Arteriovenous Malformations and Fistulas (AVM/AVF) of the Spinal Cord

Updated: Apr 21, 2022
  • Author: James S Harrop, MD; Chief Editor: Brian H Kopell, MD  more...
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Overview

Practice Essentials

Vascular lesions of the brain and spinal cord are commonly encountered in clinical practice and can lead to diagnostic, prognostic, and therapeutic challenges. Central nervous system (CNS) vascular malformations encompass a wide range of arterial and venous anomalies with various presentations, a variable clinical course, and a variety of complication rates. Spinal vascular malformations consist of an abnormal connection between normal arterial and venous pathways. These malformations do not benefit from intervening capillaries. As a result, venous pressure is increased and the individual is predisposed to ischemia or hemorrhage.

The etiology of vascular malformations of the spinal cord has not been clearly defined. Intradural parenchymal malformations often occur in a younger patient population and are believed to be congenital. Spinal arterial dural fistulas commonly arise in an elderly population and are believed to be due to a traumatic occurrence. These vascular malformations develop near a spinal dural artery, forming an abnormal arteriovenous communication with the venous circulation.

Due to increased utilization of imaging techniques of the craniospinal axis over past decades, more vascular malformations are being detected. This necessitates an increased level of expertise with diagnosis, characterization, and timely management of these lesions. The term “malformation” can imply a congenital (developmental) or acquired lesion, and these terms (“malformation” and “lesion”) are used interchangeably. [1]

The spinal cord is composed of neuronal pathways, glial tissue, and interwoven vascular structures that perfuse the spinal parenchyma. Spinal cord vascular malformations (arterial and venous) represent a heterogeneous group of blood vessel disorders that affect the spinal cord parenchyma either directly or indirectly. Vascular disorders of the spine are more rare than cerebral vascular entities but can result in significant morbidity. These lesions frequently demonstrate distinguishing characteristics on imaging that are imperative for the radiologist to recognize to provide proper guidance for diagnosis and treatment. [2]  Vascular malformations of the spinal cord include spinal arteriovenous malformations (AVMs), dural arteriovenous fistulas (AVFs), spinal hemangiomas, cavernous angiomas, and aneurysms. This article focuses on the most prevalent spinal vascular malformations—AVMs and AVFs.

(See the MRI below showing spinal malformation of the thoracic spine.)

Spinal malformation. This is a sagittal T2-weighte Spinal malformation. This is a sagittal T2-weighted MRI of the thoracic spine of a 68-year-old woman with a 9-month history of back pain and sensory loss, progressing to the point of loss of bowel and bladder function along with a sudden onset of paraparesis. Note the thoracolumbar junction with an edematous spinal cord and dilated serpiginous intradural venous plexus.

Anson and Spetzler classified spinal cord vascular malformations into 4 categories, which were later affirmed by Sabayan and associates [1, 3] :

  • Type 1: Dural AVF. This is the most common type of malformation, accounting for 70% of all spinal vascular malformations. [4] These fistulas are created when a radiculomeningeal artery feeds directly into a radicular vein, usually near the spinal nerve root. Dural AVFs are most commonly found in the thoracolumbar region. [5]  Patients with these malformations become symptomatic because the AVF creates venous congestion and hypertension, resulting in hypoperfusion, hypoxia, and edema of the spinal cord. Due to the slow-flow nature of dural AVFs, hemorrhage rarely occurs. Most dural AVFs are believed to occur spontaneously, but the exact etiology remains unknown. [5]  These lesions are most frequently found in men between the fifth and eighth decades of life.

  • Type 2: Glomus AVM. This malformation consists of a tightly compacted group of arterial and venous vessels (nidus) inside a short segment of the spinal cord. Multiple feeding vessels from the anterior spinal artery and/or the posterior spinal circulation typically supply these AVMs. Abnormal vessels are intramedullary in location, although superficial nidus compartments can reach the subarachnoid space. [5]  Glomus AVMs are the most commonly encountered intramedullary vascular malformations, representing about 20% of all spinal vascular malformations. They usually present in younger patients with acute neurologic deterioration secondary to their location, which is usually the dorsal cervicomedullary region. The mortality rate related to these lesions has been reported to be 17.6%. After initial hemorrhage, the rebleed rate is 10% within the first month and 40% within the first year.

