Surgical technique: transnasal endoscopic resection of ventral meningioma of the posterior cranial fossa
Surgical treatment of meningiomas in the slope, rock-slope area and occipital foramen area is more difficult; although the development of microscopic techniques has greatly improved the postoperative prognosis of meningiomas in these areas, they still face a high mortality rate and neurological deficits. The application of transnasal endoscopic expanded access in the treatment of skull base tumors has brought a new light to the treatment of ventral meningiomas in the posterior cranial fossa. Yi-Kun Peng, Department of Otolaryngology, People’s Hospital of Guizhou Province
Recently, Dr. André, a neurosurgeon at the University of São Paulo, Brazil, combined his experience with transnasal endoscopic treatment of ventral meningioma of the posterior cranial fossa and a review of the literature to present surgical techniques and considerations for transnasal endoscopic resection of meningioma of the posterior cranial fossa in the journal Neurosurgery Clinics of North America.
Indications and contraindications for surgery
A comprehensive evaluation of the patient’s condition, disease characteristics, and the operator’s experience is required prior to surgery. Strict selection of suitable cases is the key to successful nasal endoscopic resection of ventral meningiomas in the posterior cranial fossa.
1. Indications
Symptomatic meningiomas of the posterior cranial fossa are indications for surgery; asymptomatic giant tumors and tumors with persistent growth on imaging follow-up also require surgical treatment. When the tumor is located ventral to the posterior cranial fossa, especially near the midline, an expanded transnasal endoscopic approach is appropriate for resection. The proximity of the tumor to the midline, the posterior displacement of the brainstem and the lateral displacement of the cranial nerves are optimal conditions for performing a ventral approach to remove the tumor.
Figure 1. Schematic diagram of the posterior cranial fossa surgical approach. The transsphenoidal ramp approach has obvious advantages in exposing ventral meningioma of the posterior cranial fossa
(1) Slope meningioma
The base of the slope meningioma is located in the midline, and the tumor displaces the V nerve laterally and superiorly; the VI nerve laterally and posteriorly; the VII, VIII, IX, X, and XI nerves posteriorly; the XII nerve posteriorly and inferiorly; and the brainstem posteriorly pushes.
(2) Meningioma of the oblique region
The base of the meningioma is located in the oblique fissure of the rock and pushes out the surrounding tissues. It causes posterior displacement of V, VII, VIII, IX, X, and XI nerves; medial displacement of VI nerve; and posterior and medial displacement of the brainstem. Since meningiomas in the oblique region of the rock often originate in the partial midline, the transnasal endoscopic approach sometimes cannot completely remove the tumor and requires a combination of other surgical approaches.
(3) Meningioma of occipital foramen magnum
Meningioma of the greater occipital foramen can be divided into cranial type and spinal cranial type; spinal cranial type originates from below the greater occipital foramen, so the cranial nerve and vertebral artery are often pushed to the upper pole of the tumor. The cranial type can originate anywhere in the foramen magnum region and is associated with displacement of different structures.
Cranial tumors originating from the anterior border of the greater occipital foramen are suitable for resection via a nasal endoscopic approach. Their origin is located medial to the inferior lingual nerve and the greater occipital foramen, displacing the cranial nerve posteriorly or laterally. However, tumors growing toward the neck are not suitable for transnasal endoscopic approach because the craniocervical stability is compromised due to surgical resection of the anterior arch of C1 and C2 dentata.
(4) Meningioma of the jugular foramen area
Meningiomas in the jugular foramen area have a posterolateral origin and cause posterior displacement of the cranial nerves in the jugular foramen area; the VI nerve is displaced upward. Only individual tumors that grow medially can be resected by nasal endoscopy.
2. Contraindications
Patients with severe complications and cannot tolerate prolonged anesthesia; meningioma base located on the lateral side of the dura mater; tumor encapsulating vascular structures; transsphenoidal approach cannot reveal tumor anatomy; lack of multidisciplinary team support; lack of appropriate surgical equipment and instruments.
