Endovascular Treatment of Spinal Vascular Malformations

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CHAPTER 396 Endovascular Treatment of Spinal Vascular Malformations

Vascular lesions of the spinal axis have engendered many classification schemes in the past century.¹ Although these conditions were first recognized in the late 19th century by Hebold² and Gaupp,³ for the first half of the 20th century our knowledge of spinal vascular malformations was limited to a variety of case reports and autopsy findings. The first recorded successful operation on a dural arteriovenous fistula (AVF) was performed in 1914 by Charles Elsberg.⁴ It was not until the 1960s that angiographic diagnosis and endovascular treatment were first reported by Djindjian and colleagues⁵ and Doppman and associates.⁶^–⁹ Since then, advances in imaging modalities, angiographic techniques, and endovascular material have contributed significantly to a better understanding of the pathology of these diseases, as well as better outcomes for patients.

Vascular Anatomy

Crucial to understanding the pathophysiology behind spinal vascular malformations is a thorough comprehension of the spinal cord’s vascular architecture. The spinal cord’s blood supply is provided by one anterior spinal artery (ASA) and paired posterior spinal arteries (PSAs). The ASA runs in the anterior median sulcus and supplies the anterior two thirds of the spinal cord, including the anterior horn cells, the anterior and lateral corticospinal tracts, and the spinothalamic tract. It is formed at the craniovertebral junction by the convergence of two smaller paired ASAs that arise from the vertebral arteries distal to the origin of the posterior inferior cerebellar arteries and ends at the conus medullaris. In the cervical spine, blood flow to the ASA is augmented by radiculomedullary feeders from the vertebral and subclavian arteries, most notably, by the artery of cervical enlargement at C5 or C6. A total of 6 to 10 additional radiculomedullary feeders that derive from the aorta and iliac arteries reinforce the ASA at the thoracic and lumbar levels. The most prominent of these is the great anterior segmental medullary artery, referred to eponymously as the artery of Adamkiewicz, which usually arises from the left between T8 and L2. On an anteroposterior projection, this artery is 0.5 to 1 mm in diameter and has an ascending segment that makes a classic “hairpin” bend into the descending ASA in the midline. Angiographic identification of this artery is crucial before the embolization of middle thoracic to upper lumbar malformations. The main watershed area of the spinal cord is at the upper thoracic levels, and any ischemia of the ASA risks paralysis.¹⁰^–¹⁶

The PSAs arise from the vertebral arteries and descend along while supplying the posterolateral cord, including the posterior columns. They are augmented by radiculomedullary vessels from the vertebral, cervical, intercostal, and lumbar arteries, which also make the “hairpin” turn as they join the PSAs, albeit lateral to the midline and thinner in diameter. At the conus medullaris, the paired PSAs merge with the ASA to form the cruciate anastomosis.¹⁰^–¹⁷

The non-neural elements of the spinal axis are also supplied by branches of the vertebral, intercostal, and lumbar arteries. The blood supply to the anterior and lateral aspects of the vertebral bodies of the cervical spine are provided by branches of the vertebral arteries and the thyrocervical trunk. In the thoracic and lumbar region, however, the bodies are predominantly fed by branches of the intercostal and lumbar radicular arteries, respectively. More distal branches of the radicular arteries circumscribe the vertebrae, thereby forming a vascular arcade anterior to the posterior longitudinal ligament, which in turn supplies the posterior aspect of the vertebral bodies. Even further radicular branches form an arcade in the posterior epidural space that supplies the lamina and a portion of the posterior spinous process, as well as the dorsospinal artery, which supplies the outer surface of the lamina and the posterior spinous process.^10,^12,^15,¹⁶

Venous drainage of the cord involves intradural and extradural portions and follows a distribution similar to that of the arteries. The intradural portion is further subdivided into the intramedullary and pial veins, whereas the extradural veins include veins of the spinal column and Batson’s plexus. The anterior and posterior spinal veins are the two major midline longitudinal trunks and are filled by sulcal veins. The ventral portion of the cord is drained by anteromedian and anterolateral veins, and posteromedian and posterolateral veins drain the posterior funiculi and dorsal horns.

