Transcranial Approaches to the Optic Apparatus

Transcranial approaches to the optic apparatus include both traditional approaches, such as frontotemporal (pterional) and bifrontal craniotomies, as well as more recent minimally invasive keyhole approaches, including the supraorbital eyebrow approach and minipterional approach. Given advances in understanding anatomy, instrumentation, and, perhaps most importantly, the addition of high-definition endoscopy, the use of these smaller, minimally invasive approaches is becoming increasingly incorporated into routine neurosurgical practice at many centers for pathology involving the optic apparatus and parasellar area.

This chapter focuses on the use and limitations of the supraorbital and minipterional transcranial approaches for tumors affecting the optic apparatus, with minimal attention to the traditional larger approaches. Because it is addressed in other chapters, we do not discuss the use of the transorbital approach, which is being increasingly applied to skull base pathology. Attention is directed to the most common tumors approachable by these two keyhole routes, including meningiomas, craniopharyngiomas, intrinsic lesions of the optic apparatus and lamina terminalis, and metastatic tumors. Because the optic apparatus is located centrally in the skull base, in close proximity to the circle of Willis, cavernous sinus, hypothalamus, infundibulum, and pituitary gland, many patients with tumors in this region may present with nonvisual symptoms, such as diplopia, endocrinopathy, personality changes and/or headache. In such patients, the goal of surgery is maximal safe tumor removal while preserving the integrity of the optic apparatus.

The keyhole supraorbital and minipterional transcranial approaches, when used and executed appropriately and with ideal neuroanesthetic techniques, low-profile instrumentation, and endoscopic assistance, offer excellent exposure to the optic apparatus. These two approaches have proven quite versatile and effective even for many large tumors that affect the optic nerves, chiasm, and optic tracts, although in some cases of very large tumors, a conventional large pterional or bifrontal craniotomy may be preferred.

The advantage of these smaller keyhole approaches is that they provide access to the parasellar area, anterior, and middle cranial fossae with minimal exposure of the cerebral surface, without the need for brain retraction. a

a References .

As such, these approaches allow access to the critical neurovascular structures of the skull base while reducing approach-related morbidity. Although the window to the area of interest is limited, the use of endoscopy as an adjunct to the microscope allows the surgeon to gain high-definition visualization, illumination, and access into the deep cisternal spaces. b

b References .

Choice of Approach: Supraorbital Versus Minipterional Versus Endonasal Versus Conventional Craniotomy

Several keyhole and conventional approaches allow surgeons to reach tumors affecting the optic apparatus. From a purely anatomic standpoint, given the midline location of the optic chiasm and optic nerves, most lesions that affect the optic apparatus can be approached from the supraorbital or endonasal routes. For tumors affecting the optic chiasm from below (predominantly pituitary adenomas, Rathke cleft cysts, and retrochiasmal craniopharyngiomas), the preferred approach is endonasal. For tumors impacting the optic apparatus from above, laterally, or those that push the optic chiasm posteriorly (predominantly meningiomas), the supraorbital approach is often preferred. For tumors that in part encircle an optic nerve or the chiasm, either an approach from above or below may be reasonable depending on the pathology and the goals of the surgery. For tumors with a significant extension or origin lateral to the optic nerves and tracts that grow into the middle fossa and the lateral orbit, a minipterional approach may be optimal. Finally, for very large, extensive tumors that involve both the middle and frontal fossae, orbit and that may impinge into the suprasellar cistern, a conventional frontotemporal (pterional) craniotomy may be preferred. We discuss some of the nuances in choosing the optimal approach for specific tumors with an emphasis on the use of the supraorbital and minipterional routes.

Use of Supraorbital and Minipterional Approaches

Indications and Choice of Approach

Intra-axial and extra-axial tumors of the anterior cranial fossa, anterior aspect of the middle fossa, and frontal lobe can be accessed through the supraorbital and minipterional keyhole approaches. As detailed in Table 35.1 , the most common tumors affecting the optic apparatus (including the intracranial optic nerves, chiasm, and optic tracts) and that are appropriate for the supraorbital or minipterional craniotomy are parasellar meningiomas, craniopharyngiomas, some gliomas, pituitary adenomas, and metastatic brain tumors.

