Tentorial Dural Fistulas: Endovascular Management and Description of the Medial Dural-Tentorial Branch of the Superior Cerebellar Artery

Classification and Venous Features

In descriptions of TDAVFs, the term “incisura” is used ambiguously. For example, Ono et al²² called it “tentorial edge” and Wollschlaeger and Wollschlaeger,²¹ “free margin of the tentorium.” Literally it means “opening,” which would restrict the term to the hiatus and not the adjacent tentorium or its anterior free edge.⁷ To avoid confusion, we have simply divided TDAVFs into midline and mediolateral lesions and used conventional anatomic landmarks to separate the former.

In contrast to the more frequent DAVF of the lateral sinus located within the sinus wall^2,5 and associated with sinus thrombosis,^4,8 TDAVFs are typically located in the dura not directly connected to major sinuses.^2,5 However, among our patients, all 4 torcular TDAVFs showed some degree of major sinus thrombosis. This observation may indicate that they had or have direct connections with the sinus and explain their more benign angioarchitecture, with 2 of the 4 being classified as Cognard type II a + b. It may also be a clue to their etiology, because prior sinus thrombosis is a probable cause for the development of lateral and sigmoid sinus DAVFs.⁸ Torcular TDAVFs could thus represent an intermediate entity between sinus and purely tentorial fistulas. Two torcular TDAVFs were graded as Cognard type IV, which may indicate a stage in the development of the fistula with extension into the reflected dural layers of the tentorium. In any event, all the torcular TDAVFs drained both infra- and supratentorially, so the location appears to act as an anatomic venous epicenter connecting both supra- and infratentorial compartments.

In this series, TDAVFs of the falx cerebelli were remote from a major sinus, consistently drained infratentorially, and were graded Cognard type III or IV. The extent of the falx cerebelli varies from a midline thickening of the dura to a septum extending from the inferior surface of the straight sinus to the foramen magnum. In its more complete form, it rarely extends farther anteriorly than the midpoint of the straight sinus, which is often the site of cerebellar veins connecting to the sinus (eg, declival veins). This variability could explain overlap with torcular-located lesions at 1 extreme and the solely infratentorial drainage of TDAVFs when the falx cerebelli is fully formed. It also explains the frequency of hemorrhage at presentation in the latter situation. Furthermore, because TDAVFs of the falx cerebelli were not associated with major sinus thrombosis, their etiology may differ from that of torcular and petrotentorial TDAVFs.^5,19 One possible cause is trauma, because one of our patients had had an occipital craniotomy. One can speculate that the superior attachment of the falx cerebelli could act as a stress point for traumatic vessel injury.

Our observed venous drainage routes support the classification of TDAVFs proposed by Picard et al,⁵ who divided TDAVFs into 3 groups: 1) the tentorial marginal type with the fistulas located along the free edge of the tentorial hiatus and draining both supra- and infratentorially, 2) the tentorial lateral type adjacent to the lateral sinus and draining to supratentorial veins, and 3) the tentorial medial type with fistulas sited adjacent to the torcular and straight sinuses draining to the infratentorial draining system. Lawton et al⁷ proposed a classification with 6 anatomic locations and venous drainage patterns. This classification was based on a larger series of 31 patients and separates midline lesions into Galenic, straight sinus, and torcular. The last 2 have drainage patterns similar to those of our falx cerebelli and torcular subtypes.

Arterial Characteristics

The tentorium derives its principal arterial blood supply from tentorial branches of the ICA and dural branches of the vertebral artery and external carotid artery. These arterial systems are meningeal or dural and are usually identifiable on angiography in the nonpathologic state. Additional transarachnoid supply from the primarily pial arteries, such as the cortical branches of the PICA and contributions to the subarcuate plexus at the internal acoustic meatus from the AICA,⁹ is less evident, unless abnormally hypertrophied. Whether such transarachnoid arterial traffic is “normal” or only develops in response to the development of a DAVF is difficult to establish in vivo, and we generally rely on postmortem descriptions. The identification of a tentorial branch of the posterior cerebral artery, the ADS, was only after its enlargement in response to pathology.²¹ We assume that this explains the development of the medial dural-tentorial branch of the SCA identified in our patients. Both arteries appear to supply medially sited DAVFs. The characteristics of the arterial supply to DAVFs in this study helped to define both midline and mediolateral fistulas as tentorial and proved crucial for endovascular therapy planning. Thus, we suggest that the type of feeding artery and the venous drainage should be used to categorize subtypes of TDAVF.

Medial Dural-Tentorial Branch of the SCA

The course and cerebral structures supplied by the SCA are described in standard anatomic texts and comprehensive reviews, but these do not include a supply to the tentorial dura.^{19⇓⇓–22} Although a transcisternal branch to the dura of the tentorial hiatus has been inconsistently described in anatomic studies,^{22⇓⇓–25} this vessel has been mentioned in case reports of TDAVFs without being fully characterized.^2,4,7,19,26 Recently, a branch from the SCA was observed during surgery for a TDAVF but could not be identified on angiography.²⁷

Various names being given to this poorly defined medial dural-tentorial artery of the SCA include “meningeal artery” or “tentorial artery” of the SCA,^2,4,7,19,26 and some authors appear to have confused this artery with the ADS.⁷ The ADS is described as arising from the posterior cerebral artery¹⁹ and, therefore, runs above the tentorium, so it should be easily differentiated from the medial dural-tentorial branch of the SCA. However, artery-to-artery anastomoses can develop between these arteries within the tentorium (Fig 4) due to the rich dural arterial network.²⁸ Such connections are common in this vascular network, and in 2 patients, we further observed connecting branches between the marginal tentorial artery and the medial dural-tentorial branch of the SCA.

The medial dural-tentorial branch of the SCA was a common feature of midline TDAVFs. It is surprising that it has not found its own nomenclature in anatomic descriptions of the blood supply to the tentorium. Its omission from reviews focusing on the anatomic organization of the dura^19,23 may be due to its absence or small size in the nonpathologic state. It arises from the superior or rostral trunk of the SCA and traverses the ambiens cistern under the tentorium, which it enters in the posterior half of the incisura. In our patients, it showed a characteristic course on lateral DSA, running in an almost straight or only slightly concave downwards curve from anterior to posterior, and on anteroposterior DSA, it curves medially to reach the midline posterior to the incisura. Its course thus parallels that of the marginal tentorial artery. Fistulous contributions by transarachnoid anastomoses from primarily cerebral arteries are well-recognized,⁵ and we considered whether these might occur superficial to the tentorium (ie, that the artery runs inferior to the tentorium directly to the fistula site, but the observed anastomosis with the ADS [Fig 4] demonstrates a substantial transdural course).

Regardless of the explanation for its under-recognition, this study suggests that the medial dural-tentorial branch of the SCA should be expected to supply midline TDAVFs posterior to the incisura. Selective angiography may be necessary for confirmation of its origin from the SCA and, as in one of our patients, can be used for embolization (Fig 4). Thus, recognition of its contribution is essential for diagnosis, endovascular treatment planning, and possibly in posterior fossa surgery.²⁷