Arteriovenous malformations or AVMs of the brain are abnormal tangles of arteries and veins which usually date back to birth but can also result from head injury. Normally, blood from the heart moves from arteries to veins through a fine network of small blood vessels called a capillary bed. Here the nutrients from the blood are released into the brain tissue and the pressure of the blood flow reduces before it enters the veins on its journey back to the heart, In an AVM the arteries and veins are connected directly without a capillary bed, exposing the thin walled veins to high pressures which puts them at risk for rupturing and bleeding into the brain.