Spinal Arteriovenous Malformations


John Ratliff, MD and Edward Connolly, MD
Department of Neurosurgery; Louisiana State University Medical Center and Ochsner Clinic
New Orleans, Louisiana

[Introduction | Case Presentation | Discussion | Type I AVM | Type II AVM | Type III AVM | Type IV AVM | References]

Introduction:
Vascular malformations of the spinal cord and dura compose 3-4% of spinal cord masses. While these tumors are rare, they remain important clinical entities and may yield severe neurological deficits and even death if not recognized and treated appropriately. Due to their rare nature, these lesions are not often considered in the differential diagnosis of patients presenting with spinal cord masses or progressive myelopathy. At the Ochsner Medical Foundation, two patients recently presented with the most common variety of spinal arteriovenous malformation (AVM), the dural arteriovenous fistula. In this article, we will briefly present the clinical history and evaluation of one of these patients, and then review the different types of spinal AVMs. Spinal arteriovenous malformation should be included in the differential diagnosis of any patient presenting with progressive myelopathy and a cord mass recognized on imaging studies.

Case Presentation:
TW is a 66 year old right handed white male who presented with a 6 months history of progressive lower extremity weakness. By the time of neurosurgical evaluation, the patient required assistance of a cane or walker for ambulation. The weakness was symmetric, with both proximal and distal musculature affected equally. TW also noted bilateral lower extremity "tingling," most notable over the hips and anterior thighs, with onset approximately 3 months prior to presentation. There was no significant associated pain. There was no history of either cranial or spinal trauma. The patient noted no bowel or bladder difficulty.

Examination revealed diminished motor strength (4- to 4+/5) bilaterally, with proximal musculature affected more than distal group. The patient reported diffuse decrease in sensitivity to pin prick and light touch, although no dermatomal distribution to the sensory loss was evident. There was no sensory level. Proprioception and vibratory sense were impaired in both lower extremities. Deep tendon reflexes were hyperactive (3/4) in both lower extremities. Babinski reflexes were present bilaterally. No clonus was evident. The patient's gait was grossly spastic, and a walker was required for ambulation. No fasiculations or fibrillations were evident. There was no gross muscular atrophy. Rectal exam was heme negative with good tone. The remainder of the neurological exam revealed no abnormalities.

The patient was evaluated initially with MRI of the cervical, thoracic, and lumbar spine. The only abnormality seen in this exam was increased signal on T2 weighted images within the thoracic cord, extending from approximately T-4 to T-12. The cord was noted to be slightly enlarged over this region, with very mild to no contrast enhancement- see Figure 1. No other pathology was evidenced. Differential diagnosis of this lesion included focal demyelination, post-infectious or post-inflammatory myelitis, cord ischemia or infarction, sarcoidosis, and primary or metastatic neoplasm. Repeat MRI 2 months later revealed only a slight increase cord diameter from T-6 to T-8; there was no contrast enhancement.


Figure 1: Sagital T2-weighted thoracic MRI scan of the patient presented showing the signal abnormality (hyperintensity) in the thoracic cord extending from about T4 toT12 levels (maximum from T6-10). Also note the subtle serpengious abnormality in the dorsal aspect of the spinal canal at approximately T9-10 (red marker). In retrospect, this was the venous dilatation of the dural arteriovenous fistula. Download a larger image of above.

Due to the inconclusive MRI findings, thoracic myelography and post-myelogram CT were obtained. Vascular dilatation was evidenced posteriorly within the thecal sac on both exams, although on plain myelogram the lesion was most evident (Figure 2).


Figure 2: Thoracic myelogram (AP view) showing the serpengious filling defect inthe thoracic cord characteristic of a dilated venous structure. This abnormality (red marker) is evident from about T6 upto T11. Download a larger image of above.

Thoracic spinal angiography revealed a spinal dural arteriovenous fistula at T-10 with prominent superior venous dilatation (Figure 3).


Figure 3: Thoracic spinal angiogram showing the dural fistula (red marker) at T10 along with the dilated vein. The fistula is being fed by a prominent radicular artery from the leftside.

Due to the large size of the feeding radicular artery and fear of cord infarction with interventional radiologic occlusion of the lesion, the patient was taken to the OR for ablation of the fistula. Venous dilatation was marked at the time of exploration (Figure 4A), and a single feeding vessel was located at the site of dural nerve root entry (Figure 4B).


