Dural Arteriovenous Fistulas

An abnormal shunt within the brain’s tough outer covering — highly treatable, though some types need prompt attention.

In brief

A dAVF is a direct connection between arteries and veins within the dura
Symptoms range from pulsatile tinnitus to, rarely, bleeding
Risk depends on the pattern of venous drainage (its classification)
Diagnosed with MRI/MRA and confirmed on catheter angiography
Treatment is often endovascular, and sometimes surgical

What is a Dural Arteriovenous Fistula?

A cerebral dural arteriovenous fistula (dAVF) is an abnormal direct connection — or shunt — between arteries and veins that lies within the tough outer covering of the brain (the dura mater), rather than within the brain tissue itself. The feeding arteries are usually small branches that supply the dura, arising from the larger arteries of the neck and skull base. Instead of first passing through the fine network of tiny vessels — the capillaries within the dura, not those of the brain — that normally separates the arterial and venous systems, blood flows directly into a vein or into one of the large venous channels (the venous sinuses) that carry blood away from the brain. Unlike an arteriovenous malformation (AVM), there is no tangle of abnormal vessels within the brain itself. This abnormal flow can increase pressure within the brain’s venous system, with consequences ranging from mild symptoms to life-threatening haemorrhage. dAVFs are acquired during life rather than being present from birth (congenital), and account for approximately 10–15% of all intracranial vascular malformations. They occur most commonly in adults in the fifth to sixth decades of life. Their sex distribution varies by location: transverse–sigmoid and cavernous sinus fistulas occur more often in women, whereas tentorial and petrosal fistulas — which are more often high-grade — occur more often in men. One feature matters more than any other in determining how a dAVF behaves: whether its abnormal high-pressure flow is forced backwards into the brain’s own surface veins — the cortical veins that normally drain the brain itself. This backward flow is called cortical venous drainage (CVD). These delicate veins are not designed to withstand arterial pressure, so a fistula that drains into them carries a substantially higher risk of bleeding and progressive neurological damage, whereas a fistula without cortical venous drainage generally follows a far more benign course. This single feature underlies the pattern of symptoms, the level of risk, and the decision whether to treat, each discussed below.

What Causes a Dural Arteriovenous Fistula?

The precise cause of dAVFs is not fully understood. They are thought to develop as a result of abnormal new blood vessel formation (angiogenesis) within the dura, often triggered by an underlying event that disrupts the normal venous drainage of the brain. Recognised associated factors include:

Prior cerebral venous sinus thrombosis (clot in the brain’s draining veins) — the most consistently identified association; sinus thrombosis is thought to stimulate new vessel formation as part of a healing response

Head trauma or prior neurosurgery — scarring and disruption to venous drainage
Hormonal factors — may contribute to the female predominance seen in cavernous sinus dAVFs
Thrombophilic conditions — abnormalities in blood clotting have been found in some patients with dAVFs

In many cases, no precipitating cause is identified, and the condition is considered idiopathic.

What are the Symptoms?

The symptoms of a dAVF are highly variable and depend on its location within the skull, the pattern of venous drainage, and — most importantly — whether abnormal blood flow is redirected toward the brain’s cortical veins. Broadly, dAVFs can be divided into those that are “low risk” (without cortical venous drainage) and “high risk” (with cortical venous drainage). This distinction is the most important factor in determining how a dAVF behaves and whether treatment is required.

Low-risk dAVFs (no cortical venous drainage) typically cause:

Pulsatile tinnitus — a rhythmic whooshing or heartbeat-like sound in the ear, synchronous with the pulse; the most common presenting symptom
Orbital symptoms — redness of the eye, proptosis (forward bulging of the eye), or double vision; particularly common with cavernous sinus dAVFs
Headache

These lesions generally follow a benign course and rarely cause haemorrhage. Even so, a small proportion change over time, so it is important that patients report any change in these symptoms — for example, pulsatile tinnitus that becomes louder, alters in character, or disappears, or any new neurological symptom — to their medical team promptly, as such a change may indicate that the fistula itself has changed and requires re-assessment.

High-risk dAVFs (with cortical venous drainage) may cause any of the above, but additionally can cause:

Intracranial haemorrhage — sudden severe headache, reduced consciousness, focal weakness, or seizure; bleeding is the first presentation (at the time of diagnosis) in approximately 40–65% of the highest-grade fistulas (Cognard type III–IV) and in about one-third of Borden type III lesions — as distinct from the ongoing annual bleeding risk described below
Seizures
Progressive neurological deficits — weakness, speech disturbance, or memory problems, caused by venous congestion and impaired blood flow to brain tissue
Raised intracranial pressure and papilloedema (swelling of the optic nerves, risking vision)
Visual loss or deterioration — reduced or worsening vision (particularly with a cavernous sinus dAVF) is not a benign feature; it indicates a more aggressive lesion and warrants urgent assessment and treatment
Cognitive decline or dementia — a recognised but underappreciated presentation, caused by chronic venous hypertensive encephalopathy
Spinal cord dysfunction — rare but serious; dAVFs draining into spinal perimedullary veins can cause progressive myelopathy (leg weakness, bladder and bowel dysfunction)

How are Dural Arteriovenous Fistulas Classified?

