Endoscopic far-lateral supracerebellar infratentorial approach for resection of dumbbell-shaped trigeminal schwannoma: surgical techniques and preliminary results

Trigeminal schwannomas (TSs) are mostly benign tumors. However, dumbbell-shaped TSs are most challenging for surgeons and pose a high surgical risk. We describe the technique of the purely endoscopic far-lateral supracerebellar infratentorial approach (EFL-SCITA) for removing dumbbell-shaped TSs and further discuss the feasibility of this approach and our experience. EFL-SCITA was performed for resection of 5 TSs between January 2020 and March 2023. The entire procedure was performed endoscopically with the goal of total tumor resection. During the operation, the tumor was exposed in close proximity and multiple angles under the endoscope, and the peri-tumor nerves were carefully identified and protected, especially the normal trigeminal fiber bundles around the tumor. All the tumors of 5 patients involved the middle and posterior cranial fossa, of which total removal was achieved in 2 patients and near-total removal in 3 patients. The most common preoperative symptoms were relieved after surgery. Two patients had postoperative mild facial paralysis (House-Brackmann grade II), and 1 patient had abducens palsy; both recovered during the follow-up period. Two patients experienced new postoperative facial hypesthesia, and 1 experienced mastication weakness, which did not recover. There was no tumor recurrence or residual tumor growth during the follow-up period in any of the patients. EFL-SCITA is a new and effective alternative for the surgical treatment of TSs. For dumbbell-shaped TSs, this approach provides sufficient surgical field exposure and freedom of operation.


Introduction
Trigeminal schwannomas (TSs) are the second most common intracranial schwannomas and account for 0.8-8% of all these tumors [15].They most commonly originate from the Gasserian ganglion and occur at the trigeminal nerve root and peripheral branches.They are usually histologically benign and generally do not infiltrate surrounding structures [16].One-stage gross total resection (GTR) is still the optimal treatment for TSs because of the tumor's benign nature and its high recurrence rate [1].
Advances in microsurgical techniques and the introduction of new approaches to the skull base have significantly increased the tumor resection rate and reduced surgical complications [3,6].However, complete tumor removal while preserving neurological function remains a challenge.
In recent years, endoscopic techniques have been widely used in transcranial surgery, and the ventricles, cisterns, parafalcine, and the area around the tentorial incisura have become good natural channels for endoscopic surgery [4,8,12,19,31,33].Since the concept of supracerebellar infratentorial approach (SCITA) was first introduced by Horsley, it has elicited a variety of access routes, such as median, paramedian, lateral, and far-lateral SCITA [17,18,24].The natural gap between the tentorium and cerebellum can be used to expose the posterolateral pontomesencephalic junction and area around the posteromedial tentorial incisura [5].Transcranial endoscopy provides a wider exposure for this traditional surgical channel while reducing the retraction of brain tissue and the disturbance of important nerves and blood vessels [10,14,36].The application of endoscopy in this natural space has made this approach even more attractive.
In this clinical series, we presented an endoscopic farlateral supracerebellar infratentorial approach (EFL-SCITA) for the resection of large TSs occupying both the middle and posterior fossae and evaluated the feasibility and clinical efficacy of this approach.We also introduced our surgical techniques and experience.

Patient population
We retrospectively analyzed the clinical data of patients with TSs who underwent EFL-SCITA in our center between January 1, 2020, and March 30, 2023.A senior professor of neurosurgery performed all the operations.Informed consent was obtained from the patients before the operation, and the study protocol was approved by the Ethics Review Committee of Zhongshan Hospital, Fudan University.
Preoperative T1/T2-weighted and gadolinium-enhanced MR imaging was performed in all patients.All the operations were performed for the purpose of total tumor resection.The surgeon determined the scope of resection and evaluated the rate of resection intraoperatively, which was further confirmed by postoperative MRI.All the patients underwent CT and/or MRI within 48 h postoperatively.Follow-up data and radiological images were regularly examined and assessed.This case series has been reported in line with the PROCESS Guideline [2].
The classification of Ramina et al. based on the anatomical location of the lesion was employed in this study and categorized TSs into 6 groups: type A, mainly an extracranial tumor with small extension in the middle fossa; type B, an intracranial tumor mainly located in the middle fossa with extracranial extension; type C, a tumor in the middle fossa; type D, a tumor in the posterior fossa; type E, a tumor occupying both the middle and posterior fossae; and type F, a tumor occupying the extracranial, middle and posterior fossae [21].The population enrolled in this study should meet the following criteria: patients with TSs diagnosed as Ramina's subtype D or E, who are willing to receive surgery and are in a condition to tolerate surgery.Patients whose diagnosis was not considered TSs or with other types beyond Ramina's type D or E were excluded.

