34 Endoscopic Resection of Extra-Axial Tumors
34.1 Introduction
The key concept of endoscopic surgery is improved target visualization to augment tumor resection while minimizing tissue injury. This thought process underpinned the development of endoscopy in the first half of the twentieth century. In 1910, a Chicago-based urologist, L’Espinasse, attempted to treat hydrocephalus in two infants by coagulating the choroid plexus using a modified cystoscope, resulting in the death of one infant, and the survival of the other infant for 5 years.1 Dandy and Mixter later showed the suitability of using endoscopy in visualizing the ventricular system, termed “ventriculoscopy,”2 and treating noncommunicating hydrocephalus through an endoscopic third ventriculostomy using a urethroscope and flexible probe.3 However, endoscopy did not gain traction until the latter half of the twentieth century, mirroring technological advancements in endoscopes, including rigid and flexible, versatile scope working channels and instruments, and scope miniaturization. Additionally, advancements in optics, such as improved lighting sources and delivery, higher-quality and high-definition imaging, and a variety of lenses with larger angle views have enhanced the endoscope.4 The advent of image-guided surgery systems and three-dimensional (3D) endoscopy has also improved the safety of endoscopy. As a result, the indications for endoscopy have expanded from treating ventricular pathology, such as tumors and hydrocephalus, to the resection of intra-axial and extra-axial lesions, treating degenerative spine and peripheral nerve pathologies, and pediatric malformations such as craniosynostosis.5,6
Procedurally, endoscopy is described as either pure endoscopic, endoscope-controlled, or endoscope-assisted, reflecting the mode of operative visualization. Pure endoscopic surgery is where the endoscope is the only mode of visualization, and instruments are introduced coaxially along an endoscopic sheath. In endoscope-controlled surgeries, the endoscope is the only mode of visualization, and instruments are introduced extra-axially alongside the endoscope. In contrast, endoscope-assisted surgeries combine an open or microsurgical approach with endoscopy for visualization, with instruments introduced extra-axially as well. Most procedures fall under the endoscope-assisted category, thereby capitalizing on the strengths of the microscope and the advantages of the endoscope.
The advantages of the endoscope are threefold. First, visualization of the target is clearer than with the microscope. This is as a direct result of the introduction of light into the surgical field, thus minimizing light dispersion, as opposed to the microscope, where the light source is further away from the target lesion. Second, the higher magnification and quality of the images assist in identifying the interface between normal and abnormal tissue. And third, the use of wide-angled lenses enhances the view of the surgical cavity. As a result, lesions behind corners and critical neurovascular structures, which are not in microsurgical view, are more readily identifiable with the endoscope. The combination of these features results in enhanced visualization.
Along with the advantages introduced by the endoscope, it is important to appreciate and understand the current limitations. Critically, the endoscopic view is limited by the entry point; therefore, structures behind or to the side of the target cannot be viewed by turning the endoscope within the surgical cavity. Such slashing movements would result in devastating intraoperative complications, thus the only acceptable maneuver is a fencing movement, whereby the endoscope is inserted in a specific direction, and subsequent angulation requires the removal of the endoscope from the surgical cavity, then reinsertion at the required angle of interest. In optics, the endoscope has two interrelated limitations, including the two-dimensional view of the surgical cavity, and a different sense of depth in comparison to the microscope. To overcome this, the surgeon needs to appreciate the shadows and lighting in the surgical view, fix important landmarks, and know the relevant anatomy. Another limitation, which is a consideration for any new technique or tool, is the learning curve associated in achieving the necessary safe standard outcome with a maximal benefit for the patient. As a result, the length of the procedure will be increased initially. Moreover, dedicated endoscopic instruments are essential, including angled instruments to remove lesions hidden from the microsurgical view. Finally, intraoperative complications, such as bleeding, are more challenging to manage, thus emphasizing the importance of knowing your instruments, spending the time learning the techniques, understanding your anatomy, learning from the mistakes noted in the literature, and gradually building up the complexity of cases where the endoscope is applied.6,7,8
34.2 Anatomy
34.2.1 Anterior Cranial Fossa
The central portion of the anterior cranial fossa is formed by the ethmoid bone, with the midportion containing the cribriform plate, marked anteriorly by the crista galli. The olfactory nerve arises through the perforations of the cribriform plate. Anterolaterally, the anterior ethmoid artery and nerve arise from the cribroethmoid foramen. The frontal bone contains the anterolateral portion of the skull base, with the falx cerebri attaching to the midline frontal crest. The falx cerebri contains the superior and inferior sagittal sinuses, with the superior sagittal sinus located posterior to the frontal sinus. The floor is composed of the frontal bone orbital process, and is closely associated with the inferior portion of the frontal lobe. The inferomedial portion of the frontal lobe, the gyrus rectus, is considered a “silent area” of the brain.
