General Description

Intracranial dural arteriovenous fistulas (dAVFs) are rare pathological anastomoses between meningeal arteries and dural venous sinuses or dural veins. The fistula is located within the dural leaflets and consists of a direct communication between an artery and a venous sinus or dural veins. dAVFs account for 10%–15% of all intracranial vascular malformations and most frequently affect patients in their fifth to sixth decade of life without a clear sex predilection or genetic susceptibility. Most dAVFs are located at the junction of the transverse and sigmoid sinuses, followed by the cavernous sinus (with a benign natural history), superior sagittal sinus, anterior cranial fossa, tentorium, and other locations. The pathological mechanisms underlying dAVF formation are still debated. The majority of dAVFs are of an acquired nature, from traumatic head injury, infection, previous craniotomy, tumors, or dural venous sinus thrombosis. Also, risk factors for venous thrombosis, such as antithrombin, protein C, and protein S deficiencies, have been associated with dAVFs occurrence.

Symptoms are usually related to increased blood flow through a venous sinus or sinuses and are linked to shunt location and venous drainage patterns. Pulsatile tinnitus is a common symptom for transverse and sigmoid sinus lesions, whereas cavernous sinus dAVFs present with ocular symptoms. Severe presentations include intracranial hemorrhage and nonhemorrhagic neurologic deficits, such as seizures, parkinsonism, cerebellar symptoms, apathy, failure to thrive, and cranial nerve abnormalities. Hemorrhagic presentations are more frequent in high-grade dAVFs. Radiologic evaluation includes computed tomography angiography and magnetic resonance imaging. However, all cases must be confirmed with conventional catheter-based angiography, which is the gold standard for detection and classification of dAVFs.

Indications

Historically, endovascular management was palliative because feeding artery occlusion was followed by recruitment of additional arterial blood supply. Curative embolization can be achieved only when the microcatheter is positioned close enough to the nidus so that the embolic material occludes the fistula as well as the draining vein; this can be achieved transarterially or transvenous route. Liquid embolic agents such as Onyx (Medtronic) and N-butyl-2-cyanoacrylate (n-BCA) have emerged as a safe and effective technique for the management of dAVFs. In general, symptomatic grade I and all grade II-V dAVFs should be considered for treatment.

Neuroendovascular Anatomy

dAVFs are distinguished from parenchymal or pial arteriovenous malformations by the presence of a dural arterial supply and the absence of an interposed parenchymal nidus. The most commonly used classification for dAVFs is the Borden classification. It distinguishes dAVFs based on venous drainage characteristics, especially the absence of cortical venous drainage (CVD). Borden type I fistulas have dural arteries that drain exclusively into a dural sinus with antegrade venous flow. Type II fistulas drain into a dural sinus with venous flow that is both antegrade into the dural sinus and retrograde into cortical veins. Type III fistulas drain exclusively into cortical veins in a retrograde fashion. Another commonly used classification, the Cognard score, is based on shunt location, venous drainage characteristics, and venous outflow angioarchitecture. In this classification, type I lesions drain antegrade into a dural sinus, and type II lesions drain retrograde into a venous sinus, both without CVD. Type IIb fistulas drain antegrade into a venous sinus and have venous reflux into cortical veins, whereas type IIa fistulas drain retrograde into a dural sinus and have CVD. Type III dAVFs drain directly into cortical veins, and type IV fistulas drain directly into cortical veins and display venous ectasia. Type V fistulas drain exclusively into the spinal perimedullary veins.

Borden types II and III fistulas as well as Cognard types IIb through V fistulas are considered high grade and have an aggressive natural history.

Endovascular Approaches

Endovascular therapy has become the first line of treatment for most intracranial dAVFs. The aim of treatment is complete obliteration of the arteriovenous shunt. Endovascular treatment can be divided into three main categories: transarterial embolization (TAE), transvenous embolization (TVE), and combined approaches. TAE involves superselective catheterization of arterial feeders. The microcatheter tip should be “wedged” in the feeding artery, and the embolic agent should penetrate the fistulous connection and proximal aspect of the venous recipient. TAE is suggested when a dAVF drains into a parallel channel within a patent dural sinus that is compartmentalized and when the dAVF drains into a venous pouch immediately adjacent to a major patent dural sinus that is not compartmentalized. TVE is performed by retrograde catheterization of the involved dural sinus or cortical vein followed by deposition of coils and/or liquid embolic agents adjacent to the shunt. The aim of endovascular treatment is occlusion of the arteriovenous fistula and/or disconnection of leptomeningeal or cortical reflux with preservation of normal venous drainage.