Table 35.1

Most Common Pathologies Approached by Supraorbital Craniotomy

Extra-Axial Tumors Intra-Axial Tumors
Meningioma Germinoma
Craniopharyngioma Glioma
Pituitary adenoma Metastatic carcinoma
Rathke cleft cyst Lymphoma
Arachnoid cyst

Meningiomas arising from the tuberculum sellae and posterior planum region, anterior clinoid process, and in some instances, the medial sphenoid wing are ideally approached from the supraorbital route. Some invasive parasellar meningiomas that have extensive sellar, cavernous sinus, and/or Meckel cave growth may need to be approached from the supraorbital route for optic apparatus decompression; however, many if not most of these can also be effectively debulked from the endonasal route. Regarding tuberculum sellae meningiomas, for which both endonasal and transcranial approaches are feasible, the three key factors in choosing the approach are tumor size, sellar depth, and lateral tumor extension. As we have previously described, ideal meningiomas for the endonasal route include those less than 3 cm without lateral extension beyond the optic nerves and supraclinoid carotid arteries and a deepened sella. The remainder of tuberculum sella and anterior clinoidal meningiomas, including those that in part involve the medial sphenoid wing, can often be removed by a supraorbital or conventional frontotemporal craniotomy.

The supraorbital approach, which places the ipsilateral optic nerve directly in line with the route, creates a relative surgical blind spot along the undersurface of the ipsilateral optic nerve ( Fig. 35.1 ). Meningiomas of the tuberculum sella elevate the nerve and chiasm and can often grow into the optic canal. Compression of the chiasm can cause a bitemporal hemianopsia, and stenosis of the optic canal by tumor can cause loss of visual acuity as well as variable visual field loss. This poses a challenge to the surgeon, as decompression of the optic canal may be required. Although the supraorbital approach does allow for bony decompression of the optic canal roof, the area inferior to the optic nerve ipsilateral to the approach is not visible in the line of sight of the microscope. Leaving this tumor behind risks continued visual disturbance and increases the likelihood of recurrence. Angled endoscopes and instruments allow the surgeon to visualize and reach under the ipsilateral nerve and remove tumors in this location.

Fig. 35.1

Drawing of supraorbital craniotomy showing range of exposure ( blue shading ) and three potential blind spots (highlighted in orange ), including the anterior cribiform plate area, the inferior to the ipsilateral optic nerve, and the ipsilateral sphenoid wing area. Notably, all of these areas can typically be well visualized and accessed with the aid of angled endoscopy.

Craniopharyngiomas pose a particular challenge as they can have a variable relationship with the optic nerves and chiasm. Given that most craniopharyngiomas are retrochiasmal in location, resulting in a prefixed chiasm ( Fig. 35.2 ), they are best approached from an endonasal route to minimize manipulation of the optic apparatus. This approach also typically puts the surgical trajectory along the long-axis of the tumor. Craniopharyngiomas that arise anterior to the optic chiasm (postfixed chiasm), or elevate it superiorly ( Fig. 35.3 ), or which have suprachiasmatic anterolateral extensions into the frontal and middle fossae, can often be approached transcranially (in most cases via the supraorbital approach rather than the minipterional approach). Pituitary adenomas with exophytic supradiaphragmatic extensions can similarly be removed from the supraorbital approach, whereas the minipterional route is rarely needed.

Fig. 35.2

Drawing of a retrochiasmal craniopharyngioma causing a prefixed chiasm ( yellow ).

Fig. 35.3

Sagittal postcontrast magnetic resonance imaging scan showing an elevated chiasm displaced by a suprasellar craniopharyngioma.

Gliomas, lymphomas, germinomas, and metastatic lesions affecting or intrinsic to the optic apparatus are often ideally approached via the supraorbital or minipterional route given that these tumor types are typically above the plane of the optic apparatus (and in the case of gliomas and metastases are intra-axial). As shown in the case example later in this chapter, germinomas often extend up along the infundibulum and may invade the lamina terminalis; as such, the supraorbital route with endoscopic assistance is ideal for accessing this area. In our practice, the supraorbital route, relative to the minipterional route, is favored for accessing anterior cranial fossa and parasellar lesions, as well those extending into the proximal sylvian fissure and medial temporal lobe, and is used almost five times as often as the minipterional route.

