An arteriovenous malformation (AVM) is an abnormal tangle of blood vessels that connects arteries and veins in the brain or other parts of the body. AVMs can form anywhere in the body, but they are most common in the brain and spinal cord. The cause of AVMs is not clear, but most people are born with them. Some people with AVMs experience signs and symptoms, such as headaches, seizures, and changes in thinking or neurological function. AVMs can damage the brain or spinal cord through three basic mechanisms: by reducing the amount of oxygen reaching neurological tissues, by causing bleeding (hemorrhage) into surrounding tissues, and by compressing or displacing parts of the brain or spinal cord. Complications of brain AVMs include brain bleed/stroke, seizures, aneurysm, and brain damage that affects thinking, mental processing, memory, or understanding speech. Once diagnosed, an AVM can be treated to prevent complications, such as brain damage or stroke.