Endovascular Agents

Available embolic agents include particles, coils, ethanol, n-BCA, and Onyx. Particles should generally be avoided because they often lead to incomplete and impermanent fistula obliteration. Coils can be used as an adjunct to liquid embolic agents in high-flow dAVFs to reduce high flow and prevent Onyx emboli distal into nondesired veins or sinus. Coils are not curative when used alone, whereas n-BCA can promote progressive occlusion of residual flow seen on immediate posttreatment angiography with excellent cure rates. A major advantage of Onyx is its ability to cure complex fistulas through a single pedicle.

Periprocedural Medications

Full heparinization during embolization is essential to prevent thromboembolic complications. The procedure can be performed under conscious sedation but general anesthesia may be necessary if patients do not hold still or experience discomfort or pain during the liquid embolic injection.

Specific Technique and Key Steps

1. 
   

   After the femoral angiogram has been performed to confirm the absence of any irregularity or dissection, a guide catheter is placed over a curved wire and a diagnostic catheter (0.035-inch angled Glidewire, Terumo), and the catheter system is advanced into the aorta under fluoroscopic guidance.

   
   
2. 
   

   The guide catheter should be placed in the extracranial (external carotid artery [ECA], common carotid artery or vertebral artery) vessel of choice utilizing roadmap navigation ( Fig. 33.1–33.4, Video 33.1–33.4 ).

   
   
3. 
   

   The use of an intermediate catheter is recommended in cases of moderate to severe ECA tortuosity. The intermediate catheter (Sofia catheter, MicroVention or distal access catheter [DAC], Stryker) is connected to a heparinized flush and introduced through the guide catheter ( Video 33.1–33.4 ).

   
   
4. 
   

   A dimethylsulfoxide (DMSO)-compatible microcatheter (Headway DUO, MicroVention; Excelsior XL-10, Stryker; or Apollo detachable tip, Medtronic) is connected to the heparinized flush and introduced through the intermediate or guide catheter.

   
   
5. 
   

   Optimum working views of the fistula are identified on magnified anteroposterior and lateral fluoroscopy.

   
   
6. 
   

   The microwire and microcatheter are navigated as close as possible to the fistula nidus. For a transverse or sigmoid dAVF, the initial arterial route to approach the fistula nidus is the middle meningeal artery or one of its branches ( Fig. 33.2, 33.3, Video 33.2, 33.3 ).

   
   
7. 
   

   Superselective angiography via the microcatheter is performed to confirm microcatheter location.

   
   
8. 
   

   The Onyx 18 or 34 formulation is typically used. The embolic agent is drawn up in syringes in aliquots of 1 mL. For more distal penetration, Onyx 18 is utilized because it is less viscous. DMSO is also drawn up in a DMSO-compatible syringe.

   
   
9. 
   

   Before the Onyx injection, the microcatheter is purged with DMSO. DMSO can be caustic and therefore must be infused at a rate of 0.1 mL/min. Thus, microcatheters typically have 0.3 mL dead space, so the first 0.2 mL of DMSO can be pushed into the catheter in a controlled fashion followed by slow pushing of the last 0.1 mL.

   
   
10. 
    

    The Onyx is then connected to the microcatheter in a meniscus-to-meniscus fashion (Tuohy valve removed). A timer is set. Onyx is slowly injected at a rate of 0.1 mL/min for 3 min with the last minute of the injection observed under subtracted working views until the tantalum component of the Onyx is visible (remember, this is pushing the dead space out of the catheter).

    
    
11. 
    

    The first embolization attempt into the fistula nidus is usually the most productive and successful. Carefully watch for nidal penetration, venous or arterial filling, or arterial reflux. An extracranial Onyx injection can tolerate more arterial reflux than intracranial injections. Filling of the venous pedicle is the goal; however, if the sinus is patent, Onyx injection of the sinus is not desired ( Video 33.1–33.4 ).