Posteriorly, the sphenoid bone forms the floor of the anterior cranial fossa. Posterior to the cribriform plate, the posterior ethmoid artery arises. Additionally, the planum sphenoidale is located posterior to the cribriform plate, and forms an important landmark for the sphenoid sinus and the optic nerve. Of note, the olfactory bulb and tract are superior to the cribriform plate and planum sphenoidale. Postero-laterally, the roof of the optic canal is formed by the lesser sphenoid wing. Medially, the anterior clinoid process is also formed by the lesser sphenoid wing, which marks the internal carotid artery (ICA) and optic nerve. The ophthalmic artery originates medial to the anterior clinoid process, from the ICA, and is medial to the optic nerve until it enters the orbit and becomes lateral and superior to the optic nerve.
34.2.2 Posterior Cranial Fossa
The anterior wall of the posterior fossa is formed by the petrous bone laterally, and the clivus medially. Importantly, the inferior petrosal sinus is located anteriorly between the clivus and petrous apex. Furthermore, the posterior petrous temporal bone transmits the opening of the internal acoustic canal (IAC), which contains cranial nerves VII and VIII, nervus intermedius, and branches of the anterior inferior cerebellar artery (AICA) to the inner ear. The vestibular aqueduct is posteroinferior to the IAC and contains the endolymphatic duct.
The floor, posterior walls, and lateral walls of the posterior fossa are formed by the occipital bone, while the tentorium cerebelli forms the roof. The falx cerebelli attaches to the internal occipital crest, which is located from the foramen magnum to the internal occipital protuberance. The transverse sinus is located on each side of the internal occipital protuberance. The sigmoid sinus runs anteroinferiorly, terminating at the jugular foramen. Additionally, cranial nerves IX to XI, the inferior petrosal sinus, and the posterior meningeal branch of the ascending pharyngeal artery enter the jugular foramen. The hypoglossal canal is medial and inferior to the jugular foramen, containing cranial nerve XII, the meningeal branch of the ascending pharyngeal artery, and the hypoglossal venous plexus.
The blood supply to the posterior fossa arises from the vertebral arteries, which enter through the foramen magnum and are ventral to the roots of cranial nerves IX, X, and XII. Several important structures are found within the foramen magnum, including the medulla oblongata, spinal branches of the accessory nerve, posterior spinal arteries, and the dens apical ligaments. Prior to the vertebral arteries joining to form the basilar arteries, the posterior inferior cerebellar arteries (PICA) are given off at the lower border of the pons. The basilar artery gives off the AICA, which runs along the inferior surface of the cerebellum and is closely associated with cranial nerves VII and VIII at the cerebellopontine angle, as mentioned previously. The basilar artery then gives off the labyrinthine and pontine arteries, the superior cerebellar arteries (SCA), which are closely associated with the trochlear nerves, and the posterior cerebral arteries (PCA), which are closely associated with the oculomotor nerves.
