Cauda Equina and Filum Terminale Arteriovenous Fistulas: Anatomic and Radiographic Features

SUMMARY: Intradural AVF below the conus medullaris may develop either on the ﬁ lum terminale or the cauda equina (lumbosacral and coccygeal radicular nerves). Although not a few ﬁ lum terminale AVFs are found in the literature, only 3 detailed cauda equina AVFs have been reported. Here, we analyze the angiographic and MR imaging ﬁ ndings of our cauda equina and ﬁ lum terminale AVF cases, supplemented with literature research to characterize the radiologic features of the 2 entities. On angiography, ﬁ lum terminale AVFs were invariably supplied by the extension of the anterior spinal artery accompanied by a closely paralleling ﬁ lum terminale vein. Cauda equina AVFs were fed by either a radicular or a spinal artery or both arteries, often with a characteristic wavy radicular-perimedullary draining vein. On thin-section axial MR imaging, the ﬁ lum terminale AVF draining vein joined the spinal cord at the conus medullaris apex, but that of the cauda equina AVF joined above the conus medullaris apex.

T he spinal canal below the conus medullaris accommodates lumbar, sacral, or coccygeal nerve roots (ie, the cauda equina) 1 and the filum terminale (FT). Intradural vascular lesions in this region may affect the cauda equina or the FT. Since the pioneering work by Djindjian et al, 2 the FT AVF has gained wide recognition, but the cauda equina AVF is scarcely known. [3][4][5] As such, few radiologic findings differentiating the 2 diseases are known. 5 We present our cauda equina and FT AVF cases supplemented by previous reports and discuss the angiographic and MR imaging features of the 2 diseases.

Patients and Imaging Features
The study followed the policies of each institution's Research Ethics Committee. The cases were retrieved from a prospectively collected spinal vascular malformation database between November 2012 and April 2017. Four patients with cauda equina or FT AVF were identified. All patients underwent spinal angiography, including bilateral internal iliac and all appropriate segmental arteries.
To observe the ascending draining vein, a precontrast axial T2-weighted turbo spin-echo (section thickness, 4 mm; TE, 105 ms; TR, 5000 ms) sequence and postcontrast axial T1weighted turbo spin-echo (section thickness, 4 mm; TE, 9.30 ms; TR, 650 ms) sequence 5 minutes after a 10-mL IV injection of gadobutrol (Gadovist; Bayer Schering Pharma) were studied on a 3T scanner (Magnetom Skyra; Siemens). Whereas the radicular vein accompanies a nerve root that is apart from the spinal cord at the conus medullaris apex, the FT vein anastomoses with the anterior spinal vein at the apex. Therefore, a draining vein off the spinal cord at the conus medullaris apex was interpreted as radicular, whereas the vein on the cord was an FT vein.

RESULTS
Patient demographics, angiographic and MR imaging characteristics, and treatment outcomes are listed in the On-line Table. We diagnosed 3 cauda equina AVFs (cases 1, 2, 3) and 1 FT AVF (case 4). The FT and 2 cauda equina AVFs were treated by endovascular embolization and 1 cauda equina AVF by direct surgery. All the procedures were completed without clinical sequelae. One embolized cauda equina AVF (case 1) resulted in residual fistula, but the patient declined further treatment (case 1).

Spinal Angiography
Spinal angiography detected all the AVFs between the L2 and L4 vertebral levels. In 2 AVFs, caudal extension of the anterior spinal artery (ASA) was the single feeder (cases 3 and 4, Figs 1 and 2A), and a single right S1 radicular artery was the feeder in 1 (case 2, Fig 3A). Both the extension of the ASA and left S1 radicular artery were the feeders in 1 (case 1, Fig 4A, -B). In cases 1 and 3, the ASA made changes in the course and caliber at the conus medullaris apex (Figs 1 and 4, large arrow), whereas in case 4, the ASA made a straight descent without changing its caliber.
The ascending draining vein showed a wavy pattern not paralleling the feeding artery in 2 cases (cases 1 and 3, Figs 4A and 1) and a straight ascent in 1 (case 2, Fig 3A) and closely paralleled the feeding artery in 1 (case 4, Fig 2A).
Case 2 was complicated by multiple AVFs described in detail in Fig 3.

MR Imaging Features
Thin-section axial T2WI and enhanced T1WI detected the ascending draining vein in cases 1, 2, and 4 (Figs 4C, 3C, and 2B, arrows). In cases 1 and 2, the vein was off the spinal cord at the conus medullaris apex (Figs 4C and 3C,arrowhead) and joined it at the L1 level (Figs 4C and 3C, large horizontal arrow) and thus was interpreted as radicular. In case 4, the vein united with the cord at the conus medullaris apex (Fig 2B, arrowheads) and hence was an FT vein. In case 3, the MR imaging failed to detect the vein.

DIAGNOSIS
Based on the angiographic and MR imaging findings, the diagnosis of cauda equina AVF was made in cases 1 and 2 and FT AVF in case 4. In case 3, surgical findings confirmed the diagnosis of cauda equina AVF. 5

Literature Review
The literature review yielded 3 3,5 and 31 2,4,6-25 surgically confirmed cauda equina and FT AVF cases, respectively. In all the 3 cauda equina AVF cases, the fistula was fed by a single extension of the ASA without radicular artery involvement. The ASA changed its course and caliber at the conus medullaris apex. In 2 cases, the draining vein showed a wavy pattern, but the other case paralleled the feeding artery. 3 In the 31 FT AVF cases, the ASA invariably fed the fistula and maintained its straight course and robust caliber at the conus medullaris apex. All FT AVF draining veins showed the parallel feeder-drainer pattern. To date, only 4 surgically proved cases with additional radicular artery supply have been reported, 4,10,16,24 and no FT AVF with exclusive radicular artery supply is known.