An Arterio-Venous Malformation, or AVM, is an abnormal collection of blood vessels. Normally, 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. The "used" (deoxygenated) blood then passes back to the heart 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.