Arteriovenous malformations (AVMs) are vascular anomalies that comprise tangles of arteries connected directly to veins without intervening capillaries. AVMs cause abnormal shunting of the blood, an expansion of the vessels, and a space-occupying effect. The vessels within an AVM have thin walls because of poorly developed muscle tissue in the media. If brain tissue is involved, it is usually nonfunctional. AVMs can range in size from a fraction of an inch to more than 2.5 inches in diameter.
Epidemiology and Risk Factors
Neurological AVMs affect about 1% of the population in the U.S., occurring in males and females of all racial and ethnic backgrounds. In women, pregnancy and childbirth can cause an initial onset or worsening of symptoms, due to changes in blood volume and pressure. AVMs are about 1/10th as common as aneurysms. About 12% of those people with AVMs will suffer symptoms of varying severity, and each year, about 1% will die as a direct result of these lesions. Familial cases are rare, indicating that AVMs probably arise spontaneously during embryonic or fetal development or soon after birth. Though AVMs can develop in many areas of the body, those in the brain and spinal cord have particular systemic impacts.
Most cerebral AVMs present with intracranial hemorrhage, occurring initially in a person's 20s through 40s. If AVMs do not show symptoms by the time the patients are in their late 40s or early 50s, the lesions tend to remain stable and asymptomatic. However, asymptomatic lesions frequently show signs of previous bleeding at necropsy. The risk of rebleeding is 6-7% in the first year, 2% after 5 years, and 1-2% thereafter. Initial bleeding is associated with a 10% mortality rate, which increases with subsequent bleeds. With each recurrence, the chance of additional hemorrhaging increases. Other common manifestations are partial or total seizures (occurring in 30% of patients with an AVM) and migraine-like headaches. Additionally, they can cause a wide range of neurological symptoms such as ataxia, muscle weakness, apraxia, papilledema, aphasia, bruit, and paresthesia. These symptoms can be caused by bleeding (smaller AVMs are more likely to hemorrhage than larger ones); reducing the amount of oxygen to the tissues through abnormal blood flow; or in the case of large lesions, by compressing or displacing adjacent brain tissue.
Diagnosis and Treatment
As mentioned, 90% or more cases of AVM go undetected. In suspected cases, there are several imaging technologies used to diagnose AVMs. CT and MRI are frequently used for detection, and a new application of MRI technology-magnetic resonance angiography (MRA) can record the pattern and velocity of blood flow through vascular lesions. The gold standard for characterizing an AVM is four-vessel angiography-it shows the most accurate pictures of vessel structures. The procedure is somewhat invasive, because dye is injected into the patient, and there is a small risk of stroke; however, the recent development of more accurate dye delivery techniques has decreased the risks.
There are no standard guidelines regarding the treatment of AVMs. Each case is generally considered individually because of uncertainties concerning the history of unruptured AVMs, as well as the efficacy associated with interventional therapy. The definitive treatment is surgical excision of the central portion of the AVM and the fistula, which is helped by using an operating microscope. Conventional surgery works best with relatively small lesions that are located in superficial areas of the brain, but AVMs deep in the brain generally cannot be approached this way because of the risk of damage to essential tissue. The main complications of surgical removal are loss of normal brain tissue and associated neurological function and breakthrough phenomenon, which is massive brain swelling and intracranial hemorrhage occurring postoperatively because of changes in blood flow patterns. AVM surgery is associated with about an 8% risk of serious complications or death.
Factors such as age, degree of neurological dysfunction, and location of the AVM should be considered in treatment decision making. Customarily, patients younger than 55 years old are treated more aggressively, with surgical resection, if the lesion is accessible. Older patients or those whose lesions are in vulnerable areas of the brain can be treated with focused gamma rays or proton-beam radiation, which works by damaging the tangle of blood vessels that progressively occlude during the months after treatment. However, it is only effective in lesions less than 3 cm in diameter. In endovascular embolization, the surgeon uses a catheter to plug up the fistula with a substance like inert glue, titanium coils, or tiny balloons. Embolization of the feeding arteries is rarely performed as the sole therapy, but it is frequently used in conjunction with either surgery or radiation therapy. Medical management options include strict blood pressure control and avoiding anticoagulant and antiplatelet drugs.
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