Classification

The terms dural arteriovenous malformation and dural arteriovenous fistula are often used interchangeably. DAVFs are classified according to their venous drainage, which determines both the natural history and treatment recommendations. Houser and colleagues first established the association of clinical symptoms with patterns of venous drainage in a series of 28 patients with DAVFs.⁶ Although several classification systems exist for DAVFs,⁷^–⁹ we refer primarily to the Borden¹⁰ and Cognard¹ classifications, which have been described in detail elsewhere and are summarized in Tables 391-1 and 391-2. According to the Borden classification, type I fistulas have anterograde drainage into a dural venous sinus or meningeal vein. These fistulas are benign, often asymptomatic or characterized by a cranial bruit, and have a high rate of spontaneous remission. Borden type II fistulas have both anterograde venous sinus drainage and retrograde drainage into subarachnoid veins. The venous sinus may be patent but largely defunctionalized because of high-flow venopathy causing reversal of flow into arterialized leptomeningeal veins.

TABLE 391-1 Borden Classification of Dural Arteriovenous Fistulas

Type I	Anterograde drainage into the dural sinus/meningeal vein
Type II	Anterograde drainage into the dural sinus plus retrograde drainage into the subarachnoid veins
Type III	Isolated retrograde drainage: 1 Directly into the subarachnoid veins 2 Trapped segment of sinus with reflux into the subarachnoid veins 3 Venous varix/dural venous lake with reflux into the subarachnoid veins

TABLE 391-2 Cognard Classification of Dural Arteriovenous Fistulas

Type I	Anterograde drainage into the main sinus
Type IIa	Drainage into the main sinus with reflux into another sinus or sinuses
Type IIb	Drainage into the main sinus with reflux into cortical veins
Type IIa + IIb	Drainage into the main sinus with reflux into another sinus or sinuses plus cortical veins
Type III	Direct cortical venous drainage without venous ectasia
Type IV	Direct cortical venous drainage with venous ectasia (>5 mm and 3× larger than the diameter of the draining vein)
Type V	Drainage into spinal perimedullary veins

Borden type III DAVFs have exclusive retrograde drainage into arterialized subarachnoid veins at or on the wall of dural venous sinuses. Such drainage is frequently but not always due to occlusion of the arterialized dural sinus. When the sinus is patent, the point of the fistula is located either between the meningeal artery and leptomeningeal vein or between the meningeal artery and a segment of arterialized dural venous sinus that is thrombosed at either end or somehow isolated from the rest of the sinus, thereby causing reversal of drainage into leptomeningeal veins (Fig. 391-2).

FIGURE 391-2 Lateral angiogram of a Borden type III dural arteriovenous fistula of the right transverse sinus that was thrombosed distally. The catheter is situated in the proximal left transverse sinus, which has multifocal severe stenoses that functionally isolate the arterialized right transverse sinus pouch and force venous drainage retrograde. Contrast material refluxes into the superior sagittal sinus and supratentorial veins, as well as into the galenic system, occipital sinuses, and cerebellar veins.

Clinical Manifestations

Patients with DAVFs are frequently asymptomatic, but the most common initial symptoms are pulse-synchronous tinnitus and headache. Proptosis is frequent with carotid-cavernous DAVFs. Pulsatile tinnitus from a DAVF is nearly always exacerbated by a Valsalva maneuver. A bruit is often auscultated over the mastoid process ipsilateral to the arterialized transverse/sigmoid sinus and can be diminished by compression of the ipsilateral common carotid/occipital arteries. Patients with exclusive retrograde leptomeningeal venous drainage universally have significant venous hypertension, often remote from the site of the fistula, and delayed venous clearance of contrast material. The resultant encephalopathy can be clinically indistinguishable from dementia. Gait ataxia, seizures, myelopathy, cerebral edema, ischemia, subarachnoid hemorrhage, or any combination of these signs or symptoms may also be present. The sudden disappearance of a bruit usually indicates thrombosis of the draining vein or veins and resolution of the fistula. It can also represent more ominous shunting of venous blood into cortical venous channels sufficiently distant from the middle ear such that pulsatile tinnitus is no longer appreciated by the patient. Thus, any change in the nature or intensity of the bruit warrants repeated angiographic evaluation of the DAVF.

