Epidural and subdural interdural approach to the lateral wall of the cavernous sinus for preserving the laterocavernous sinus in trigeminal schwannoma

Evacuation of middle fossa trigeminal schwannomas (TS) warrants a subtemporal interdural approach through the lateral wall of the cavernous sinus (CS). The dura comprises the dura propria, which follows the trigeminal nerve and develops into the epineurium, and periosteal layer. The interdural approach involves peeling off the dura propria and exposing the epineural sheath. The venous route around the CS is often obstructed due to TS progression. The interdural approach based on venous route preservation remains to be discussed. The laterocavernous sinus (LCS) is formed in these layers, draining to either the medial or lateral route. In the lateral route, the LCS drains to the pterygoid plexus via the middle cranial fossa foramen. Exposure of the interdural space disturbs the lateral route’s venous flow. We describe an operative strategy for venous route preservation in TS via the LCS lateral route. The venous route can be preserved by peeling off the dura propria from the posterior end of the foramen ovale short of the venous drainage route to the pterygoid plexus epidurally and then cutting from the middle cranial fossa dura posterior to the venous route subdurally to the exposed interdural space. This technique helps in avoiding postoperative venous complications.


Introduction
Typical trigeminal schwannomas (TSs) arise from Schwann cells within Meckel's cave [11] and are categorized into six types based on their location. Type M (middle fossa tumors, 38.5%) and Type MP (middle and posterior fossa, 28.3%) tumors are the most common types of TS [10]. These classifications are useful for selecting the appropriate surgical approach. Type M tumor resection includes removing both single and multiple components. Epineural sheath exposure of the trigeminal nerve embedded in the lateral wall of the cavernous sinus (CS) via the subtemporal interdural approach is vital. Type P (posterior fossa) tumors necessitate the lateral suboccipital or anterior transpetrosal (ATP) approach.
Important membrane anatomical knowledge with respect to this approach is as follows. The dura comprises two layers: dura propria (inner meningeal layer) and periosteal layer (outer dural layer). The periosteal layer of the dura is continuous with the outer periosteum, which covers the external surface of the cranial bone through the suture lines and foramina for nerves and vessels. The dura propria faces the brain surface covered by the arachnoid. The dura propria and arachnoid typically follow the cranial nerves at varying distances as they leave the cranial cavity. The dura propria that follows each cranial nerve forms the epineurium, whereas the pia-arachnoid continues as the perineurium that supplies each nerve fascicle. These two layers tightly fuse, except where they are separated to provide space for the dural venous sinuses and cranial nerves that pass through the parasellar region [9,11].
To expose the epineural sheath of the lateral wall of the CS, the cleavage plane between the dura propria and periosteal dura was identified at the skull base foramen (i.e., the superior orbital fissure (SOF), foramen ovale (FO), and foramen rotundum (FR)) and developed by the dissection and continued peeling of the dura propria layer from the epineural sheath of the trigeminal nerve (ophthalmic (V1), maxillary (V2), and mandibular (V3) trigeminal divisions). Although the range of peeling of the dura propria (i.e., between the SOF, FR, and FO) depends on TS size and location, to the best of our knowledge, no studies have focused on preserving the venous route.
The venous route from the superficial middle cerebral vein (SMCV) to the CS is classified into four groups (medial, intermediate, lateral, and absent). In the medial group, the venous route courses medially under the lesser sphenoid wing and terminates in the anterosuperior aspect of the CS (13.8-19.5% of cases). In the intermediate group (laterocavernous sinus: LCS), the venous route runs medially toward the CS and continues posteriorly as an LCS enclosed within the lateral CS wall (24.1-34% of cases). In the lateral group (paracavernous sinus), the venous route follows a more lateral trajectory within the dural floor of the middle cranial fossa (32.8-46.5% of cases). The SMCV is absent in 19-29.3% of cases [6,8]. In these four groups, especially for LCS cases, the venous route (which arises from the SMCV) runs along the interdural space between the epineural sheath and dura propria of the lateral wall of the CS. In the evacuation of TSs in both single and multiple-compartment Type M LCS cases, the possibility of venous route disturbance occurs when the dura propria is peeled off from the epineural sheath of the trigeminal nerve in the lateral wall of the CS, since the LCS runs along the same interdural space (which was peeled off).
Although intraoperative venous route disturbances cause venous complications, conclusive evidence has not been presented to date [7]. In petroclival meningioma, tumor progression causes rich vascularity [5], venous route obstruction (including the CS), and bypass venous route formation (including greater superficial anastomosis) [2,3]. When these bypass routes cannot recruit sufficient intracranial venous flow, venous congestion occurs. This can be evaluated using the dilatation difference between the diseased and healthy sides of the basal vein of Rosenthal through susceptibility-weighted imaging (SWI). Furthermore, studies have reported that venous congestion leads to postoperative Karnofsky Performance Status (KPS) deterioration [1,4].
Similar to petroclival meningioma, TS progression involves the possibility of venous route obstruction with bypass venous route formation and deep venous congestion. Type MP TS arises from the Schwann cells of the trigeminal nerve around the CS lateral wall, and the CS involves one of the main drainage pathways of the intracranial venous route; it is important to preserve the remaining intracranial venous route to avoid postoperative venous complications.
To the best of our knowledge, no study has focused on the operative strategy in TS with the LCS, although the epidural interdural approach for such cases may cause postoperative venous complications because the LCS exits the lateral wall of the CS.
Hence, we present three cases of Type MP TS, including bypass venous route formation in TS progression, descriptions of venous congestion owing to venous route obstruction, and a surgical strategy to avoid disturbing the venous route from the SMCV.

