General Description

Arteriovenous malformations (AVMs) represent an abnormal connection between arterial feeders and veins with an intervening nidus. The treatment of certain AVMs can be challenging. Numerous grading scales have been developed to help the interventionist select which lesions should undergo intervention. The classification most frequently used is the Spetzler–Martin (SM) with supplemental criteria. The postoperative risk of neurological deficit ranges from 8% for SM grades 1–2 lesions and 18% for SM grade 3 lesions to 32% for SM grade 4 lesions. Patients with AVMs can present with headaches, seizures, hemorrhage, or incidentally. The lesions carry a 3%–5% estimated yearly risk of hemorrhage. Deep locations, deep venous drainage only, high-flow lesions, and venous stenosis are among the factors that can increase the risk of rupture. Intranidal and prenidal flow-related aneurysms, especially in the posterior fossa, are prone to rupture and should always be treated. Lifetime hemorrhage risk is often calculated by way of the Ondra et al. ¹ equation of 105 minus the patient’s age.

The use of liquid embolic Onyx (Medtronic) has revolutionized the treatment of AVMs. Onyx is an ethylene vinyl copolymer requiring a dimethylsulfoxide (DMSO) carrier for delivery. It is a nonadhesive polymer that is different than N-butyl-2-cyanoacrylate (n-BCA; glue), which was previously utilized for embolization. Ready-to-use vials of Onyx are available in 6% and 8% concentrations named Onyx 18 and Onyx 34, respectively. (Onyx 500 is no longer available.) Initial studies utilizing Onyx for embolization of AVMs date back to 2001. Since that time, numerous studies have supported the use of Onyx for the occlusion of small AVMs or staged embolization of larger AVMs before surgery or radiation therapy. Onyx embolization can reduce the overall AVM size and the amount of blood lost at the time of surgery. Pedicles not easily accessed at the time of surgery can be embolized with Onyx. Furthermore, because we perform AVM Onyx embolizations in an awake patient, awake superselective intra-arterial Wada testing can be performed with Amytal (amobarbital sodium) and lidocaine prior to pedicle embolization.

Indications

The role of Onyx embolization depends on the treatment plan. Embolization can be used as curative therapy for small, nonsurgically accessible AVMs; preoperatively as a precursor to complete curative surgical resection; for targeted therapy to obliterate a source of hemorrhage; as a precursor to radiotherapy; and for palliative embolization to relieve symptoms of high-flow shunting. Onyx embolization can also be utilized for occluding nidal and prenidal aneurysms and to reduce surgical morbidity in patients with coagulation disorders. Studies of Onyx embolization have documented complete obliteration rates in up to 50% of small AVMs and an average 75% reduction in AVM size. ²

Neuroendovascular Anatomy

The angioarchitecture and surrounding anatomy of an AVM have to be studied and understood perfectly before an embolization procedure. A diagnostic cerebral angiogram is mandatory to understand the anatomy and physiology of an AVM. Arterial feeders to AVMs should be carefully distinguished from en passage vessels. When planning an embolization procedure, the length, size, and tortuosity of the feeder arteries are important anatomical considerations, because these will determine how easy or difficult it will be to navigate a microcatheter into the AVM nidus and the amount of Onyx reflux that can be tolerated by the catheter in the vessel. Onyx can be injected from arterial pedicles or through a transvenous approach. Frontal lesions will have supply from the anterior cerebral artery and the middle cerebral artery (MCA), depending on how medial they are; they may also receive supply from deep lenticulostriate feeders. Parietal lesions will parasitize MCA vessels and posterior cerebral artery (PCA) vessels, such as the parieto-occipital branch. Temporal lesions can have direct internal carotid artery feeders for more medial lesions or be supplied by direct branches from the MCA and choroidal arteries. Posterior fossa lesions typically receive flow from PCAs, the anterior inferior cerebellar artery, or the posterior inferior cerebellar artery. External carotid artery branches should also be investigated because some AVMs can have a dural fistula-like component. Spinal AVMs can be treated with embolization of radicular feeders; however, in our experience, these AVMs are more responsive to surgical resection. Ventricular and deep lesions recruit choroidal vessels and require great care when planning for interventions. Flow-related aneurysms occur at pedicle branch points and within the nidus. Careful review of oblique angiograms can aid in the diagnosis of nidal aneurysms.

