The type I AVF represents the most common type of spinal vascular malformation and should be in the differential diagnosis in an adult presenting with gradually worsening myelopathy. This lesion, which
Table 16.1. Classification
New classification (Spetzler et al.1)
Neoplastic vascular lesions:
Hemangioblastoma Cavernous angioma
Vascular malformations: Epidural arteriovenous fistula
Intradural (pial) arteriovenous fistula
Extradural/ intradural AVM
Neoplastic vascular lesions:
Hemangioblastoma Cavernous malformation
Arteriovenous fistulas: Extradural
Arteriovenous malformations: Extradural/ intradural
Compact Diffuse Conus medullaris
Paraspinous vascular malformation:
Vertebro-vertebral AV fistula
Dorsal extramedullary AVM Peri-medullary AVF
Juvenile AVM, metameric AVM
Classic AVM, glomus AVM
Cord compression (enlarged veins), vascular steal, venous congestion
Venous hypertension/ compression; hemorrhage is rare Compression (enlarged veins/venous varix), hemorrhage, vascular steal, venous hypertension
Cord compression, hemorrhage, vascular steal
Hemorrhage, compression, vascular steal
Venous hypertension, compression, hemorrhage
Pain, progressive myelopathy
Acute myelopathy, MRI, pain, progressive angiography myelopathy
Progressive myelopathy, radiculopathy
MRI, angiography most authors have classified as a dural malformation, is subdivided into type A (single arterial feeder) and type B (multiple arterial feeders). The most common location for these malformations is between T4 and L3, with the peak incidence between T7 and T12.3 Malformations very uncommonly occur above the level of the heart, possibly owing to the helpful effect of gravity on venous drainage above the level of the right atrium. This lesion is composed of a direct fistula between the dural branch of a radicular artery (only rarely of a radiculo-medullary artery) at the level of the proximal nerve root and a radicu-lomedullary vein (type A, Figure 16.1), or several abnormal connections between branches of adjacent radicular arteries and a radiculomedul-lary vein (type B).
The arterialized radiculomedullary vein then transmits the increased flow and pressure to the valveless coronal venous plexus and longitudinal spinal veins. The radiculomedullary vein is usually enlarged and tortuous as a consequence. The mean intraluminal venous pressure is increased to 74% of the systemic arterial pressure.4 The normal venous pressure in the coronal venous plexus is approximately 23 mmHg, which is almost twice that of the epidural venous plexus; this gradient is necessary for venous drainage. In one series, the mean venous pressure in the coronal venous plexus was measured at 40 mmHg.1 The consequent venous hypertension causes progressive myelopathy, often leading to paraplegia and bowel, bladder, and sexual dysfunction, with gradual worsening over months to a few years.
The majority of patients become severely disabled within 3.5 years.1 The overwhelming majority of patients (79-85%) are men, and 86% of patients are 41 years of age or older at presentation.2,3 The mean age at presentation is 55, with patients as young as 26 reported as presenting with this kind of malformation. The most common presentation is progressive paraparesis of the lower extremities with sensory changes also.
Complaints of back and leg pain are common. Although the progression is usually continuous, it can also present in a stepwise fashion, or a waxing-waning course with gradual progression. Between 10 and 20% of patients can present with an acute exacerbation. The symptoms can be exacerbated by any physical activity that increases intraabdominal pressure, and thus central venous pressure, as well as by an upright posture (venous drainage hindered by gravity).
Figure 16.1. Dural arteriovenous malformation (fistula). (A) Schematic illustration of a dural arteriovenous malformation: 1, descending aorta; 2, lumbar artery; 3, dural artery; 4, dorsal somatic artery; 5, nerve root sleeve, 6, nerve-arteriovenous malformation complex; 7, radiculomedullary vein, 8, dorsal longitudinal vein. (B) Contrast-enhanced T1-weighted magnetic resonance image of a dural arteriovenous fistula (DAVF), a nonspecific diffuse enhancement, and swelling of the spinal cord (arrow). (C) Three-dimensional TOF magnetic resonance angiogram in coronal plane shows congested radiculomedullary vein (arrow) and dorsal median vein (open arrow). Superselective angiogram of an intercostal artery (D, arrow) shows (E) the DAVF (curved arrow), the retrograde draining and congested radiculomedullary vein (open arrow), and the congested dorsal median vein (heavy black arrow).
Van Dijk et al.5 reported a series of 49 consecutive patients treated between 1986 and 2001. The mean age of the population was 63, and 80% were men. Almost all these patients (98%) exhibited myelopathy, with 96% displaying leg weakness and/or paraparesis. Ninety percent had sensory numbness or paresthesias, and 55% had pain either in the lower back or lower extremities. Eighty-two percent had urinary incontinence/retention, and 65% complained of bowel dysfunction.
Atkinson et al.6 reported a second series of 94 patients treated between June 1985 and December 1999. All the patients had lower extremity weakness with or without perineal or bowel/bladder dysfunction. Five patients also had upper extremity symptoms, all of whom had high T2 signal within the cervical cord. Eighty-eight patients reported sensory loss, and 61 patients had bowel/bladder dysfunction. A very interesting finding in this series was an essentially 50-50 split among patients with symmetric versus asymmetric lower extremity symptoms; in addition, approximately 50% of patients demonstrated worsening of symptoms with erect posture/Valsalva maneuver and improvement with recumbent position. This effect was not as prominent in the group of patients with the most severe symptoms. Eight of the patients included in this series had posterior fossa dural arteriovenous shunts with drainage into the medullary venous system, which is a well-described phenomenon and necessitates the injection of the posterior fossa and external carotid arteries in completion of a total spinal angiogram. The most common misdiagnosis for these lesions was transverse myelitis.
The surgical treatment for type I malformations has been well described and essentially consists of performing one or more laminectomies and surgical disconnection of the draining vein, just distal to the fistulous site. In experienced hands, this is a very effective technique. Atkinson et al.6 reported a 97.9% success rate for obliteration of the fistula, with morbidity equal to that of patients undergoing decompressive laminectomy [one superficial wound and two deep venous thromboses (DVTs)].
The endovascular treatment of these lesions has also been well described. Before the availability of acrylate products ("glue"), treatment consisted of selective microcatheterization of the feeding artery, with particulate embolization of the fistula by means of polyvinyl alcohol (PVA) particles. Despite high rates of angiographic success immediately after treatment, this technique was associated with a high recurrence rate (<83%), owing to recanalization of the arterial feeding pedicles. With the availability of acrylate products, the recurrence rate has significantly diminished.
The consensus among interventional neuroradiologists at this time is that successful treatment of these malformations consists of penetration of the fistula and the proximal radicular draining vein to obviate the need for future surgery (Figure 16.1). The treatment protocol used in the series of patients presented by Van Dijk et al.5 used endovascular therapy as the first line of treatment because it is noninvasive, has a low complication rate, and offers the ability to obtain immediate angio-graphic control and confirmation of obliteration of the malformation. Using their endovascular treatment criteria, which included both the abil ity to penetrate the fistula and proximal portion of the draining vein, as well as the ability to treat the malformation in a single session, only 11 (25%) of the patients were treated via the endovascular route, all of whom demonstrated a clinical success rate and stability equivalent to that of surgery (mean follow-up of 32.3 months) with no permanent complications. Under less stringent criteria, other endovascular specialists using acrylate have reported success rates of up to 90%, but with recurrence rates of up to 23%.
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