  • Type 3: Juvenile metameric AVM. This malformation is an arteriovenous abnormality of the spinal cord parenchyma that is fed by multiple vessels. These extensive lesions have abnormal vessels that can be both intramedullary and extramedullary in location. They are typically found in young adults and in children.

  • Type 4: Spinal pial AVF. This malformation is an intradural extramedullary AVF on the surface of the cord that results from direct communication between a spinal artery and a spinal vein without an interposed vascular network. These lesions are usually seen in patients who are between their third and sixth decades of life.

Spinal arteriovenous malformations

Spinal arteriovenous malformations (AVMs) are a rare form of spinal blood vessel defect in which blood is received from the spinal feeding arteries, resulting in vessel engorgement that leads to clinical signs secondary to mass effect and ischemia. [6]  They account for about 10-15% of all spinal vascular shunts. [7]  Because of the rarity of spinal AVMs, studies regarding their diagnosis and treatment are limited. In addition, the various classifications that have been proposed historically make it difficult for young neurosurgeons to understand this disease. Because delayed initial diagnosis leads to irreversible damage to the spinal cord, neurosurgeons should always consider spinal AVM as part of the differential diagnosis. To understand the pathologic condition of spinal AVM, it is important to learn its basic classifications. Spinal AVM is classified as intradural, dural, and epidural. Spinal digital subtraction angiography (DSA) is the gold standard for diagnosis of spinal AVM and is an indispensable tool for treatment planning. [8]

For many patients with spinal AVM, the symptoms are nonspecific. Therefore, we consider it critical to detect signal flow voids in enlarged spinal veins by using magnetic resonance imaging (MRI). An accurate understanding of the vascular structures is indispensable for deciding appropriate treatment strategies. Hence, performing an angiography is essential. Regarding treatment, whether to select surgical or endovascular treatment for AVF depends largely on institutional protocols. [9]

Dural arteriovenous fistulas

Spinal dural arteriovenous fistulas (AVFs) are produced by direct communication between arterial and venous systems of the spinal cord, causing hypertension in the latter with spinal cord dysfunction. This is a rare pathology with unknown etiology and nonspecific clinical symptoms that usually lead to delayed diagnosis. Often, radiologists are the first to guide the clinician toward an adequate diagnosis. Characteristic findings can be seen on MRI or magnetic resonance angiography; these modalities may locate the fistula in a high percentage of cases, although the pathology must be confirmed by spinal angiography. [10]

Spinal dural AVFs represent rare pathologic communication between arterial and venous vessels within the spinal dural sheath. Clinical presentation includes progressive spinal cord symptoms such as gait difficulty, sensory disturbances, changes in bowel or bladder function, and sexual dysfunction. These fistulas are most often present in the thoracolumbar region. Diagnosis of AVF is commonly missed, possibly due to a low index of suspicion, nonspecific symptoms, and challenging imaging. [11]

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Background

 

 

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History of the Procedure

Spinal vascular malformations have been recognized as a potential cause of myelopathy for more than 100 years. In 1914, Charles Elsberg performed the first successful operation on a spinal cord malformation.

In the 1960s, significant advances were made in the technique of spinal angiography, which led to improved understanding of normal spinal vasculature and the pathophysiology of spinal cord malformations. Kendall and Loque used these modern imaging modalities to define a distinct subgroup of spinal AVMs classified as dural spinal AVFs. Kendall and Loque treated these lesions by performing the less invasive technique of directly ligating the fistula origin along the dural sleeve and achieved good results. [12]

Treatment approaches for spinal cord malformation are being expanded through the use of interventional neuroradiology. With improvements in spinal angiography and endovascular techniques, these lesions may be embolized either as primary treatment or as a complement to open microsurgical techniques.