It is important to note that the transition of a median vein into a radicular vein has the same “hairpin” turns as the arteries described earlier. Drainage of blood from the spine occurs through the valveless internal and external venous vertebral plexus, which is connected to the azygos and hemiazygos venous systems.^11,^13,¹⁵^–¹⁷

Diagnostic Imaging of Spinal Vascular Malformations

Magnetic Resonance Imaging

Digital subtraction angiography remains the “gold standard” as the most reliable and accurate method for detecting spinal vascular malformations. Spinal angiography, however, is invasive, has associated risks, and should therefore be reserved for patients who are likely to have spinal vascular malformations and for those who require endovascular therapy. This is particularly true since the advent of magnetic resonance imaging (MRI), which is currently the best modality for noninvasive evaluation.^11,¹⁸

MRI findings of spinal cord edema coupled with the demonstration of flow voids secondary to dilated arteries or veins within or on the surface of the spinal cord are characteristic of spinal vascular malformations and suggest the diagnosis. In addition, MRI is useful in evaluating associated findings such as cord atrophy, hematomyelia, thrombosis, and cavitation.^11,¹⁹ Neither the location of pathologic vessels nor the intramedullary imaging findings seem to be related to the level of the fistula.¹¹ Localizing the fistula, therefore, may be very difficult and often necessitates spinal angiography. Newer noninvasive diagnostic techniques such as contrast-enhanced magnetic resonance angiography (MRA) with relative fast acquisition protocols and larger fields of view have been developed to assist in fistula localization.^11,¹⁹

Computed Tomography

Recently, there have been several small case series investigating the value of multidetector computed tomographic (CT) spinal angiography in diagnosing spinal vascular malformations. The benefits of this modality over contrast-enhanced MRA include a shorter acquisition time with increased scanning range and spatial resolution.²⁰ In addition, there is limited experience in using immediate postembolization CT scans as a tool for predicting permanent fistula occlusion and the prognosis for outcome.²¹

Digital Subtraction Angiography

Angiography confirms the diagnosis and determines the feasibility of either surgical or endovascular treatment. Preferably, spinal angiography is performed with the patient under general anesthesia and with controlled respiration to avoid motion artifact. This increases the resolution of images, an advantage important in avoiding complications, such as missing a small spinal cord artery, which can lead to disastrous results during embolization.^12,²²

The goal of angiographic evaluation is to demonstrate the angioarchitecture of the pathology and the normal spinal cord blood supply above and below the lesion. The majority of this information can be obtained by straight anteroposterior views. Occasionally, lateral and oblique views are required. For cervical (and complex thoracic and lumbar) lesions, the bilateral vertebral arteries, ascending and deep cervical arteries, and supreme intercostal arteries need to be evaluated. Occasionally, for larger lesions in the cervical region, the occipital and ascending pharyngeal arteries may provide an indirect supply. It is also important that the evaluation be continued until the blood supply for the normal spinal cord is demonstrated. For thoracic and upper lumber lesions, the bilateral intercostal arteries and lumbar arteries are studied until the entire blood supply to the lesion and the normal spinal cord above and below the lesion is demonstrated. If spinal cord arteries are not seen on the lumbar artery angiogram, the lateral and median sacral arteries should be studied because the spinal cord artery can be derived from them via the filum terminale.

The basic technique of spinal angiography is the same for all diseases involving the spinal cord. A high-volume injection is not necessary; however, prolonged filming (>30 seconds) is mandatory for spinal dural AVFs because of their slow-flow nature.

If a spinal dural AVF is not discovered despite bilateral angiography from the supreme intercostal artery to the internal iliac artery, the external carotid arteries and the vertebral arteries should be studied to rule out an intracranial pial or dural arteriovenous malformation (AVM) draining to the spinal cord.²³

Principles of Endovascular Therapy

The remainder of this chapter focuses on endovascular therapy for arteriovenous lesions using the most recent classification described by Spetzler and colleagues (see Table 396-3 and Chapter 395).²⁴ Because cavernous malformations are angiographically occult, endovascular therapy has no role in their treatment.^24a

Techniques

The goal of endovascular treatment of spinal vascular malformations is obliteration of either the fistula in AVFs or the nidus in AVMs. Endovascular therapy for spinal vascular malformations should be performed with the patient under general anesthesia and with controlled respiration, as described earlier for spinal angiography. Many different catheters are available for cannulation of the required vessels. Systemic heparinization is initiated before microcatheter navigation to achieve an activated clotting time two to three times the normal clotting time. Once the pathology is identified, selective and superselective catheterization of the feeding pedicle can be performed with a hydrophilic microcatheter, which in most cases is used with a hydrophilic microwire. The value of performing superselective catheterization is that it allows occlusion of the fistula or AVM nidus without compromising supply to the normal cord by going beyond the branch that supplies the cord, it allows direct access to the abnormality, and it permits penetration with embolic material. In addition, superselective catheterization facilitates characterization of the anatomic structure of the lesion. Furthermore, direct access to the pathology is important because proximal occlusion without reaching the pathology has no long-lasting effect and makes further treatment difficult.

