Extradural AVFs

Extradural AVFs have a direct connection between a branch of a radicular artery and the epidural venous plexus. These rare fistulas are characterized by engorgement of epidural veins which propagates medullary venous congestion and may cause compression of the spinal cord or nerve roots.

The most recent subclassification of these lesions was presented by Rangel-Castilla et al.7 In this scheme, type A spinal extradural AVFs exhibit drainage into both the epidural venous plexus and a vein draining into the perimedullary venous plexus(figure 1A,B). By comparison, type B lesions drain only into Batson's plexus. Type B1 lesions compress the thecal sac secondary to an engorged epidural venous plexus and type B2 lesions lack such compression.

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Figure 1

(A) Extradural arteriovenous fistula (AVF) arising from a vertebral artery branch (arrow). (B) Posterior view demonstrating engorgement of the epidural venous plexus producing compression of the spinal cord and nerve roots. (C) Intradural dorsal AVF demonstrating radicular feeder and fistula along the right-sided nerve root sleeve (arrow). (D) Posterior view demonstrating dilation of the coronal venous plexus producing venous hypertension. (E) Ventral intradural AVF depicting a fistulous connection between the anterior spinal artery (ASA) and coronal venous plexus (arrow). (F) Anterior coronal view showing the fistula along the anterior spinal cord. (G) Intramedullary arteriovenous malformation with a compact nidus supplied by a feeder arising from the ASA. (H) Posterior coronal view demonstrating additional supply from the posterior spinal artery. Used with permission from [blinded for review]. Used with permission from Barrow Neurological Institute, Phoenix, Arizona.

Intradural dorsal AVFs

Intradural dorsal AVFs are the most common type of spinal vascular malformation (figure 1C,D). These lesions consist of a direct connection between a radicular feeding artery and a medullary vein at the dural nerve root sleeve. Propagation of venous hypertension to the coronal venous plexus engenders venous congestion and progressive myelopathy.

Intradural ventral AVFs

Intradural ventral AVFs exhibit a direct fistula between the ASA and coronal venous plexus (figure 1E,F). These midline lesions are located in the ventral subarachnoid space and are divided into three subtypes. Type A fistulas are single-feeder lesions with slow blood flow and mild venous hypertension, type B fistulas are progressively high-flow lesions with multiple feeding arteries and type C lesions are characterized by a giant fistula with a markedly distended venous network. As the size and flow of the fistula increases, symptoms of vascular steal and spinal cord compression become pronounced. High-flow lesions are often associated with flow-related aneurysms.

Extradural-intradural AVMs

Extradural-intradural AVMs are also known as juvenile or metameric AVMs. They often occur in children in association with genetic syndromes. These lesions can involve bone, muscle, skin and cord along an entire metamere.

Intramedullary AVMs

Intramedullary or glomus AVMs are characterized by a classic nidus located within the cord parenchyma (figure 1G,H). These lesions are supplied by branches of the ASA and PSA and may be either compact or diffuse. They are high-pressure and high-flow and often exhibit both nidal and feeding artery aneurysms.

Conus medullaris AVMs

Conus medullaris AVMs represent a separate category characterized by multiple direct shunts from the ASA, PSA and radicular arteries. These lesions also generally exhibit a true nidus, which is typically based in the pia. Conus AVMs are located at the conus medullaris or cauda equina but may extend along the entire filum terminale.

Extradural AVFs

Extradural AVFs may be treated endovascularly, usually through a transarterial approach. Occasionally a transvenous approach may provide improved access to the fistula.7

Intradural dorsal AVFs

Intradural dorsal AVFs may be treated by embolization, surgery or both. Prior to embolization, microcatheter injections are used to verify the absence of normal spinal cord supply. If a spinal artery is visualized, surgical treatment is recommended.

The goal of embolization is obliteration of the fistulous site as well as the proximal portion of the intradural arterialized draining vein (figure 2). n-Butyl cyanoacrylate (nBCA) was historically preferred as it provides a more permanent occlusion than particles. Several recent reports have described ethylene-vinyl alcohol copolymer embolization for sDAVF.8 Following fistula embolization, collateral supply must be ruled out by microcatheter injections at the same level on the contralateral side, as well as two levels above and below the fistula.

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Figure 2

(A) Selective injection of a left T7 segmental artery reveals a spinal dural arteriovenous fistula at the nerve root sleeve with caudally directed venous drainage (arrow). (B) Supraselective microcatheter angiography demonstrating fistula supply as well as reflux to the distal intercostal artery. (C) Unsubtracted image demonstrating the extensive Onyx cast filling both the fistula and the proximal draining vein as well as reflux along the distal intercostal artery supply to the rib. (D) Post-embolization thoracic aortogram revealing no residual fistula. Used with permission from [blinded for review].

