Spinal Dural Vascular Malformations

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Chapter 104 Spinal Dural Vascular Malformations

The most common type of spinal cord arteriovenous malformation (AVM) is the spinal-dural arteriovenous fistula (SDAVF), also known as a type I spinal AVM. First described by Gaupp1 in 1888 as “hemorrhoids of the pia mater,” spinal-dural AVMs have recently become better recognized and understood with the advent of modern superselective neuroangiography. As a distinct subtype of spinal AVMs, these lesions require specific treatments that differ from those for intradural or intraparenchymal vascular malformations. At present, these AVMs are best treated surgically, although endovascular techniques may play an increasing role in the future.

Spinal Vascular Anatomy

A comprehensive knowledge of the vascular anatomy of the spinal cord is necessary to understand the pathologic and clinical aspects of SDAVFs and their differentiation from other spinal AVMs. The spinal cord receives its blood supply from three separate longitudinal vessels: one anterior spinal artery and two posterior spinal arteries (Fig. 104-1).

The anterior spinal artery is formed by the convergence of branches from each of the distal intradural vertebral arteries and descends in the anterior median sulcus. Additional contributions are received from radiculomedullary arteries branching from the vertebral, ascending cervical, intercostal, and lumbar arteries. These arteries make a characteristic hairpin turn as they join with the anterior spinal artery. The largest of these is the artery of Adamkiewicz, or arteria radicularis magna. Usually arising from a lower intercostal artery on the left side, this vessel supplies the ventral two thirds of the thoracic spinal cord and conus medullaris. Another large radicular artery from the C5 or C6 level often predominates in the cervical region and is known as the artery of cervical enlargement. As they enter the dura mater at the level of the nerve root sleeve, the radiculomedullary arteries give off small branches that supply the dura. These are the vessels that form the enlarged arterial feeders to SDAVFs.

The posterior spinal arteries course along the dorsolateral aspect of the spinal cord behind the dorsal nerve roots. They also receive supply from radiculomedullary arteries. The two posterior spinal arteries supply the dorsal one third of the spinal cord, including the posterior columns and portions of the lateral columns of the spinal cord. They join with the distal anterior spinal artery at the end of the conus medullaris to form the cruciate anastomosis.

The venous drainage of the spinal cord is via small radial veins that run from the center to the periphery of the cord and into the coronal venous plexus that ascends and descends along its dorsal surface. These surface veins converge to form medullary veins that exit at the root sleeve. The coronal veins along the dorsal spinal cord surface become dilated and tortuous in patients with SDAVFs, often forming a convoluted vascular mass along the dorsal aspect of the spinal cord.

Classification

Although this chapter addresses only SDAVFs, the classification system for spinal AVMs should be understood to appreciate the differences between these lesions and other types of AVMs. Recognizing and properly categorizing spinal AVFs, particularly distinguishing between dural and intramedullary lesions, is important for treatment decisions. Historically, spinal-dural AVMs were first referred to as angioma racemosum venosum by Wyburn-Mason2 in his 1943 monograph. This was later shortened to just angioma racemosum by Bergstrand et al.3 and Krayenbuhl et al.4 Malis5 later referred to them as long dorsal AVMs. Currently, dural AVF or type I spinal AVM is the most appropriate term.6,7

Type II spinal AVMs, also known as glomus AVMs, represent intramedullary AVMs with a true compact nidus.2,5 Type III spinal AVMs are also known as juvenile AVMs and are much less common. They are larger, more extensive lesions that often involve intramedullary, extramedullary, and extradural spaces over more than one spinal level.5,8 Last, type IV AVMs are intradural extramedullary AVFs that were first described by Djindjian et al.9 and later classified as type IV lesions by Heros et al.10 Unlike type I dural AVFs that arise from dural branches, these lesions are fed from the anterior spinal artery or, less commonly, from the posterior spinal artery. They flow directly into an enlarged venous outflow tract, lie outside the spinal cord and its pia mater, and vary in size and flow.11

Pathophysiology

It is important to understand that the clinical signs and symptoms develop because of venous hypertension of the spinal cord. The fact that the patient has a small AVM of the dura is inconsequential. What is of utmost importance, however, is that the venous outflow of this AVM is into the coronal venous plexus of the spinal cord. This leads to venous congestion of the plexus, stagnation of arterial flow through the spinal cord, decreased perfusion pressure, ischemia, and edema formation.

It is easiest to think of SDAVFs as consisting of two relevant compartments: a vascular malformation (AVM) nidus located in the spinal dura and the medullary vein and coronal venous plexus draining the AVM. Usually, a single radiculomedullary artery enters the dural root dorsolaterally at the dural root sleeve. This artery supplies an AVM that is typically embedded within the dura mater around the proximal nerve root sleeve and/or adjacent spinal dura (Fig. 104-2). The venous outflow of the AVM is then via retrograde flow through a medullary vein that has anastomosis with the coronal venous plexus. This medullary vein and coronal venous plexus is obvious on the superselective spinal angiogram. This medullary vein and coronal venous plexus are normal but dilated from the flow through the AVM lying in the dural wall.

The radiculomedullary arterial feeder tends to be separate from the branch that normally penetrates the dura to supply the anterior or posterior spinal arteries. Occasionally, however, a single vessel supplies both the malformation and the anterior spinal artery.