Surgical techniques and steps
1. Pre-operative preparation
(1) Imaging evaluation
Pre-operative imaging examination can provide anatomical information of the tumor and enable the operator to develop the best surgical plan. Preoperative imaging should include CT and MRI for evaluation of the bone and soft tissue surrounding the meningioma.
(2) Base of the tumor
Determining the base of the tumor is critical in determining how the tumor grows and its relationship to the surrounding neurovasculature, and can predict the difficulties that may be encountered intraoperatively.
T1 enhancement sequences are the best imaging method to determine the base of meningiomas. Although, MRI provides excellent soft tissue anatomy, determining the presence of calcification, osteophytes and the anatomy of associated bony structures still requires the aid of CT. Often, osteophytes at the base of the tumor are seen, and in addition, the assessment of bony structures by CT provides a basis for the selection of surgical access and the extent of bone resection.
(3) Vascular anatomy
Vascular imaging can evaluate the relationship between blood vessels and tumor; clarify whether there are tumor-encapsulated blood vessels, blood supply arteries and drainage veins of tumor.
It is necessary to clarify the presence of tumor-encapsulated vessels before surgery to facilitate safe resection of tumor. Vascular stenosis may be an indication that the tumor has encroached on the outer membrane of the vessel, and if this artery cannot be sacrificed, it is difficult to achieve total resection of the tumor. Angiography can show the collateral circulation of the encapsulated artery and the patient’s tolerance to the balloon occlusion test to determine whether the encapsulated artery can be sacrificed.
Venous imaging is required if the meningioma in the posterior cranial fossa is in close proximity to any of the venous sinus vessels. Preoperatively, if the associated venous sinus vessel is occluded, it appears that the segmental venous sinus vessel can be sacrificed intraoperatively.
Figure 2. MRI shows a slope meningioma compressing the brainstem, and an enlarged transnasal approach is a contraindication to surgery because the tumor wraps around the right vertebral artery
(4) Cranial nerve anatomy
MRI (steady-state free motion imaging and steady-state acquisition fast imaging) can clearly evaluate the anatomy of cranial nerves.
(5) Hydrocephalus
Meningioma of the posterior cranial fossa can cause obstructive hydrocephalus. Patients with markedly dilated ventricles and significantly elevated intracranial pressure require temporary or permanent cerebrospinal fluid shunts prior to removal of the tumor. Preoperative cerebrospinal fluid shunts in these patients help to remove the tumor during surgery and prevent the occurrence of postoperative cerebrospinal fluid leaks.
(6) Evaluation of paranasal sinuses
Once the decision to perform a transnasal endoscopic approach is made, an evaluation of the paranasal sinus structures is required. Thin-section CT of the paranasal sinuses and skull base is important for surgical guidance.
(7) Equipment and instruments
Adequate surgical instrumentation is a prerequisite for transnasal endoscopic resection of slopes and posterior cranial fossa meningiomas, and lack of appropriate instrumentation should be considered a contraindication to surgery. Intraoperative surgical instruments required include: high-definition endoscopes (0 and 45 degrees), video equipment, endoscopic bipolar electrocoagulation forceps, microdrills, separation instruments, ultrasonic aspirators, and hemostatic materials.
2.Patient position
The patient is placed in a supine position with a 30-degree elevation, neck flexion and head extension and turned toward the operator.
Neurophysiological monitoring is mandatory for the removal of posterior cranial fossa meningioma via nasal endoscopic approach. The VI nerve, motor evoked potentials and somatosensory evoked potentials are routinely monitored. Monitoring of other nerves is determined by the location and size of the tumor.
Preoperatively, the nasal cavity is filled with 1:1,000 epinephrine soaked brain cotton for 10 minutes; the nasal septum is infiltrated with lidocaine mixed with 1:100,000 epinephrine.