Figure 4: A (left pict): Intraoperative dorsal view of the spinal cord showing the prominent arterialized venous dilatation (white marker). Download a larger image above. B (right pict): Intraoperative magnified view of the dorsolateral aspect of the spinal canal and cord showing the feeding artery to the dural arteriovenous fistula (white marker). The feeding vessel was located at the site of the nerve root entry. This is characteristic of a Type I spinal arteriovenous malformation (AVM). Download a larger image of above. See text for full explanation of the classification of spinal AVMs.

The patient tolerated the procedure without difficulty, and is now undergoing physical therapy for rehabilitation of persistent lower extremity weakness.

Discussion:
This case illustrates some of the diagnostic difficulties encountered in evaluation of spinal vascular pathology, and also evidences the delay in diagnosis which often complicates these lesions. We will now review the four different types of spinal AVM's, note their different epidemiologic and clinical aspects, and cover different treatment options. Readers interested in a more thorough review of endovascular therapy in spinal AVM's are referred to a recent review by Hodes et al (see references below).

Type 1 Spinal AVM: Dural Arteriovenous Fistula
Dural arteriovenous fistulas are the most common variety of spinal cord AVMs, comprising 80-85% of spinal AVMs. These lesions show a male predominance (80-90%) and generally present in late adulthood, ages 40-60. Presentation is generally with radiculomyelopathy, followed by slow but progressive neurological deterioration. Subarachnoid hemorrhage is very uncommon in dural AV fistulas, and acute deterioration in neurologic function is unlikely.

Site of pathology in these lesions is within the dural root sleeve, where a direct arteriovenous fistula develops, generally with a single dural artery feeder vessel (Figure 4 above). Additional small feeding vessels from adjacent levels may also penetrate the dural and contribute to venous outflow. Hence these lesions are not true AVMs but instead AV fistulas. Venous drainage of the AV fistula is by a high-pressure, low-flow arterialized vein intradurally. Venous dilatation may extend rostrally and caudally from the fistula site. Eventual drainage is to the coronal venous plexus. Increased pressure in the coronal venous plexus yields spinal cord dysfunction, via chronic venous hypertension and loss of autoregulatory tone. Increased venous pressure yields chronic spinal cord ischemia, cell loss, and cord atrophy. Impaired autoregulation yields direct transmission of changes in systemic arterial pressure to the spinal cord without the normal dampening effect of the venous plexus.

Goal of treatment is isolation and obliteration of the fistula and draining veins, which normalizes venous pressure and corrects venous hypertension. Two different therapeutic modalities are possible: embolization of the feeding vessel, via endovascular techniques, and direct surgical ligation, through laminectomy and direct intradural exposure of the fistula. Fistula obliteration yields correction of the lesion; resection of arterialized veins is not necessary and may actually be traumatic to the spinal cord. Embolization in well chosen cases is a safe and effective therapy for dural AV fistulas. In cases where the site of origin of the AV fistula is a large radicular artery and embolization may be difficult or dangerous, direct surgical exposure is preferred.

Type II Spinal AVM: Intramedullary AVM
This is a true intramedullary arteriovenous malformation of the spinal cord. These lesions are characterized by a compact intramedullary nidus, with feeding vessels arising from the anterior or posterior spinal arteries, or both, and drainage into an arterialized coronal venous plexus. Figure 5 depicts a type II spinal AVM with an intramedullary nidus. In contradistinction to spinal AV fistulas, flow within these lesions is high pressure and low flow, with rapid filling on angiogram and early venous drainage.


Figure 5: Spinal angiogram (AP; intermediate phase) showing the intraparenchymal nidus (red marker) as well as the feeding and drainage vessels. This type of AVM is supplied by branches of the spinal arteries and has a propensity for intramedullary hemorrhage.

Intramedullary AVMs have equal incidence distribution between men and women, and generally present at a much earlier age (average age of presentation: 24). The clinical course of these lesions is marked by progressive and fluctuating myelopathy, often overlaid by periods of acute neurologic deterioration secondary to hemorrhage within the AVM. Sudden apoplectic presentation, often with profound neurologic impairment and possible transverse myelopathy, is common in spinal AVMs. SAH often occurs in these lesions, occurring in 50% of cases. While dural AV fistulas are more commonly found in the lower thoracic and lumbar spine, true intramedullary AVMs occur throughout the cord, and hence presentation with upper extremity symptoms is possible.