    
    
12. 
    

    Once satisfied with the nidal embolization, remove the microcatheter and perform another angiogram run to ensure no unwanted events and examine the awake patient.

    
    
13. 
    

    The above steps can be performed for multiple pedicles. A staged approach with one or two pedicles embolized at a time is often safer than a more extensive embolization during a single session. The microcatheter must be disposed of after each Onyx injection.

    
    
14. 
    

    In a similar fashion, a TVE is performed. A guide catheter is placed at the sigmoid sinus. An intermediate catheter is navigated through the transverse sinus, straight sinus, or superior sagittal sinus. From there, a microcatheter is navigated through the draining vein in to the fistula. DMSO and Onyx are injected in a similar fashion.

    
    
15. 
    

    TVE tolerates less embolic agent reflux, and the interventionist should be aware of this ( Fig. 33.4, Video 33.4 ).

    
    
16. 
    

    Once the stopping point has been reached, a final angiogram should be performed and all catheters removed.

Device Selection

The following are common setups and devices used for Onyx AVM embolization.

• 
  

  6–8 French (F) sheath.

  
  
• 
  

  90–100 cm long 6F guide catheter.

  
  
• 
  

  0.044-0.058 intermediate catheter (Sofia, Microvention; DAC or Navien, Medtronic; Catalyst 5, Stryker).

  
  
• 
  

  0.035-inch angled Glidewire.

  
  
• 
  

  DMSO-compatible 0.016-inch microcatheter (e.g., Headway DUO, MicroVention; SL-10, Stryker) or detachable tip microcatheter (Apollo catheter, Medtronic).

  
  
• 
  

  0.014-inch microwire (Synchro 2, Stryker).

  
  
• 
  

  DMSO.

  
  
• 
  

  Onyx 18 or 34.

  
  
• 
  

  Intracranial balloons (Scepter, MicroVention) can be utilized if injecting near a vital vessel.

  
  
• 
  

  Continuous heparinized flush.

Pearls

• 
  

  Appropriate patient selection and a complete understanding of the anatomy and physiology of the fistula and the morbidity profile of each treatment strategy are keys for successful dAVF treatment.

  
  
• 
  

  Extracranial vessel microcatheter entrapment carries less risk of vessel rupture than intracranial microcatheter entrapment but nevertheless requires slow continuous and firm tension to remove the microcatheter.

  
  
• 
  

  Recognize the vessel anatomy that predisposes catheter entrapment. In general, the smaller the feeding vessel, the more tortuous the approach; the longer the segment of Onyx reflux, the more difficult it will be to remove the microcatheter.

  
  
• 
  

  We recommend utilizing an intermediate catheter for distal vessel navigation and support because of the risk of the microcatheter becoming wedged in the Onyx. The intermediate catheter can allow countertraction when removing the microcatheter ( Video 33.1–33.4 ).

  
  
• 
  

  Venous access can be utilized for dAVF access; however, care should be taken not to disrupt the venous pedicle before the fistula is completely obliterated ( Fig. 33.4, Video 33.4 ).

  
  
• 
  

  A small amount of liquid embolization into the normal venous sinus is generally tolerated.

  
  
• 
  

  DMSO can result in vessel injury if it is injected too quickly, so inject it slowly.

  
  
• 
  

  Do not lose track of how much Onyx has refluxed along the catheter and keep the catheter in the Onyx for less than 35 minutes. Both techniques will help decrease the incidence of trapped microcatheters.

  
  
• 
  

  Do not pause the injection for more than 2 minutes, as this can cause the Onyx to solidify in the catheter.

  
  
• 
  

  Neurological injury can occur if embolizate inadvertently occludes normal draining veins or if collaterals between the ECA and internal carotid artery or vertebral arteries are unrecognized.

  
  
• 
  

  Avoid using the ascending pharyngeal artery as a conduit to access a dAVF. Embolization of branches of this artery could cause lower cranial nerve neuropathies.

  
  
• 
  

  Avoid using the petrosal branch of the middle meningeal artery to access a dAVF. Partial or complete occlusion of this branch could cause facial nerve palsy.