62 Spinal Arteriovenous Fistulas

10.1055/b-0038-162191

62 Spinal Arteriovenous Fistulas

Eduardo Martinez-del-Campo, Bradley A. Gross, Leonardo Rangel-Castilla, Peter Nakaji, and Robert F. Spetzler

Abstract

Spinal arteriovenous fistulas (AVFs) represent 60 to 80% of spinal vascular malformations and include dural or pial arteriovenous shunts between a radicular artery and intradural vein, and a pial artery and a vein. Spinal AVFs are classified in four different types: dural (type I), glomus intramedullary (type II), juvenile extradural–intradural (type III), and pial (type IV). Due to their low-flow nature, venous hypertension is the main cause of symptoms. Spinal AVFs have nonspecific progressive symptoms, but patients usually present with sensory and gait disturbances, bladder and bowel dysfunction, motor weakness, and less frequently subarachnoid hemorrhage. Spinal MRI and MRA are the initial imaging evaluation; spinal cord edema with or without flow voids is the key finding when suspecting spinal AVFs. Up to 80% of patients with spinal AVFs are initially misdiagnosed with other conditions. Digital subtraction angiography is the mandatory imaging modality for diagnosis and classification. Due to the progressive nature of the disease, asymptomatic and symptomatic patients should undergo treatment unless medically unfit. Microsurgical resection/ligation and endovascular embolization are two excellent management alternatives. In either modality, the fistulous portion of the AVFs is the target to be obliterated. If diagnosis and treatment occur before permanent neurological deficits exist, the outcome of spinal AVFs is good to excellent. Clinical and radiological long-term follow-up is mandatory in all patients.

Introduction

Spinal arteriovenous fistulas (AVFs) include dural arteriovenous shunts between a radicular artery and intradural vein (type I) and pial shunts between a pial artery (most often the arterial spinal artery [ASA]) and vein (type IV). The incidence of spinal AVFs is low, with only 5 to 10 new cases per million inhabitants every year. Nonetheless, spinal AVFs represent 60 to 80% of spinal vascular malformations. Dural AVFs (dAVFs), also known as type I or intradural dorsal AVFs, primarily cause neurological symptoms as a result of myelopathy from venous hypertension. Rarely, cervical lesions may hemorrhage if they possess intracranial drainage. They have a well-known male sex predilection and tend to affect persons who are middle-aged or older, with a predisposition for the thoracolumbar spine. Pial AVFs (i.e., perimedullary, type IV, intradural extramedullary, and intradural ventral AVFs) may cause venous hypertension or spinal hemorrhage.

Major controversies in decision making addressed in this chapter include:

Whether or not treatment is indicated for all spinal AVFs.
Differential diagnosis between spinal AVFs and spinal arteriovenous malformations (AVMs).
Role of venous drainage and need for treatment.
Open microsurgery versus endovascular treatment for ruptured and unruptured spinal AVFs.
Endovascular transarterial versus transvenous approach.

Whether to Treat

Treatment must be individualized to the specifics of each situation. Patients suffering from myelopathy should undergo disconnection to halt the progression of symptoms and potentially mitigate their existing deficits. Hemorrhagic lesions should be promptly disconnected to prevent potentially devastating rehemorrhage ( 1 , 2 in algorithm ). There is no reliable medical literature that clearly delineates the natural history of untreated, asymptomatic spinal AVFs. Given the potential for progressive, possibly irreversible, myelopathy, treatment is recommended even for asymptomatic lesions in medically fit patients.

**Algorithm 62.1** Decision-making algorithm for spinal arteriovenous fistulas.

Anatomical Considerations

Despite the variable distribution of spinal vessels, the major patterns of the vasculature are often well preserved. The spinal cord has three main spinal pial arteries (one anterior and two posterior) that run parallel to the spinal cord. These vessels supply spinal pial (type IV) AVFs. The caliber of the supplying pial vessel can have a significant impact on the safety and feasibility of transarterial embolization.