Two treatment modalities are available: endovascular and surgical therapy. Endovascular treatment has improved over the years and offers the advantages of a less invasive approach; therefore, it is usually chosen as primary therapy. [10]  However, treatment should always be based on an accurate diagnosis. [9]  Eradication can often be incomplete, and risk for spinal cord ischemia may be increased. Stereotactic radiosurgery can be curative in some cases or can facilitate shrinking of the lesion. [7]

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Pathophysiology

Spinal malformations can be separated into 2 subgroups: AVMs and AVFs. Spinal arteriovenous malformations (AVMs) are a rare form of spinal blood vessel defect that results in vessel engorgement leading to clinical signs secondary to mass effect and ischemia. [6]  These lesions represent an abnormal connection between the spinal radicular artery and the medullary vein of the spinal cord. This type of fistula creates a slow-flow vascular malformation that typically develops over months to years. High-pressure arterial flow from the radicular artery dilates the perimedullary venous system, causing venous stasis and hypertension. Venous hypertension results in a decreased arteriovenous gradient. The end result consists of venous outflow obstruction, hypoperfusion, and hypoxia of the spinal cord. Neurologic compromise is thought to occur secondary to this venous engorgement and resultant spinal cord ischemia.

Arteriovenous malformations are congenital high-flow vascular lesions that account for about 10-15% of all spinal vascular shunts. They may occur sporadically or in the setting of genetic syndromes. Intramedulary glomus-type AVMs consist of an intramedullary nidus of shunting vessels usually located in the anterior half of the spinal cord fed by 1 or more spinal arteries that drain into spinal veins. Most AVMs are confined to the thoracic spine; intranidal or arterial aneurysms are common and are responsible for subarachnoid hemorrhage or hematomyelia, which is experienced in more than half of patients with AVMs. [7]

The second subgroup is the spinal  arteriovenous fistulas (AVFs), which are congenital lesions that consist of abnormal vasculature. Spinal dural AVFs are produced via direct communication between arterial and venous systems of the spinal cord, causing hypertension in the latter with spinal cord dysfunction. This is a rare pathology with unknown etiology and nonspecific clinical symptoms, and diagnosis is usually delayed. [10]  These lesions recruit arterial blood vessels and have thin-walled venous vessels. Hemorrhage occurs when the high-flow arterial system overcomes the capacity of the abnormal venous vessels.

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Presentation

Patient history and presentation are important factors in distinguishing spinal vascular malformations from other neurologic disorders. Patients with the more common dural AVFs typically have presentations that are different from those of patients with intradural AVMs.

Typical characteristics of patients with dural AVF (type 1)

Patients with AVF are typically older than 40 years. Arteriovenous fistulas occur much more frequently in males than in females. Symptoms increase over an extended period of months to years and include progressive weakness of the legs and concurrent bowel or bladder difficulties. Typically, pain is located in the distal posterior thoracic region over the spine, without a significant radicular component. However, painful radiculopathy may be present. Activity or a change in position may exacerbate symptoms in the thoracic or lumbar region and can result in thoracic spinal cord venous congestion and lower extremity weakness.

These lesions can be mistakenly diagnosed as spinal stenosis and neurogenic claudication. The history of a patient with spinal claudication does not usually include lower extremity weakness, but it can include a significant pain component similar to that of spinal dural AVF.

Foix-Alajouanine syndrome is an extreme form of spinal dural AVF that affects a minority of patients. Patients present with rapidly progressive myelopathy due to venous thrombosis from spinal venous stasis.

Typical characteristics of patients with intradural AVM (types 2-4)

The typical patient is younger than 30 years and presents with subarachnoid or intraparenchymal hemorrhage, vascular steal phenomenon, and, rarely, mass effect on the spinal cord.

