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 heterogenous group of blood vessel disorders that affect the spinal cord parenchyma either directly or indirectly. This group consists of spinal arteriovenous malformations (AVMs), dural arteriovenous fistulas (AVF), spinal hemangiomas, cavernous angiomas, and aneurysms. The focus of this article is the most prevalent spinal vascular malformations, AVMs and AVFs.
AVMs and AVFs are rare disorders that may cause neurologic deterioration. An 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 further insight into the anatomy and pathophysiology of these lesions. In addition, less-invasive treatment options such as neuroendovascular surgical approaches are presently being further explored.
In 1992, Anson and Spetzler classified spinal cord vascular malformations into the following 4 categories: 
Type 1: This dural AVF is the most common type of malformation, accounting for 70% of all spinal vascular malformations.  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.  Patients with type 1 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 type 1 AVFs, hemorrhage rarely occurs. Most dural AVFs are believed to occur spontaneously, but the exact etiology is still unknown.  Type I lesions are most frequently found in men between the fifth and eighth decades of life.
Type 2: Referred to as a glomus AVM, type 2 malformations consist 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. The abnormal vessels are intramedullary in location, although superficial nidus compartments can reach the subarachnoid space.  Type 2 AVMs are the most commonly encountered intramedullary vascular malformations, representing about 20% of all spinal vascular malformations. These lesions 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 type 2 malformation is reported at 17.6%. After initial hemorrhage, the rebleed rate is 10% within the first month and 40% within the first year.
Type 3: These malformations are arteriovenous abnormalities of the spinal cord parenchyma fed by multiple vessels. These juvenile malformations are extensive lesions with abnormal vessels that can be both intramedullary and extramedullary in location. These lesions are typically found in young adults and children.
Type 4: Also known as pial AVFs, these malformations are intradural extramedullary AVFs on the surface of the cord that result from a direct communication between a spinal artery and a spinal vein without an interposed vascular network. They are usually seen in patients who are between their third and sixth decade of life.
Spinal vascular malformations can also be classified into 2 general groups. One group consists of the spinal dural fistulas (type 1), and the other group has intradural pathology (types 2-4).
Spinal dural arteriovenous fistulas (SDAVFs) are rare pathologies, with a yearly incidence of 5-10 new cases/million, constituting 60-80% of spinal arteriovenous malformations. Clinical symptoms include progressive paraparesis, paresthesias, bladder, and bowel disturbances. 
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 techniques of spinal angiography, which produced further 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. In 1977, Kendall and Loque treated these lesions with the less-invasive technique of directly ligating the fistula origin along the dural sleeve, with good results. 
The treatment of spinal cord malformations is being further expanded with the use of interventional neuroradiology. With further improvements in spinal angiography and endovascular techniques, these lesions may be embolized either as a primary treatment or as a complement to open microsurgical techniques.
Spinal vascular malformations consist of an abnormal connection between the normal arterial and venous pathways. These malformations do not benefit from intervening capillaries. As a result, venous pressure increases 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 arise in a younger patient population and are believed to be congenital. However, spinal arterial dural fistulas commonly arise in an elderly population and are believed to be due to a traumatic occurrence. These AVF malformations develop near a spinal dural artery, forming an abnormal arteriovenous communication with the venous circulation.
Spinal malformations can be separated into 2 subgroups.
The first subgroup is spinal AVFs, which are believed to be acquired lesions. They represent an abnormal connection between the spinal radicular artery and the medullary vein of the spinal cord. This fistula creates a slow-flow vascular malformation that typically develops over months to years. The 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 is venous outflow obstruction, hypoperfusion, and hypoxia of the spinal cord. Neurologic compromise is thought to occur secondary to this venous engorgement and to the resulting spinal cord ischemia.
The second subgroup is spinal intradural AVMs/AVFs, which are congenital lesions that consist of abnormal vasculature. 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.
Patient history and presentation are important factors in distinguishing spinal vascular malformations from other neurologic disorders. Patients with the more common dural AVF typically have presentations that are different from those of patients with intradural AVMs.
Typical characteristics of patients with dural AVF (type 1)
Patients with AVFs are typically older than 40 years. These AVFs 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 typical 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 a spinal dural AVF.
Foix-Alajouanine syndrome is an extreme form of spinal dural AVF that affects a minority of patients. These patients present with a 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 a subarachnoid or intraparenchymal hemorrhage, vascular steal phenomenon, and, rarely, mass effect on the spinal cord.
Patients with spinal intradural malformations typically present acutely either after intraparenchymal or subarachnoid hemorrhage. Patients with subarachnoid hemorrhage may experience sudden onset of a 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, a 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 enlargement of the vascular malformation compresses the surrounding neural tissue, impairing neurologic function.
These intradural spinal vascular malformations (types 2-4) develop during embryogenesis and, therefore, are present in an even distribution throughout the spinal cord. Therefore, patients with intradural AVMs may present with upper- or lower-extremity difficulties, as opposed to patients with dural AVFs, who typically have only lower-limb–extremity involvement.
Physical examination findings and the type of spinal malformation are 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 disturbances - Dural AVF
In order 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 quite variable and inconsistent, but the 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 is the artery typically involved in the formation of a spinal arteriovenous fistula (AVF) by its connection to the medullary spinal veins. This medullary artery bifurcates into anterior and posterior divisions, which then merge and form the spinal arteries. The spinal cord has 3 main spinal arteries (1 anterior and 2 posterior), which parallel the spinal cord.
The blood supply to the spinal cord can be divided into 3 anatomic regions:
The first is the cervicothoracic region, which receives segmental blood vessels from the vertebral arteries and the great vessels of the neck (ie, aorta, subclavian and carotid arteries).
The second is the midthoracic region, which receives most of its segmental blood supply from the aorta. This region of the spinal cord receives most of its blood supply from collateral circulation (superior and inferior arteries) and, therefore, is susceptible to infarction as a watershed area. For example, an aortic dissection or aortic atherosclerotic disease can send emboli to the anterior spinal artery (ASA); the patient presents with a sudden onset of painless lower-extremity paralysis with intact sensation. The 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.
The third is the thoracolumbar region, which 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 with other venous plexuses in the body. This network of venous vessels does not have valves and thus does not prevent retrograde venous flow. Therefore, 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.
Only relative contraindications exist for surgical obliteration of the fistula site, no absolute contraindications. Some of these relative contraindications include a hemodynamically unstable patient, active infection, and cardiac instability, among others.
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