As opposed to the spinal cord AVMs, spinal DAVFs are acquired shunts located within or adjacent to the dura along the spinal canal. Anatomically, a spinal DAVF is an abnormal arteriovenous shunt in the dura, which can be located anywhere along the dura but is most commonly situated near the nerve root exit.[31,37,48,52] The arterial supply usually arises from a dural branch of the dorsospinal artery, draining into a usually single, very dilated, tortuous vein that can pierce the dura quite far from a nerve root, eventually reaching the perimedullary venous system and producing venous hypertension that leads to a slowly progressive mixed motor and sensory myelopathy that can progress to irreversible myelopathy.[3,4,12,37,52] The chronic venous hypertension reduces the arteriovenous pressure gradient and decreases tissue perfusion, resulting in progressive hypoxia to the spinal cord, producing intramedullary vasodilation, further decreasing blood flow, with possible exhaustion of autoregulation in the affected areas. This will decrease tissue perfusion and generate edema and progressive loss function.
Spinal DAVFs are the most frequent arteriovenous shunts in older adults, usually presenting after the 4th or 5th decade of life with a heavy male predominance (5:1). The most frequent location is in the midthoracic spine. A cervical localization is rare, but one should be aware that dural shunts located at the level of the foramen magnum or above can present with spinal cord myelopathy due to spinal cord venous drainage.[13,34,55] The clinical presentation is characterized by progressive motor and sensory symptoms, with spasticity, paresthesias, pain, bladder, and bowel disturbances, and sexual dysfunction.[37,46,48,53] Hemorrhage is extremely rare. A lag between initial symptoms and diagnosis is common, on average 10.5 months in our series.
In a recent paper, Geibprasert et al. made an important contribution to our understanding of these fistulas. These authors were able to connect cranial and spinal DAVFs based on embryological and developmental aspects of the venous drainage of the central nervous system, classifying them according to their venous drainage into ventral epidural, dorsal epidural, and lateral epidural, generating 3 groups with very different demographics, presentation, and outcomes. As they pointed out in their paper, the decision to treat is still based on the presence of venous reflux into cortical or perimedullary veins, but as with spinal cord AVMs, grouping these lesions based solely on anatomical and angiographic characteristics ignores very important differences and hinders the advancement of our understanding.
Magnetic resonance imaging is the initial investigative modality of choice, usually showing signal changes within the cord, no mass effect, and nonspecific slight enhancement with Gd, suggesting a nonneoplastic or nondemyelinating cause of the symptoms.[25,35] The identification of abnormal perimedullary flow voids points to a vascular abnormality. These findings on MR imaging are nonspecific, and spinal angiography is necessary to confirm the diagnosis and establish the exact location of the nidus and the point at which the vein pierces the dura reaching the intradural space. Improvements in MR imaging technique now allow the establishment of an area of interest and help to focus spinal angiography at a specific segmental level (Fig. 6). During angiography, it is important to assess the blood supply to the spinal cord, particularly the origin of the ASA. The origin of the ASA can be located at the same level as the radicular supply to the dural fistula, a contraindication for endovascular treatment (Fig. 7). Bimetameric supply (a location of the shunt in the dura between 2 adjacent nerve roots resulting in dual arterial supply) can occur in a percentage of cases, with important implications for treatment and postembolization angiographic control.
Images obtained in a 76-year-old man with a slowly progressive neurological deficit over several months. A T2-weighted MR image (A) demonstrating increased signal within the cord (arrow). Contrast-enhanced sagittal MR image (B) showing early filling of the perimedullary venous system (arrows), whereas a 3D reconstructed coronal view (C) demonstrates the likely level of the dural shunt (arrow), confirmed at spinal angiography (D) and at surgery (E) when the enlarged radicular vein (arrow) was clipped and disconnected.
Images obtained in an 80-year-old man with a slowly progressive neurological deficit. Coronal CT angiography view (left) identifies the probable level of the DAVF (arrow). An angiogram (right) demonstrating the radiculomedullary artery (ASA, black arrows) arising from the same radicular artery as the dural shunt (white arrow) precluding safe endovascular treatment.
Considering that endovascular and surgical treatments have low risk and good results as well as the known natural history of a progressive neurological deficit if untreated, we favor treatment for virtually all spinal DAVFs, with the goal of occluding the draining vein as it exits the dura, disconnecting the fistula from the spinal cord venous system. An attempt at endovascular treatment is always performed as a first treatment choice, usually at the time of diagnostic angiography, done after induction of general anesthesia. A permanent, low-viscosity liquid adhesive agent, NBCA, is used when attempting to reach the nidus and the proximal vein. Particulate agents, such as polyvinyl alcohol or Gelfoam are contraindicated because of very high recanalization and recurrence rates.[27,38] The fistula is considered treated only if NBCA reaches and occludes the proximal portion of the draining vein. Our initial success rate with endovascular occlusion of spinal DAVFs is 25%, with no evidence of recanalization. Complications are rare and are usually associated with the failure to recognize a common origin for the spinal DAVF supply and the anterior or posterior spinal arterial systems or inadvertent deposition of the embolic material beyond the nidal-venous junction, reaching the perimedullary venous system.
If endovascular therapy is unsuccessful, surgery is indicated. We usually perform the surgical disconnection during the same hospital stay. Although in the past excision of the dura at the site of fistula was proposed,[38,52] we, as most groups, treat spinal DAVFs by simple venous disconnection.[1,40,53] Outcomes are usually good, with the majority of patients showing clinical improvement or at least stabilization of symptoms after interruption of the abnormal venous drainage. The best results are seen in patients treated soon after onset of symptoms. Motor and deep sensory symptoms are more likely to improve than superficial sensation and sphincter disturbances.[8,36,50,53] Patients submitted to surgical disconnection should have a postoperative angiogram confirming the interruption of the intradural venous drainage before leaving the hospital. If there is no clinical improvement or recurrent symptoms occur, additional MR imaging and angiography is advised. It is important to remember that signal changes within the cord can persist for a while even in patients with clinical improvement. Therefore, absence of improvement on MR imaging does not necessarily reflect residual or recurrent spinal DAVF.
Although multiplicity of spinal dural arteriovenous shunts is extremely rare as is the metachronous occurrence of such lesions, any development of new or worsening neurological deficit should warrant additional investigation with MR imaging to exclude such diagnosis or to exclude venous thrombosis.
Neurosurg Focus. 2009;26(1):E6 © 2009 American Association of Neurological Surgeons
Cite this: Spinal Cord Vascular Shunts: Spinal Cord Vascular Malformations and Dural Arteriovenous Fistulas - Medscape - Jan 01, 2009.