Individuals with subarachnoid hemorrhage may experience sudden onset of severe headache, meningismus, or photophobia. Acute subarachnoid hemorrhage with excruciating back pain is referred to as coup de poignard. A spinal AVM should be considered in the differential diagnosis of any patient with a subarachnoid hemorrhage who has negative cerebral angiography results.

If the hemorrhage is intraparenchymal, the patient presents with sudden neurologic deterioration, sudden onset of pain, and a distinct spinal level of neurologic dysfunction. Rarely, patients present because of vascular steal phenomenon, in which oxygenated arterial blood shunted through the AVM causes the surrounding normal parenchyma to become hypoperfused.

Lastly, patients with intradural lesions can present with mass effect caused by growth of the AVM. The enlarged vascular malformation compresses surrounding neural tissue, thereby impairing neurologic function.

These intradural spinal vascular malformations (types 2-4) develop during embryogenesis and are present in an even distribution throughout the spinal cord. Therefore, patients with intradural AVM may present with upper or lower extremity difficulties, as opposed to patients with dural AVF, who typically have only lower extremity involvement.

Physical examination findings in patients with spinal vascular malformation

Arteriovenous malformations (AVMs) and arteriovenous fistulas (AVFs) are rare disorders that may cause neurologic deterioration. Accurate diagnosis is important because these lesions may represent a reversible cause of myelopathy. Improvements in spinal cord imaging, such as MRI and angiography, have provided insight into the anatomy and pathophysiology of these lesions. In addition, less invasive treatment options such as neuroendovascular surgical approaches offer promise for improved patient outcomes.

Specific physical examination findings are associated with different types of spinal malformations, as follows:

  • Bruit over spinal cord: Intradural AVM

  • Hyperreflexia caudal to lesion: Dural AVF and intradural AVM

  • Upper motor signs: Dural AVF and intradural AVM

  • Weakness: Dural AVF and intradural AVM

  • Increased tone: Dural AVF and intradural AVM

  • Saddle region sensory disturbance: Dural AVF

  • Gait disturbance: Dural AVF

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Relevant Anatomy

To understand and treat these arterial and venous malformations, knowledge of the normal spinal cord vascular supply is imperative. Unfortunately, the distribution of these spinal vessels is highly variable and inconsistent, but major vessels are more consistent.

The aorta contributes to blood flow through the segmental arteries, which, in turn, supply the spinal medullary and radicular arteries. The radicular artery provides circulation to the nerve root dural sleeve. This artery is typically involved in the formation of a spinal AVF via its connection to the medullary spinal veins. This medullary artery bifurcates into anterior and posterior divisions, which then merge to form the spinal arteries. The spinal cord includes 3 main spinal arteries (1 anterior and 2 posterior), which parallel the spinal cord.

Blood supply to the spinal cord is provided through 3 anatomic regions:

  1. Cervicothoracic region: Receives segmental blood vessels from the vertebral arteries and the great vessels of the neck (ie, aorta, subclavian and carotid arteries).

  2. Midthoracic region: Receives most of its segmental blood supply from the aorta. This region of the spinal cord receives most of its blood supply via collateral circulation (superior and inferior arteries) and therefore is susceptible to infarction as a watershed area. For example, an aortic dissection or an aortic atherosclerotic disease can send emboli to the anterior spinal artery (ASA); the patient presents with sudden onset of painless lower extremity paralysis with intact sensation. Spinal cord infarction affects the anterior motor portion because the ASA supply is lost, but the posterior spinal arteries still perfuse the posterior spinal cord and sensory tracts.

  3. Thoracolumbar region: Receives segmental vessels from the abdominal aorta and the iliac arteries. The largest segmental vessel—the artery of Adamkiewicz—may be variably located between levels T9 and L2 and, in most cases, arises from the left side of the vertebral column.

The venous plexus in the spinal column—the Batson plexus—is unique compared to other venous plexuses in the body. This network of venous vessels does not include valves and thus does not prevent retrograde venous flow. This valveless system allows an arterial fistula from the radicular artery to create congestion through the entire venous plexus, which can manifest as spinal cord ischemia.

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