An AVM is a congenital defect of the brain vasculature where a mass of arteries and veins connect directly without a intermediate capillary bed, called a nidus.  The direct vessel connection creates a blood flow shunt through one section of the brain, robbing other areas of the brain adequate blood flow.   Eventually, the nidus weakens and ruptures from the high blood flows.  The result is an unpredictable and life threatening hemorrhage.