Intradural ventral AVFs

Treatment of intradural ventral AVFs differs depending on the lesion subtype. In type A fistulas, the ASA is minimally dilated and arteriovenous transit is slow. Effective embolizate penetration is limited by the small caliber of the ASA, and surgical treatment is generally favored. In type B fistulas, supraselective catheterization of feeding vessels near the fistulous point is often possible, and embolization is feasible (figure 3). In type C lesions the fistulous pouch may be accessed either transarterially or transvenously. Fistula occlusion is then performed using liquid embolics with or without coil embolization.

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Figure 3

(A) Spinal angiogram showing an intradural ventral arteriovenous fistula supplied by a branch of the left T4 segmental artery with further supply from two small feeders from the anterior spinal artery (ASA) (arrows). (B) Supraselective catheterization of the T4 feeder in the region of the fistula was followed by n-butyl cyanoacrylate embolization penetrating the draining vein (arrow) (C). (D) Post-embolization injection reveals a small residual fistula supplied by the ASA feeders (arrow). (E) Repeat angiography obtained 5 days after treatment showing complete fistula obliteration with preserved ASA flow. Used with permission from [blinded for review].

Extradural-intradural ACMs

Extradural-intradural AVMs typically exhibit multiple high-flow feeders and encompass both intramedullary and extramedullary components. Complete obliteration of these lesions is associated with a significant risk of neurological morbidity, and treatment goals thus generally include reducing the malformation size to minimize mass effect and ameliorate symptoms of ischemic steal. Staged embolization using coils, particles or liquid embolic agents is often followed by surgical resection.

Intramedullary AVMs

Intramedullary AVMs are treated by a combination of embolization and surgery. Almost all of these lesions exhibit a main feeder from the ASA; however, the posterior spinal artery also contributes supply in many cases. Many authors believe, as with brain AVMs, that surgical resection is the mainstay of treatment for these lesions.9 However, many centers advocate presurgical embolization and embolization may represent definitive treatment in select cases. Endovascular cure requires catheterization close to the nidus, with significant intranidal embolizate penetration.

Conus medullaris AVMs

Optimal treatment of conus medullaris AVMs involves aggressive embolization followed by microsurgical resection.10

Extradural AVFs

To our knowledge, only a single series of extradural AVFs treated endovascularly has been reported (table 1). In this report, Rangel-Castilla et al treated seven patients with eight extradural AVFs treated with Onyx in all cases, in conjunction with coils in one patient and nBCA in two.7 Surgery was required in a single case. Complete obliteration was achieved in all cases and persisted at a mean of 18 months. Four patients made an excellent recovery while three patients experienced persistent bladder/bowel dysfunction. In these authors’ hands, Onyx embolization provided an effective treatment strategy for these rare lesions.

Intradural dorsal AVFs

While microsurgery has historically been considered the treatment of choice for intradural dorsal AVFs, a number of endovascular series have emerged. Early series used polyvinyl alcohol (PVA) but, in general, patients treated in this manner demonstrated a high rate of recanalization.11 Given these high recurrence rates, most authors currently favor liquid embolics.12–15 The latest large series of 63 patients with sDAVF treated with surgery and embolization over a 20-year period was recently published.16 Thirty-nine patients underwent initial endovascular embolization with a 69% obliteration rate. By comparison, 24 patients underwent surgery with an 83% obliteration rate. Endovascularly-treated patients exhibited a reduction in length of stay (3.1 vs 9.8 days). The authors concluded that both surgery and endovascular treatment may be effectively applied in the treatment of these lesions.

While studies directly comparing surgical and endovascular treatment are lacking, a meta-analysis of sDAVF treatment was published by Steinmetz et al in 2004.17 In that analysis, 98% of those patients treated with microsurgery exhibited complete fistula obliteration following initial treatment compared with 46% of patients following embolization, as determined by both radiographic and clinical follow-up. 89% of patients demonstrated improvement or stabilization of neurologic symptoms after surgical treatment. Although many of the endovascular series used both PVA and liquid embolic agents, series using only cyanoacrylates reported recurrence rates of 30–75%. Complication rates were low in both series. The authors conclude that surgery may be a more effective treatment strategy for sDAVF.

Although most modern series have reported acrylic embolization, Onyx embolization is currently emerging as an effective treatment option for DAVFs. The first report in 2003 detailed two patients treated in this manner.18 One patient exhibited a small residual fistula and was planned for surgical intervention. The second demonstrated complete occlusion in addition to symptomatic improvement without evidence of recurrence at 7 months of follow-up. An additional report of three patients was published in 2008.19 Two patients were cured following a single procedure while a third patient required two procedures to achieve cure. No complications were noted and no evidence of recurrence was observed on long-term follow-up.

Intradural ventral AVFs

An analysis of endovascular treatment of ventral AVFs is clouded by the small number of reported series and the lack of uniformity of classification systems presented.

