What is an arteriovenous malformation?

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Arteriovenous malformations are abnormalities in blood vessels that can affect the arteries or the veins. They can be found in all parts of the body including the brain, lungs, the extremities and in organs, such as the kidney and liver.
An arteriovenous malformation, better known by its acronym AVM, occurs in less than 1 percent of the general population. Why they occur is not very clear, although brain AVMs are the most common and are usually congenital, meaning you are born with them. 

An AVM is a tangle of arteries and veins in your brain that likely develop before your are born. Blood normally enters your brain through arteries, which branch into smaller blood vessels. The brain then uses oxygen removed from the blood in your capillaries.

In a brain AVM, your blood passes directly from your arteries to your veins, bypassing capillaries. The arteries and veins in an AVM can rupture, causing bleeding into the brain.
 
Arteriovenous malformations (AVMs) are defects of the circulatory system that are generally believed to arise during embryonic or fetal development or soon after birth. AVMs can develop in many different sites, those located in the brain or spinal cord can have especially widespread effects on the body. Most people with neurological AVMs experience few, if any, significant symptoms. These malformations tend to be discovered incidentally. But for about 12 percent of the affected population these abnormalities cause variable symptoms.
Arteriovenous malformations (AVMs) are defects of the circulatory system that arise during embryonic or fetal development or soon after birth. They consist of snarled tangles of arteries and veins. Arteries carry oxygen-rich blood away from the heart to the body's cells, while veins return oxygen-depleted blood to the lungs and heart. The absence of capillaries?small blood vessels that connect arteries to veins?allows the blood to pass directly from the arteries to the veins. The presence of an AVM disrupts this vital cyclical process. Although AVMs can develop in many different sites, those located in the brain or spinal cord-the two parts of the central nervous system-can have widespread effects on the body.
AVMs of the brain or spinal cord (neurological AVMs) are believed to affect approximately 300,000 Americans. They occur in males and females of all racial or ethnic backgrounds at almost equal rates.
This answer from the National Institute of Neurological Disorders and Stroke has been reviewed and/or edited by Dr. William D. Knopf.