Fistula, Dural Arteriovenous

Article Author:
Stephanie Zyck
Article Editor:
Grahame Gould
Updated:
10/27/2018 12:31:33 PM
PubMed Link:
Fistula, Dural Arteriovenous

Introduction

Dural arteriovenous fistulas (dAVF) are vascular abnormalities in which arteries arising from branches of the carotid or vertebral arteries drain directly into the dural leaflets of the venous sinuses. They are sometimes referred to as dural arterio-venous malformations. Dural arteriovenous fistulas are more commonly supratentorial than infratentorial in location. The transverse-sigmoid junction is the most common location for dAVFs, with a slight left-sided predominance. They can also be found at tentorial, petrosal, ethmoidal, sylvian, cavernous sinus, spinal dura, and superior sagittal sinus locations. When present at the cavernous sinus, they are referred to as carotid-cavernous fistulas.[1][2]

The clinical behavior of dAVFs, including a risk of intracranial hypertension and hemorrhage, predominantly depends on the venous drainage patterns. Cortical venous drainage predisposes to a more aggressive clinical course. In one meta-analysis, tentorial location was found to be associated with most aggressive behavior followed by sylvian/middle fossa and ethmoidal/anterior fossa locations.[3] The Borden and Cognard classifications are the most well-known classification systems for predicting the aggressiveness of dAVFs.[4][2]

The Borden classification system describes lesions as the following types, based on the direction of flow and presence of cortical venous drainage[4]:

  • Type I dAVFs have anterograde flow into a dural venous sinus or meningeal vein and usually have a benign natural history.
  • Type II dAVFs have anterograde flow into a dural venous sinus, however, also have retrograde cortical venous reflux. These are considered high-grade lesions.
  • Type III dAVFs, also high-grade lesions, have direct retrograde flow from the fistula into cortical veins, thereby causing venous hypertension. Further subclassification with an "a" denoting a single fistula or a "b" denoting multiple fistulas is also used.

In Cognard classification, dAVFs are divided into the following seven categories[5]:

  • Type I lesions exhibit normal antegrade flow into a dural venous sinus and have a benign clinical course. 
  • Type IIa lesions have drainage into a venous sinus with retrograde flow within the sinus. Of these type IIa lesions, 20% have intracranial hypertension. 
  • Type IIb dAVFs have drainage into a venous sinus with retrograde flow into cortical veins. This group has a 10% risk of hemorrhage induced by venous reflux.
  • Dural arteriovenous fistulas with drainage into a venous sinus with retrograde flow both within the sinus and into cortical veins are classified as Cognard type IIa+b lesions. These have an aggressive natural history with a 66% risk of hemorrhage with or without intracranial hypertension. 
  • Type III dAVFs have direct drainage into a cortical vein without venous ectasia. These have a 40% risk of hemorrhage. 
  • Type IV dAVFs have direct drainage into a cortical vein with venous ectasia and are associated with a hemorrhage risk of 65%. 
  • Lastly, type V dAVFs have direct drainage into spinal perimedullary veins and present with progressive myelopathy in 50% of cases.

Etiology

Due to a few cases occurring in the pediatric population, these were traditionally thought to be congenital; however, there is evidence for acquired lesions from causes such as thrombosis, trauma, infection, and prior craniotomy.[6]

Epidemiology

One population study in Minnesota over a 27-year period reported a detection rate of 0.15 per 100,000 persons per year.[7] Another Japanese study showed a detection rate of 0.29 per 100,000 persons per year.[8] Dural arteriovenous fistulas can present at any age but are most commonly diagnosed between the ages of 40 and 50.[2]

Pathophysiology

Some dAVFs remain asymptomatic or even involute spontaneously[9]; however, some lesions present with symptoms such as headache, seizures, cranial neuropathies, pulsatile tinnitus, or bruits.[2][9] Venus hypertension from cortical venous drainage can also cause intracranial hypertension, papilledema, glaucoma, hydrocephalus, and intracerebral hemorrhage.[2][9]

Histopathology

A histopathological study performed by Hamada and colleagues found the main abnormality in dural arteriovenous fistulas to be a connection between the dural arteries and veins within the venous sinus wall via small vessels that are approximately 30 micrometers on average.[10]

History and Physical

History and physical exam should include a thorough neurological examination as well as an evaluation of the overall medical status for surgical risk stratification.

