A New Sign in Spinal Dural Fistula?
Abstract & Commentary
Synopsis: A number of magnetic resonance imaging (MRI) features of spinal dural AVF have been described in recent years, facilitating the early diagnosis and treatment of this disorder, which is essential to prevent permanent disability. In some cases, however, the classic MRI features may be absent or subtle.
Source: Hurst RW, Grossman RI. Peripheral spinal cord hypointensity on T2-weighted MR images: A reliable imaging sign of venous hypertensive myelopathy. AJNR Am J Neuroradiol 2000;21:781-786.
Nonhemorrhagic venous hypertensive myelopathy (VHM) results from a group of vascular lesions affecting the spinal cord that are characterized by arteriovenous shunting. VHM most commonly results from spinal dural arteriovenous fistulas (SDAVFs), acquired lesions in which blood is shunted from radicular arteries in the dural sheath around a nerve root into radicular veins and then into the perimedullary coronal venous plexus. This leads to venous hypertension, which is presumed responsible for the progressive neurologic deficit associated with these lesions. Parenchymal hemorrhage is not a feature of these lesions, unlike other spinal vascular malformations such as arteriovenous malformations or cavernous malformations, which may present with parenchymal hemorrhage. The diagnosis of an SDAVF is frequently delayed as the clinical presentation may be confusing. However, prompt recognition of SDAVF is essential as many of the neurologic deficits may reverse if the patient is diagnosed and treated in a timely manner.
The MRI features reported with SDAVF have included some combination of spinal cord enlargement, parenchymal enhancement, parenchymal T2 prolongation, and engorgement of medullary veins, with parenchymal T2 prolongation most common and engorgement of medullary veins most specific for the diagnosis. Hurst and Grossman describe peripheral hypointensity of the spinal cord in SDAVF on T2-weighted images in 11 consecutive cases of angiographically confirmed symptomatic SDAVF and four cases of intracranial DAVF with spinal drainage. This finding was sometimes subtle on fast spin echo T2-weighted images, and more conspicuous on true T2-weighted and gradient echo images. As pathologic studies in spinal venous hypertension have shown dilatation involving pial surface veins that extend retrogradely into the venous system within the cord parenchyma, it is hypothesized that the observed T2 shortening is the result of slow flow of blood containing deoxyhemoglobin within a distended spinal cord capillary and venous system. This would account for the increased conspicuity on imaging sequences that are more sensitive to paramagnetic effects. A second mechanism that was considered was accumulation of a paramagnetic substance such as iron and/or calcium, but this was considered unlikely based on imaging features and known pathological studies of SDAVF. Frank hemorrhage was also considered unlikely due to the imaging appearance and the fact that parenchymal hemorrhage is not a feature of SDAVF. Hurst and Grossman conclude that this finding, in conjunction with the presence of increased central cord signal, may suggest a more specific diagnosis of VHM as a cause of spinal cord dysfunction.
Comment by Nancy J. Fischbein, MD
SDAVF is a lesion that typically affects patients in their fifth to seventh decades, with men more commonly affected than women. Signs and symptoms have often been present for months to years before the diagnosis is made and include back pain, leg numbness, and muscle weakness, often progressing to disturbance of bladder and bowel function by the time of clinical presentation.1 Uncommonly, severe signs and symptoms may occur within days, presumably related to thrombosis of draining veins and acute elevation in cord parenchymal pressure. The diagnosis of SDAVF was previously made by myelography, which showed enlarged and tortuous coronal venous plexus, and confirmed by spinal angiography. The diagnosis of SDAVF has been significantly improved by the availability of MRI, which directly demonstrates spinal cord edema and dilatation of perimedullary veins on the spinal cord surface. Catheter angiography is still essential for confirmation of the diagnosis and for surgical planning. It also affords the opportunity for therapeutic transcatheter embolization in appropriate cases.
Certain MRI features of SDAVF are quite nonspecific, such as cord expansion and edema. The presence of enlarged pial veins is the most specific MR feature. Visible pial venous enlargement has been reported in 45-60% of cases,2,3 but with improvements in MR technology, this seems to be more commonly seen and in fact was observed in 10 out of 11 patients by Hurst and Grossman. The addition of peripheral spinal cord hypointensity on T2-weighted images may prove to be another useful sign indicating the presence of SDAVF and helping to improve sensitivity and specificity for what has historically been a difficult diagnosis to make. The pathophysiology behind the low signal on T2-weighted images is unclear and will require further study. Accumulation of deoxyhemoglobin in dilated, and slow-flowing peripheral veins is certainly a possibility. Disrupted axonal transport and free radical accumulation are other theories that have been applied to subcortical low signal on T2-weighted images in the brains of patients with early cortical ischemia,4 and these mechanisms could also play a role in the genesis of these signal changes. It must be cautioned that the apparent peripheral spinal cord hypointensity could conceivably represent a visual phenomenon due to the silhouetting of the periphery of the cord between the bright CSF in the subarachnoid space and the high signal of the abnormal cord, as pointed out in an editorial accompanying this article,5 so the use of this sign awaits further validation.
References
1. Grote EH, Voigt K. Clinical syndromes, natural history, and pathophysiology of vascular lesions of the spinal cord. Neurosurg Clin N Am 1999;10:17-45.
2. Gilbertson JR, et al. Spinal dural arteriovenous fistulas: MR and myelographic findings. AJNR Am J Neuroradiol 1995;16:2049-2057.
3. Bowen BC, et al. Spinal dural arteriovenous fistulas: Evaluation with MR angiography. AJNR Am J Neuroradiol 1995;16:2029-2043.
4. Ida M, et al. Subcortical low intensity in early cortical ischemia. AJNR Am J Neuroradiol 1994;15:1387-1393.
5. Quencer RM. Is peripheral spinal cord hypointensity a sign of venous hypertensive myelopathy? AJNR Am J Neuroradiol 2000;21:617.
All of the following are MRI features typically associated with spinal dural fistula except:
a. spinal cord parenchymal T2 prolongation.
b. contrast enhancement.
c. spinal cord swelling.
d. focal spinal cord parenchymal hemorrhage.
e. enlarged perimedullary veins.
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