Arteriovenous Malformations

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The normal pattern of blood supply is that large arteries leave the heart carrying oxygen-rich blood. They branch into smaller and smaller arteries reaching every point in the body. Capillaries, or very thin-walled blood vessels, slow down the flow of blood and perform the crucial step of oxygen and nutrient exchange with the body's tissues. The depleted blood moves into veins which gather together like streams into a river, carrying the blood back to the heart and lungs where it is replenished with oxygen and the cycle begins again.

In arteriovenous malformation, a tangle of large arteries and veins forms, bypassing capillaries and sending blood directly back into the venous system. This prevents the blood from slowing down and releasing its load of oxygen -- starving nearby tissues. Arteriovenous malformations can form throughout the body, but they are especially problematic when they are located in the brain or spinal cord.

In areas near the malformation, brain tissue deteriorates and sometimes dies for lack of oxygen. The defect also alters the normal flow of blood and the balance of fluids throughout the brain. The fistula, or area where arteries and veins connect, has very high rates of blood flow along with unusually high blood pressure. Arteries leading to the AVM can become enlarged and veins leading away may become tightened and narrowed (stenosis). Blood vessel walls become weakened and distended and may burst (aneurysm). The size of the AVM can also cause problems, putting pressure on nearby structures or blocking the normal flow of cerebrospinal fluid through and around the brain.

Symptoms

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Some AVMs cause no symptoms at all and may never be discovered, but most become symptomatic at some point during the patient's life. Some AVMs can cause seizures or persistent headaches, sometimes as painful as migraines. Other symptoms depend largely on the location of the malformation and can include weakness or paralysis on one side of the body, trouble speaking, loss of coordination, numbness, tingling or pain. If the AVM is large or is blocking the flow of cerebrospinal fluid, severe headaches due to hydrocephalus can develop.

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Causes

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AVMs are present at birth (congenital) and appear to result from incomplete blood vessel development early in fetal life. Rarely, AVMs can run in families, so researchers suspect a genetic link is at work. Physicians do not attribute AVMs to environmental influences during pregnancy.

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Diagnosis

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Virtually all AVMs can be detected on MRI images but an arteriogram is the standard test for AVM. After feeding contrast dye into an artery leading to the brain, the radiologist will take a series of head x-rays. The structure of the AVM and the flow of blood through its arteries and veins show up clearly in the resulting images. Sedation or anesthesia may be necessary if a child is young and cannot tolerate the discomfort of having the procedure done with local anesthesia. CT angiography (CTA) and MR angiography (MRA) are additional imaging procedures that can be used to define the three dimensional qualities of the malformation, and may be requested by a neurosurgeon to aid in planning treatment.

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Treatment

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As with most other brain lesions, the choice of treatment depends heavily on the location. If the AVM is accessible and in a relatively "non-eloquent" part of the brain, surgery is likely to be the best option. If the lesion is supplied by many blood vessels, some of which are difficult to reach and block surgically, your neurosurgeon may recommend endovascular embolization prior to an operation. Stereotactic radiosurgery can be employed in certain cases to treat AVMs that are too risky to remove by a direct operation.

Embolization employs the body's natural defenses to help it close off the malformation. By feeding particles or tiny coils into the AVM, doctors slow the flow of blood and encourage a clot to form which can shut down part of the blood flow within the malformation. By itself, embolization is rarely curative, but by reducing blood flow, it may make subsequent surgery easier and safer.

In stereotactic radiosurgery, a neurosurgeon uses a linear accelerator, proton beam, or gamma knife to generate beams of high-energy radiation which can be focused on the malformation while leaving nearby tissues unaffected. After the treatment, the affected blood vessels gradually wither and close down. This approach can only treat small AVMs (less than 3 centimeters) and also requires two to three years to reach its full effect, and for that reason is not the first choice of treatment after an AVM has bled. In eloquent areas of the brain where surgery could cause damage to vital functions, radiosurgery is often the only treatment option which can be used.

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Follow up

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Whatever treatment plan is chosen, a follow-up arteriogram or MRI can show whether the defect has been completely or partially eliminated. In children, AVMs can occasionally partially recur, the often the arteriogram or MRI will be repeated one year after surgery. When an AVM has caused neurological damage, follow up will focus on treating those symptoms and helping the child and family cope with any resulting disability. In the best cases -- where the malformation was caught before neurological damage was done and the AVM completely removed -- the patient can lead a normal life.

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