Brain Dural Arteriovenous Fistula (BDAVF)
The word “fistula” means abnormal connection between two structures that are normally not connected. An arteriovenous fistula is an abnormal connection between an artery and a vein. Any part of the body can have a fistula. The brain is surrounded by a tough fibrous cover called “dura mater” or “dura”. When a fistula forms between an artery and a vein within the dura, it is called Brain Dural Fistula, or Brain Dural Arteriovenous Fistula, or BDAVF, etc. The following images from a brain angiogram show what a typical fistula behind the ear (sigmoid sinus fistula) looks like. Red arrows point to some of the arteries going into the fistula, and blue ones show the outflow veins (sigmoid sinus and jugular vein)
When a fistula forms, blood from an artery under relatively high pressure and flow goes directly into a vein, normally a low pressure structure. Even though the fistula is not directly on or within the brain, it can lead to brain dysfunction by congesting the brain venous system, interfering with normal brain circulation.
Etiology (why fistulas happen)
The reason brain dural fistulas form is usually unclear. Sometimes, a patient has clear history of significant trauma or surgery in the area which later develops a fistula. Some physicians believe that fistulas form in places of previous thrombosis or clotting. In the end, there is no clear explanation why they form. There is, of course, a cause; we just don’t know it yet. Typically, the patients with fistula are in 50 years of age or older, and more often are women, but many exceptions to this are seen and really any age or gender can have a fistula.
Brain dural fistulas can cause trouble in several ways. Much depends on their location and size. A particularly important symptom — when the patient can actually hear blood flow in a fistula located just behind their ear as a characteristic whooshing to-and-fro sound that is always in rhythm with the heartbeat – is called “pulsatile tinnitus”.
When dural fistulas form near the ear, the patient can actually hear the sound made by the blood rushing from the arteries through the fistula into the veins, as was the case with the patient in the above picture. Many dural fistulas form exactly in such a location, in the back of the head behind the ear, near a large brain vein called “sigmoid sinus”. Sometimes the patient is told that they have a “sigmoid sinus fistula”. Many patients can hear the noise of the fistula, which follows the pulse because of the different pressures the vessels experience during the cardiac cycle. Often, the fistula can be heard by people other than the patient – a doctor usually can hear it by listening with stethoscope behind the ear. Sometimes the sound is so loud that it can be heard by a person next to the patient without the stethoscope or a microphone.
Of note, most patients who have pulsatile tinnitus do not have a dural fistula, but some other condition — a venous stenosis, for example, or atherosclerotic disease of the carotid artery, or even a cause that remain unclear even after an exhaustive search.
Sometimes fistulas can form in a large venous lake behind the eye called “cavernous sinus”. Because the cavernous sinus is normally connected to the veins that serve the eye, the fistula can produce high pressures within the eye veins and eye itself. When this happens, the eye can get large and red and painful. The medical terms for this are orbital proptosis (bulging), orbital chemosis (redness), and ocular hypertension (high pressure). Sometimes, the eye swells so quickly that there is no mistaking something very bad is happening. Other times the process is more gradual. Needless to say, most patients are very anxious to get the problem taken care of. Delaying closure of the fistula risks permanent eye damage from high ocular pressures, among other issues.
Perhaps the worst type of fistula is one that at first produces no symptoms at all. Because the fistula is located on the covering of the brain (the dura), the blood going through the fistula usually empties into the same veins that are normally used by the brain. When the fistula is small, it usually does not have enough flow to cause brain problems. However, larger or growing fistulas result in so much added flow and pressure in the veins that it can interfere with normal venous drainage of the brain, producing venous congestion. At this point, the patient may develop various types of brain dysfunction, such as seizures or strokes. As with pulsatile tinnitus, most patients with seizures or strokes don’t have a dural fistula, but these are some ways a fistula can present itself. Many patients, however, remain symptom-free even at the stage where venous pressures in the brain are elevated.
