An arteriovenous malformation (AVM) is a tangle of abnormal blood vessels connecting arteries and veins in the brain. AVMs are a rare disease (approximately 1-5 out of 10,000 patients) of the blood vessels in the brain. In healthy individuals, arteries that branch into smaller and smaller arteries carry high-pressure oxygenated blood to the brain. The exchange of oxygen occurs in tiny blood vessels called capillaries. Blood that is now low-pressure and has exchanged oxygen for carbon dioxide is then collected by small veins that then form larger veins and carry blood back to the heart and subsequently lungs. In patients with brain AVMs, these connections are incorrect, and arteries in the brain connect directly to veins without the usual interspersed capillaries. This means that high-pressure blood ends up in veins, which are not built to withstand this pressure for a long time. In addition, the arteries usually are abnormally enlarged and tortuous, which is called the ’nidus’ of the AVM.