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ARTERIOVENOUS MALFORMATIONS DESCRIPTION |
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Arteriovenous Malformationss represent an abnormal interface
between arteries and veins. Normally, arteries carry oxygenated
blood to the body's tissues through progressively smaller blood
vessels. The smallest are capillaries, which form a web of blood
vessels (the capillary bed) through the body's tissues. The arterial
blood moves through tissues by these tiny pathways, exchanging its
load of oxygen and nutrients for carbon dioxide and other waste
products produced by the body cells (cellular wastes). The blood is
carried away by progressively larger blood vessels, the veins. Arteriovenous Malformationss
lack a capillary bed and arterial blood is moved (shunted) directly
from the arteries into the veins. Arteriovenous Malformationss can occur anywhere in the body and have been found in the arms, hands, legs, feet, lungs, heart, liver, and kidneys. However, 50% of these malformations are located in the brain, brainstem, and spinal cord. Owing to the possibility of hemorrhaging, such Arteriovenous Malformationss carry the risk of stroke, paralysis, and the loss of speech, memory, or vision. An Arteriovenous Malformations that hemorrhages can be fatal. Approximately three of every 100,000 people have a cerebral Arteriovenous Malformations and roughly 40-80% of them will experience some bleeding from the abnormal blood vessels at some point. The annual risk of an Arteriovenous Malformations bleeding is estimated at about 1-4%. After age 55, the risk of bleeding decreases. Pre-existing high blood pressure or intense physical activity do not seem to be associated with Arteriovenous Malformations hemorrhage, but pregnancy and labor could cause a rupture or breaking open of a blood vessel. An Arteriovenous Malformations hemorrhage is not as dangerous as an aneurysmal rupture. (An aneurysm is a swollen, blood filled vessel where the pressure of the blood causes the wall to bulge outward.) There is an approximate 10% fatality rate associated with Arteriovenous Malformations hemorrhage, compared to a 50% fatality rate for ruptured aneurysms. Although Arteriovenous Malformationss are congenital defects, meaning a person is born with them, they are rarely discovered before age 20. A genetic link has been proposed for some Arteriovenous Malformationss, but studies are only suggestive, not positive. The majority of Arteriovenous Malformationss are discovered in people age 20-40. Medical researchers estimate that the malformations are created during days 45-60 of fetal development. A second theory suggests that Arteriovenous Malformationss are primitive structures that are left over from the period when fetal blood circulating systems began to develop. However they form, Arteriovenous Malformationss have blood vessels that are abnormally fragile. The arteries that feed into the malformation are unusually swollen and thin walled. They lack the usual amount of smooth muscle tissue and elastin, a fibrous connective tissue. These blood vessels commonly accumulate deposits of calcium salts and hyalin. The venous part of the malformation receives blood directly from the artery. Without the intervening capillary bed, the veins receive blood at a higher pressure than they were designed to handle. This part of the malformation is also swollen (dilated) and thin walled. There is a measurable risk of an aneurysm forming near an Arteriovenous Malformations, increasing the threat of hemorrhage, brain damage, and death. Approximately 10-15% of Arteriovenous Malformationss are accompanied by saccular aneurysms, a type of aneurysm that looks like a small sac attached to the outer wall of the blood vessel. Although the malformation itself lacks capillaries, there is often an abnormal proliferation of capillaries next to the defect. These blood vessels feed into the malformation, causing it to grow larger in some cases. As the Arteriovenous Malformations receives more blood through this "steal," adjacent brain tissue does not receive enough. These areas show abnormal nerve cell growth, cell death, and deposits of calcium in that area (calcification). Nerve cells within the malformation may demonstrate abnormal growth and are believed to be nonfunctional. |
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| ARTERIOVENOUS MALFORMATIONS RELATED ITEMS | ||