Surgical procedure and EF-SCITA technique
Before surgery, a lumbar drain was placed to facilitate CSF drainage and brain relaxation intraoperatively.After successful anesthesia, the patient was placed in a lateral oblique position with the upper body elevated 30°.The head was slightly flexed upward and rotated posteriorly to facilitate gravitational retraction of the cerebellum away from the tentorium.The layout of the operating room and the position of the patient are described in our previous clinical report [34] (Fig. 1).
A straight skin incision measuring 5-7 cm in length was performed, starting from 2 cm behind the asterion at the level of the pinna to that of the tip of the mastoid process.Then, suboccipital craniotomy (3 cm × 3 cm) was performed, and the lateral aspect of the craniotomy was necessary to expose the posterior margin of the sigmoid sinus while the superior margin exceeded the superior margin of the transverse sinus.A nearly semicircular dural incision was made, pedicled on the transverse sinus, with a few millimeters left below the sigmoid sinus.The following procedure was performed with two-handed microdissection under rigid 4-mm endoscopes with different angles (Karl Storz, Germany).The endoscope was fixed on the pneumatic holder, and the depth and direction could be adjusted according to the requirements.Gravitational sagging of the cerebellum inferiorly and posteriorly increased exposure of the infratentorial supracerebellar and posteropetrosal anterocerebellar space.The endoscope was placed along the infratentorial supracerebellar space.First, the upper part of the posterior cranial fossa of the tumor was exposed and intratumorally decompressed using the Cavitron Ultrasonic Surgical Aspirator (CUSA, Integra, USA), and then the component of the tumor in the middle cranial fossa could be decompressed through the tumor cavity.After sufficient decompression of the tumor, the superior petrosal vein and trigeminal nerve were separated, and intratumoral decompression and piecemeal resection were further performed (Figs. 2 and 3).For tumors in the middle cranial fossa, angled suction (3 mm in diameter) and angled ring curettes (5 mm and 7 mm in diameter, Storz, Germany) were more effective in separating them from the wall of Meckel's cave; double suction devices were sometimes used.The trigeminal nerve was often tightly adherent to the tumor, especially at the entrance of Meckel's capsule; therefore, careful separation and removal of as many nerve fibers as possible from the tumor was necessary to preserve nerve function (Figs. 2 and 3).Generally, other cranial nerves (cranial branches III, IV, VI, VII, VIII) had no obvious adhesion to the tumor and could be well recognized and dissected from the tumor while maintaining a clear surgical field.However, special attention should be given to the recognition and protection of the abducens nerves.Intraoperative electrophysiological monitoring played an important role in the protection of the trigeminal nerve and facial nerve.After tumor resection, the middle fossa, especially the inferolateral and anterior inferior parts, was explored with a 30° or 45° angle endoscope, and the residual tumor was removed with an angled microinstrument (Fig. 3).Then, the tumor cavity was perfused with warm saline, hemostasis was achieved, the suture in the dura mater was watertight, the bone flap was returned, and the incision was sutured in a standard fashion (Video).

Patients and tumor characteristics
Five patients were included, and the mean age was 36.2 years (21-64 years) (Table 1).All the lesions grew across the middle and posterior cranial fossa, and the brain stem and cerebellum were obviously compressed and displaced.According to Ramina's classification, all cases were type E. The average diameter of the tumors was 51.2 × 42.8 × 42 mm.Among them, 2 patients who had previously undergone tumor resection in other institutions had tumor recurrence, and the other 3 patients had primary tumors.The mean follow-up period was 26.8 months (10-35 months).

