Posterior Fossa Dural Arteriovenous Fistulas with Subarachnoid Venous Drainage: Outcomes of Endovascular Treatment

Treatment Considerations and Classification

When intervention is indicated, the cornerstone of dAVF treatment is the disconnection of the vein foot. Proximal draining veins must be occluded at the end of the treatment.⁶ Due to their rarity, endovascular treatment of posterior fossa dAVFs has been less often described. Nevertheless, the arterial approach is the most common one even if a transvenous or combined approach can be a safe and effective option.^7,8 As described in the literature,⁷ in our series the transarterial route was the first-line approach. However, regarding the 3 patients treated by a transvenous approach, alone or combined, we detected no clinical complications, and the mRS was stable or improved at 6 months for those patients. Hence, even if we cannot support any conclusion with such a small number of patients, the transvenous approach could be an option for some selective cases.

Furthermore, in agreement with the literature,^6,9,10 the MMA was the most commonly selected artery to navigate to the shunt, even if it was not the main feeding artery, because of the less tortuous route and the lower rate of dangerous anastomosis. We report 2 main complications: extended venous thrombosis causing hydrocephaly and cerebellar ischemia unrelated to the MMA navigation.

In the present series, complication rates linked to the procedure were low. However, the complication rates reported in the literature were even lower. Indeed, Maimon et al⁶ reported a complication rate of 5.8%, and Cognard et al,¹¹ a rate of 6.7%. However, those rates do not specifically concern posterior fossa dAVFs but all dAVFs with cortical venous drainage, including dAVFs that are considered safer to treat. Besides, Zhou et al,⁴ in a series of 5 tentorial dAVFs, reported 1 complication (20%) due to a cerebellar infarctus, whereas Motebajane and Choi,¹² in a series of 12 dAVFs of the foramen magnum treated with n-BCA, reported no complications as Liang et al¹³ in a series of 5 dAVFs of the foramen magnum treated with Onyx. Besides, tentorial fistulas are more challenging because there is usually a dural supply from both the internal and external carotid arteries, increasing the procedural risk of intracranial embolic events.⁹ Furthermore, because the venous drainage pattern is linked to the clinical symptoms,^14,15 this slightly higher complication rate regarding fossa posterior fistulas might be linked to the specificity of posterior fossa drainage. Likewise, posterior fossa fistulas are located in a very eloquent zone.

Finally, the complication rate of the present series might also be linked to a more intensive endovascular treatment because 100% of the dAVFs were occluded, whereas Maimon et al⁶ reported a rate of 88.3%, and Cognard et al,¹¹ a rate of 80% of total occlusion.

Anatomic Considerations and Classification

We analyzed 26 dAVFs of the posterior fossa with infratentorial venous drainage. Because of the importance of venous drainage in the clinical presentation,¹⁵ special attention should be given to the venous outflow to improve the understanding of the shunt and the flow. Venous outflow can be defined according to the main draining vein at the level of the shunt. Thus, based on their venous outflow, cases have been classified into 5 groups, considering the shunt, the arterial feeders, and the treatment strategy.

The 5 groups noted were the following: group 1, vein outflow into the basal vein; group 2, vein outflow into the torcular; group 3, vein outflow into the cerebellar cortical vein; group 4, vein outflow into the lateral tentorial sinus; and group 5, vein outflow into the perimedullary venous plexus.

Other classifications exist but are not adapted to the endovascular approach because they are not specifically based on the outflow. Indeed, Lawton et al¹⁶ established a very detailed classification dividing 31 tentorial dAVFs into 6 groups. Nevertheless, this classification is anatomic and does not examine the venous outflow pattern. It is mainly to prepare the surgical access but is not adapted for an endovascular approach. There is another classification based on the location of the shunt and not on the outflow, thus including a posterior fossa shunt with occasionally supratentorial drainage.¹⁷ Hence, the proposed classification is more adapted to the endovascular approach and based on venous outflow, which is the most clinically impactful.¹⁵ However, due to the rarity of the cases, this classification has to be strengthened by further studies including more patients.

All of the dAVFs draining into the basal vein were petrosal and had a venous shunt into the superior petrosal vein. One other petrosal dAVF drained into the tentorial sinus, and another petrosal dAVF drained into perimedullary veins. Hence, for petrosal dAVFs, the anatomic location of the shunt did not predict the outflow and thus the symptoms, enhancing the need for a classification considering the venous outflow. In fact, our results were consistent with the pattern of drainage of the petrosal dAVF, which was described by Li et al¹⁸ and Matsushima et al.¹⁹ They classified petrosal dAVFs into 4 groups based on the draining area: the posterior mesencephalic group, anterior pontomesencephalic group, petrosal group, and tentorial group. They showed that all patients from the posterior mesencephalic group and some patients from the pontomesencephalic group had supratentorial drainage via a connection to the basilar vein. However, in the present series, we focused only on infratentorial drainage, so we do not present any pontomesencephalic dAVFs in our series.

Regarding the perimedullary drainage, the 5 dAVFs with a shunt into the lateral medullary vein were located on the foramen magnum and fed by the PMA of the vertebral artery or the APA as classically described in the literature.^3,12

Limitations

Limitations should be noted in the present series. First, it is a retrospective analysis with inherent bias due to this method. Second, the cohort is relatively small because posterior fossa dAVFs are rare, and patients were not randomized into transarterial, transvenous, or combined approaches, which does not allow comparison among the different endovascular approaches.

Finally, due to their rarity, not many studies focusing on dAVFs of the posterior fossa can be found in the literature. To our knowledge, our series is the largest in the literature specifically concerning dAVFs of the posterior fossa.