Natural History

Borden type I DAVFs have a benign clinical course and a high rate of spontaneous remission. Up to 10% of Borden type II and III DAVFs with cortical venous reflux may spontaneously close as well.¹¹ The risk for intracranial hemorrhage and stroke is dictated by the presence of cortical venous reflux, galenic drainage,^11,¹² and venous congestion. Lasjaunias and associates proposed that the source of intracranial hemorrhage in these cases is not the fistula itself but rather ectatic, thin-walled arterialized veins.¹³ Persistence of cortical venous reflux in intracranial DAVFs carries an annual risk for hemorrhagic or nonhemorrhagic neurological deficits of 15% along with an annual mortality rate of 10.4%.¹¹ Cognard and coworkers noted intracranial hemorrhage in 40% of patients with type III and in 65% with type IV DAVFs.¹

Little is known about the rebleeding rate for DAVFs, but in small case series it is as high as 43% within the first few days of hospitalization ¹⁰ and up to 35% in the initial 2 weeks after the first hemorrhage.¹⁴ Davies and coauthors reported a 20% annual morbidity and mortality rate after an initial aggressive neurological manifestation.¹⁵

Treatment

Observation

Borden type I DAVFs can be observed, especially since they may spontaneously thrombose, which has been noted with cavernous sinus DAVFs.¹⁶ Factors associated with spontaneous regression of DAVFs are unknown, but sinus thrombosis may play a role.¹⁷ Occasionally, treatment is offered if a patient cannot tolerate symptoms such as a constant bruit. If conservative treatment is chosen, patients are advised to avoid antiplatelet or anticoagulant agents, if possible, to prevent interference with spontaneous thrombosis of the DAVF. High-flow venopathy in sinuses not involved in the fistulous drainage, however, may predispose to undesirable venous sinus thrombosis; such patients are advised to take at least 81 mg of aspirin daily.

Compression Therapy

Compression therapy is seldom used currently, except in patients with Borden type I fistulas as a possible first step before neuroendovascular therapy. Compression of the ipsilateral carotid or occipital artery (if the latter vessel is a known feeder to the fistula) is performed for 30 minutes at a time, three times a day. If compression of the carotid artery is chosen, patients are instructed to use the contralateral arm. In case hemispheric ischemia ipsilateral to the compressed artery and paresis of the contralateral upper extremity develop as a result of overzealous compression, the arm will fall away and the compression will necessarily be self-limited. In 25% of simple DAVFs of the transverse/sigmoid sinuses supplied by the occipital artery, compression of the occipital artery results in complete thrombosis of the DAVF within several weeks.¹⁸ In patients with subjective improvement or disappearance of pulsatile tinnitus, a follow-up cerebral angiogram can be performed to evaluate whether thrombosis of the DAVF has occurred since the last evaluation.

Transvenous Therapy

Because neuroendovascular techniques have evolved considerably over the past decade, open surgery for cure of DAVFs has become a secondary treatment. Transvenous coil embolization plus occlusion of the recipient venous pouch is the mainstay of endovascular therapy and offers the best chance of cure.¹⁹ It is particularly useful for Borden type II DAVFs, which tend to be high flow and may have multiple associated fistulas. The risk of occluding the sinus/arterialized pouch and causing venous infarction is low because the sinus is usually defunctionalized. The use of balloon test occlusion has been recommended for assessing the impact of embolizing a sinus in communication with normal cortical veins^1,^20,²¹; however, this is not our practice because all transvenous/transarterial embolizations for DAVFs at our institution are performed with the patient under general anesthesia.

As Cognard and colleagues pointed out, it is essential to differentiate between type IIb and type III DAVF (according to their classification) when considering the option of transvenous coil embolization. Transvenous treatment is appropriate for a Cognard type IIb fistula, in which there is drainage into the sinus and then reflux into a cortical vein, but it is not appropriate for a Cognard type III fistula, in which there is drainage into a cortical vein and then the sinus. In the latter case, transvenous occlusion of the sinus blocks eventual venous egress, thus further elevating pressure in the arterialized subarachnoid vein.¹

If patent, venous access is via the transfemoral ipsilateral internal jugular vein and sigmoid sinus route. A triaxial system consisting of a 6 French shuttle sheath advanced to the jugular bulb, a 4 or 5 French vertebral catheter, and a microcatheter provides maximum support. Differentiation of the recipient arterialized venous pouch from the sinus responsible for normal cortical venous drainage is crucial because in the latter drainage may occur via the sinus wall instead of the sinus.³ The arterialized sinus or fistulous pouch is then fully packed with thrombogenic fibered or platinum coils (Fig. 391-3). Inadequate embolization of a sinus with leptomeningeal venous drainage or embolization of the parent sinus without occlusion of the parallel venous channel receiving arterial inflow²² may aggravate the venous hypertension by impeding venous egress and diverting arterialized blood to the subarachnoid veins.^10,¹⁹