3.Surgical Approach
A transcranial endoscopic approach to the posterior cranial fossa was used. The slope consisting of the posterior part of the pterygoid body and the anterior part of the occipital bone can be divided into upper, middle and lower parts: the upper part is bounded by the superior part of the pterygoid sinus, which consists of the pterygoid body and the dorsal part of the saddle; the middle part consists mainly of the occipital bone, which is located above the line of the end of the oblique fissure; the lower part consists of the caudal part of the occipital bone.
To reach the posterior cranial fossa via the middle and upper 2/3 of the slope, the pterygoid sinus needs to be opened; to reach the posterior cranial fossa via the lower part of the slope, only the rostral part of the pterygoid needs to be removed. The mid-superior 2/3 of the slope is adjacent to the pontine brain. The outer cranial portion of the slope protrudes toward the pharynx at the mid-inferior junction of the slope, and the upper part is adjacent to the top of the nasopharynx. The oblique fissure formed by the temporal bone rock forms the boundary of the mid-superior portion of the slope. The basal venous plexus is located between the dura mater at the upper end of the slope and is closely related to the posterior wall of the dorsal saddle and pterygoid sinuses; it communicates with the lateral part of the infratentorial sinus, the upper part of the cavernous sinus and the lower part of the limbic sinus and epidural venous plexus.
4. Surgical steps
Using an anterior septoplasty incision, create bilateral mucous cartilage flaps; remove most of the septal cartilage and bony structures, preserving L-type cartilage to support the dorsum and tip of the nose; turn up the mucous flaps and search for bilateral pterygoid sinus openings; create a mucous flap of the septum fed by the pterygopalatine artery for later skull base reconstruction; rotate the mucous flap so that it is placed in the posterior nasal tract or maxillary sinus; enlarge the anterior wall of the pterygoid sinus and grind the beak of the pterygoid bone with an abrasive drill. The sinus mucosa covering the slope area is carefully separated to expose the slope bone.
These steps require 2 operators to operate instruments simultaneously in both nostrils, and the creation of a tipped tissue flap facilitates later reconstruction of the skull base defect; the mucosa of the nasal septum on one side is preserved to prevent perforation of the septum.
Depending on the characteristics of the tumor, the surgical access can be modified on a subbasis, and the production of the tipped tissue flap needs to be determined by the size and shape of the skull base defect.
(1) Transslope access
After the slope bone is exposed, the bone is excised with grinding drill and biting forceps. The extent of excision needs to be decided according to the size and location of the tumor, but care needs to be taken not to exceed the following boundaries.
Above: saddle base; below: foramen magnum; lateral: internal carotid artery, hypoglossal nerve canal, occipital condyle.
The outer layer of the dura is first incised to expose the basal venous plexus. Bleeding from the basal venous plexus is not safely stopped by cautery, but can be stopped by compression with hemostatic material. Large tumors can break through the dura and compress the plexus structures, but if the tumor is not very large or if the plexus is not fully compressed, rapid hemorrhage often occurs.
VI The intradural segment of the nerve is often located here and requires special attention when operating on the lateral side and neurological monitoring should be performed. Once the dural lining has been opened, even minor bleeding should be stopped with bipolar hemostasis, after which the following structures should be identified with a 0-degree endoscope: vertebral artery, basilar artery and its branches, anterior inferior cerebellar artery, superior cerebellar artery, posterior cerebral artery, course of the intradural III-VI nerves, brainstem, and papillae. The use of angled endoscopy also allows identification of the following structures: the pontocerebellar horn of the cerebellum, the VII-XII nerves, and the dorsal saddle area.
Once all anatomical structures are identified, careful resection of the tumor is initiated. For the clarity of the operative field, it is necessary to ensure complete hemostasis in the nasal cavity and butterfly sinus to prevent contamination of the lens. Frequent irrigation and suctioning will keep the field clear.
The microsurgical principles used for tumor removal are as follows: identify the boundary between the tumor and normal brain tissue; perform intratumoral resection with microsurgical scissors and ultrasonic suction; and perform separation along the tumor perimeter, preserving the arachnoid interface.