Venous drainage is the next step in AVM anatomy assessment, including the presence of deep or superficial draining veins, number and size of draining veins, venous hypertension, cortical venous reflux, and stenosis of draining veins. Attention should be paid to venous outflow obstructions and venous thrombosis because these findings can make the AVM more likely to rupture. AVMs can drain superficially by way of the cortical veins or through deep veins such as the internal cerebral veins, ependymal veins, basal vein of Rosenthal or the pontomesencephalic veins, or the petrosal vein of the posterior fossa. Venous access can be important for the completion of an embolization procedure if some of the arterial pedicles have been “locked out” by Onyx.

Last, the compactness of the nidus is important. AVMs with a diffuse nidus are challenging and possibly more suitable to embolization than surgery because some eloquent areas of the brain cannot undergo resection without morbidity. Conversely, AVMs with a compact nidus are ideal for surgery, and if superficial, they may require no embolization at all.

Periprocedural Medications

AVM embolization can be performed under awake (conscious) sedation or general anesthesia. Our preference is awake (conscious) sedation to facilitate a neurological examination and more accurate Wada testing. Systemic heparinization is recommended for the embolization of all unruptured AVMs and for most ruptured ones unless there is an absolute contraindication. Verapamil is kept on hand for catheter-induced vasospasm. Antiseizure medications should also be available.

We perform awake superselective Wada tests during all embolization procedures. Once the microcatheter has been navigated into the desired location and its position confirmed with microcatheter angiography, we proceed with infusion of 75 mg of preservative-free Amytal (amobarbital), followed by 30 mg of lidocaine, followed by neurological examination. Amytal and lidocaine are used to inhibit the γ-aminobutyric acid (GABA) receptors in gray matter and the sodium (Na) channels in the white matter, respectively.

Specific Technique and Key Steps

1. 
   

   After obtaining femoral arterial access and performing an angiogram to confirm the absence of any irregularity or dissection, the guide catheter is mounted over a curved wire/diagnostic catheter (0.035-inch angled Glidewire, Terumo), and the system is advanced into the aorta under fluoroscopic guidance.

   
   
2. 
   

   The guide catheter should be placed in the extracranial vessel of choice utilizing roadmap navigation.

   
   
3. 
   

   Once in the large vessel of interest (e.g., ICA), cranial anteroposterior and lateral baseline angiograms with high frame rates per second should be obtained and carefully reviewed for flow dynamics ( Fig. 31.1–31.7, Video 31.1–31.7 ).

   
   
4. 
   

   A microcatheter is navigated over a microwire into the desired arterial feeders or into the AVM nidus, if possible. The goal is to navigate to a pedicle as close as possible to the nidus to limit embolization of normal brain tissue ( Fig. 31.1–31.7, Video 31.1–31.7 ). If navigating far distally (e.g., beyond the M2 or A2 segments), we recommend utilizing an intermediate catheter for support and stabilization (see Pearls).

   
   
5. 
   

   Superselective angiography via the microcatheter is performed again, followed by awake superselective Wada testing. If no major symptoms are encountered with the physiological testing, the pedicle is deemed safe to embolize.

   
   
6. 
   

   Onyx 18 or 34 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.

   
   
7. 
   

   Before Onyx injection, the microcatheter is purged with DMSO. DMSO-compatible microcatheters must be utilized (Headway DUO, MicroVention; SL 10, Stryker; Apollo, Medtronic) ( Video 31.1–31.7 ). DMSO can be caustic to cerebral vessels 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 ( Fig. 31.2, 31.4 ).

   
   
8. 
   

   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 × 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).

   
   
9. 
   

