Contents of This Section:

1. What is a dural arteriovenous fistula (DAVF)?

A dural arteriovenous fistula (DAVF) is a site of abnormal connectivity between arteries and veins, where the abnormal connection or fistula lies exclusively in the leathery covering of the brain or spinal cord known as the dura. Sometimes there may be one or more fistula in the same patient, the plural of fistula being fistulae.

DAVF can occur in the brain or in the spinal cord: the former is referred to as a cranial DAVF; the latter is referred to as a spinal DAVF.

• 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. The fistula usually resides in the convexity dura overlying the brain hemisphere or in the tentorial dura between the forebrain and hindbrain. Rarely, it lies in a deep region of the brain known as the cavernous sinus. There is usually a prominent "draining vein" that can be large and curving or tortuous leaving the fistula site. There may be dilatations in this vein known as varices (single is varix) that can look like "venous aneurysms". Frequently the blood flow in a DAVF is very high, and it may cause blood to flow in the opposite direction to normal over the brain's surface, an event comprising abnormal cortical venous drainage -- this is a worrisome finding on a cerebral angiogram (see below).
• A Spinal DAVF is supplied by dural arterial branches arising from the aorta's segmental (spinal) arteries. The fistula itself is in the spinal nerve root dural sleeve. Typically, a spinal DAVF occurs somewhere in the lower half of the spine. The vein draining the fistula is usually a long, tortuous or "serpentine" arterialized vein (redder than normal because of the high-flow fistula blood shooting along it). This vein frequently drains upwards along the spinal cord into what is referred to as the coronal venous plexus.

Dural Arteriovenous Fistula:

Figure 1 shows a cranial DAVF as seen on a side-on or lateral view of a cerebral angiogram. The small fistulae are in the dura (red circles). The arterial supply is marked by the red arrows (branches of the external and internal carotid arteries). In this case, there is no distinct draining vein, but there is a thrombosed dural venous sinus in the vicinity of the fistulae. The blocked or thrombosed portion of the sinus is shown by dark-blue arrow heads, the normal open portion is shown by light-blue arrow heads.

2. How common is a dural arteriovenous fistula (DAVF)?

They are very rare. Interestingly, cranial DAVF are more commonly diagnosed in women over the age of 40 years while spinal DAVF are more commonly diagnosed in men over the age of 40 years.

3. Why does a dural arteriovenous fistula (DAVF) develop?

Unlike AVMs, which are thought to be present from birth, cranial DAVF most often develop later in life following blockage or thrombosis of a cranial dural venous sinus. Cranial dural venous sinuses are relatively large-caliber blood-containing structures that exist in-between the leaflets of the brain's dural covering. These sinuses usually move large volumes of venous blood from the brain, back towards the base of the brain where they form the internal jugular vein on each side of the head/neck junction. When a venous sinus blocks off for whatever reason, the brain can try to compensate by moving venous blood across other parallel or collateral pathways. In this process, however, a fistula may form, representing an abnormal collateral pathway to drain blood away from the brain. Why would a venous sinus block off or thrombose? Reasons include chronic central nervous system infection, brain trauma, or a patient with some form of hypercoagulability state (tendency to experience thrombosis).

In the spine, a DAVF may form from birth. This is however a matter of debate. Some neurosurgeons feel that a primitive artery-vein connection known as vascular peloton that normally disappears by the time of birth, may in fact persist in some persons (for reasons unknown), leading to spinal DAVF formation at that persistent site(s).

4. What are the symptoms of a dural arteriovenous fistula (DAVF)?

This depends on the location of the fistula:

• A cranial DAVF may present with pulsatile tinnitus (whooshing sound heard by the patient), or pulsatile proptosis (abnormal eye bulging and pulsation, possibly associated with staining of the eye referred to as chemosis, and impairment of vision and/or eye movement). It may present with isolated but persistent or progressive headache, or symptoms and signs of a brain hemorrhage including sudden severe headache, neurological impairment and/or collapse, or with symptoms and signs of an ischemic stroke ( take me to the Brain Attack section now).
• A spinal DAVF may present with slow but progressive loss of function in the limbs (more often the legs than the arms), with impairment of leg movement and sensation, bowel and bladder dysfunction including incontinence, and progressive erectile dysfunction in males. Together, these constitute a syndrome referred to as progressive myelopathy from spinal cord swelling and damage. Sudden loss of limb function from hemorrhage in the spinal canal or spinal cord from a DAVF is regarded as a very rare event (such a presentation is more common with spinal AVMs, in which case there may be sudden, severe back pain, too).

5. What are the complications of a dural arteriovenous fistula (DAVF)?

The most feared complication of a cranial DAVF is brain hemorrhage. This is more likely to occur if the cerebral angiogram shows "cortical venous drainage" (see above). Brain hemorrhage can cause permanent neurological disability and death. There may be seizures following a brain hemorrhage.

