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SPINAL CORD ARTERIOVENOUS MALFORMATION (AVM)

OVERVIEW

Spinal Arteriovenous Malformations (AVM's) are rare disorders. These defects of the circulatory system consist of an abnormal connection between the normally higher intravascular pressure arterial system and the lower pressure venous pathways. Arteries carry oxygen-rich blood away from the heart to the body's cells; veins return oxygen-depleted blood to the lungs and heart. Under ordinary circumstances there is a pressure regulatory system in place that reduces the arterial blood pressure through progressively smaller vessels until the capillary portion is reached. This capillary network is vital in order to permit the transfer of nutrients and oxygen to the surrounding tissue as well as reducing the intravascular pressure before the blood is transferred to the relatively thin walled veins.

An AVM can occur when the intervening capillary system is absent. There are several potential consequences to the presence of an AVM.

    Haemorrhage into surrounding tissue is a real threat since the thin walled vein portion of this malformation may be unable to sustain the higher intravascular blood pressure that is directly shunted from the thicker muscular walled arteries. Haemorrhage can be as little as a "leak" or as destructive as an explosive disruption within the tissue that surrounds it.

    Ischemia (lack of blood born oxygen) is a frequent problem in the absence of the capillary system which is designed to release oxygen and nutrients to these tissues. As pressure within the venous system increases, the vein dilates resulting in slowing of blood flow. Slow blood flow leads to ischemia. Additionally, the surrounding tissue (the Neurons and Fibre Tracts) is subjected to increased localized pressure from these dilated veins. This local pressure produces ischemia as well. As a result, the tissue suffers from several sources of decreased blood perfusion and is then also predisposed to hemorrhage.

ETIOLOGY

The precise cause of Spinal Cord Arteriovenous Malformations has never been clearly defined. Malformations that arise within the Dural Sheath and within the parenchyma (substance) of the Spinal Cord are usually found in younger patients. These are generally believed to be Congenital in origin.

When Arteriovenous Malformations are found in older patients, they are usually associated with some form of trauma. The term used for this malformation is an Arteriovenous Fistula (AVF). These AVF malformations develop when a Spinal Radicular (Nerve Root) Artery forms an abnormal arteriovenous communication (known as a "fistula") with the venous Medullary Vein circulation of the Spinal Cord. This is a relatively "slow-flow" malformation that usually develops over a period of months to years.

The high-pressure arterial blood flow from the Radicular Artery causes a dilatation of the Spinal Cord's venous system which is weaker and thinner walled compared to the artery. This results in slower flow within the veins, a condition known as venous stasis. In turn this venous outflow obstruction then causes ischemic (lack of oxygen and nutrient supply) changes to the surrounding and adjacent Spinal Cord tissue. This hypo-perfusion results in impaired neural function. The progressive venous engorgement, as well as the Spinal Cord ischemia, appears to conspire to produce Neurologic compromise of Spinal Cord function.

Congenital intradural Spinal AVMs consist of a tangle of abnormal vessels. There appears to be a progressive recruitment of arterial blood vessels and thin-walled venous vessels. Once the high-flow arterial system overcomes the capacity of the abnormal venous vessels, hemorrhage occurs.

CLASSIFICATION

Spinal Vascular Malformations have been classified into four categories:
  • Type 1 is the most common type of malformation. This is a Dural AVF arising at the dural nerve root sleeve. These malformations are created when a single arterial feeder develops a fistula to the spinal venous circulation. Patients with type 1 malformations become symptomatic because the AVF causes hypo-perfusion of the Spinal Cord from the venous congestion and hypertension within the malformation.
  • Type 2 is called a Glomus AVM which consists of a tightly compacted abnormal group of arterial and venous vessels (referred to as a "nidus") residing inside a short segment of the substance of the Spinal Cord (intramedullary.)
  • Type 3 malformations are called "Juvenile" Arteriovenous Malformations. They are extensive lesions fed by multiple vessels with abnormal vessels located within the Spinal Cord substance (intramedullary) as well as on the surface of the Spinal Cord (extramedullary.)
  • Type 4 malformations are intradural extramedullary AVFs on the surface of the Spinal Cord.

SYMPTOMS

Type 1 (AVF) patients with are usually over 40 years of age and occur more frequently in males than in females. Symptoms tend to increase over an extended period of months and years manifested by progressive weakness in the lower extremities as well as bowel or bladder control difficulties. Pain, located in the distal posterior thoracic spine with or without a significant radicular component, can be a prominent feature. Symptoms may be worsened by strenuous activity or a change in body position.

These lesions can have the same or similar clinical presentations as Spinal Stenosis although confusion between Spinal Stenosis and neurogenic claudication can usually be differentiated by some of the characteristics unique to each entity.

There is a particularly rare form of Spinal Dural AVF known as Foix-Alajouanine syndrome. In this case there is a rapidly progressive myelopathy believed to be due to venous thrombosis from spinal venous stasis.