Treatment of intramedullary spinal AVMs involves initial embolization of feeding vessels using particulate matter. Immediate clinical improvement is often noted after embolization, through reduction in arterial steal and improved cord perfusion; however, recanalization may occur over time, with continued risk of hemorrhage. Hence, surgical resection of residual nidus after embolization is generally considered.

Type III Spinal AVM: Juvenile AVM
Juvenile spinal AVMs are extremely rare lesions. These lesions are again true AVMs, with an intramedullary nidus which may occupy the entire spinal canal at the involved level. Cord tissue is present within the AVM interspaces. Extramedullary and even extraspinal extension of the lesion is possible (Figure 6).


Figure 6: Diagram of a Type III spinal AVM demonstrating the extensive involvement of the spinal cord as well as the intra- and extradural spaces. The bone can also be involved by these AVMs. The AVM is supplied, to some extent, by all spinal arteries at multiple levels. This AVM is very difficult to treat and has a poor prognosis. This diagram is reprinted with permission (Oldfield EH, Doppman JL: Spinal arterioenous malformations. Clin Neurosurg 34:161-183, 1988). Download a larger image of above.

Juvenile AVMs are large and complex lesions, with multiple arterial feeding vessels often arising from different cord levels. Hemodynamically, this lesion manifests both high flow and high pressure, often yielding an auscultatable spinal bruit over involved levels. They occur most commonly in adolescents and young adults. Presentation and treatment are similar to type II AVMs; however, prognosis for these lesions, considering their size and vascular complexity, is understandably very poor.

Type IV Spinal AVM: Perimedullary Arteriovenous Fistula
These rare lesions are similar to type I spinal AVMs, in that they are not true AVMs but instead arteriovenous fistulas. The fistulous connection in this lesion is intradural but extramedullary, with feeding vessel or vessels arising from the anterior spinal artery. Venous drainage is via an enlarged coronal venous plexus. No small vessel network or glomus is evident connecting arterial and venous halves of the circuit (Figure 7). These lesions were first described by Djindjian, and then classified as a type IV spinal AVMs by Heros et al.


Figure 7: Diagram of a Type IV spinal AVM showing the direct fistula between the anterior spinal artery and an adjacent vein. These lesions are typically located anterior to the spinal cord and the fistulous connection is intradural, but extramedullary. These AVMs most often occur at the level of the conus medullaris and are subclassified into three groups (type IVA,B,C) depending on size and complexity of the fistula.This diagram is reprinted with permission (Ojemann RG, Heros RC, Crowell RM: Surgical Management of Cerebrovascular Disease. Baltimore, MD, Williams and Wilkins, 1988, p 462). Download a larger image of above.

These lesions may present in young adults, but presentation in the third to sixth decade is more likely. SAH is possible with intradural AV fistulas, with subsequent acute neurologic deterioration. A gradual but progressive neurologic deterioration is common.
Three subcategories of intradural spinal AV fistulas have been recognized, with different treatment options appropriate for each. The simplest of these, the type IVa, features a single feeding vessel, often the artery of Adamkiewicz, with low flow through the arteriovenous shunt and moderate venous enlargement. Endovascular techniques are difficult with these lesions, due to the small size of feeding vessels. Surgical excision is therefore often mandated. Type IVb AV fistulas are intermediate in size, often with multiple feeding vessels, and more marked venous enlargement. Venous ectasia may develop at the site of shunting. Embolization in these lesions is easier, due to the increased size of feeding vessels. In cases of incomplete shunt obliteration with an endovascular approach, direct surgical excision may be necessary. The largest of the intradural AV fistulas are the type IVc, which feature giant, multipediculated fistulas, high blood flow, and large, tortous draining veins. Spinal ischemia may develop in these lesions secondary to vascular steal. Due to the size of these lesions, surgery is technically difficult and may jeopardize the spinal cord. Treatment is hence through combination of endovascular ablation, followed by surgical excision of retained elements.

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