Depending on the location of the fistula, an unruptured fistula can cause progressive neurological impairment, including visual impairment resulting in blindness. In the spine, a DAVF typically causes progressive myelopathy (see above). Fortunately, prompt treatment of a spinal DAVF can lead to considerable neurological improvement.

6. How is a dural arteriovenous fistula (DAVF) detected?

Any of the symptoms and signs mentioned above may lead to investigation of patient. Unfortunately, in the case of spinal DAVF, the diagnosis may be missed or delayed because such lesions occur so rarely, and when they do occur, their symptoms and signs may be mistaken for spinal stenosis or disc disease, or spinal cord inflammation (myelitis).

For a cranial DAVF, regular CT scanning is not very helpful in diagnosing a fistula, but it may show a hemorrhage from a fistula. CT angiography (CT arteriography + CT venography) is helpful in defining a cranial fistula, as are MRI techniques including MR angiography and MR venography. However, the gold-standard for detection and characterization of a DAVF is cerebral angiography. Here, a DAVF often appears as a complex vascular abnormality made up of abnormally large and tortuous dural arteries, and a large serpentine draining vein. There may be cortical venous drainage, and a thrombosed dural venous sinus may also be seen.

For spinal DAVF, the gold-standard is selective spinal angiography. Here, a catheter is used to inject multiple spinal segmental arteries arising from the aorta, one-by-one, till the fistula is found (see below) and characterized. Spinal MRI can be used to screen a patient for the presence of a DAVF, in addition to contrast-bolus MR angiography of the spine.

7. How is a dural arteriovenous fistula (DAVF) treated?

At present, the two main ways of treating a DAVF are through open surgery or endovascular surgery. The need for treatment becomes more pressing if cortical venous drainage is seen on the cerebral angiogram or if a patient has experienced rupture or other significant neurological effects of the DAVF.

1. Open surgery: The goal of surgery is to physically disconnect the fistula in the dura, with particular attention to obliterating the draining vein. A craniotomy is required for surgical disconnection of a cranial DAVF and a laminectomy or laminotomy is required for surgical disconnection of a spinal DAVF. Open surgery for a DAVF typically has a very high success rate.
2. Endovascular surgery: This involves a catheter-based technique for "squirting" of a "glue" (or a similar particle-composite or resin) into the lumen of arteries feeding the DAVF, or directly into the vein draining the DAVF. This process is known as embolization. Sometimes, embolization is used alone to obliterate the fistula, or it may be used as a helpful additional option prior to open surgery, to help shut down as much of the fistula as possible prior to the operation.

In the ideal circumstance, the decision as to how to best treat a DAVF is made in joint consultation between the patient, a microneurosurgeon and an endovascular surgeon or interventional neuroradiologist. Note that radiation techniques including stereotactic radiosurgery (SRS) have not been proven to be helpful in the treatment of DAVF, but they have been shown to be helpful in AVM treatment ( take me to the section on AVM now).

8. Images of a brain dural arteriovenous fistula (DAVF).

Figure 2 (above collage) shows a cranial (brain) DAVF. Top left: Axial CT scan shows a ruptured DAVF (circled in red) located in the right paramedial cerebellum and located in the dura surrounding a very high-risk venous structure known as the torcula herophili. The patient presented with abrupt-onset headache and impaired consciousness. Bottom left: Preoperative cerebral angiogram (right external carotid artery injection) showing the actual fistula (circled in red) with its blood supply arising mainly from the occipital artery (OA) via many arterial channels (red arrow heads). The fistula is drained by an abnormal sac-like draining vein (DV). Top right: Intraoperative photograph showing the fistula being surgically disconnected by the placement of titanium microclips via a neurovascular clip applier (CA) across the draining vein. Bottom right: Postoperative cerebral angiogram (right external carotid artery injection) following surgical disconnection of the fistula. The angiogram shows complete obliteration of the fistula (nothing left to see in the red circled area!). The patient was discharged from hospital neurologically intact.

9. Images of a spinal dural arteriovenous fistula (DAVF).

Figure 3 (above) shows a spinal DAVF during selective spinal angiography. The catheter through which the contrast is injected is marked by the light-blue arrow heads. The fistula itself (F; red circle) is in the spinal nerve root dural sleeve. The arterial supply is marked by the red arrow and the draining vein is marked by the dark-blue arrows.

Figure 4 (above) shows the draining vein complex of a spinal DAVF immediately following open surgical disconnection of the fistula. Here, the backs of some of the adjoining spinal column bones (spinal laminae) have been unroofed to reveal the spinal dura (which has been opened surgically and is being held open by the many black tack up sutures shown). The draining vein complex, a tangle of red-purple veins, can be seen. Normally, there should be no such tangle of veins on the spinal cord surface.