Case series
This study was approved by the institutional review board of our hospital, and patients were enrolled with an option of opting out.

Case 1
A 50-year-old woman presented with sudden unconsciousness. She was admitted with an indwelling lateral ventricular drainage tube due to hydrocephalus. Brain magnetic resonance imaging (MRI) revealed a mass with a cyst centered at the left Meckel's cave measuring 60 × 65 × 55 mm in size (Fig. 1A). Three-dimensional computed tomography angiography (3D-CTA) and digital subtraction angiography (DSA) revealed SMCV obstruction (Fig. 1B, C). Consequently, anastomosis of the superficial cerebral veins (i.e., the Sylvian vein and Labbe vein) occurred, and the venous route from the ophthalmic vein to the facial vein was formed as a bypass route (Fig. 1D). Dilatation of the left basal vein of Rosenthal was observed on SWI ( Fig. 1E), which was thought to be caused by venous congestion. However, the venous flow was not disturbed owing to the exposure of the interdural space of the lateral wall of the CS from the FO to the SOF because the venous route had already been obstructed. The tumor was resected using the ATP approach, with an interdural approach toward the CS lateral wall. A portion of the tumor cyst wall adhering to the brain stem was retained (Fig. 1F). Brain edema due to venous congestion did not occur (Fig. 1G). The patient recovered consciousness but experienced left facial numbness (V1-V3) postoperatively.

Case 2
A 68-year-old woman presented with left trigeminal neuralgia (V1). MRI revealed a mass with a cyst centered at the left of Meckel's cave measuring 19 × 10 × 10 mm in size ( Fig. 2A). 3D-CTA and DSA showed that the venous flow from the SMCV connected the CS via the LCS and drained into the inferior petrosal sinus (Fig. 2B, C). The venous route from the SMCV drained from the anterior portion to the medial portion of the tumor. The tumor size was small, and the range of exposure of the interdural space of the lateral wall of the CS was sufficient around the FO. Therefore, the tumor could be resected using an interdural approach without venous route disturbance via the ATP approach toward the lateral wall of the CS (Fig. 2D). Brain edema due to venous congestion did not occur (Fig. 2E). Left trigeminal neuralgia improved postoperatively.

