Spinal Dural Arteriovenous Fistula (SDAVF)
“Fistula” means abnormal connection between two structures that are normally not connected. An arteriovenous fistula is therefore an abnormal connection between an artery and a vein. Any part of the body can have a fistula. A spinal fistula, particularly spinal dural fistula, is an abnormal connection between an artery and a vein that are located near the covering of the spinal cord. This covering is called dura, and so the full name “Spinal dural arteriovenous fistula.” When a fistula forms, blood from an artery under high pressure and flow goes directly into a vein, which is a low pressure and low flow structure. Even though the dural fistula is usually not directly on or within the spinal cord, it nevertheless causes dysfunction by interfering with normal spinal cord circulation, eventually producing severe and sometimes irreversible problems.
Etiology (Why fistulas happen)
The reason spinal dural fistulas form is usually unclear. They appear to develop spontaneously, that is without any reason we can identify. There is in fact a cause, we just don’t know it yet. Typically, the patients with fistula are in 50 years or older, and more often men (maybe as much as 5 times more common), but many exceptions to this are seen and really any age or gender can have a fistula.
Pathophysiology (How fistulas cause trouble)
The way fistulas cause trouble is by preventing normal spinal cord blood circulation. Normally, spinal cord is nourished by a number of arteries, from which blood goes to the capillaries from which it passes into small venules and finally into larger veins that run along the surface of the cord. These surface veins then drain blood out via connecting veins that go through the dura into still bigger veins which finally return blood to the heart. Because the veins which drain the spinal cord are connected to veins that sit on the dura, a spinal dural fistula vein can be connected, in a reverse way, to spinal cord veins. The veins near the fistula are under higher pressure and higher flow than they are used to. This causes congestion (backup) of fistula blood into spinal cord veins. The backup of blood prevents the spinal cord from draining the blood that goes to it, since spinal cord blood has to overcome higher pressures created by the fistula to exit the cord. This causes the spinal cord to swell and malfunction. As time goes on, the situation gets progressively worse. The veins which drain the fistula eventually become diseased and often close down. Failure of these draining veins worsens already existing spinal cord congestion and makes symptoms worse. Irreversible damage to the spinal cord (myelopathy) eventually results. The patient may end up paraplegic (unable to move legs), impotent, and unable to control bowel and bladder.
The length of time between formation of the fistula and symptoms can be quite variable. Initially, the fistula is probably asymptomatic (no symptoms). As spinal cord swelling gradually develops, most patients start to have some problems with leg weakness, back and leg pain, and issues related to their bladder and bowel. Weakness can be of different types (upper vs. lower motor neuron, or both), and the patient may feel either stiff in the legs, or wobbly, or any other number of descriptive terms. The bowel and bladder problems can also be vague, — a person may feel they cannot start urinating as well as they could, of feel that they cannot empty their bladder as well as they should, or may not feel the urge to urinate and therefore have some incontinence. Constipation or fecal incontinence can happen also. Other problems patients report are impotence, abnormal sensations in the legs, which can be very variable, such as pins and needles, numbness, burning etc. The problem with diagnosing the fistula early is that at first the symptoms are usually nonspecific – meaning that they don’t for sure imply that the patient has a fistula. In fact, most people with symptoms of numbness in the legs or urinary incontinence don’t have a fistula. Likewise, because most patients with dural fistulas are elderly men, their complaints of problems with urination often result in investigations of prostate dysfunction. Many patients have symptoms of the fistula for one or more years before they are diagnosed. A high degree of suspicion and experience is necessary.
The most common way a fistula is eventually discovered is by MRI, which usually shows characteristic findings of large veins surrounding the spinal cord (red arrows on image below). The cord may show evidence of swelling (edema, marked by yellow arrows), as in this typical MRI picture of a patient with a fistula. A normal MRI of the cord is shown on the left for comparison:
Notably, even MRI can miss small fistulas, and we see patients who bring us old MRI studies where the fistula cannot yet be seen, but must have been there in retrospect. Also, the quality of MRI study has to be good. Not all MRI machines are equal, and newer machines, such as ones at The diagnostic radiology center of NYULMC, operated by competent technicians are vastly superior to older ones.
Once the diagnosis of fistula is made, the patient is referred for more evaluation and treatment, which is the time when we first see the patient. The next step in evaluation of the fistula is a catheter spinal angiogram. MRI studies do not show where the fistula is, only that one is likely present. To find the fistula, a minimally invasive procedure, — a spinal angiogram, is necessary. This is a procedure where a skinny but relatively long catheter is inserted into the aorta and many vessels which can give rise to the fistula are catheterized (selectively entered) under x-ray guidance. Many vessels may need to be catheterized to find the fistula. For this reason, and because the patient has to be completely motionless during angiogram injections, we do spinal angiograms under general anasthesia (patient is completely asleep). This spinal angiographic images show an advanced dural fistula; even though the fistula is located in the low back area, there is so much venous congestion that the vein can be seen ascending all the way into the head, as seen on image on the right.
Once the fistula is found, the anatomy of the fistula is evaluated to see if it can be treated. Again, because the MRI cannot fully identify the type and location of the fistula, spinal angiography sometimes established a more specific and even rarer diagnosis, such as a Spinal Epidural Fistula, or Spinal Pial Fistula.
The majority of fistulas can be treated either through catheters by embolization, or by surgery. The goal of treatment is to close the fistula by either plugging it up with glue (embolization) or by surgical disconnection. Embolization can be often done at the same time as the spinal angiogram, so that both diagnosis and treatment can be carried out during the same procedure. To treat a fistula, a very small catheter is inserted into the fistula or very close to it, and the fistula is plugged up with a glue-like substance, as seen in the following images of the same patient, whose fistula is shown above. A Microcatheter is used to enter an artery as close as possible to the fistula and inject a glue-like substance, closing the fistula hole. Notice how the glue cast on the right image looks similar to the pre-treatment injection on the left.
In the following case, a spinal dural fistula is supplied by the S1 segmental branch of the internal iliac artery in the pelvis, a relatively rare location that should not be overlooked.
Treatment with nBCA glue is again performed, closing the fistula:
If endovascular (embolization) treatment is not possible, for one of several reasons, the patient can have the fistula closed by having special spine surgery. Overall, the vast majority of fistulas we see are cured through one of these two ways. Whether the fistula is cured by endovascular (catheter) or surgical means, the treatment should only be undertaken at specialized centers having extensive experience with this particular condition, as details of treatment are usually quite complex and are best discussed with the specialist who will be treating the fistula.
Prognosis and Recovery
Once the fistula is closed, spinal cord blood circulation improves, but usually does not go back to normal because a number of veins draining the cord have already been damaged. The patient’s symptoms may go away completely, partially, or not at all. The extent of eventual improvement depends on duration and severity of pre-treatment symptoms, individual anatomy, and other considerations. Most patients will get better. On the other hand, without treatment, virtually all patients get worse. For this reason, we strongly recommend that essentially all patients with dural fistulas be evaluated for treatment.
Our Role: Spinal dural fistula treatment is best undertaken in a multidisciplinary fashion. Our role, as neurointerventional radiologists, consists of performing the diagnostic cerebral angiogram and embolization treatment of the fistula. We function as part of a team consisting of diagnostic radiologists, neurologists, neurosurgeons, and other staff. To refer a patient or make an appointment, you can contact the Bernard and Irene Schwartz Interventional Neuroradiology Center at 212-263-6008