FIGURE 391-3 A, Left anteroposterior oblique angiogram of the left ascending pharyngeal artery, with numerous branches supplying the left marginal sinus. A 5 French vertebral catheter is situated in the left jugular bulb. This Borden type I fistula was treated to abolish a persistent, bothersome, left-sided pulsatile bruit. B, Lateral angiogram of the left internal jugular vein. The entire left sigmoid sinus (arrowheads) and distal left marginal sinus (arrow) have a trabeculated appearance suggestive of previous thrombophlebitis. C, Microcatheter angiogram from the distal left marginal sinus. D, Coil embolization of the left marginal sinus and fistulous pouch with the arterial road map used for guidance. E, Post-embolization lateral angiogram of the left ascending pharyngeal artery demonstrating ectatic arterial branches but no early venous drainage.

Transvenous embolization for a Borden type III fistula is successful only if the catheter can be carefully manipulated across the thrombosed or trapped segment of the sinus over a guidewire into the arterialized recipient pouch. There is a small risk of perforation and particularly subdural extravasation with this approach (Fig. 391-4).²¹

FIGURE 391-4 Subdural extravasation of contrast material into the right transverse sinus secondary to perforation of a thrombosed distal sinus by the catheter. Note the hazy opacification of the right transverse sinus, indicative of extravascular contrast material. There is retrograde reflux of contrast into the superior sagittal sinus and left hemispheric veins.

Sinus thrombosis is not an absolute contraindication to passage of the microcatheter,^23,²⁴ especially with the current availability of high-performance, small-caliber, hydrophilic guidewires and catheters. When entry to the diseased sinus is not feasible because of venous thrombosis, venopathy, or tortuosity, access can sometimes be secured from the contralateral transverse sinus across the torcular Herophili (Fig. 391-5).²⁵ Recanalization of thrombosed transverse-sigmoid sinuses with balloon angioplasty plus closure of the intramural septations in the wall of the proximal sinus forming the fistula by overlapping stents has been reported. This technique may prove to be useful for eradicating DAVFs of the lateral sinus.²⁶^–²⁸ Care should be taken to observe for alterations in venous drainage patterns after balloon angioplasty.²⁹ Primary stent placement in a stenotic sinus to normalize venous outflow after treatment of the DAVF may also relieve venous congestive symptoms,^2,³⁰ although long-term venous stent patency rates are not yet known.

FIGURE 391-5 A road map of the left transverse sinus demonstrates stenoses of the mid left transverse and distal left sigmoid sinuses (arrows). There is early reflux of contrast material into an ectatic left vein of Labbé and other leptomeningeal veins before antegrade opacification of the distal left sigmoid sinus and jugular bulb. Access to a distally occluded right transverse sinus is achieved across the torcular with a hydrophilic 0.035-inch angled Glidewire (Terumo, Tokyo, Japan).

Complex torcular or paratorcular DAVFs may be impossible to approach from either transverse sinus and might require a bur hole over the torcular Herophili. Direct torcular puncture is then performed for the introduction of microcatheters and embolic material into the fistulous transverse sinus. Coil embolization/occlusion of the torcular or distal superior sagittal sinus (SSS) is, of course, ill advised. If the goal is to preserve sinus patency, a slow-setting liquid adhesive such as the Onyx Liquid Embolic System (Micro Therapeutics, Inc., Irvine, CA) is a potential option. Onyx often streamlines along the venous intima in a controlled, laminar fashion and occludes points of the fistula without causing hemodynamic compromise. However, Onyx within a patent vein is a platform for thrombus formation and distal embolization, and many neurointerventionalists will prescribe a short (4 to 6 weeks) course of daily aspirin for such patients.

Transvenous embolization with other liquid nonadhesive agents (Eudragit E mixture) has also been described.³¹ In DAVFs with a dilated venous channel, creating a “nest” of coils before transvenous injection of a liquid embolic agent may prevent unwanted distal embolization.³² Proper injection technique should be used with liquid embolic agents to avoid retrograde embolization into the leptomeningeal veins, which can produce venous infarction.¹⁹

If residual opacification of the tightly packed venous pouch is noted, thrombosis can be enhanced with a 1-g/hr intravenous infusion of ε-aminocaproic acid for 24 to 48 hours, followed by a control angiogram.³³ Angiographic resolution of venous hypertension correlates with the disappearance of pulsatile tinnitus and venous congestive symptoms, even if minor residual DAVF is present. However, the risk for rehemorrhage is not eliminated unless the DAVF is angiographically cured.¹

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