Type A and B fistulas have traditionally been treated by microsurgery and endovascular reports are limited. A recent series by Oran et al described the treatment of five patients with type A fistulas.20 Using flow-directed catheters and nBCA, the authors were able to achieve complete fistula obliteration in four of the five cases. The last patient required surgery. There were no complications and all patients improved clinically. The authors conclude that embolization is an option for the treatment of these lesions. Lundqvist and colleagues reported two patients with type B fistulas who underwent PVA embolization.12 Both patients showed clinical improvement but no long-term radiographic follow-up was documented.

Type C lesions pose a particular treatment challenge. Surgical treatment is high-risk given the risk of intraoperative hemorrhage, and endovascular treatment is generally favored. Both transarterial and transvenous treatment routes as well as direct venous puncture have been described.21–23 Ricolfi et al presented a series of 12 consecutive giant perimedullary AVFs.23 Two patients early in the series were treated with gelatin sponge particles. Ten patients were treated by balloon occlusion, leading to eight anatomic cures and six good clinical results. One patient treated with balloons deteriorated following migration of a balloon through the fistula to the venous side and one patient with a cervical fistula treated with gelatin sponge died. This series is not comparable to present-day techniques using liquid embolic agents.

Several larger series classify these lesions as perimedullary fistulas (type IV) in the older classification scheme.1 ,24 ,25 The largest series by Rodesch et al details 32 spinal cord AVFs.24 Embolization with nBCA was undertaken in 86% of macro AVFs with a cure rate of 67%, with all others demonstrating 75% occlusion. All patients improved on follow-up. By comparison, 48% of micro AVFs were treated, with 75% resulting in complete occlusion. All patients improved clinically on follow-up. There were transient complications in 22% of patients, without permanent morbidity, mortality or rebleeding. In 36% of micro AVFs a surgical approach was undertaken for treatment as embolization was deemed impossible.

Extradural-intradural AVMs

The endovascular treatment of extradural-intradural AVMs is limited to single case reports. Spetzler et al reported a patient with a large type III malformation treated using a staged approach with preoperative and intraoperative PVA embolizations combined with surgical resection.26 The patient experienced a transient worsening but returned to baseline within 6 weeks, with angiographic evidence of lesion obliteration. Although these lesions have historically been considered incurable, in select cases preoperative embolization followed by microsurgical resection can result in acceptable outcomes.

Intramedullary AVMs

Reports of embolization of intramedullary AVMs are rare.27 In 1990 Biondi et al reported 35 patients with thoracic AVMs who underwent a total of 158 PVA embolizations.27 Although clinical improvement was seen in 63% of patients, recanalization was frequently observed. Neurological decline following embolization was observed in seven patients (20%). Despite these limitations, the authors suggest that particulate embolization of intramedullary AVMs may be a safe and efficient tool for treatment, although the effect is temporary.

More recently, Onyx embolization of intramedullary AVMs has been reported and preliminary evidence suggests that it may be more effective than particles. Corkill et al published their series of 17 patients with intramedullary AVMs treated exclusively with Onyx over a 4-year period.28 Thirteen patients underwent a single procedure and four patients underwent two sessions. Complete obliteration was observed in six patients (37.5%), subtotal obliteration in five patients (31.25%) and partial obliteration was observed in five patients (31.25%). Improved neurological function was achieved in 82% of cases.

Conus medullaris AVMs

Conus AVMs constitute a distinct category of spinal vascular lesions. The first series detailing the multidisciplinary treatment of conus AVMs was recently published.10 Eight patients underwent preoperative embolization followed by definitive microsurgical resection while eight patients underwent microsurgery only. Of the patients treated with surgery, one was angiographically occult and seven were not amenable to embolization. For those patients treated endovascularly, the rate of complete obliteration was 88% and, at a mean follow-up of 70 months, 43% were neurologically improved, 43% were stable and 14% had worsened. Recurrences were detected in three patients, two of whom underwent additional embolization and resection. This series establishes multimodality treatment with embolization followed by microsurgery as an effective treatment for these lesions.

Spinal cord AVM series

Several of the largest series of endovascularly-treated sAVMs analyzed results for all subtypes in a single series.24 ,29 In 2000 Nimii et al reported 108 sAVMs consisting of 38 pediatric and 70 adult cases, including 81 nidal AVMs and 27 fistulous AVMs.29 Seventy-five cases were treated with embolization alone, 10 with surgery and embolization, two with embolization following radiation and 12 with surgery alone. Nine patients received no treatment. A total of 156 embolization sessions were performed. In 79 of 87 patients undergoing embolization, acrylic was used alone in 49 cases or in combination. Complete obliteration was achieved in 17 cases. Hemorrhage was observed in only two embolized cases over 34 months of follow-up. Neurological worsening was observed following 21 sessions, 10 of which resulted in permanent deficits.