Figure 1A (Left): Spinal Cord Angiogram demonstrating the arterial origin of this AVM.

Figure 1B (Right): Operative photograph demonstrating Type 1 Spinal Cord AVM in a 40 year old male. The Arrow indicates the Artery responsible for Dural AVF arising at the Dural Nerve Root Sleeve.

Types 2-4 patients are usually younger than 30 years of age. They are often identified as a result of a hemorrhage (either subarachnoid or intra-parenchymal), vascular steal phenomenon (a form of ischemia), or, more rarely, pressure effect by their "mass" upon the Spinal Cord.

Patients with Subarachnoid Hemorrhage usually experience sudden onset of a severe headache, stiff neck (meningismus), and photophobia (pain associated light). A Spinal AVM should be considered in the differential diagnosis of any patient with a Subarachnoid Hemorrhage who has negative results from a cerebral angiogram.

Haemorrhage within the Spinal Cord substance (intraparenchymal) is usually associated with sudden neurologic deterioration, sudden onset of pain, and a distinct spinal level of neurologic dysfunction.

More rarely, patients come to attention because of "vascular steal" phenomenon in which oxygenated arterial blood that is shunted through the AVM causes hypo-perfusion of the surrounding normal Spinal Cord tissue.

"Mass effect" caused by growth and progressive enlargement of the intradural AVM can cause compression of the surrounding neural tissue which results in impairment in neurologic function.

Since Types 2-4 intradural spinal vascular malformations develop during embryogenesis they can be located in any region of the Spinal Cord.

Patients with intradural AVMs may present with upper or lower extremity difficulties, whereas patients with Type 1 dural AVFs, typically have lower limb extremity involvement only.

DIAGNOSIS

An accurate diagnosis is important since these lesions often represent a reversible cause of Spinal Cord dysfunction (myelopathy.)

There are some potentially distinguishing features related to each type of malformation. These include:


NEUROIMAGING

MRI (including MR Angiography) as well as CT Angiography are among the initial neuroimaging investigations that help to identify these problems. Ultimately Spinal Cord Angiography is prerequisite to accurately and definitively identify the precise anatomy, configuration of both the lesion as well as the feeding and draining vessels. Myelography and Post-myelographic CT scanning are additional Neuroimaging techniques that are occasionally used to identify the extent of the lesion.


Figure 2A (Above): Spinal MYELOGRAM demonstrates the extent of this Type 4 AVM extending throughout the Thoracic and upper Lumbar Spinal levels.

Figure 2B (Below): Operative Photograph of the same patient demonstrating an extensive multilevel intradural Spinal Cord AVM which was entirely removed using Microvascular Techniques.


TREATMENT

Medical therapy

There are no effective pharmacological means available to treat Spinal Vascular Malformations.

It is common to use "Steroids" in an attempt to acutely improve the patient's neurologic function since steroids can decrease the swelling (edema) that often accompanies an injury to the Spinal Cord. There is no place for the long-term use of steroids since they do nothing to treat the underlying pathology of the disorder and have deleterious side effects such as gastric ulceration, elevated blood glucose levels, and suppression of the immune system.

Surgical therapy

Treatment must be custom tailored to each patient since each Spinal Vascular Malformation is a unique lesion. The present surgical treatment options include open microsurgical ligation and/or resection of the malformation. Some can be treated using Minimally Invasive Microendoscopic techniques.

Other treatment options

Endovascular occlusion using various methods of "embolization" is a common and often preferential treatment method for many of these lesions.

Radiosurgery is also an important method of treatment for some lesions. Spinal radiation is not without risk however the advances in radiosurgery are making it increasingly safer.

Combined Therapy

It is common for Neurosurgeons to recommend a combination of the techniques described above. The use of endovascular embolization followed by microsurgical resection is frequently an effective combination.

Type 1 Dural AVFs can be treated with either open or endovascular ligation. Both techniques yield excellent results, with successful occlusion rates reported as higher than 80%. Endovascular technique is generally regarded as being less invasive. If the patient has multiple sites of fistula formation, direct microsurgical ligation is more appropriate since all feeding vessels can be ligated under direct vision.

Direct microsurgery is also necessary in cases where Endovascular access to the arterial feeding vessel is impossible due to the extreme tortuosity of the vascular supply to the AVM or where the arterial blood supply to healthy regions of the Spinal Cord is difficult to spare when attempting the selective obstruction of the blood supply to the AVM.

Types 2-4 Intradural AVMs are often best treated with Endovascular therapy; however, microsurgical resection may be appropriate for some lesions. The "Juvenile" variety is almost only treated with endovascular therapy. Radiosurgery is appropriate for some lesions as well.



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This page last edited on 2/19

All content ©2016 by Neurosurgical Consultants, P.A.
Author, Martin L. Lazar, MD, FACS
All Rights Reserved. See Usage Notices.