An arteriovenous malformation, or AVM, is an abnormal tangle of vessels in the brain or spinal cord in which one or more arteries are directly connected to one or more veins. Arteries carry blood from the heart to the tissues and veins take blood back from the tissues to the heart. In an AVM there is a direct connection between one or more arteries and veins, which gives rise to many problems. The most serious problem is that veins are typically thin-walled vessels that cannot accept high-pressure blood flow for extended periods. The result is that AVMs can rupture and bleed into the brain. We estimate that 0.6% of people are born with a brain or spinal cord AVM. Although AVMs are congenital (which means that patients are born with it), they are not hereditary (which means that they are not passed from parents to children). Most AVMs (71%) declare themselves by bleeding in young adults, typically younger than 40 years old. Some AVMs declare themselves by causing seizures (24%) or headaches (5%). Currently, we attempt whenever possible to identify and eliminate AVMs before they bleed. We typically first find the AVM in a CT scan or, more commonly, in an MRI scan. If we find an AVM by CT or MRI scan, we then obtain an angiogram or arteriogram. An angiogram (also called arteriogram) is a special test in which a neuroradiologist injects dye into the blood vessels in the brain and obtains images of the blood vessels. At this point, the angiogram is the test that most accurately shows the AVM and its relationship to the surrounding arteries and veins. At Johns Hopkins, we treat AVMs using a combination of three methods, depending on the type of AVM. These three methods are (1) microsurgical resection, (2) stereotactic radiotherapy, and (3) endovascular embolization. Microsurgical resection is the more established of the three techniques. During microsurgical resection, we perform a craniotomy and using the microscope remove the AVM from the brain or spinal cord. Stereotactic radiotherapy is a more recent technique for the treatment of AVMs. It is also know as "the gamma knife" and "stereotactic radiosurgery." During stereotactic radiotherapy, we deliver a concentrated dose of radiotherapy to the core of the AVM in one session. Over the course of 2 to 5 years, the vessels of the AVM clot off and the AVM shuts down. Endovascular embolization is also a more recent technique for the treatment of AVMs. It is typically used in preparation for either microsurgical resection or stereotactic radiotherapy. During endovascular embolization, we pass a catheter through the groin up into the arteries in the brain that lead to the AVM and inject a material into these arteries. This injection shuts off that artery and reduces the flow of blood through the AVM. Endovascular embolization by itself typically does not eliminate the AVM and is therefore almost always used as a preliminary step in preparation for either microsurgical resection or stereotactic radiotherapy. At Johns Hopkins, we have a group of specialists that evaluate each AVM patient and decide the best treatment for the patient's specific AVM, namely microsurgical resection, stereotactic radiotherapy, endovascular embolization, or a combination of the three. Johns Hopkins is one of the largest referral centers for the treatment of AVMs in the country.