The behaviour and risk of a dAVF are determined primarily by its pattern of venous drainage. Two classification systems are in routine clinical use, both based on this principle:

The Borden classification (Borden, Wu and Shucart, 1995) divides dAVFs into three types:¹

Type I — drainage directly into a dural venous sinus or meningeal vein, with no cortical venous involvement; generally benign
Type II — drainage into a dural sinus with retrograde reflux into both the sinus and cortical veins; intermediate to high risk
Type III — drainage directly into cortical veins only, with no sinus involvement; high risk

The Cognard classification (Cognard et al., 1995) provides a more detailed seven-category system, incorporating the direction of flow within the sinus (normal or reversed), the presence of cortical vein reflux, venous ectasia (ballooning of draining veins), and spinal drainage. Cognard types IIb through V all carry elevated risk of haemorrhage or neurological injury.²

Both classification systems are well validated and consistently show that cortical venous drainage (CVD) is the critical determinant of risk. Fistulas without CVD carry a very low annual risk of haemorrhage (less than 2%). Once CVD is present the risk rises substantially: across the largest natural-history series the annual risk of haemorrhage for such lesions is broadly of the order of 8–10%, with an annual risk of a non-haemorrhagic neurological deficit of around 7% and an annual mortality approaching 10%. This average conceals a wide range, however — the risk is lower (around 2% per year) for lesions found incidentally with few or no symptoms, and considerably higher for the most aggressive lesions, rising to around 20% per year when the draining veins are dilated (venous ectasia) and higher still in the first year after a fistula has already bled.^3-5

How are Dural Arteriovenous Fistulas Diagnosed?

Magnetic Resonance Imaging (MRI) and MR Angiography (MRA)

MRI is often the first imaging investigation performed when a dAVF is suspected. It may demonstrate enlarged or abnormally prominent cortical veins, venous ectasia, white matter signal changes (reflecting venous congestion), or in the acute setting, evidence of haemorrhage. However, MRI can be normal, particularly for low-grade dAVFs, and a normal MRI does not exclude the diagnosis. MR angiography provides a non-invasive assessment of brain vessels and may identify the fistula, though its sensitivity is lower than formal catheter angiography.⁶

CT Angiography (CTA)

CT angiography is a rapid test that can detect many dAVFs, particularly larger or higher-grade lesions with prominent draining veins. It is often performed as an urgent first investigation in the setting of acute haemorrhage. It does not provide the dynamic flow information available from catheter angiography.

Digital Subtraction Angiography (DSA)

Conventional catheter-based digital subtraction angiography remains the gold standard for diagnosing and characterising dAVFs. A six-vessel cerebral angiogram — injecting contrast sequentially into both carotid and both vertebral arteries, as well as the external carotid arteries — is required to fully define the fistula, its arterial supply, the pattern of venous drainage (including whether cortical venous reflux is present), the patency of draining sinuses, and the angioarchitecture required for treatment planning. DSA also provides real-time dynamic information about blood flow direction that no other imaging modality currently replicates. It should be performed at a centre experienced in cerebrovascular interventional neuroradiology.⁶

Should a Dural Arteriovenous Fistula be Treated?

Not all dAVFs require treatment. The decision is based on the type and grade of the fistula, its location, its symptoms, and the patient’s overall health. Low-grade dAVFs (Borden Type I / Cognard Types I and IIa) without cortical venous drainage generally follow a benign natural history, with a very low annual risk of haemorrhage. In the largest series of conservatively managed benign fistulas, the great majority — around 98% — remained stable over long-term follow-up, and only about 2% later developed cortical venous drainage, a change that mandates ongoing clinical review. For these lesions, conservative management — including clinical observation and interval imaging — is appropriate for most patients. Treatment may be offered for lesions causing intolerable symptoms such as severe pulsatile tinnitus or disabling orbital symptoms.⁷ High-grade dAVFs (Borden Types II and III / Cognard Types IIb–V) with cortical venous drainage carry a substantially elevated risk of haemorrhage and neurological injury. Treatment is strongly recommended for all such lesions, as the risk of observation clearly exceeds the risk of treatment in the hands of experienced operators.^3,4 When a high-grade fistula has presented with a haemorrhage, the risk of early re-bleeding is high, and the lesion should be treated during the same hospital admission rather than deferred. As with all cerebrovascular conditions, all patients should be discussed in a multidisciplinary setting involving a neurosurgeon and an interventional neuroradiologist.

How are Dural Arteriovenous Fistulas Treated?