As the fistula continues to dump large amounts of arterial blood into veins which are not used to this much flow or this much oxygen, the veins of the fistula become diseased. This usually means that they get progressively smaller and many eventually shut down completely. While this may seem like a good thing, in reality it is the opposite. Once these veins are gone the fistula has to find other veins to drain, and this usually means draining directly into veins of the brain itself. At this point, the chance of stroke (a bleeding type of stroke usually) or seizures is much higher. Treatment of fistulas at this stage can also be more difficult. In this patient, the primary venous outlets that used to drain the fistula are now closed (white arrows), so that all of fistulous blood is now directed into brain veins (blue arrows)
Finally, many patients who have dural fistulas of all sizes and locations can have a myriad of symptoms such as different kinds of headache, tiredness, loss of concentration, memory problems, even hallucinations, psychosis or other psychiatric problems. Unfortunately, in these patients the diagnosis of dural fistula can be significantly delayed because dural fistulas are relatively rare
The diagnosis of the fistula can be made in various ways, depending on the location, symptoms, etc. Often, the patient can diagnose their own fistula when they hear it or see it (as above). Sometimes, the fistula is found incidentally because of an MRA obtained for some other reason, though this is relatively rare since MRI and CT are not good tests for findings dural fistulas. New MRI techniques, such as time-resolved contrast MRA, are promising to change that. Likewise, new dynamic (sometimes also called 4-D) CT angiographic methods have been used with success in diagnosis of dural fistulas, although one important drawback of dynamic CT is the relatively higher dose of radiation required to perform this examination, while MR exams, such as MRI or MRA, are entirely radiation-free.
The definitive way of making a diagnosis of the fistula is by catheter cerebral (brain) angiography. During catheter angiography, a skinny but long hollow tube called catheter is placed into the patient’s artery (usually in the leg) and then directed under x-ray guidance into different arteries which go to the brain. A contrast dye is injected through the catheter and the flow of dye through arteries and veins is then imaged by real-time x-ray cameras. The fistula is seen as an area of premature appearance of fistula veins, when normally the dye should still be in the arteries. An angiogram like this usually takes about 1 hour, but practically involves a one day’s hospital stay. There is usually no need to put the patient to sleep for the angiogram, however this varies depending on the facility and preference of the physician performing the procedure.
The purpose of the treatment is to close the fistula. Two main treatment options are available: catheter embolization and radiation. Catheter embolization is a procedure similar to an angiogram, but more involved and longer. The patient is usually put to sleep by an anesthesiologist. A catheter or catheters are again introduced through an artery and often vein in one or both legs. The catheters are now guided into or near the region of the fistula. Through these catheters, different agents such as metal coils, plastic particles, glue-like substances, and occasionally stents or other devices are delivered to close the fistula, as in this patient, where glue and a stent (yellow arrows) were used to selectively close the fistula, while keeping the venous sinus open. This is the same patient whose fistula was shown in the first picture on this page.
This kind of treatment usually involves several days of hospital stay.
Some dural fistulas can be observed conservatively, depending on factors such as size, location, and overall patient condition. The details of any treatment are best discussed with the specialist performing the procedure.
Sometimes, catheter embolization is not possible for various reasons. In these cases, a highly precise form of radiation treatment (usually gamma-knife) can be used to target and close the fistula. Radiation treatments take time to work (months to years) but can be remarkably effective at safely and effectively closing very complicated fistulas for which no other treatment exists. Again, the details of treatment are discussed with the specialist performing the procedure.
Finally, in some select, complex cases, fistulas require an open surgical procedure – either to directly close the fistula or to facilitate access to it through microcatheters when getting there through arteries or veins in the legs and other places is not anatomically possible.
Prognosis depends on symptoms and degree of dysfunction. Pulsatile tinnitus that is caused by the fistula is permanently cured by closing the fistula. Eye problems are usually completely cured or made much better. Seizures often stop or markedly improve. Strokes already caused by the fistula cannot be cured, but new dysfunction can be prevented. Ultimately, prognosis is established individually based on the kind of fistula the patient has.
The physicians at the Bernard and Irene Schwartz Interventional Neuroradiology Center specialize in angiography and embolization of brain vascular malformations, including brain dural fistulas, where we have extensive experience. As all centers of excellence, we function as part of a group of neurosurgeons, radiation specialists, neurologists, and other physicians and support staff. The center can be contacted by calling 212-263-6008.