Dural arteriovenous fistula

Dural arteriovenous fistulas (dAVF) are a heterogeneous collection of conditions that share arteriovenous shunts from dural vessels. They present variably with haemorrhage or venous hypertension and can be challenging to treat.

Epidemiology

Most dural arteriovenous fistulas present in adulthood and account for 10-15% of all cerebral vascular malformations ⁶.

Clinical presentation

Clinical presentation is highly variable and depends on the location of the supplying and draining vessels, as well as the presence of complications (see below). Presentations include ⁴:

• pulsatile tinnitus
• cranial nerve palsies
• seizures
• orbital symptoms (see caroticocavernous fistula)
• symptoms of venous hypertension
  • raised intracranial pressure
  • focal neurological deficits

Complications

The likelihood depends on venous drainage (reflected in both the Cognard and Borden classification systems), and not arterial supply.

• haemorrhage
  • subdural haemorrhage
  • intracerebral haemorrhage
  • subarachnoid haemorrhage
• venous congestion/hypertension and oedema
  • intracranial hypertension
  • ​spinal myelomalacia

Pathology

Dural arteriovenous fistulas are usually acquired and in most instances are idiopathic ⁶. In patients with a documented antecedent cause, most occur as the result of neovascularisation induced by a previously thrombosed dural venous sinus (typically the transverse sinus). Other causes include trauma and previous craniotomy. It is likely that at least some patients with apparently idiopathic fistulae had prior asymptomatic thrombosis, particularly as inherited prothrombotic conditions (e.g. antithrombin, protein C, and protein S deficiencies) have been associated with development of dAVFs ⁶.

Typically, they are supplied by multiple feeders from arteries that supply the relevant part of the meninges and regional scalp vessels that often give transosseous branches:

• supratentorial and lateral: (external carotid artery)
  • middle meningeal artery
  • superficial temporal artery (transosseous branches)
• anterior cranial fossa: (internal carotid artery)
  • ethmoidal branches of the ophthalmic artery
• cavernous sinus: (internal and external carotid arteries)
  • meningohypophyseal trunk and inferolateral trunk
  • accessory meningeal artery
• posterior cranial fossa: (vertebral and external carotid arteries)
  • vertebral arteries (both dural and muscular branches)
  • occipital and ascending pharyngeal arteries

Location

• transverse/sigmoid sinus
  • most common
  • least likely to have retrograde leptomeningeal venous drainage
• cavernous sinus (indirect caroticocavernous fistula)
• superior sagittal sinus
• straight sinus
• other venous sinuses
• anterior cranial fossa
  • typically only ICA supply due to meningeal supply of this region
  • frequently associated with retrograde leptomeningeal venous drainage
• tentorium: frequently associated with retrograde leptomeningeal venous drainage

Association

• intracranial aneurysms ⁷

Radiographic features

CT

Diagnosis can be difficult on non-contrast CT but should be thought of when an intracranial haemorrhage is in an unusual location or age group. With contrast and particularly CT angiography a diagnosis can often be made if care is taken. Findings that may be evident:

• abnormally enlarged and tortuous vessels in the subarachnoid space, corresponding to dilated cortical vein
• an enlarged external carotid artery or enlarged transosseous vessels
• abnormal dural venous sinuses including arterialisation of contrast phase in the affected sinus due to arteriovenous shunting

MRI

On routine non-vascular imaging, the diagnosis can be difficult in patients without retrograde leptomeningeal venous drainage although sometimes enlarged arterial feeders or dilated pial vessels in the subarachnoid space, best seen on T2 weighted imaging can be a clue to the diagnosis. 

MR angiography is, however, fairly sensitive and specific with both time of flight angiography and time-resolved angiography able to imply or directly visualize arteriovenous shunting into dural sinuses or cortical veins as well delineating enlarged feeding arteries ^8-9,9. 

When interpreting time of flight angiography, care must be taken, however, to avoid misinterpreting superiorly flowing venous blood as arterialized flow. This is particularly an issue in the left jugular bulb where retrograde (cranial) venous flow can ascend into the sigmoid sinus and inferior petrosal sinus mimicking arteriovenous shunting ¹⁰. 

In patients with retrograde leptomeningeal venous drainage, oedema is present in approximately half of the patients, although it may also be seen in patients who do not have retrograde leptomeningeal drainage on angiography ³. These regions of white matter oedema may also enhance and are indicative of an aggressive fistula with a high rate of haemorrhage ³.

Angiography (DSA)

Catheter digital subtraction angiography demonstrates arteriovenous shunting most typically with multiple feeders and with no intervening nidus. DSA remains the gold standard in both diagnosis and accurate classification of dAVF (see below), allowing not only systematic evaluation of feeding vessels (and thus planning for potential intervention) but also demonstrating the presence and extent of retrograde leptomeningeal venous drainage. Due to often extensive supply, a six vessel selective angiogram is required (bilateral ICA, ECA and vertebral artery injections).

Classification

Two classifications are most commonly used ⁶:

1. Cognard classification
2. Borden classification

Both classifications revolve around the knowledge that venous drainage pattern correlates with increasingly aggressive neurological clinical course. The single most important feature for prognosis is the presence of retrograde leptomeningeal venous drainage (ie, into the cortical veins or deep cerebral veins). The Borden classification is a simplified version of the Cognard system but loses granularity ^5,6.

Treatment and prognosis

Treatment largely depends on the classification of the fistula and the age and comorbidities of the patient, as well as the presence of symptoms directly attributable to the fistula.

• conservative (especially Borden type I and Cognard types I and IIa)
• higher grades (Borden types II and III, Cognard types IIb-V) have an annual mortality rate of ~10% and an annual risk of intracranial haemorrhage of ~8% ⁶, so treatment should be considered:
  • endovascular
  • surgical resection
  • stereotaxicstereotactic radiosurgery

​See also

• pial arteriovenous fistula (pAVF)
• vein of Galen aneurysmal malformations (VGAM)
• spinal dural arteriovenous fistulas