There is more to the brain than nervous tissue. Many other structures lie within or near the brain that help it stay healthy and function well.
Two of the most important supportive structures are connective tissue and blood vessels. Both support and sustain organs throughout the body, and the brain is no exception.
To better understand dural arteriovenous fistulas, it is important to know something about these support systems and how the systems can fail.
The most prominent connective tissue in the brain is called the dura mater, or “dura” for short. Dura mater is Latin for “tough mother”; it is a tough outer membrane that encases the brain and spinal fluid.
Classically, the membrane consists of three layers. One layer is attached to the skull and is a thick fibrous membrane; another, called the arachnoid layer, directly envelops the brain and provides cushioning to the brain; and the third layer is the delicate pia mater directly applied to the brain’s surface.
The dura, however, is actually more than just connective tissue. Like virtually all other tissues in the body, the dura contains an elaborate network of blood vessels—both arteries and veins.
Arteries deliver oxygen and nutrients to the organs; veins carry oxygen-poor blood and waste products away. The cerebral veins empty into larger conduits that carry blood to the lungs, to pick up oxygen, and to the liver and kidneys, to eliminate waste. Connecting the arteries and veins are capillaries—tiny blood vessels that are in direct contact with tissues, exchanging oxygen and nutrients for waste products.
The dura has several functions. One is to house the fluid, known as cerebrospinal fluid, that serves to cushion the brain. Together with the cerebrospinal fluid, the dura keeps the brain from moving too much within the skull.
In addition to cushioning the brain and housing the cerebrospinal fluid, it turns out that the dura plays an important role in blood flow from the brain. The layers of connective tissue that make up the dura contain spaces known as dural sinuses. The sinuses are large veins that are the conduits, receiving blood from the brain’s cortical veins.
Unfortunately, sometimes things go wrong with blood vessels, including those in the brain and its supporting structures. Occasionally an abnormal connection develops between an artery and a vein. One sort of abnormality is known as an arteriovenous fistula—often abbreviated as “AVF.”
With an AVF, blood flows directly from an artery to a vein, bypassing the normal capillary connection. When this happens, the flow of blood through the involved vein increases and often becomes so great that the vein’s capacity is overwhelmed.
This results in increasing pressure in the involved veins. The abnormal flow can damage the vein, leading to inflammation and scar formation. Also, other veins nearby may be similarly affected as blood flow is diverted to them.
AVFs can occur anywhere in the body, and they sometimes happen in the dura. Dural AVFs (DAVFs) can happen after an injury, an infection or surgery. Sometimes, patients with dural fistulas cannot identify a point where they were injured. Dehydration or an increased propensity of the blood to form clots is also a risk for development of a DAVF.
Many patients, particularly if the DAVF is mild, have no symptoms. It depends on the location of the fistula and its proximity to the body’s sensory systems.
More severe DAVFs may cause symptoms: ringing in the ears (pulsatile tinnitus), problems seeing (double vision or blurred vision), even redness or bulging of the eyes. These eye and ear symptoms occur because DAVFs are commonly located not far from the eyes and ears, as well as from the nerves that supply them.
However, there can be more subtle signs and symptoms that can occur for many reasons and are more difficult to ascribe to a DAVF. These include headache, dizziness, nausea and vomiting.
A patient may experience ataxia (poor coordination), dementia (confusion and stroke-like symptoms) or even seizures. A major complication of severe, untreated DAVF is intracranial hemorrhage, or bleeding through the abnormal vessels into the brain, causing a hemorrhagic stroke.
How do doctors diagnose a DAVF? Most important, doctors need to simply consider DAVF as a diagnosis because it is relatively uncommon and can easily be overlooked or missed if not specifically considered. If a doctor suspects the patient may have a DAVF, some form of non-invasive brain imaging, such as a CT or MRI scan, is obtained.
In many cases, the DAVF, its effects on the brain or both may become apparent, but not always. The most specific test is a catheter angiogram performed by specialists to study the brain vasculature in detail. With this test, physicians can then diagnose a DAVF with certainty and assess the level of risk that the fistula poses to the patient.
Once diagnosed, what is the treatment? For a fistula considered “low risk” based on the angiogram, doctors may recommend observation alone with repeat non-invasive imaging at intervals or if the patient’s symptoms change. However, DAVFs typically grow, resulting in greater risk over time.
When DAVFs do cause serious symptoms or appear “high risk” for complications based on the angiogram, the treating physician has other options. Although sometimes surgery and radiation are used, the most common treatment is a procedure called endovascular embolization.
In this method, the doctor inserts a thin tube called a catheter into a blood vessel, usually in the leg, and the tube is threaded through the body’s vessels until it reaches the DAVF.
Through a catheter, the physicians can then deliver a treatment to block the flow of blood through the area—often a tiny coil, or a liquid embolic agent that acts as a “glue.” In most cases now, endovascular cure or palliation can be accomplished without the need for head-open surgery.
In addition to closing off the blood vessels directly, endovascular embolization helps the body’s own defenses get moving to develop a blood clot in the area, which also helps stop the abnormal flow. Careful use of these techniques provides effective treatment of the DAVF while allowing the “good” blood vessels to function properly and support the brain.
Endovascular embolization is commonly performed by neurointerventionalists, specializing in minimally invasive treatment of cerebral vascular diseases. Dr. Philip Meyers of Columbia University Medical Center/NewYork-Presbyterian Hospital is a neurointerventionalist who treats DAVFs.
Dr. Meyers has extensive experience and training in treatment of diseases of the blood vessels affecting the brain. He regularly shares his expertise with fellow neurointerventionalists by lecturing at continuing medical education meetings. Most recently he spoke about the treatment of DAVF at the meeting of the Society of Vascular and Interventional Neurology.
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