(2) Skull base reconstruction
Reconstruction of the dura mater at the skull base is difficult and the procedure is as follows.
(i) If the defect is large, the first choice requires sealing with abdominal fat, followed by covering with fascial grafts or dural synthetic material.
(ii) Finally, these grafts are covered with a tissue flap with the pterygopalatine artery, and the use of fibrin glue helps to prevent displacement of the graft and the tipped flap.
(iii) The surface of the tipped flap is covered with another layer of gelatin sponge; a silicone rubber tube is inserted into the nostril on the side with the graft to promote regeneration of the epithelial cells.
(iv) Gauze was stuffed as support; lumbar pool drainage was not used as a routine.
⑤ broad-spectrum antibiotics for 10 days or longer.
5. Details of surgical technique
①The pterygoid nerve is an important marker to identify the ruptured foramen segment of the internal carotid artery; ②Neural navigation can help to determine the limits of bone resection, especially in patients with poor pterygoid sinus pneumatization and reoperation; ③Careful evaluation of the parapharyngeal internal carotid segment is required before operating in the area of the lower end of the ramp; ④When performing deep bone grinding of the ramp, it is recommended to preserve a thin layer of bone next to the ramp to protect the internal carotid artery; ⑤Before opening the dura sufficient bone needs to be removed so that adequate intradural exposure can be achieved; 6) early confirmation of the VI nerve is essential for safe dural opening and tumor resection.
The angle of the slope needs to be considered during skull base reconstruction; when the angle of the slope is too large, the nasal septal flap may not completely cover the lower part of the skull base defect; multilayer reconstruction of the skull base of the posterior cranial fossa is necessary; hydrocephalus associated with meningioma in the posterior cranial fossa needs to be treated before resection of the tumor.
Complications
Complications can be classified into mild and severe complications according to their severity; early and late complications according to the time of occurrence.
Mild complications rarely lead to death, have little impact on patients’ lives, and can be treated conservatively for improvement. Severe complications have a high rate of disability and mortality; intracranial complications include damage to the brain, cranial nerves, blood vessels and venous sinuses.
Cerebrospinal fluid leaks can cause symptoms directly, or susceptibility to meningitis and occupational effects through the pneumocranium. The risk of bleeding is increased during surgery because several important vessels are involved. Common vessels include: the pterygopalatine and maxillary arteries and their branches; the internal carotid artery; the basilar and vertebral arteries and their branches; and the venous sinus at the base of the skull.
Early complications are mainly cerebrospinal fluid leakage, bleeding in the operative area, brain damage and nerve damage; late complications include meningitis, bleeding, adhesions and infection.
Postoperative management
Postoperative recovery is not only related to intraoperative operations, but postoperative management is also important. Broad-spectrum antibiotics should be given intraoperatively and continue for 10 days postoperatively; nasal gauze stuffing should be removed at 10-14 days; the occurrence of cerebrospinal fluid leakage should be observed; and the nasal cavity should be irrigated with 0.9% saline.
Typical cases
Case 1.
Female, 50 years old, presented with persistent headache for 4 months and intracranial occupying lesion on cranial MRI. The cranial CT and MRI showed that the tumor was located in the dura mater with significant enhancement and the base was located in the middle part of the slope; it compressed the ventral part of the brainstem and the lateral ventricles were mildly dilated.
Figure 3. A: preoperative sagittal T1 enhancement sequence; B: preoperative horizontal T2 sequence; C: immediate postoperative sagittal T1 sequence; D: immediate postoperative horizontal T2 compression lipid sequence
The tumor was resected through a slope endoscopic approach and skull base reconstruction; intraoperatively, the tumor wrapped around the left VI nerve, resulting in excessive nerve stretching; finally, the tumor reached near-total resection, with the remaining part of the left inferior lateral tumor. Postoperatively, the patient had VI nerve palsy and cerebrospinal fluid leakage; skull base reconstruction and ventriculoperitoneal shunt were performed again.
