TSs are benign tumors that mainly arise from some fibers of the trigeminal nerve, with some normal fibers usually being compressed around the tumor[13, 30]. Total removal of the tumor has been the goal of treatment[11, 28]. Samii et al. divided TSs into four groups based on the anatomical location of the tumor body[24]. Ramina et al. further modified the classification based on Sammi's, which are important to assure the best surgical approach for each type of lesion and classified TSs into 4 categories: 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 occupied both middle and posterior fossa; and Type F, a tumor occupied extracranial, middle and posterior fossa[19]. According to this classification, all the five patients included in this paper were type E.
Limited by the narrow Meckel’s cave, tumors involving the middle and posterior fossa usually present a typical "dumbbell shape", which is the most challenging type of surgical treatment[24, 25]. The component in the middle cranial fossa of this type of tumor is located in the interdural, while the part of the posterior cranial fossa is located in the subdural[27]. Thus far, selecting a safe surgical approach for complete resection of this type of tumor is still controversial. Yoshid and Kawase suggested the anterior subtemporal approach for resection of this type[34]. Samii et al. supported the adoption of a retrosigmoid sinus approach for resection of this type of tumor under microscopy combined with neuroendoscopy[23]. 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 [21, 26]. Based on cadaveric anatomical practice, our team explored the scope of the endoscopic transnasal-pterygoid approach to expose the relevant anatomical structures around Meckel's cave, and then used this approach to resect TSs in Meckel's cave region[9]. However, some disadvantages, such as limited exposure of the posterior cranial fossa, narrow surgical corridor, risk of internal carotid artery and postoperative cerebrospinal fluid leakage, limit the wide use of this approach[20].
Increasingly, neurosurgeons are using endoscopes in traditional skull base surgery, finding that endoscopes can provide a deeper and wider surgical field[12, 23]. Since the concept of SCITA was first introduced by Horsley, it has elicited a variety of access routes, such as median, paramedian, lateral, and far lateral SCITA[15, 16, 22]. Transcranial neuroendoscopy provides a wider exposure for these traditional surgical channels while reducing the traction of brain tissue and the disturbance of important nerves and blood vessels[10],[35].Since 2018, 10 cases of giant petroclival meningiomas have been safely resected by EFL-SCITA in our center, during the operation, the channel could directly expose the entrance of Meckel's cave from the posterior side under endoscopy, and the visual angle of Meckel's internal and middle cranial fossa was also obtained[32]. Based on our experience, we explored the feasibility and safety of using this approach to remove dumbbell-shaped TSs. From March 2020, our center has safely resected 5 cases of large dumbbell-shaped TSs through EFL-SCITA. To date, this approach has not been reported for the surgical treatment of TSs. Of these cases, the tumors were large, causing the brain stem, cerebellum and cavernous sinus to be obviously compressed and displaced. It was difficult to expose and resect these tumors through conventional skull base surgery. EFL-SCITA was safely performed in all cases, and there were no operative deaths and no tumor recurrence or residual tumor regrowth during follow-up.
Because of the complex growth patterns of dumbbell-shaped TSs, the difficulty of the operation is to remove the posterior part of the tumor, 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. Based on our initial clinical experience and unpublished anatomical study on EFL-SCITA, we found that compared with the retrosigmoid approach, which requires frequent manipulation across important nerve gaps, EFL-SCITA provides more direct access to the middle tentorial incisura, petroclival region, cerebellopontine area and Meckel's cave. Directly exposing the tumor from the lateral-posterosuperior to medial-anterior inferior direction and separating the involved neurovasculature after sufficient intratumoral decompression can reduce their injury probability. The dumbbell-shaped TSs are often accompanied by the enlargement of Meckel's cave. However, the surgical field of the middle cranial fossa and Meckel's cave is still relatively limited through the retrosigmoid approach even with enlarged bone removal of the suprameatal tubercle. The lateral approach to this region requires removal of middle fossa petrous bone, retraction of temporal lobe and incision of the lateral wall of Meckel's cave. By comparison, EFL-SCITA can naturally expose 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 tumor in the Meckel's cavity. Moreover, this approach has the advantage of simple and minimal craniotomy, as well as taking full advantage of the natural infratentorial supracerebellar channel to identify and separate neurovascular structures around the tumor under direct visualization.
However, this approach has some limitations. For example, for a small number of dumbbell-shaped TSs without enlargement of the entrance to the Meckel 's cave, this approach still requires removal of the lateral portion of the bone at the entrance to the Meckel 's cave. For tumor with extracranial extension, this approach has limited exposure and needs to combine with a lateral or transnasal approach. Moreover, the approach does have a full intra-dural surgical access and require careful manipulation; it requires a learning process and a thorough knowledge of skull base anatomy and endoscopic techniques. In addition, since only 5 cases with dumbbell-shaped TSs resected using the EFL-SCITA approach were included in this study, there were limitations in terms of small number of cases and short follow-up time, and we will further verify the optimal indications and long-term clinical outcomes of this approach in more cases and longer follow-up time in the future.
Patients with TSs often experience facial anesthesia, hypesthesia, facial pain and masticatory difficulties. The current goal of surgical treatment of TSs is to protect residual trigeminal nerve function while removing the tumor[5]. Since TSs usually originate from only part of the trigeminal nerve fibers, intraoperative protection of the non-involved normal nerve fibers is particularly important. Samii et al. suggested that normal nerve fibers should be carefully identified and protected along the arachnoid plane to maintain nerve function[24]. 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 neuroendoscopic resection of all tumors. Our results also demonstrate that good exposure of the tumor origin and careful dissection are necessary for the protection of normal neurological function. In addition, intraoperative electrophysiological monitoring can also help the operator identify and protect the compressed or displaced trigeminal nerve.