The goal of treatment is complete elimination of cortical venous reflux, which removes the risk of haemorrhage and neurological injury. If complete obliteration of the fistula is not achievable in a single procedure, eliminating cortical venous drainage by reducing the fistula to a Borden Type I lesion is an acceptable intermediate goal and substantially reduces risk.⁷ Three treatment modalities are available: endovascular embolisation, microsurgical disconnection, and stereotactic radiosurgery. These may be used alone or in combination, and the choice between them depends on the fistula’s anatomy, location, and angioarchitecture. All treatment for dAVFs should be performed by a neurosurgeon or interventional neuroradiologist with specific experience in cerebrovascular conditions, at a centre with appropriate expertise.

Endovascular embolisation — First-line treatment

Endovascular embolisation is the first-line treatment for the majority of dAVFs. A small catheter is inserted into the groin and guided under X-ray to the fistula site, where a liquid embolic agent — most commonly Onyx (ethylene vinyl alcohol copolymer) — is injected to seal the abnormal connection. Both transarterial (via the feeding arteries) and transvenous (via the draining veins) approaches may be used, individually or in combination. In a large endovascular series of 260 dAVFs (Gross et al., Journal of Neurosurgery, 2017), the overall initial angiographic occlusion rate was approximately 70%, with favourable outcomes in the large majority of treated patients. Transvenous embolisation with coils targeting the primary draining sinus has shown particularly high rates of complete occlusion for cavernous sinus and transverse-sigmoid dAVFs. Recurrence after angiographically confirmed complete occlusion is uncommon (approximately 3–13% depending on series), though long-term imaging follow-up is recommended given the possibility of late recurrence.⁸ Potential complications of endovascular treatment include ischaemic stroke, haemorrhage from vascular perforation, cranial nerve injury, and access-site complications. Serious permanent complication rates are low in experienced hands.

Microsurgical disconnection

Open surgery is reserved for dAVFs that are not amenable to complete endovascular treatment, or where surgical access to the fistula and draining vein is straightforward. The goal of surgery is direct disconnection of the cortical venous outflow from the fistula, eliminating the risk of haemorrhage. This is particularly valuable for anterior cranial fossa dAVFs (ethmoidal location): here the feeding vessels arise from the ophthalmic artery, so endovascular embolisation places the ophthalmic artery — and therefore the vision — at risk, whereas surgical disconnection through a frontal craniotomy is reliable and carries no such risk to sight. When cortical venous reflux is successfully disconnected surgically, cure rates are high. Surgery carries general operative risks including bleeding, infection, stroke, and risks specific to the location of the fistula.⁶ Surgery and endovascular treatment are also sometimes used in combination: embolisation can substantially reduce the arterial inflow but may not fully control or completely disconnect the fistula, and microsurgery can then divide the remaining venous connection — an approach particularly useful for fistulas in the posterior fossa, such as those around the petrous bone.

Stereotactic radiosurgery

Stereotactic radiosurgery (Gamma Knife or CyberKnife) delivers focused radiation to the fistula site, inducing gradual thrombosis and closure of the abnormal vessels. It is not a first-line treatment for high-grade dAVFs with cortical venous reflux, as there is a latency period of approximately 2–3 years before obliteration occurs, during which haemorrhage risk persists. A multicentre international study of 147 dAVF patients treated with Gamma Knife radiosurgery reported an annual post-radiosurgery haemorrhage risk of 0.84% during the latency period, with no haemorrhage after confirmed obliteration. Radiosurgery is most appropriate for lower-grade dAVFs causing troublesome symptoms, or as adjunctive treatment for residual high-grade dAVFs after embolisation or surgery has eliminated cortical venous reflux.⁹

What Happens After Treatment?

Following successful treatment — whether endovascular, surgical, or radiosurgical — the risk of future haemorrhage is very low, approaching zero after confirmed complete obliteration of the fistula by follow-up angiography. Symptoms attributable to venous hypertension (including pulsatile tinnitus, orbital symptoms, cognitive difficulties, and neurological deficits) typically improve or resolve after successful treatment, though recovery of established neurological deficits may be gradual. Imaging follow-up is required after all forms of treatment, and the schedule depends on how the fistula was treated. In surgical cases, a catheter angiogram (DSA) is performed immediately after the operation, often under the same anaesthetic, to confirm that the fistula has been completely disconnected. For both surgical and endovascular cases, a further DSA is then recommended at 6–12 months to confirm a durable cure and to exclude any late recurrence. If the fistula is confirmed to be obliterated at this 6–12 month angiogram, most patients can be discharged from routine follow-up and simply advised to return promptly should any symptoms recur; some centres additionally recommend a non-invasive scan (such as an MRA) approximately every five years thereafter.⁸ Fistulas treated with stereotactic radiosurgery — an uncommon situation — are instead monitored with non-invasive imaging such as MRI during the latency period, and once the fistula appears obliterated this is confirmed with a catheter angiogram (DSA).

References

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Dural Arteriovenous Fistulas — patient information sheet PDF · prepared by Mr Jeremy Russell

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Content on this page is for education and information only and is not a substitute for advice from your doctor. Medically reviewed by Mr Jeremy Russell.