Surgical results
Tumor resection using EFL-SCITA was safely performed in all the patients.Gross-total resection (GTR) was achieved Fig. 1 Layout of our operating room and far-lateral supracerebellar infratentorial approach (EFL-SCITA).Schematic illustration showing the locations and characteristics of the incision and bone window in this approach.(Reproduced from [34]) in 2 patients, and near-total resection (NTR) was achieved in 3 patients (Table 1).NTR was achieved in 2 patients with TSs (cases 1 and 3) because of the close connection to the brainstem and the difficulty in separating them for resection.In a patient with recurrent TSs (case 4), NTR was achieved, indicating that part of the tumor extended to the infra-anterior corner of the middle cranial fossa and a small part of the tumor remained.We did not observe tumor recurrence or residual tumor regrowth during the follow-up period.
Preoperative symptoms included facial anesthesia (n = 4), facial pain (n = 2), facial hypoesthesia (n = 2), masticatory weakness (n = 2), headache (n = 4), diplopia (n = 2), hearing loss/tinnitus (n = 4), vision loss/visual field defect (n = 2), drinking cough (n = 1), walking instability (n = 1), and limb weakness (n = 1).Diplopia, facial pain, headache, cough while drinking, hearing loss/tinnitus, vision loss/visual field defect, walking instability, and limb weakness were relieved after the operation.Of the 3 patients with facial anesthesia, 1 experienced relief, and 2 patients with recurrent TSs did not experience relief.Two patients experienced mild facial nerve palsy (House-Brackmann grade II), and 1 patient experienced abducens palsy postoperatively, which were considered to be temporary dysfunctions caused by traction of the facial nerve and the abducens nerve during tumor resection, and these symptoms were basically relieved after 6 weeks of followup.Two patients experienced new postoperative facial hypoesthesia (cases 2 and 5, V1-2 hypoesthesia) and one had mild masticatory weakness (case 2), which did not recover during the follow-up period and may have been related to neurological impairment due to injury to some normal nerve fibers during tumor dissection.One patient had postoperative secondary hydrocephalus which was relieved by endoscopic third ventriculostomy.No decrease in the Karnofsky functional prognosis score (KPS score) was observed in any of the patients, and all the patients had KPS scores between 90 and 100 at the most recent follow-up examination (Table 1).