The spinal dural supply is derived from radicular branches that supply dAVFs (type I; ▶ Fig. 62.1a ). In the cervical spine, these most often arise from the vertebral artery or, less often, from the thyrocervical or costocervical trunk branches. In the thoracolumbar spine, they arise from segmental branches. Meticulous attention should be paid to the opacification of radiculomedullary branches on segmental injections that opacify the fistula, as it can substantially affect the risk and feasibility of embolization (▶ Fig. 62.1b ). The valveless venous plexus in the spinal column is particularly relevant in the context of fistula formation, because arteriovenous shunting can easily cause congestion, venous hypertension, and resultant myelopathy.

**Fig 62.1 (a)** A segmental artery injection demonstrating a spinal dural arteriovenous fistula (type I or intradural dorsal) (anteroposterior view). **(b)** A segmental artery injection supplying both a spinal dural arteriovenous fistula (that was later surgically disconnected) and a radiculomedullary branch (anteroposterior view). (Used with permission from Barrow Neurological Institute, Phoenix, AZ.)

The type IV, or spinal pial AVF, was first described by René Djindjian as an “intradural extramedullary spinal arteriovenous malformation” that is fed by the anterior spinal artery (ASA); it was subsequently dubbed the “type IV” spinal AVF by Heros et al. These AVFs are often referred to as spinal perimedullary fistulas. Type IV AVFs are supplied by a pial artery (▶ Fig. 62.2 ). A recent review reported the presence of feeding artery aneurysms in 10% of cases (▶ Fig. 62.2 ). The fistula point has a proclivity to exist ventral to the spinal cord because it is most often supplied by the ASA, and Spetzler et al referred to these fistulas as intradural ventral lesions. This approach reinforces their anatomical location, as the emphasis on their ventralal location captures the added surgical challenge they present compared with intradural dorsal lesions (spinal dAVFs or type I AVFs).

**Fig. 62.2** A superselective injection of a radiculomedullary branch demonstrating a spinal pial arteriovenous fistula (type IV or intradural ventral); a feeding artery aneurysm (a, mid-arterial phase), and the draining vein **(b)** (anteroposterior view). Note that the level of the shunt site is remote from the level of the origin of the radiculomedullary artery, which opacifies the anterior spinal artery and the fistula. (Used with permission from Barrow Neurological Institute, Phoenix, AZ.)

Pathophysiology/Classification

The most common classification scheme for spinal arteriovenous shunts stratifies them into four types: dAVFs (type I; ▶ Fig. 62.3 ) ( 3, 4 in algorithm ), glomus intramedullary AVMs (type II; ▶ Fig. 62.4 ) ( 5 in algorithm ), juvenile extradural–intradural AVMs (type III; ▶ Fig. 62.5 ) ( 6 in algorithm ), and pial AVFs (type IV; Fig. 62.6 ) ( 7 in algorithm ). This chapter focuses on type I and type IV fistulas. Type I lesions have been substratified into IA (single-hole; Fig. 62.3a,b ) and IB (multiple-hole) fistulas (▶ Fig. 62.3c,d ), which are low-flow AVFs. Because of the low-flow nature of spinal dAVFs, venous hypertension is the main cause of symptoms in patients; the shunting blood from the radicular artery to the radicular vein within the dural sleeve increases pressure in the coronal plexus and radial veins. This hypertension causes reduction of the arteriovenous gradient and tissue perfusion; consequently, the spinal cord is affected by progressive hypoxia and progressive loss of intramedullary vasodilation that results in the loss of vessel autoregulation in affected areas.

**Fig. 62.3** Artist′s illustration depicting a type I dural arteriovenous malformation. (Used with permission from Barrow Neurological Institute, Phoenix, AZ.)

