An Arterio-Venous Malformation, or AVM is an abnormal collection of blood vessels. Normally, red oxygenated blood is pumped by the heart through branching tubes called arteries to the brain where it enters a fine network of tiny vessels called capillaries. It is in these capillary beds where the blood nourishes the tissues. Blue, deoxygenated blood then passes back to the heart back through branching thin walled tubes called veins. Arterial-Venous-Malformations are areas that lack the tiny Capillaries. The location of the connection between the artery and the vein is called the shunt. The area of tissue is called a nidus of the AVM. An AVM can be thought of as a "Short Circuit" where the blood does not go to the tissues but is pumped through the shunt and back to the heart without ever giving nutrients to the tissues.

What causes AVMs is unknown. People are born with AVMs, although they do not appear to inherit them from their parents nor do they give them to their children. It appears that AVMs may be caused by a rupture or clotting of a blood vessel that happens during development before one is born. It is usually not associated with either other maldevelopments or with other AVMs.

Usually, patients do not know they have an AVM. A number of the patients with AVMs have seizures or persistent headaches. An AVM can put additional strain on the blood vessels and the surrounding tissues. This is usually well tolerated in the first 2 decades of life. But the increased flow of blood caused by the shunt weakens the blood vessels. These weakened blood vessels can rupture. This is known as a hemorrhage or bleed. If an AVM bleeds, the patient almost always has an extremely severe headache, and most commonly has a stroke and often dies. The chances of an AVM bleeding is 4 % per year. Larger AVMs occasionally may cause progressive neurological problems by putting pressure on the brain or altering the blood flow.