Dural AV Fistula

Dural arteriovenous fistulas (DAVFs) are direct connections between meningeal arteries and a dural venous sinus or cortical vein, located within the dura. Most are acquired, and are associated with sinus thrombosis, trauma, surgery or venous hypertension. Common locations are the transverse–sigmoid sinus, the cavernous sinus, the tentorium, the anterior cranial fossa (ethmoidal), the superior sagittal sinus and the craniocervical junction.

Classification is based on venous drainage. Borden type I drains into a sinus or meningeal vein with normal (antegrade) flow; type II drains into a sinus with retrograde flow into cortical veins; type III drains directly into cortical veins^[1]. The Cognard classification further separates the direction of sinus flow and adds venous ectasia (type IV) and spinal perimedullary drainage (type V)^[2].

Benign fistulas without cortical venous drainage rarely bleed, and some close spontaneously. Fistulas with cortical venous drainage carry a substantially higher annual risk of haemorrhage and non-haemorrhagic neurological deficit, and this risk is higher still after a haemorrhage or when the patient has venous-congestive symptoms^[3]. A fistula can also convert from a benign to an aggressive pattern if the draining sinus thromboses, so benign lesions need follow-up.

Benign transverse–sigmoid fistulas cause pulsatile tinnitus and an audible bruit. Cavernous-sinus fistulas cause chemosis, proptosis, ophthalmoplegia and raised intraocular pressure. Aggressive fistulas present with intracranial haemorrhage (often subdural, subarachnoid or intraparenchymal), seizures, focal deficits or a progressive dementia from venous congestion. Craniocervical-junction fistulas may drain into the spinal veins and present with progressive myelopathy.

CT and MRI may show haemorrhage, venous congestion with oedema, or dilated pial veins; MR and CT angiography can suggest the diagnosis but are not sensitive enough to exclude it. Selective six-vessel catheter angiography, including the external carotid arteries, is required to define the arterial feeders, the exact fistulous point and, above all, the venous drainage pattern.

Fistulas with cortical venous drainage should be treated. Benign fistulas are treated for intolerable symptoms or threatened vision. The goal is to occlude the fistulous point and the very first segment of the draining vein; occluding arterial feeders alone leads to recruitment of new feeders and recurrence.

• Transarterial embolisation with liquid embolic agents is the most common first-line treatment.
• Transvenous embolisation is used when the involved sinus segment is not needed for normal brain drainage.
• Microsurgical disconnection of the arterialised draining vein at its exit from the dura is curative and is often first-line for anterior-cranial-fossa (ethmoidal) and some tentorial fistulas, where endovascular access is difficult or dangerous.
• Stereotactic radiosurgery has a role for selected benign fistulas, but obliteration is delayed.

Surgery is planned on the angiogram: the target is the single draining vein where it leaves the dura. Intraoperative indocyanine-green angiography confirms the arterialised vein and its disconnection. Ethmoidal fistulas are reached by a frontal or pterional approach along the floor of the anterior fossa; tentorial fistulas require approaches tailored to the tentorial location (supracerebellar, subtemporal, petrosal). After disconnection the vein typically turns from red to blue. A postoperative angiogram is needed to confirm cure.