Brain dural arteriovenous fistulae (dAVFs) are characterised by abnormal anastomoses between meningeal arteries and dural venous sinuses or cortical veins. Their dural arterial supply and lack of a parenchymal nidus differentiates brain dAVFs from brain arteriovenous malformations. Brain dAVFs are uncommon lesions, composing only 10-15% of all intracranial vascular malformations. Most frequently, brain dAVFs affect patients in their middle-to-later years of life, though have been known to affect children. Neither sex nor family history has been shown to be a reliable risk factor.
While the majority of dAVFs arise idiopathically, a smaller percentage occur after from head trauma, infection, previous craniotomy, tumours, or dural venous sinus thromboses. As venous sinus pressure increases, meningeal arteries develop fistulous connections with the dural sinus or cortical veins, possibly via angiogenesis or via enlargement of preexisting physiological shunts, ultimately giving rise to a complex venous network under arterial pressure. With worsening venous hypertension, normal antegrade venous flow is reversed, resulting in retrograde flow through cortical veins (cortical venous drainage) that can produce venous hypertension within the surrounding brain. This may progress to recruitment of further external carotid artery feeders result in the formation of a dAVF.
Complications of brain dAVFs are capable of producing intracranial haemorrhage or non-haemorrhagic neurological deficits, which can lead to substantial neurological morbidity and mortality. Intracranial haemorrhage is thought to occur from rupture of fragile arterialized parenchymal veins due to continuous venous reflux and hypertension. The precise pathophysiology of non-haemorrhagic neurological deficits is not completely understood but is hypothesised to be due to inadequate arterial delivery of oxygen and impaired removal of metabolic waste products to vital tissues, producing functional impairment of the involved cerebral tissue. Cortical venous drainage is thus an imaging marker indicating increased risk of intracranial haemorrhage or non-haemorrhagic neurological deficits.
Commonly used brain dAVF classifications are the Borden-Shucart and Cognard schemes, based on shunt location and the presence of cortical venous drainage and hypertension.
Borden-Schucart Classification of Brain dAVFs
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Type
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Venous Drainage Site
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Cortical Venous Drainage (Y/N)
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“Benign”
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I
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Dural Sinus
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No
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“Aggressive”
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II
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Dural Sinus
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Yes
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III
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Cortical Vein
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Yes
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Cognard Classification of Brain dAVFs
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Type
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Venous Drainage Site
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Sinus Flow Pattern
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Cortical Venous Drainage (Y/N)
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“Benign”
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I
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Dural Sinus
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Antegrade
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No
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IIa
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Dural Sinus
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Retrograde
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No
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“Aggressive”
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IIb
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Dural Sinus
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Antegrade
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Yes
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IIa + IIb
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Dural Sinus
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Retrograde
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Yes
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III
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Cortical Vein
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Yes
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IV
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Cortical Vein
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Yes (+ venous ectasia)
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V
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Cortical Vein (with spinal perimedullary drainage)
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Yes
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Management of dAVFs is currently guided by symptoms and presence of aggresive features/ dAVF grading, as well as patient preference. dAVF without high-risk features can be managed conservatively with a low rate of serious complications, but should undergo clinical and imaging follow-up given the risk of conversion to an aggressive lesion. Patients with aggressive lesions or severe clinical presentations warrant urgent treatment. Treatment options include stereotactic, endovascular, and surgical treatments. Given the recent achievements in endovascular treatment for dAVFs, surgical repair is often reserved for lesions that fail or are not amenable to endovascular therapy. Stereotactic radiosurgery is also an acceptable alternative though inherently results in a delayed closure of the fistula given the nature of treatment, which may pose unacceptable risks to particularly aggressive lesions.
Endovascular embolisation of dAVFs is achieved using particulates, microcoils, or liquid embolic agents. Transarterial, transvenous, or dual access may be employed to localise and treat the lesion. The goal of endovascular therapy is closure of the fistulisation between feeding arteries and draining veins.
Current outcomes of liquid embolic agents (Onyx) for treatment of dAVFs have been favourable, with several small studies reporting closure rates of 79-85%, with only one study reporting significant morbidity from intervention. While generally well tolerated and safe, embolisation inherently carries the risk of occlusion in adjacent structures with the potential for causing blindness, stroke, or intracranial haemorrhage. Embolisation may also occlude the feeding artery but subsequently cause recruitment of adjacent arterial feeders that may necessitate further treatment. Despite this, endovascular embolisation is currently the preferred treatment for brain dAVFs.