The traditional pterional craniotomy is a workhorse approach in neurosurgery, allowing access to the entire circle of Willis, sylvian fissure, optic apparatus, pituitary gland and infundibulum, and the base of the skull in the anterior and middle cranial fossae. The minimally invasive variant of this approach allows similar access to all of these structures, with the only limitation access to the distal sylvian fissure. As detailed in Table 35-2 , the minipterional approach is particularly well suited for lesions that are predominantly in the middle fossa with extension to the ipsilateral optic nerve and chiasm, as well as tumors that extend into the orbit or traverse the superior orbital fissure. Meningiomas are by far the most common tumor approached via the minipterional route, which is particularly effective for sphenoid wing meningiomas that invade the orbital apex and/or optic canal. Bony decompression of the optic canal, orbital apex, and orbit can be accomplished via the minipterional route. Sphenoid wing meningiomas are relatively easy to access with this approach, as the lateral aspect of the wing is drilled and the tumors present themselves close to the surface, as seen in the later example. An anterior clinoidectomy can also be performed extradurally or intradurally. The limitation of the minipterional approach, as with most keyhole approaches, is the difficulty getting light to the deep structures. This is overcome by introducing the endoscope to look up close at the neurovascular structures of the skull base, as well as around corners and into blind spots. This can aid the surgeon in finding small remnants of tumors and improve the likelihood of a complete resection. Fig. 35.4 indicates the overlap and the difference between minipterional and supraorbital approaches. The minipterional approach is better suited for lateral orbital apex and optic canal lesions, whereas the subraorbital approach is ideal for lesions involving the entire optic apparatus.

Table 35.2

Most Common Pathologies Approached by Minipterional Craniotomy

Extra-Axial Tumors Intra-Axial Tumors
Meningioma Glioma
Optic nerve sheath tumor Metastatic carcinoma
Orbital tumors Lymphoma

Fig. 35.4

Drawing showing the relative positions and overlap of the supraorbital ( blue ) and minipterional ( orange ) incisions and craniotomies, as well as relevant landmarks including the superior temporal line and supraorbital nerve.

Nuances of Endoscopy and Endoscope-Assisted Transcranial Tumor Removal

For most tumor resection with either endoscopy or endoscope-assisted approaches, the microscope provides excellent visualization and illumination, and in some instances, the endoscope is of little value. However, for certain anatomic regions and to better understand key neurovascular-tumor relationships, the endoscope is invaluable. The most common utility for the panoramic angled visualization provided by the endoscope is visualization of the region behind or under the optic nerve or chiasm or overlying brain without retraction. Provided a potential intracranial space has been created, the endoscope can be brought into this space and illuminate what cannot be seen by the microscope. In most instances, this means that most of the tumor has already been debulked, the brain is relaxed, and there is sufficient space to maneuver effectively and safely to determine if additional tumor can be safely removed. Using the endoscope at this stage of the surgery in most instances allows one to see these spaces without directly retracting the optic apparatus or the brain, which is otherwise necessitated if one is using only the line-of-sight visualization of the microscope. For an already compromised optic nerve or chiasm, even minimal manipulation may lead to permanent vision damage.

The endoscope thus allows visualization of parts of the skull base around the optic apparatus that are otherwise too risky to clearly visualize with the microscope alone. This up-close endoscopic view, however, comes with several of its own risks. First, given the small opening, there is the potential for instrument conflict and poor maneuverability. Second, the actual heat of the endoscope light must be appreciated and proximity to the optic apparatus and associated vasculature must be respected. Decreasing the light intensity and frequent irrigation can help mitigate this effect. Third, the surgical team must continually be cognizant of the passage of instruments in and out of the field. In laparoscopic or thoracoscopic surgeries, a port is placed that enables safe passage of instruments without traumatizing superficial tissues. This is not possible in cranial surgery, as the instruments and the endoscope are advanced through the same opening. This requires extra caution and coordination on the part of the surgical team to maintain a coaxial view of the instruments upon entry through the craniotomy. This maneuvering is complicated by the need to maintain a two-handed surgical technique. Although various endoscope holding arms are available, the authors recommend that a skilled assistant should drive the endoscope, as this allows for dynamic focus to be maintained over the entire surgical field both superficially and in the deep parts of the field, using synchronized movements of the endoscope, as instruments are inserted and removed throughout the procedure.

