Pathophysiology: Arteriovenous malformations (AVMs) are defects of the circulatory system that are generally believed to arise during embryonic or fetal development or soon after birth. They are comprised of snarled tangles of arteries and veins. Arteries carry oxygen-rich blood away from the heart to the body’s cells; veins return oxygen-depleted blood to the lungs and heart. The absence of capillaries—small blood vessels that connect arteries to veins—creates a short-cut for blood to pass directly from arteries to veins. The presence of an AVM disrupts this vital cyclical process. Although AVMs can develop in many different sites, those located in the brain or spinal cord—the two parts of the central nervous system—can have especially widespread effects on the body. The purpose of this text is to provide patients who are diagnosed with central nervous system vascular malformations with some background information regarding the nature of their problem and the possible alternatives in terms of treatment. It is written in terminology that is designed to minimize medical jargon and technical terms. When possible, simple diagrams are used to help explain points. The intent of this text is to provide basic background information. Specific details regarding any particular patient's vascular malformation should be obtained through discussions with their physician. The text is divided into the four major types of vascular malformations that are known to occur.