Although most SDAVFs have a single arterial feeder, some may have two arterial feeders that enter at separate levels.5,12 The additional feeders appear to travel within the dura mater to the fistula nidus located in the wall of the dura, where they converge and communicate with the intradural efferent medullary vein. No valves are present within the radial veins or coronal plexus and, therefore, the increased pressure is transmitted to the spinal cord parenchyma. It is critical to recognize the additional feeding branches when these are present, because failure to obliterate all inflow channels can lead to recurrence of the AVF.13

On angiography, the nidus appears as a small area of fine vessels near the neuroforamen. From there, outflow of the fistula passes intradurally through the medullary vein and then into the dorsal venous plexus along the spinal cord surface. This plexus becomes dilated and tortuous because of the arterialized venous pressure and may extend over the full length of the cervical, thoracic, and lumbar spine.

Clinical Characteristics

Most patients with type I dural AVFs are between the ages of 40 and 70, with few showing symptoms before age 30. Over 80% of patients are male, and no familial tendency has been identified.14,15 This differs from types II and III spinal AVMs, which typically appear in patients younger than age 40 and have less male predominance. This age discrepancy suggests that type I lesions may be acquired rather than congenital.

The typical pattern of symptoms and clinical course was first described by Aminoff and Logue,16,17 and this description has been supported by other more recent reports.131518 The most common symptom associated with dural AVFs is pain, which may be local, radicular, or nonspecific. Most patients also experience leg weakness and sensory changes by the time of diagnosis.14,15 Spastic paraparesis, along with loss of pain and temperature sensation, is the most common neurologic pattern. Most patients have a distinct sensory level corresponding to the level of the vascular nidus. Disturbances of bladder, bowel, and sexual function are less common initially but become more frequent over time.

Most patients experience a gradual onset of symptoms and a slowly progressive clinical deterioration.16,17 Only 10% to 15% of patients experience an acute onset of symptoms, in contrast to patients with types II and III AVMs that lead to an acute onset of symptoms in more than 50% of patients. The progressive neurologic deterioration occurring with these lesions was first documented by Aminoff and Logue.17 At 6 months after onset of symptoms, only 56% of patients had unrestricted activity, and 19% were severely disabled. At 3 years after onset, only 9% had no restrictions, and 50% were severely disabled.

Because of the infrequency and gradual course of SDAVFs, symptoms are often present long before the diagnosis of SDAVF is made. In the series of 55 patients studied by Symon et al., only 33% were diagnosed within 1 year of symptom onset, and 66% were not diagnosed for more than 3 years.15 In fact, given the large amount of edema found on T2-weighted MRI, many patients will have undergone spinal cord biopsy in search of a tumor prior to proper diagnosis. On rare occasions, onset of symptoms can be acute, caused by thrombosis within the draining medullary veins. This produces a catastrophic, acute necrotizing myelitis that is often referred to as Foix-Alajouanine syndrome.19 Subarachnoid hemorrhage (SAH) is extremely uncommon with SDAVMs.14,15 In contrast, other types of spinal AVMs, particularly type II lesions, have a significant incidence of SAH.

Exercise and certain postures can exacerbate symptoms in patients with dural AVMs.1416 Because almost all dural AVMs have rostrally directed venous outflow, the greater venous hydrostatic pressure in the upright position may explain why symptoms worsen with standing.20 Types II and III AVMs, which have both rostral and caudal venous drainage, do not produce symptoms that change with position. Worsening symptoms have also been associated with physical activity, probably because of increased draining venous pressure during systemic hypertension.21

The key to making a timely diagnosis of an SDAVF lies with a physician being aware of this condition and having a high clinical suspicion for the presence of this lesion.

Radiologic Evaluation

Since the first diagnosis of spinal AVM was made by myelography in 1927,20 most patients have undergone myelography as part of their radiologic evaluation. Although the typical findings of tortuous channels outlined by intrathecal contrast are almost pathognomonic for spinal AVM, in recent years myelography has largely been replaced by MRI as the initial imaging study.22 Irregular, serpentine flow void signals suggest vessels can often be seen along the dorsal surface of the spinal cord (Fig. 104-3). MRI can also differentiate type I from type II and type III lesions, and it is the test of choice for visualizing spinal cord cavernous malformations. Moreover, T2-weighted MRI images often suggest extensive edema of the cord.23,24

The definitive radiologic study for SDAVFs is selective spinal angiography. Aortography may demonstrate the general location of the AVM; however, this large-volume contrast injection may limit the extent of the superselective injections available because of contrast load reasons. Generally, bilateral selective injections of radiculomedullary branches are performed in both anteroposterior and lateral views to demonstrate the precise location, extension, hemodynamic characteristics, and venous drainage of the lesion. Multiple levels above and below the nidus must be studied to identify any additional feeding vessels. It is also essential to visualize the anterior spinal artery above and below the AVM to determine whether it has a supply in common with the AVM. Although this is a rare configuration with dural AVFs, it is a critical factor in planning treatment. Most dural AVFs are located along the dorsal aspect of the spinal cord, although 15% of patients may have dilated veins ventral to the spinal cord, and almost all of these lesions are found in the midthoracic to lower thoracic or thoracolumbar region.5,14,15

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