Case 3
A 27-year-old woman presented with headache, dysarthria, dysphagia, left upper monoparesis, and right trigeminal neuralgia (V2 and V3). MRI showed an enhancing mass with a cyst centered at the right Meckel's cave measuring 68 × 43 × 39 mm in size (Fig. 3A). DSA showed SMCV drainage to the pterygoid plexus (Fig. 3B). On 3D-CTA, lack of connection was observed between the superficial cerebral veins (Fig. 3C). SWI revealed lack of dilatation of the basal vein of Rosenthal. Although 3D-CTA and SWI findings indicated the absence of venous congestion, a venous bypass route was not formed, and preserving the venous route from the SMCV to the pterygoid plexus via the LCS to avoid postoperative venous congestion was considered crucial. The SMCV runs far lateral to the middle cranial fossa, mainly connects the LCS, runs from the anterior lateral to the lateral side of the TS, and drains to the pterygoid plexus via the FR (Fig. 3D-F). In this case, exposing the interdural space of the CS lateral wall was necessary (with exposure ranging from the FO to the SOF) to remove the TS effectively owing to the large tumor size. However, the venous route of the LCS would have been disturbed if the interdural space of the CS lateral wall were to be exposed as usual. Therefore, the TS was removed using the epi-and subdural approach to expose the interdural space of the CS lateral wall and preserve venous flow from the SMCV to the pterygoid plexus via the FR. A portion of the tumor cyst wall adhering to the brain stem was retained (Fig. 3G, H). Brain edema due to venous congestion did not occur (Fig. 3I). Right facial numbness (V2) remained postoperatively.

Technical note (case 3, video 1)
After general anesthesia administration, a spinal drain was placed to control the cerebral spinal fluid during the operation. The patient was positioned in the supine lateral position. Her head was immobilized using a 3-pin Doro® QR3 Skull Clamp (Malmö, Sweden), angled 10° above the chest and rotated 90° to the right. A question mark-shaped skin incision was made anterior to the tragus, swinging posteriorly above the pinna. Frontotemporal craniotomy with a small frontal component was performed.
Subsequently, the middle cranial fossa dura was elevated, and the middle meningeal artery was cauterized and cut at the foramen spinosum. Thereafter, the greater superficial petrosal nerve was preserved. Drilling out of Kawase's triangle was unnecessary as the petrous apex osteolysis had caused tumor compression.
Peeling off the periosteal dura from the FO to the SOF to expose the interdural space of the CS lateral wall was necessary due to the tumor size in Case 3. However, if the periosteal dura was to be peeled off from the FO to the SOF, the venous route would have been disturbed because the LCS ran through the intradural space. Therefore, the periosteal dura was peeled off from the posterior end of the FO and stopped just short of the FR draining into the pterygoid plexus (Fig. 4, arrow 1). The middle cranial fossa dura was cut subdurally posterior to the venous drainage route; this cut line connected to the point that had already exposed the interdural space of the lateral wall of the CS (Fig. 4, arrow 2). Lastly, the dura propria was cut along the lateral side of the TS (Fig. 4, arrow 3), as was the cerebral tentorium to expose and remove the TS.