Monitoring

Many authors advocate the adjunctive use of physiologic monitoring with somatosensory evoked potentials (SEPs) and motor evoked potentials (MEPs) during embolization of spinal cord vascular malformations.²⁵^–³⁰ In these situations, a continuous infusion of propofol and fentanyl is used rather than halogenated agents or muscle relaxants. This allows the detection of any iatrogenic abnormal spinal cord function that may be related to the embolization; if an abnormality is detected, the physician can either abandon the procedure or change the position of the catheter. Because the changes detected by physiologic monitoring may occur in a delayed fashion after the insult, chemical provocative testing can be performed under SEP and MEP monitoring. The chemical provocative test is performed by injecting 50 to 75 mg of amobarbital sodium (Amytal Sodium), followed by 20 to 40 mg of lidocaine (Xylocaine).^26,^28,²⁹ Amobarbital is used to test functional changes in neurons, and lidocaine is used to test axonal function. If a decrease in amplitude (of 50%) or an increase in latency (>10%) of SEPs or disappearance of MEPs is induced by the chemical provocative test, embolization from this catheter position is aborted.^26,^28,²⁹ Recovery of SEPs and MEPs to baseline values usually occurs within 5 to 10 minutes. Further superselective catheterization of the same pedicle after advancing the catheter closer to the nidus—or another pedicle if the former approach still fails—is performed after searching for a safer catheter position.

Niimi and colleagues have recently reported their experience in monitoring embolization of spinal arteriovenous lesions. In their experience of 60 provocative tests during 84 angiographic procedures in 52 patients with intended endovascular embolization, they found only one false-negative result (negative predictive value of 97.6%), in which the patient had a transient increase in spasticity after embolization with n-butyl cyanoacrylate (NBCA) through an ASA feeder despite a negative provocative test.²⁸

After embolization, the patient should be admitted to a neurosurgical intensive care unit and undergo continuous monitoring for neurological changes. Corticosteroids are frequently used to minimize spinal cord swelling in patients with spinal cord AVMs; those with AVFs often do not require steroids. In patients with large AVFs or when there is significant preexisting venous congestion, postprocedure heparinization should be considered to prevent progressive venous thrombosis and potential worsening of the patient’s neurological condition.

Serial follow-up angiograms are indicated to monitor for lesion progression. If complete occlusion of the malformation was achieved, the angiogram is repeated at 3 months, and if unchanged, again at 1 year and finally at 3 years. If complete treatment of the lesion was not achieved or possible, angiographic follow-up is based on clinical grounds. Progression or recurrence of neurological symptoms warrants repeated imaging.

Embolic Material

Different embolic agents are available to treat spinal cord vascular malformations, and there are different opinions regarding the best embolic agent. The choices include polyvinyl alcohol (PVA) and microspheres (of polyacrylamide and gelatin), liquid adhesives, and a variety of coils. The advantage of PVA and microspheres is their ease of use; however, the occlusive effect tends to be temporary and is frequently associated with recanalization.^10,^12,^22,³¹^–³³ Liquid adhesives such as NBCA are currently the best agents in terms of a long-lasting effect, but they are cumbersome to use and require that the microcatheter be advanced up to the nidus or fistula.^10,^12,^22,³¹^–³³ Moreover, the use of liquid adhesives requires a greater level of expertise because of their complex properties. Coil embolization is often used only for large fistulas and aneurysms. Onyx is a relatively new liquid embolic agent that is licensed in the United States for presurgical endovascular treatment of cerebral AVMs, but it is not yet approved by the Food and Drug Administration for the treatment of spinal AVMs. It is a mixture of ethylene vinyl alcohol copolymer dissolved in dimethyl sulfoxide (DMSO) that contains micronized tantalum powder for fluoroscopic visualization. Onyx is delivered in liquid phase via a microcatheter to the target lesion, where it transforms into a solid polymer as the DMSO diffuses away. In the largest patient series (n = 17) of spinal intramedullary AVMs treated exclusively with Onyx, Corkill and coauthors reported promising results in which total AVM obliteration was achieved in 6 patients (35.3%), subtotal obliteration (tiny and insignificant remnant) in 5 patients (29.4%), partial obliteration (substantial residual nidus) in 5 patients (29.4%), and procedure abortion in 1 patient (5.9%) secondary to intraprocedural rupture.³⁴ Improvement in neurological or functional status, or both, was noted in 14 patients for an 82% rate of overall good clinical outcome. No long-term studies exist, however, in which persistent obliteration of the lesion is monitored.