34.3 Anterior Cranial Fossa and Upper Basal Cistern Tumors
34.3.1 Indications/Contraindications
The approach to extra-axial anterior cranial fossa tumors can be achieved through various routes, including a supraorbital eyebrow craniotomy, a minipterional craniotomy, or an extended endoscopic endonasal approach. Meningiomas involving the olfactory groove, planum sphenoidale, and anterior clinoid are commonly accessed through these various techniques. Indications for a supraorbital eyebrow or minipterional craniotomy include anterior cranial fossa tumors with a significant component located superior to the optic chiasm, and with significant vascular involvement. A minipterional craniotomy would be favored for tumors with a significant middle cranial fossa component. Tumors that displace the optic chiasm superiorly and posteriorly can be resected endonasally, but a prefixed chiasm is a relative contraindication for the endonasal approach. Importantly, experience in endoscopic techniques and managing their complications can also factor into the decision-making process for the approach. Below, we discuss each approach, relevant anatomy, and complications associated with the technique.
34.3.2 Supraorbital Approach (Case 1)
This 43-year-old male patient presented with confusion. A magnetic resonance imaging (MRI) scan illustrated a large olfactory groove meningioma, with lateral extension beyond the mid-pupillary point, and posteriorly to the tuberculum sellae (Fig. 34.1a,b). A keyhole supraorbital approach was performed, with complete resection of the tumor (Fig. 34.1c,d). The patient had an uncomplicated recovery and was discharged home on postoperative day 3. He had no neurologic deficits.
Operative Technique
The supraorbital eyebrow craniotomy, described in detail previously,9,10,11,12,13 is suitable for most extra-axial tumors involving the anterior cranial fossa. However, the microscope is limited in midline tumors of this area due to the depression between the orbital roofs; as a result, an angled endoscope is critical to overcome this anatomical barrier.
Preoperative Planning
Correct surgical positioning is necessary to maximize adequate exposure of the target lesion, and allows gravity to replace the need for fixed retractors. The patient is supine, head extended and rotated to such a degree that the target area is in a perpendicular line to the point of entry. In cases of anterior cranial fossa tumors, this is typically between 45º to 60º contralateral to the side of the approach. Adequate extension of the head opens the subfrontal pathway as well as the sylvian fissure as the frontal lobe will fall away with gravity and the temporal lobe will be retained by the sphenoid wing. The ipsilateral eyelid is closed with a temporary tarsorrhaphy. It is critical to assess the extent of the frontal sinus preoperatively with neuronavigation, as a large lateral extension may make the placement of a supraorbital craniotomy quite difficult, and an alternative trajectory would be more feasible.
Description of the Procedure
The eyebrow incision made is limited medially by the supraorbital notch, in order to avoid damage to the supraorbital nerve, and extends to the lateral aspect of the eyebrow, limited by the frontalis branch of the facial nerve. Subgaleal dissection is performed, and a U-shaped pericranial flap is cut and reflected inferiorly. The temporalis muscle is opened to expose the keyhole area, followed by a small bur hole, and a craniotomy is fashioned with the footplate that is flush with the orbital roof. This is necessary, as the craniotomy needs to be as low as possible to the anterior cranial floor in order to create an optimal surgical corridor. Dura is also opened in a U-shaped manner and fixed inferiorly.
Microsurgically, and without retractors, cerebrospinal fluid (CSF) is first released from the prechiasmatic, opticocarotid, and carotid-oculomotor cisterns, in addition to the proximal sylvian fissure to allow for brain relaxation. Moreover, arachnoid at the base of the frontal lobe and within the sylvian fissure is dissected. The surgical corridor created allows the identification of the ipsilateral oculomotor nerve, optic nerves, optic chiasm, lamina terminalis, supraclinoid carotid arteries, in addition to branches of the anterior cerebral arteries (ACA). The tumor is then resected using the microscope as completely as possible, taking meticulous care of the surrounding neurovascular structures.
In the anterior cranial fossa, 0º as well as angled endoscopes are inserted to provide a comprehensive examination of the resection cavity, specifically around corners that would otherwise not be visible with the microscope, and the residual tumor is resected with angled instrumentation. Of note, endoscopic insertion should be cautiously performed, with the scope advanced in a straight line towards the opticocarotid complex and stabilizing the shaft against the craniotomy. The rod–lens and camera are rotated simultaneously to maintain an upright image, without displacing the shaft. Sudden movements of the endoscope in this region can have devastating consequences, and as mentioned previously, only straight-in and -out movements should be performed with the endoscope and not side-to-side movements.
