Dural arteriovenous fistulae that affect the brain (BDAVF) arise from abnormal connections that develop between the blood supply tot he coverings of the cranial cavity (scalp, skull, dura) and the circulation that service the brain, two systems that communicate minimally under normal circumstances
Development of BDAVF
It is believed that DAVF are acquired rather than being present throughout life. Thrombosis, the formation of a blood clot appears a key step in their formation. As a result of thrombotic occlusion of pre-existing vascular channels in the walls of veins within the dura itself and/or chemical signalling to direct new vascular channels to appear, a new "short circuit" develops where by high-pressure blood from the arteries is diverted into the low-pressure venous system of the brain thus preventing it from functioning as it should. On careful enquiry one will often uncover a history of sinus infections, cranial trauma, surgery or hypercoagulable blood conditions.
Should the diverted blood remain confined to the veins within the lining of the brain the risk of stroke is very low although this can still produce troublesome symptoms. The panel to the left illustrates the various configurations this can take. When the blood drains into the veins on the surface of the brain, they are stressed by this additional work and may eventually give way resulting in brain haemorrhage. This pattern of drainage is referred to a cortical venous hypertension and characterised the more dangerous "high-grade" dural arteriovenous fistula.
Carotid-Cavernous Fistula
Carotid -cavernous fistulae represents a somewhat distinct disease entity from cranial dural arteriovenous fistulae with a distinct natural history although certain types are technical dural arteriovenous shunts.
The cavernous sinus is a venous space, enclosed by the bone of the skull base and through it pass the internal carotid artery as well as several cranial nerves that move the eyeball or transmit sensation from the face. There are two of these bony boxes on either side lying directly behind the medial part of the orbit.
Direct carotid-cavernous fistula develops when (usually) an aneurysm on the carotid artery ruptures into the venous blood surrounding it. The resulting hole allows arterial blood to be continuously pumped into the venous sinus and from there back into other veins draining the brain above. It may cause congestion in the orbit sufficient to threaten vision or trigger pain and double vision in the eye.
Indirect carotid-cavernous fistula more frequently complicate trauma or surgical interference in the area. A branch arising from the internal carotid artery supplying the dura lining the cavernous sinus itself is transected an bleeds into the cavernous sinus.
Symptoms and Signs.
BDAVF may remain completely asymptomatic or they can produce a vast array of neurological symptoms determined by the architecture of the particular fistula. These include but are not limited to:
- Brain haemorrhage- Most typically characterised by a sudden onset of severe headache, perhaps accompanied by nausea, vomitting, impaired consciousness or other signs of stroke.
- Double Vision- as a result of high-pressure oxygenated blood diverting through venous channel at the base of the skull or the down stream effect of insufficient oxygen reaching the brainstem. With certain types of fistula it may result from engorgement of the veins draining blood from the eye.
- Imbalance or poor co-ordination- this may result from reduced oxygen delivery to the brainstem or upper spinal cord.
- Seizures- resulting from impaired venous drainage of the hemisphere or occasionally as a direct result of haemorrhage.
- Impaired intellectual function or alertness- this may result from a global interference with brain function because of reduced function in the native venous drainage system.
- Pulsatile Tinnitus- resulting from the diversion of high-pressure oxygenated blood around the venous sinuses adjacent to the ear. Tinnitus is the perception of a noise which is not actually present.
BDAVF can change their behaviour in time either upgrading or downgrading their risk. Therefore if symptoms change the fistula should be re-evaluated. An example might be of a low-grade fistula causing tinnitus that a person chose not to treat because the disturbance to them was mild. Should that sound suddenly disappear that could reflect a change in the architecture of the fistula and it should be reassessed to ensure it has not increased its risk
What is the Risk from a DAVF?
A symptomatic high-grade fistula with cortical venous hypertension is estimated to have a risk of stroke of at least 7-8% per year meriting consideration of its treatment to prevent future stroke on the balance of risks. A low-grade fistula that is not producing symptoms however may be managed conservatively.
This example to the right shows how a draining grain has grown so dilated from years of draining high pressure blood it grew to a size which at first was mistaken for a brain tumour and then an arterial aneurysm. This patient came to medical attention after suffering a seizure. The large round structure is a massively dilated vein with blood clot within it. This partially thromboses venous aneurysm behaved as a Bain "tumour" pushing the brain away and causing the healthy brain to swell on top of the significant risk that it would bleed at some point. In a case such as that treatment is urgent.
It is usually necessary to conduct a digital subtraction angiogram to assess the architecture of a dural arteriovenous fistula and to plan a treatment.
Management of Dural Fistulae
Conservative and Non-surgical Management
Conservative management may be most approriate for an asymptomatic fistula that is not short-circuiting blood into the veins on the surface of the brain. It is recognised that occasionally such fistulae can change to a more aggressive behaviour but it may be very rare.
Symptomatic fistula too may be managed conservatively if the symptoms are tolerable to the patient. In the case of carotid-cavernous fistula self administered regimens of pressure on the jugular vein can both reduce symptoms and in time promote closure of the fistula in selected cases.
Endovascular Treatment
The goal of any DAVF treatment is to close vein forming in the dural covering that communicates with the cranial cavity. This can be achieved by delivering catheters to the vein through the blood vessels unde x-ray control. Material can then be deposited within the fistulous vein to occlude it. The arteries converging on that point will then begin to involute.
Endovascular embolisation is especially well suited to the treatment of carotid cavernous fistulae and dural arteriovenous fistulae that form in walls of venous sinuses e.g. those involving the transverse sinuses.
Microsurgical Disconnection
Microsurgical closure of a fistula can be a desirable option DAVF unrelated to venous sinuses and offer a very durable treatment with excellent safety. Fistulae arising from the base of the skull behind the nose are a good example where surgery has an excellent track-record.
In many cases there are both endovascular and surgical options each with their own strengths and drawbacks. An experienced specialist team should be able to take you through these issues and address any questions you might have before settling on the right treatment for your particular situation, taking proper account of your priorities. The process of shared decision-making applies when weighing up choices.