Figure 4. intraoperative images. a: exposure of the posterior cranial fossa dura after slope bone abrasion; b: brainstem and vertebrobasilar artery visible at the inferior margin of the tumor; c: tumor flip to the left after intratumoral decompression; d: skull base repair with bilateral nasal septal flaps. (Note: Bas, basilar artery; VI nerve, abducens nerve; ICA, internal carotid artery; Tu, tumor; vert, vertebral artery)
Case 2.
Female, 60 years old, presented with ataxia for 3 months. Imaging revealed an occupying lesion in the posterior cranial fossa. There were no positive signs on physical examination. Cranial CT and MRI showed a markedly enhancing tumor in the middle part of the intradural slope. The brainstem was ventrally compressed and the ventricles were mildly dilated.
Figure 5. A: preoperative sagittal T1 enhancement sequence; B: preoperative horizontal T1 enhancement sequence; C: exposure of the intradural structures in the posterior cranial fossa; D: residual tumor tissue in the left lateral subcranial base. (Note: AICA, anterior inferior cerebellar artery; bas, basilar artery; VI nerve, abducens nerve; Tu, tumor; vert, vertebral artery)
The tumor was resected via ramp endoscopy, with near-total resection and a small amount of residual tumor in the lower part; there was no postoperative neurological deficit, but the patient died 2 weeks later due to sepsis secondary to pneumonia.
Case 3.
Female, 63 years old, presented with intermittent headache and mild cognitive impairment for 1 year. Physical examination showed no significant positive signs, and cranial CT and MRI revealed a markedly enhancing lesion on the mid-superior intradural slope and growing dorsally to the saddle, causing significant compression of the ventral aspect of the brainstem.
Figure 6. A: preoperative sagittal T1 enhancement sequence; B: preoperative coronal T1 enhancement sequence; C: pituitary shift; D: inferior margin of the tumor and exposure of the left VI nerve; E: postoperative sagittal T1 enhancement sequence; F: postoperative horizontal T1 enhancement sequence. (Note: PG, pituitary gland)
The transslope endoscopic approach to resect the tumor requires intraoperative traction of the pituitary gland in order to be able to maximize the resection of the tumor in the dorsal saddle; eventually the tumor achieves sub-total resection, with tumor remnants remaining at the superior end; no postoperative neurological deficit, with permanent hypopituitarism at a later stage, requiring hormone replacement therapy.
Advantages and disadvantages of the transnasal endoscopic approach
The greatest advantage of the transnasal endoscopic approach is that it can remove the tumor ventral to the posterior cranial fossa through a ramp approach and avoid removing brain tissue, which reduces the possibility of cranial nerve injury. This approach allows early control of the blood supply artery of the meningioma and reduces intraoperative bleeding. In addition, this approach removes part of the bone and dura mater and allows for Simpson grade I resection of the tumor.
Although endoscopic surgery does not provide a 3-D surgical field of view, it does provide a near field of view, and multiple directions of view.
Endoscopy requires only narrow surgical access, and with important neurovascular vessels surrounding the surgical field, there is an increased risk of bleeding during surgery, and there is still the possibility of cerebrospinal fluid leakage and nerve injury.
The greatest disadvantage of transnasal endoscopy is that it is limited in removing tumors from the lateral skull base, has difficulty in repairing larger dural and cranial defects, and has a high potential for cerebrospinal fluid leakage.
Summary
Meningiomas in the slope, rock slope area and occipital foramen magnum area remain challenging conditions to treat surgically.
At present, the reasons why the technique of transnasal endoscopic resection of ventral meningioma of the posterior cranial fossa cannot be widely applied are related to the limited level of understanding of transnasal endoscopic treatment of tumors in this area, strict surgical indications, and difficulties in tumor resection and skull base reconstruction.
Careful evaluation of the imaging anatomy, the patient’s clinical condition, and the operator’s experience can guide the selection of an appropriate surgical approach.