location. Some AVM’s can be treated with a special form of radiation called stereotactic radiosurgery (SRS). Another common form of treatment is embolization. In embolization, a small tube is navigated from a groin artery to the brain AVM itself. The AVM is then “plugged” with a variety with substances. Thanks to recent technological advancements, we can now effectively treat more AVM’s with embolization than ever before. In most cases, AVM’s will require a combination of therapies, such as embolization and surgery or embolization and radiation.

It is important to note that patients with AVM may be at risk of hemorrhage for some time even after treatment. When an AVM is surgically removed or embolized, there may be the possibility of a delayed hemorrhage. This is called a perfusion pressure breakthrough hemorrhage (PPBH). This occurs because the normal brain next to an AVM has been deprived of normal blood flow for a long time. When the AVM is removed, the steal phenomena is reversed, and the brain sees normal blood flow for the first time. Unfortunately, due to a life of steal, the normal brain may lack the ability to regulate that blood flow causing leaking of normal brain capillaries. A hemorrhage may result, leading to death or disability. This canbe prevented by embolizing a large AVM in multiple steps, allowing the normal brain to gradulally become accustomed to normal blood flow.

Patients will also be at risk for hemorrhage following radiation. This is because radiation works very slowly, taking almost three years to fully obliterate an AVM. Until the AVM is completely gone, the risk of bleeding remains.