An arteriovenous malformation, or AVM, is an abnormal tangle of vessels in the brain or spinal cord in which one or more arteries are directly connected to one or more veins. Arteries carry blood from the heart to the tissues and veins take blood back from the tissues to the heart. In an AVM, the direct connection between one or more arteries and veins gives rise to many problems. The most serious problem is that veins are typically thin-walled vessels that cannot accept high-pressure blood flow for extended periods. The result is that AVMs can rupture and bleed into the brain.
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