   The first embolization attempt into the pedicle/nidus is usually the most productive and successful ( Video 31.1–31.7 ). Carefully watch for nidal penetration and unwanted venous or arterial filling or arterial reflux. If unwanted venous embolization occurs, stop immediately! Onyx is injected in a fast, pulsing fashion with the thumb. Some operators will first form a plug of Onyx around the catheter while others will just push distally. Reflux around the catheter must be noted, as this can make removal of the microcatheter difficult.

   
   
10. 
    

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

    
    
11. 
    

    These steps can be performed for multiple pedicles. For large lesions (SM III-V), 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.

    
    
12. 
    

    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.

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  6–8F sheath.

  
  
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  90- to 100-cm-long 6F guide catheter.

  
  
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  0.044–0.058 intermediate catheter (Sofia, MicroVention; DAC, Stryker; Navien, Medtronic; Catalyst 5, Stryker).

  
  
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  0.035-inch angled Glidewire.

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

  
  
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  0.014-inch microwire (Synchro 2, Stryker).

  
  
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  DMSO.

  
  
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  Onyx 18 or 34.

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

  
  
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  Continuous heparinized flush.

Pearls

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

  
  
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  Endovascular neurosurgeons should know the limitations of the embolization technique and safely try to reduce flow into large AVMs. The remainder of the AVM should be left for microsurgical resection ( Fig. 31.3, Video 31.3 ) or radiosurgery.

  
  
• 
  

  Microcatheter entrapment is the most common complication of Onyx embolization. If the microcatheter does not come out easily, it requires very slow continuous tension under fluoroscopy to prevent vessel rupture. If the catheter cannot safely be removed, it can be amputated at the groin or removed in surgery as a last resort.

  
  
• 
  

  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.

  
  
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  A particular hazard is allowing reflux to creep back along a curve in the vessel. Reflux should be limited as much as possible to straight segments of the feeding vessel.

  
  
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  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 ( Fig. 31.7, Video 31.7 ).

  
  
• 
  

  We suggest injection of verapamil through the intermediate or guide catheter to reduce vasospasm induced by mechanical traction of the entrapped vessel and microcatheter.

  
  
• 
  

  Venous access can be utilized for AVM access; however, care should be taken not to disrupt the venous pedicle before the AVM is completely obliterated ( Fig. 31.6, Video 31.6 ).

  
  
• 
  

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

  
  
• 
  

  If a venous pedicle is partially occluded, consider a heparin infusion to avoid thrombosis.

  
  
• 
  

  If the microcatheter is stuck in the Onyx, the microcatheter can be cut and the intermediate catheter advanced over the microcatheter for counter traction. Balloons can also be used for countertraction. Snares can be used for removing catheters.

  
  
• 
  

  Stentrievers can be used to retrieve unwanted embolized Onyx casts if the casts are in undesirable locations.

  
  
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  Strict blood pressure control and at least an overnight stay in the intensive care unit are essential for patients who undergo AVM embolization procedures.

  
  
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  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 stuck microcatheters.

  
  
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  Flow-related aneurysms will often decrease in size once the associated pedicle is embolized; however, we advocate treating any aneurysm associated with an AVM, especially intranidal AVMs.

  
  
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  If increased venous shunting is noted, a final angiogram is imperative because the lesion may be at risk of rupture.

References

Case Overview: CASE 31.1 Grade V Frontal Arteriovenous Malformation: Ophthalmic Artery Embolization

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  A 48-year-old female presented with seizures and was found to have a right frontal grade V arteriovenous malformation (AVM). She has no past medical history of significance. Seizure became medically refractory to three different anticonvulsants.

  
  
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  Because of severe symptomatology, the decision was to treat the AVM with staged embolization in preparation for surgical resection or radiation therapy.

  
  
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  Multiple stage embolization was planed over a period of 6 months.

Related posts:

32 Arteriovenous Embolization with N-butyl-2-cyanoacrylate 35 Carotid-Cavernous Fistula Embolization 34 Spinal Arteriovenous Fistula and Malformation Embolization 33 Endovascular Embolization of Dural Arteriovenous Fistulas 5 Direct Carotid Artery Access 29 Aneurysm Embolization with Liquid Embolic Agents

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