Spinal extradural arteriovenous fistulas with retrograde intradural venous drainage: Diagnostic features in digital subtraction angiography and time-resolved magnetic resonance angiography

Spinal extradural arteriovenous fistulas (AVFs) may be more difficult to prospectively identify than dural AVFs because they are less common than dural AVFs. The primary purpose was to further characterize the diagnostic imaging of spinal extradural AVFs with intradural retrograde venous drainage. The magnetic resonance (MR) imaging and angiographic results of 23 patients with suspected spinal dural AVFs were analyzed in order to distinguish dural and extradural AVFs. The diagnostic accuracy of MR angiography was retrospectively compared between dural and extradural AVFs. All 23 patients showed high intensity in the spinal cord on T2-weighted MR images. Eighteen out of 23 patients were diagnosed with dural AVFs, while the remaining 5 were diagnosed with extradural AVFs by angiography. Extradural AVFs were fed by a branch of the segmental artery to the vertebral body, characterized by a fistula located in the ventral extradural space, and drained retrogradely via an epidural venous pouch into intradural veins. The segmental artery was localized within 1 vertebral level using MRA in 12 out of 18 patients (67%) with dural AVFs and in 1 out of 5 patients (20%) with extradural AVFs (p = 0.09). The reasons behind the lower accuracy was mainly the image misinterpretation. Congestion of the spinal cord in spinal extradural AVFs with intradural retrograde venous drainage was similar to that in dural AVFs, whereas its angioarchitecture differed from that of dural AVFs. A clearer understanding of the imaging features of extradural AVFs is important for improving the diagnostic accuracy and clarifying treatment targets.