Discussion
Skull base lesions involving the petrous apex and Meckel's cave remain a challenge for the modern skull base surgeon given the critical confluence of neurovascular structures surrounded by complicated bony anatomy in these tightly intertwined anatomical areas.TSs are commonly benign tumors that are curable by total surgical removal [13].Limited by the narrow Meckel's cave, TSs involving the middle and posterior fossa usually present a typical "dumbbell-shaped," which is the most challenging type of surgical treatment [26,27].Previous study confirmed that the component in the middle cranial fossa of dumbbell-shaped TSs is located in the interdural region, while part of the posterior cranial fossa resides in the subdural region [29].Many different surgical strategies have been described to treat this type of TSs and the correct choice of surgical technique allows postoperative complications to be minimized.In this study, we describe a novel surgical strategy for the treatment of dumbbell-shaped TSs by using endoscopy to expose the tumor from lateral-posterosuperior to medial-anterior-inferior direction through natural supracerebellar infratentorial corridor, and successfully employed it in 5 cases.During the early stage of the surgical procedure, the component in the posterior fossa was easily exposed.After partial resection of the tumor, the endoscope could be further inserted into the cavity through the enlarged entrance of Meckel's cave to explore and resect the middle fossa component under direct vision.Follow-up data suggested that this technique could achieve encouraging results on overall tumor resection rate, incidence of recurrence, and neurological deficits.Despite the advancement in surgical technique, the selection of a safe surgical approach for complete resection of dumbbell-shaped TSs remains a topic of debate.Al-Mefty et al. recommended the transzygomatic-middle cranial fossa approach for dumbbell-shaped TSs resection.The tumors in the middle fossa are first removed from the epidural fossa, and then the component in the posterior fossa could be further resected through an enlarged Meckel's cave [3].Yoshid and Kawase suggested that the anterior subtemporal approach could be used for resection of this subtype of TSs, and component in the posterior fossa was safely removed by drilling Kawase triangle after epidural excision of tumors in the middle fossa [35].Because of the complex growth patterns of dumbbell-shaped TSs, it is relatively difficult to remove the posterior part of the tumor during the operation, especially when it is adhered to the brain stem and cranial nerves.Blind retraction of the tumor via an anterior or lateral approach may cause serious consequences.In addition, the lateral approach to this region requires removal of the middle fossa petrous bone, retraction of the temporal lobe, and incision of the lateral wall of Meckel's cave.In the current era, the wide application of endoscopy provides a new direction for the surgical treatment of TSs.Shin and Shaan reported successful removal of TSs using an endoscopic transnasal approach from the anterior-inferior direction [23,28].In 2020, Park HH et al. reported an endoscopic transorbital approach for TSs and found that this approach could achieve satisfactory accessibility and resectability for the lesion limited in the middle fossa or dumbbell-shaped tumors mainly located in the middle fossa [20].However, some disadvantages, such as limited exposure of the posterior cranial fossa, narrow surgical corridor, risk of internal carotid artery injury, and postoperative cerebrospinal fluid leakage, limit the wide use of this approach [22].
The dumbbell-shaped TSs are often accompanied an enlarged entrance of Meckel's cave.The tumor component in the posterior cranial fossa is often closely adjacent to the brainstem, cerebellum, and neurovascular structures.Successful removal of this part remains the most challenging.Samii et al. supported the adoption of a retrosigmoid approach for resection of this type of tumor under microscopy combined with endoscopy; however, the surgical procedure required frequently traversing neurovascular structures of the posterior fossa and still provided relatively limited exposure of Meckel's cave even with enlarged bone removal of the suprameatal tubercle [25].To overcome these obstacles, we introduce EFL-SCITA, which naturally expose the posterior fossa and Meckel's cave from the lateral-posterior direction.After partial decompression intratumorally, different angle endoscopes can be further placed into the cave cavity for closer visualization and removal of tumors in Meckel's cave.In addition, the surgical position we have adopted allows gravitational retraction of the cerebellum away from the tentorium and petrosa, which facilitates the manipulation and avoids retraction on the cerebellum.Based on our initial clinical experience, we found that compared with the retrosigmoid approach, EFL-SCITA provided more direct access to the middle tentorial incisura, petroclival region, cerebellopontine area, and Meckel's cave.Direct exposure of the tumor from the lateral-posterosuperior to medial-anterior-inferior direction and adequate debulking intratumorally facilitates dissection of the involved neurovascular structures under straight view, effectively reducing the likelihood of injury.Moreover, this approach has the advantage of a simple and minimal craniotomy, as well as fully exploiting the natural supracerebellar infratentorial corridor to identify and separate neurovascular structures around the tumor.
However, this approach also has some limitations.For example, for a small number of dumbbell-shaped TSs without an enlarged entrance to Meckel's cave, this approach still requires removal of the lateral portion of the bone at the entrance to Meckel's cave.For tumors with extracranial extension, this approach has limited exposure and needs to be combined with a lateral or transnasal approach.Moreover, the size of the population sample and the length of followup is an inherent limitation that will need to be addressed in the future by enrolling more cases and allowing for a longer follow-up period.
Protection of residual trigeminal nerve function during tumor removal should also be taken into account.Samii et al. suggested that normal nerve fibers should be carefully identified and protected along the arachnoid plane to maintain nerve function [26].In the process of tumor resection via EFL-SCITA, close endoscopic observation can clearly display the whole course of the trigeminal nerve along the direction of the nerve course, reducing the iatrogenic injury of normal nerve fibers.In this series, we obtained satisfactory trigeminal nerve exposure and protection during resection of all tumors via EFL-SCITA.In addition, intraoperative electrophysiological monitoring can help the operator identify and protect the compressed or displaced trigeminal nerve.

Conclusions
EFL-SCITA provides a new curative strategy for the surgical resection of TSs, which is helpful to achieve safe and radical tumor treatment.For dumbbell-shaped TSs, this technique provides a sufficient surgical exposure view and freedom of operation.

Fig. 2
Fig. 2 Total excision of a large TS in a 25-year-old female (case 2).A-B Preoperative axial and C-D sagittal T1-weighted contrast-enhanced MRI; E-F Postoperative axial and G-H sagittal T1-weighted enhanced MRI.I-J Endoscopic exposure through the infratentorial supracerebellar corridor revealed the superior-lateral part of the tumor and SPV; K-L After intratumoral debulking of the tumor, the SPV was displaced and protected with brain cotton.The tumor was resected after careful dissection and the normal trigemi-

Fig. 3
Fig. 3 Near-total excision of a large TS in a 34-year-old female (case 3).A-C Preoperative axial, D coronal, and E sagittal T1-weighted contrast-enhanced MRI; F-H Postoperative axial, I coronal, and J sagittal T1-weighted contrast-enhanced MRI.K-R Intraoperative field of EF-SCITA.K-N After opening the dura through the natural supracerebellar infratentorial surgical corridor, the posterior fossa compartment of the tumor was first exposed and internally debulked, and then dissected off the trochlear nerve (CN IV) and other sur-

Table 1
Overall outcomes in five patients with TSs occupying both the middle and posterior fossa *Sequelae of previous surgeries; F,