**Fig. 62.4** Artist′s illustration depicting a type II glomus intramedullary AVMs. (Used with permission from Barrow Neurological Institute, Phoenix, AZ.)

**Fig. 62.5** Artist′s illustration depicting a type III juvenile extradural–intradural AVMs. (Used with permission from Barrow Neurological Institute, Phoenix, AZ.)

**Fig. 62.6** Artist′s illustration depicting a type IV pial arteriovenous malformation. (Used with permission from Barrow Neurological Institute, Phoenix, AZ.)

Type IV AVFs were classified by Anson and Spetzler into three subtypes (A–C). Type IVA AVFs are small and have a single feeder; type IVB lesions are intermediate, and type IVC lesions are giant, multipedicled lesions with massively dilated venous channels. Extraordinarily high flow through these lesions leads to the phenomenon of vascular steal from the intrinsic spinal cord arterial supply and to the ischemic sequelae. Type IVC fistulas are associated with hereditary hemorrhagic telangiectasia. The Kim-Spetzler classification describes dAVFs as intradural dorsal lesions (referring to the fistula site) and describes pial AVFs as intradural ventral lesions (reflecting the proclivity toward a ventral ASA-pial vein fistula site).

All these shunts can be a source of neurological morbidity secondary to venous hypertension, whereas types II–IV and selected cervical type I lesions with intracranial drainage can cause hemorrhage. The demographic profile of patients with type I dAVFs (older male) is mirrored among patients with type IVA AVFs. Type IVC fistulas are seen in younger patients, without a clear sex predilection.

Workup

Clinical Evaluation

Spinal AVFs are usually associated with nonspecific progressive symptoms of myelopathy, making the diagnosis generally difficult in clinical practice due to its rarity and nonspecific initial presentation. The most common initial neurological symptoms that manifest are sensory and gait disturbances, followed by bladder (urinary retention and incontinence) and bowel dysfunction. Other symptoms include decreased motor strength, thoracolumbar pain with or without radiculopathy, and, in a small percentage (9%) of cases, no neurological deficits. Patients may endorse worsened symptoms with positional changes (increasing intra-abdominal pressures) or with physiological states of increased plasma volume (e.g., pregnancy) that result in venous congestion and worsening of symptoms. Most frequently, patients present with progressive symptoms over several months.

Spinal dAVFs rarely present with subarachnoid hemorrhage if intracranial drainage is present. A recent review of 213 cases noted that approximately 36% of pial AVFs present with hemorrhage.

Imaging

Patients are initially evaluated radiographically with magnetic resonance imaging (MRI) and MR angiography. Imaging usually reveals serpiginous vessels in the intradural compartment, as evidenced by flow voids on T2-weighted MRIs (86%) and vasogenic edema (60%; ▶ Fig. 62.7 ). Although MRIs are paramount in the initial identification and differential diagnosis of spinal AVFs, the prognostic value of preoperative and postoperative imaging remains controversial, with no correlation between changes on postoperative MRIs and clinical outcomes.

**Fig. 62.7** Typical appearance of a spinal arteriovenous malformation with intradural flow voids, which most often represent a pressurized draining vein, and edema (high signal) in the intramedullary spinal cord (sagittal T2-weighted magnetic resonance image). (Used with permission from Barrow Neurological Institute, Phoenix, AZ.)

When an MRI indicates a spinal AVF, a more detailed understanding of the spinal anatomy can be provided by digital subtraction angiography. Spinal arteriography is the gold standard for diagnosis of these vascular lesions. For dAVFs, it enables determination of the level of the fistula site and discernment of the number of radicular feeders. It allows the identification of the rare, but important, concomitant presence of an opacified pial branch and the fistula, revealing the presence of a radiculomedullary artery emanating from the proximal radicular branch to the fistula (▶ Fig. 62.1b ). For pial AVFs, it is crucial to identify the supplying radiculomedullary branches and to discern the level of the fistulous point itself (▶ Fig. 62.2 ).

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