Arteriovenous Fistula (AVF)
What is an arteriovenous fistula (AVF)?
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 arteriovenous malformations (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, and may also occur anywhere in the body. 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.
Two major types of AVFs
The two major types of AVFs are dural arteriovenous fistulas (DAVF) and carotid-cavernous fistulas (CCFs).
Dural arteriovenous fistulas (DAVFs) are the most rare type of fistula, and are most commonly found in woman over the age of 40. DAVFs occur in the dura mater, which is the tough fibrous membrane that envelops the brain and spinal cord outside the arachnoid and the pia mater (the other layers of membrane that protect the brain and spine). Although DAVFs can occur in any part of the location where the dura mater exists, DAVFs are commonly found in cavernous sinus (behind the eye) and transverse / sigmoid sinus (behind the ear, back of your head). A cranial DAVF is supplied by branches of the carotid artery (external and internal carotid arteries) and possibly also by branches of the vertebral artery before these arteries penetrate the dura. There is usually a prominent “draining vein” that can be large and curving or tortuous leaving the fistula site. There may also be dilatations in this vein known as varices that can look like venous aneurysms. The venous drainage pattern is the most important determinant of prognosis because in benign fistulas, drainage does not involve the cerebral veins and tinnitus or eye symptoms are the most common forms of presentation. However, in more aggressive fistulas, the blood flow in a DAVF is so high that it may cause blood to flow in the opposite direction over the brain’s surface, which is known as retrograde cortical venous drainage. This condition is very dangerous and patients tend to present with bleeding in the brain (hemorrhage) and progressive neurological deficit, sudden nausea and vomiting, seizures, or intracranial hypertension. It is possible for there to be more than one fistula in the same patient.
Carotid-cavernous fistulas (CCFs) are the most common type of fistula. CCFs result from an abnormal communication between the arterial and venous systems within the cavernous sinus in the skull. As high pressure arterial blood enters the cavernous sinus, the normal venous return to the cavernous sinus is backed up, causing engorgement of the draining veins. This presents most dramatically as a sudden engorgement and redness of the eye of the same side.
Risks of developing a Fistula
Unlike AVMs, patients are not born with AVFs, but instead acquire these lesions later in life as a result of infection, tumors, surgery, traumatic injury, and most often, for no reason at all. People with diabetes, compromised immune systems (for example, AIDS or cancer), and certain gastrointestinal diseases (Crohn’s disease, inflammatory bowel disease) have an increased risk of developing fistulas.
Some fistulas are life-threatening, some cause discomfort, and others are benign and go undetected. Headache is a symptom that can be associated with all types of fistulas. The symptoms of a fistula vary depending on the location. Most patients with DAVFs tend to present with an abnormal “wooshing” sound in one ear, known as a bruit. Patients with DAVF behind the ear tend to hear a ringing in the ears that gets louder with each heartbeat. This is known as pulsatile tinnitus. Patients with CCF usually complain of decreased vision and redness, congestion, and swelling of one or both eyes in addition to a bruit. CCFs are the most common type of fistula. All types of DAVFs can cause stroke like symptoms and seizures due to the back up of blood in the venous circulation that compromises normal blood flow to the brain.
Normal blood flow goes through the arteries, then the capillaries, and returns to the heart through veins. When a dural arteriovenous fistula is present, blood flows directly from an artery into a vein, bypassing the capillaries. If the volume of diverted blood flow is large enough, tissues downstream receive less blood supply and heart failure is possible due to the increased volume of blood returned to the heart.
Diagnosis of arteriovenous fistula (AVF)
Once diagnosed, it is important to close the AVF off as soon as possible to prevent the abnormally high pressure of the venous system from causing irreversible damage to the brain or spinal cord. At this point, a diagnostic cerebral angiogram is the test that most accurately shows the AVF and its relationship to the surrounding arteries and veins. Based on the symptoms, the physician will run tests in order to rule out everything, but in most cases of AVFs, MRI and CT scans appear normal, unless or course there is hemorrhage caused by the fistula. Please see diagnostic cerebral angiogram for more information on this procedure.
Treatment of an Arteriovenous Fistula (AVF)
Treatment options for AVFs include endovascular techniques (embolization), open surgery, or a combination of these methods. Embolization is most often used as a preparatory step prior to surgery. Each patient with an AVF is treated on an individualized basis, taking into consideration the age of the patient, presence of other significant medical conditions, the location of the lesion, and size of the AVF, whether there is a history of previous AVF hemorrhage in that patient, and the type and risks of treatment option most suitable for that AVF and person. Treatment becomes more urgent if cortical venous drainage is seen on the diagnostic cerebral angiogram or if a patient has experienced rupture or other significant neurological effects of the DAVF.
The goal of surgery is to physically disconnect the fistula in the dura, while playing close attention to obliterating the draining vein. A craniotomy is required for surgical disconnection of a cranial DAVF and a laminectomy is required for surgical disconnection of a spinal DAVF. Open surgery for a DAVF typically has a very high success rate. Please see Operative Procedures for more information on treatment options and recovery.
Endovascular Embolization is a catheter-based technique performed to inject a “glue-like” substance (usually Onyx) into the lumen of the arteries feeding the DAVF, or directly into the vein draining the DAVF. The purpose of this procedure is to close off as much of the fistula as possible. Please see Neurointerventional Procedures for more information on treatment