An arteriovenous fistula, or AVF, is an abnormal connection of vessels in the tissues around the brain or spinal cord in which one or more arteries are directly connected to one or more veins or venous spaces called sinuses. Arteries carry blood from the heart to the tissues, and veins take blood back from the tissues to the heart. In an AVF there is a direct connection between one or more arteries and veins or sinuses, which gives rise to many problems. AVFs differ from AVMs in that AVMs are found within the tissue of the brain or spinal cord, but AVFs are found in the coverings of the brain or spinal cord, such as the dura mater or arachnoid. The most serious problem associated with AVFs is that they transfer high-pressure arterial blood into the veins or venous sinuses that drain blood from the brain or spinal cord. This results in an increase in the pressure of the venous system around the brain or spinal cord. There are two major types of AVFs: dural AVFs and carotid-cavernous fistulas (CCFs). These are acquired lesions, which means that patients are not born with them but instead develop them later in life. They can be a result of infection or traumatic injuries, but most develop without any specific precipitating event. Patients with dural AVFs typically present with a rumbling noise in one ear that follows the heartbeat, which is called a bruit. Patients with CCFs typically present with swelling and redness of one or both eyes in addition to a bruit. Currently, we attempt whenever possible to close the AVFs before the increased pressure in the venous system causes irreversible damage to the brain or spinal cord. We typically find the AVF when we obtain an angiogram or arteriogram. An angiogram (also called an 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 AVF and its relationship to the surrounding arteries and veins. In the case of most AVFs, the CT and MRI scans often read as normal. At Johns Hopkins, we treat AVFs using a combination of two methods, depending on the type of AVF. These two methods are (1) endovascular embolization and (2) microsurgical resection. Endovascular embolization is typically sufficient to cure the majority of AVFs. During endovascular embolization, we pass a catheter through the groin up into the arteries in the brain that lead to the AVF and inject a material into these arteries. This injection shuts off that artery and reduces the flow of blood through the AVF. Microsurgical resection is reserved for AVFs that cannot be closed with endovascular embolization. During microsurgical resection, we perform a craniotomy and using the microscope isolate the AVF from the tissues around the brain or spinal cord. In special cases, we use both techniques in combination. We have a group of specialists that evaluate each AVF patient and decide the best treatment for the patient's specific AVF, namely endovascular embolization, microsurgical resection, or a combination of the two. Johns Hopkins is one of the largest referral centers for the treatment of AVFs in the country.