Water-tight dural closure is performed at the end of the procedure, and the bone flap is fixed with titanium plates or other appropriate fixation devices. The bone flap is always pushed upwards, and the inferior craniotomy gap is filled with a collagen sponge. Pericranium, galea, and skin are closed separately.
Risks and Rescue
During placement of the incision and craniotomy, adequate preoperative planning will reduce the risk of damaging the frontalis branch, by limiting the incision extent laterally, and the risk of entering the frontal sinus, by placing the craniotomy sufficiently lateral to the frontal sinus. If the frontal sinus is breached, a small defect can be repaired with bone wax, otherwise fat or muscle is required for larger defects, reinforced by a collagen sponge.
Complications
Complications specific to this technique include frontal paraesthesia, temporary or permanent frontalis paresis, CSF rhinorrhea, and pseudomeningocele formation.14,15 To avoid frontal paraesthesia, which is usually transient, the supraorbital nerve, which runs from the supraorbital notch at the medial border of the incision, should be identified early in the procedure. Frontalis paresis, reported between 1 to 5.5% of cases, requires careful soft tissue dissection. CSF rhinorrhea, reported less than 4% of cases, occurs due to a breach of the frontal sinus. Pseudomeningocele may occur postoperatively as the patients cough during extubation; therefore pressure is applied over the incision to reduce this risk.
Pearls
Correct positioning and early CSF release from the cisterns will provide adequate exposure for the approach, without the need for retractors, and necessary brain relaxation.
An angled endoscope is essential in overcoming the anatomical limitation of the supraorbital craniotomy for visualizing the skull base floor between the orbital roofs. Equally, angled instruments are necessary to resect the tumor in this location.
Postoperative Management
Patients postoperatively require routine monitoring in a high-dependency or intensive care unit initially. The majority of patients have uneventful postoperative courses. In cases where the olfactory nerve has been sacrificed, maintaining adequate nutrition is essential, as patients may not maintain sufficient nutritional intake due to anosmia and the lack of joy in eating. If the optic apparatus has been compromised, recovery is difficult to predict. In addition to intracranial complications, pneumocephalus and CSF leaks could occur but are uncommon with this approach, and are more relevant in endonasal cases. Other complications requiring active management include other systems, such as pneumonia and emboli. As a result, patients are mobilized early on, encouraged do breathing exercises to minimize basal atelectasis, wear compression stockings, and are discharged as soon as they are medically fit.
34.3.3 Minipterional Approach (Case 2)
This 41-year-old, with a known history of medulloblastoma treated with craniospinal radiation and several rounds of chemotherapy, has developed new bifrontal lesions, including along the crista galli on the anterior cranial fossa ( Fig. 34.2a,b : preoperative MRI). A bilateral minipterional incision was performed ( Fig. 34.2c–f : pre- and postoperative [MRI] of incision) with resection of the tumor from the right and left frontal lobes, temporal lobes, insula, and anterior cranial fossa using an angled endoscope, including the right lateral wall of the orbit and decompression of the optic nerve ( Fig. 34.2g,h : postoperative MRI). Postoperatively, the patient had transient difficulty opening his mouth completely due to disturbance of the temporalis muscle bilaterally. The neuropathology report was a WHO Grade IV desmoplastic medulloblastoma.
Operative Technique
Accessing the anterior and middle skull base through a frontotemporal approach was classically known as a pterional craniotomy, championed by Yaşargil.16,17 Since then, the approach has been modified to reduce the morbidity associated with the technique. The minipterional approach has been adapted from the standard pterional approach, providing adequate access to the skull base18,19,20 while reducing the incision size, temporalis muscle atrophy, drilling, and unnecessary brain tissue exposure.
Related posts:
30 Principles of Endoscope-Assisted Microneurosurgery
35 Endoscopy in Aneurysm Surgery
32 Endoscope-Assisted and Fully Endoscopic Microvascular Decompression
33 Endoscopic-Assisted Resection of Intra-Axial Brain Tumors
31 Endoscopic Exploration and Biopsy of Basal Cisterns
36 Use of Fluorescence in Neuroendoscopy
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