Discussion
LCS formation is considered to involve the primitive tentorial sinus, which drains the cortical blood that arises from the SMCV and migrates medially toward the CS region at the time the lateral wall of the CS forms (during the 8th week of gestation) [6]. The venous route of the CS can separate the two routes near the lateral wall of the CS. The medial route (medial to the lateral wall of the CS) includes the ophthalmic vein, CS, and inferior petrosal sinus, whereas the lateral route (lateral to the lateral wall of the CS) includes the superior petrosal sinus, paracavernous sinus, and LCS (Fig. 5a). Regarding LCS, although LCS termination into the posterior aspect of the CS and drainage to the inferior petrosal sinus occurs in 32% of cases (medial route; Fig. 5b), the venous route in most LCS cases is the lateral route with (9%, Fig. 5c) or without (59%, Fig. 5d) the anastomosis channel to the CS [8,6].
For Type MP TS cases involving the LCS (i.e., when the LCS terminates into the posterior aspect of the CS) and when the tumor is small, the interdural space in the lateral wall of the CS can be exposed to facilitate tumor resection without venous route disturbance because the venous flow from the SMCV runs from the anterior margin of the TS to the CS, as seen in Case 2 (Fig. 5e).
However, TS cases may present with a lateral LCS route (i.e., a route differing from the termination of the LCS into the posterior aspect of the CS). In these cases, the venous drainage route may be disturbed when exposing the interdural space in the lateral wall of the CS because the LCS runs through the interdural space. Therefore, such cases restrict exposure of the interdural space in the lateral wall of the CS to preserve the venous route of the LCS. In Case 3, although venous congestion did not occur, preserving the venous drainage route from the SMCV to the pterygoid plexus was important and it also did not form a bypass route. In this situation, this epi-and subdural approach to expose the interdural space of the lateral wall of the CS is useful to preserve the venous route (Fig. 5f).
However, most Type MP TS cases do not require this strategy as the LCS is usually positioned in the interdural space. TS progression gradually obstructs the LCS. Obstructing the lateral route of the LCS can lead to the Fig. 4 Illustration of the operative strategy for preserving the venous flow from the SMCV to the pterygoid plexus via the lateral route of the lateral cavernous sinus (LCS). To preserve the venous flow to the pterygoid plexus, we started to peel off the dura propria from the perineural sheath of the V3 trigeminal divisions in the posterior end of foramen ovale, just short of the venous route, thus exposing the inter-dural space (arrow 1). A cut was then made in the middle cranial dura from the posterior SMCV to the exposed interdural space (arrow 2). Finally, we cut the dura propria between the connection points (arrow 1 and arrow 2 (*)) to gain sufficient space for tumor resection (arrow 3). SMCV, superficial middle cerebral vein natural opening of the surgical corridor (Fig. 5g). Therefore, although most Type MP TS cases can be operated using an ATP approach (exposing the interdural space of the lateral wall of the CS), some cases in which the LCS lateral route must remain as the main drainage route necessitate an epiand subdural approach to prevent the venous flow of the LCS, as in Case 3.
In Case 1, the venous drainage route from the SMCV to the CS was disturbed, resulting in the formation of a bypass route using the greater anastomosis of the SMCV [2,3] and deep venous congestion [1,4], similar to the presentation in petroclival meningioma. Therefore, proper evaluation of the preoperative venous route in a range of clinical presentations includes assessing the formation of the bypass route and evaluating whether venous congestion occurred; these evaluations are likewise important in TS to avoid postoperative KPS deterioration [1,4]. In these cases, preserving the remaining venous route is more important; thus, the venous anatomy and operative strategies necessary to avoid postoperative venous complications must be understood.

Conclusions
Herein, we reported a surgical strategy to preserve the venous drainage route and elucidate the anatomical characteristics of the TS and LCS, as well as change the venous route and venous congestion due to TS progression. In Type MP TS, tumor progression may cause venous route obstruction, leading to bypass route formation and deep venous congestion, similar to petroclival meningioma. In Type MP TS cases presenting with a lateral LCS route, the epi-and subdural approach for exposing the interdural space of the lateral wall of the CS considering the venous drainage route's location The type of venous flow from the SMCV determines whether the LCS terminates into the posterior aspect of the CS (i.e., the medial route) and likewise determines whether the venous flow from the SMCV drains the lateral route with (c) and without (d) anastomosis to the CS. e Depiction of cases presenting with TS when the LCS terminates into the posterior aspect of the CS. f Depiction of cases with TS in the lateral route of the LCS. g Progression of the tumor causes obstruction of the lateral route of the LCS. BP, basilar plexus; CS, cavernous sinus; ICA, internal carotid artery; IPS, inferior petrosal sinus; LCS, laterocavernous sinus; Oph V, ophthalmic vein; SMCV, superficial middle cerebral vein; SPS, superior petrosal sinus; TS, trigeminal schwannoma helps to preserve the venous drainage route and avoid venous complications. Preoperative venous route evaluation, including the assessment of congestion using SWI, is necessary, especially for the CS around the TS tumor.