Surgical Technique: General Room Setup and Essential Instrumentation for Keyhole Surgery in the Optic Apparatus Region

Given that the endoscope will be used for at least part of the procedure, the room monitor should be set up with this need in mind ( Fig. 35.5 A, B) so that it can be swung into position easily when the endoscopes are used. Rigid-4 mm 0-, 30-, and 45-degree rod-lens endoscopes are ideal. Additionally, given the relatively restricted opening of the supraorbital and minipterional approaches, keyhole low-profile bayoneted or pistol-grip instrumentation should be used. Other essential equipment include the Doppler probe for vessel localization, two-dimensional ultrasonography for assessing completeness of tumor removal, with aneurysm clips and appliers readily available. Consideration should also be given to evoked potential monitoring, including somatosensory evoked potentials and motor evoked potentials depending upon the pathology. If it is anticipated that the frontal sinus will be entered, the right or left lower quadrant of the abdomen should be prepared to harvest a fat graft.

Fig. 35.5

Room setup for intracranial endoscopy. A, Positioning of a patient and room setup for right supraorbital craniotomy for both microscope and endoscope-assisted surgery with the monitors positioned to easily swing into position for endoscopy. B, Endoscopic portion of the operation with microscope swung out of position, surgeon sitting, and endoscope being driven by assistant.

Surgical Technique: Supraorbital Craniotomy

The supraorbital craniotomy was first described by Fedor Kraus in 1908 and was subsequently modified with a decrease in the size of the bony opening while preserving access to the skull base as described by Reisch et al. in 2003. The supraorbital craniotomy is now considered a workhorse approach to access the anterior cranial fossa and optic apparatus. Fig. 35.1 shows the broad access to the base of the skull allowed by this approach. The patient is placed supine on the operating table with the head in a Mayfield clamp. The head is turned to the contralateral side, typically 20 to 45 degrees depending on the pathology, and extended so that the malar eminence is the most prominent part of the head ( Fig. 35.6 A, B). The degree of turning is dependent on the location of the pathology; more medial pathology requires more of a head turn. For a typical tuberculum sella meningioma, a 30-degree head turn with the malar eminence prominent is ideal. Intraoperative navigation is used as an adjunct to localize the lateral border of the frontal sinus as well as intermittently throughout the procedure to localize relevant anatomy.

Fig. 35.6

A, Head positioning for supraorbital approach. B, Position of the craniotomy relative to the frontal sinus and exposure of the skull base.

The incision is made within the eyebrow to minimize a visible scar ( Fig. 35.7 A ), starting from just medial to the supraorbital notch to just beyond the superior temporal line. The lateral termination may need to extend a few millimeters beyond the termination of the eyebrow for adequate incision length. The subcutaneous tissues are opened sharply until the pericranium is reached. A stitch is placed through the skin and subcutaneous tissues of the inferior aspect of the incision for gentle retraction. The supraorbital nerve is then dissected out bluntly with scissors and carefully preserved ( Fig. 35.7 B). The pericranium is incised just above the supraorbital rim from the supraorbital nerve medially, to the superior temporal line, continuing along the temporalis fascia and muscle down to bone ( Fig. 35.7 C). An intersecting cut is made parallel and immediately lateral to the supraorbital nerve, and the pericranium directly over the site of the craniotomy is elevated superiorly. Multiple fish hooks are placed for gentle but effective superior retraction. Achieving adequate superior exposure is critical to the success of the eyebrow craniotomy. A bone flap that is less than 2 cm in height will be very restricting for the procedure and potentially unsafe to adequately maneuver and perform microneurosurgery. The high-speed drill with matchstick bit is used to create a burr hole at the keyhole just behind the frontozygomatic process. The dura is carefully stripped away from the inner table and the craniotome is used to fashion a craniotomy that is approximately 2.5 cm width and 2.0 cm high. If the frontal sinus is entered, a piece of Gelfoam (Pfizer, Groton, NY) or collagen soaked in betadine is placed inside for the duration of the procedure. Small openings can be sealed off with bone wax, but larger openings need to be obliterated and reinforced with an abdominal fat graft. The dura is dissected off the floor of the anterior cranial fossa, and the matchstick bit is again used to thin down the inner table of the frontal bone and the roof of the orbit, making a flat plane ( Fig. 35.7 D).

Jan 3, 2021 | Posted by in OPHTHALMOLOGY | Comments Off on Transcranial Approaches to the Optic Apparatus

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