Endovascular Treatment of Spinal Arteriovenous Malformations

Extradural-Intradural Arteriovenous Malformations

Extradural-intradural AVMs are uncommon and have previously been referred to as juvenile, metameric, or type III AVMs. Treatment is typically palliative because complete obliteration of these extensive lesions is rarely achieved as a result of the unacceptable risk for neurological morbidity.^10,^12,¹³ Embolization of multiple feeding arteries with the delivery of PVA, NBCA, and coils is required, often in combination. Staged embolization may be necessary to decrease the arteriovenous shunt if the symptoms are related to steal phenomena. Compression symptoms secondary to the lesion may be reduced or even eliminated by embolizing the enlarged vascular structures. Staged resection may follow the embolization.^10,^12,^13,³⁵^–³⁹

Intradural Arteriovenous Malformations

Analogous to brain AVMs, intradural intramedullary spinal AVMs are located entirely within the cord parenchyma. These lesions were previously referred to as glomus-type lesions, type II AVMs. There is a slight male preponderance, and they can occur at any level of the spinal cord. Aneurysms associated with this type of AVM are common (Fig. 396-1).

FIGURE 396-1 A, A right supreme intercostal injection shows an intradural intramedullary arteriovenous malformation (AVM). The anterior spinal artery arises just from the same arterial feeder. B, Magnified view showing two veins draining an AVM with multiple aneurysms. Aneurysms associated with this type of AVM are common.

In the majority of patients, symptoms occur before 16 years of age. MRI has significantly improved the diagnosis, with subarachnoid hemorrhage or hematomyelia being the usual initial clinical findings. Recurrent hemorrhage is seen more commonly with spinal AVMs than with cerebral malformations, and 40% of these AVMs rebleed within the first year after the initial hemorrhage. The hemorrhage is frequently accompanied by significant neurological deficits, and each ictus is associated with significant mortality of 10% to 20%. Less frequently, a patient will have acute or progressive neurological deterioration related to an ischemic cause without evidence of hemorrhage. This is usually secondary to a mass effect or venous hypertension arising from arterialization of the draining veins and resulting in their impaired function.^25,⁴⁰ Some authors believe that a progressive myelopathy can occur as a result of vascular steal,⁴¹ but others disagree.²⁵ Rarely, the diagnosis is made in patients with no neurological signs or symptoms and is based on an evaluation for unrelated symptoms or associated metameric or skin lesions.

The poor prognosis of patients with untreated spinal cord AVMs justifies aggressive intervention, especially in young patients, even if significant neurological deficits are present.^10,^24,^25,⁴⁰ In some paraplegic patients, embolization may suppress their severe radicular pain and diminish their spasticity.^10,⁴² When the neurological deficits are due to recent hemorrhage, treatment is usually delayed to permit maximal recovery before the initiation of treatment.

The diagnosis can be made with MRI, but angiography is necessary to define the exact angioarchitecture of the lesion and identify potential associated aneurysms. This allows a clear understanding of the lesion, which is vital for planning and executing treatment. At the cervical level, these AVMs can have both anterior and posterior suppliers, with multiple feeders arising from the deep cervical, upper intercostal, and vertebrobasilar arterial systems.¹⁰ The largest feeder is frequently the artery of cervical enlargement at C5 or C6.^10,⁴² Attempted catheterization of small anterior feeders may produce spasm, which can be dangerous because emboli may then stop in the ASA and cause inadvertent paralysis. The collateral blood supply, however, is extensive in the cervical spine and may obviate any neurological insult. Posterior pedicle spasm is less dangerous but may preclude adequate nidal occlusion. At the thoracolumbar levels, embolization is often possible via a pathologically enlarged artery of Adamkiewicz, with collateral flow supplied by the cruciate anastomosis.^10,⁴² Because of a more limited collateral blood supply than in the cervical neck and in situations in which the blood supply to this region arises from the AVM feeders themselves, catheterization at these levels cannot be attempted without demonstrating an alternative arterial supply to the conus medullaris.

Some authors believe that surgical resection remains the mainstay of treatment.⁴³ In Spetzler and associates’ series, complete resection was not possible in only 8% of cases.^24,⁴⁴ Many other authors believe that embolization should be considered the treatment of choice, and others have demonstrated that in select cases, embolization can be the definitive treatment.²⁵ Many centers advocate a combined technique, with presurgical embolization aimed at facilitating surgical resection. Embolization to cure the lesion requires catheterization of the primary feeder close to the nidus, with obliteration of the nidus being achieved with either particulate matter or liquid adhesives. Care must be taken to not occlude the ASA or open up collaterals, which can lead to filling of the ASA and inadvertent stroke.¹⁰