Evaluation

Imaging studies in which dural arteriovenous malformations may be identified via computed tomography (CT) angiography or magnetic resonance (MR) angiography, demonstrating dilated vessels corresponding to abnormal arteries and veins, sinus enlargement, or sinus occlusion. In addition, MR angiography may reveal associated early prominent sinus filling, dilated pial vessels and associated edema from venous hypertension seen as T2-weighted hyperintensity. Hydrocephalus can also develop if there is hypertension in the venous sinuses.[9]

Ultimately, a six-vessel cerebral angiogram is required to establish a diagnosis and plan treatment definitively.

See "Introduction" for a description of the Borden and Cognard classifications.

Treatment / Management

The decision of whether to treat dural arteriovenous fistulas is based on the patient's symptoms, medical comorbidities, and risk intracranial hypertension or hemorrhage. Lesions are asymptomatic, low-grade, tend to have a benign natural history, and are generally managed conservatively with serial monitoring.[9] Dural arteriovenous fistulas that are high-grade with cortical venous drainage or symptomatic are often candidates for intervention.

Open surgery, endovascular embolization, and stereotactic radiosurgery are the main options for intervention. The goal of treatment is to achieve complete disconnection of the fistula from its venous drainage. Incomplete disconnection may allow for recruitment of other arteries to the fistula and may not improve the risk of hemorrhage or symptoms.[11] Endovascular surgery using embolic material or coil occlusion can involve transvenous, trans-arterial, direct access, or a combination of techniques.[11] Stereotactic radiosurgery achieves excellent rates of obliteration for low-grade lesions but is less effective for higher grade lesions. Therefore, while surgery or embolization remains the first line treatment for high-grade lesions, radiosurgery is most often utilized for low-grade (Borden type I) dAVFs with persistent symptoms such as pulsatile tinnitus. It can also be used for dAVFs with unfavorable anatomy for other interventions, patients with significant medical comorbidities, or as salvage therapy for lesions incompletely treated with surgery or endovascular embolization.[12][11]

Differential Diagnosis

Differential diagnosis of cerebral vascular malformations includes arteriovenous malformations, cavernous malformations, capillary telangiectasias, and venous angiomas.

Consultations

Neurosurgical consultation is required for dural arteriovenous fistulas.

Enhancing Healthcare Team Outcomes

Dural arteriovenous fistulas are not common, and hence there are no universal guidelines for clinicians to follow. The clinical signs of the condition are heterogeneous, and a high index of suspicion is necessary for diagnosis. An integrated approach with a team of healthcare professionals is recommended to make an early diagnosis and appropriate referral. The neurologist, neurosurgeon, interventional radiologist, and critical care physician can coordinate the treatment care plan and monitoring. Neurosurgical consultation is required for management of dural arteriovenous fistulas.

Post-treatment, the nurse is vital for monitoring the neurological vitals, ambulation, and feeding. The nurse also plays a role in educating the patient and family about the disease and the potential adverse effects of treatment. The pharmacist ensures that the patient remains compliant with medications and controls his or her blood pressure.

Outcomes

Once diagnosed, the treatment is either surgery or an endovascular approach. Six-vessel catheter angiogram is the gold standard for diagnosing dural arteriovenous fistulas and for treatment planning. (Level V). One retrospective study found computed tomography (CT) angiography to have a sensitivity of only 15.4% and magnetic resonance (MR) angiography to have a sensitivity of only 50%.[13] (Level IV).

The eventual outcome depends on the degree of neurological deficit and symptoms. In most patients, the pulsatile tinnitus can be cured by closing the fistula. Both the seizures and visual problems can be improved with therapy. Stroke cannot be cured, but any new neurological deficit can be prevented. Ultimately the prognosis is based on the type of treatment. Both surgery and the endovascular approach do have their own complications which can add to the morbidity.