Endoscopic Approaches to Skull Base Lesions

Clinical Pearls

Become knowledgeable about the equipment and practice on a cadaver or under the guidance of an experienced neuroendoscopist.
Orientation and anatomy of the cranial skull base are everything. Absolutely know where you are, what you are looking at, and your orientation at all times. In this chapter, the most important neurovascular structures related to the cranial skull base approaches are defined. If you are not sure, abort the procedure.
Use the endoscope for its advantages and do not fight its disadvantages. Increase your use of angled scopes and increase the size of your visualized field.
Become comfortable with the endoscopic management of sellar lesions prior to using the endoscope for skull base neuro-oncologic applications.
Endoscopic applications are not all or nothing. Even for an experienced endoscopist, some lesions are more safely dealt with from a microscopic surgical approach. In these cases, use an endoscope-assisted approach to your advantage.
The standard endoscopic endonasal approach to the sellar region is targeted to the sella turcica. Because the sella is the epicenter at the crossroads of the sagittal and coronal planes, this approach is the starting point for most of the extended endonasal surgical modules.
Endoscopic endonasal approach has expanded its horizon allowing surgeons to access several diseases interesting different areas of the skull base—namely the suprasellar, retrosellar, ventircular, clival, infratemporal and parasellar spaces—obviating brain retraction. Indeed it offers: a superior close-up view of the relevant anatomy and an enlarged working angle with an increased panoramic vision inside the surgical area.

Introduction

The skull base is one of the most fascinating and complex areas of the human body, from both anatomic and surgical perspectives. It is located in a peculiar position, between the brain and extracranial compartment, and is composed of many anatomic structures. It can be involved in a wide variety of lesions, either neoplastic or not, primarily arising from this area or subsequently involving it. The surgical management of such pathologies may be extremely difficult, above all for deep-seated lesions. Although a variety of innovative craniofacial approaches have been adopted to access the entire skull base, these routes are often characterized by tissue disruption and neurovascular manipulation, leading to an increase of perioperative morbidity or mortality rates.

Continuous technologic innovations and surgical advances, together with progress in diagnostic imaging techniques and intraoperative neuronavigation systems, have led to a progressive reduction of the invasiveness of skull base approaches, affording the development of the transsphenoidal technique and its variations. Thanks to the introduction of the endoscope, the neurosurgery community has moved forward. Nowadays, the endoscopic endonasal transsphenoidal approach appears to be a viable technique for the management of several skull base diseases: it has been defined as the least traumatic route to the sella, and it provides an excellent visualization of the targeted area while avoiding brain retraction and neurovascular manipulation—above all of optic chiasm/optic nerve complex—also at the level of the suprasellar, retroclival, and third ventricle areas. The extended endoscopic endonasal transsphenoidal approach permits exposure of the entire midline skull base and paramedian regions through the nose without brain retraction, making it possible to treat different lesions traditionally approached transcranially (tuberculum sellae meningiomas, craniopharyngiomas, clival chordomas, etc.).

Endoscopy has brought advantages to the surgeon (wider and closer views of the surgical target area, easier treatment of recurrences) and to the patient (less nasal traumatism, no nasal packing, lower incidence of complications, less postop pain, and usually quick recovery).

Historical Overview

The first pure endoscopic procedure was performed in 1910 by Victor Darwin Lespinasse, a urologist from Chicago, who realized ventricular endoscopy for the coagulation of the choroid plexus in the treatment of a hydrocephalus. Transsphenoidal surgery began in the late 1800s thanks to the anatomy studies of the Italian surgeon Davide Giordano, which the Viennese surgeon Herman Schloffer relied on to run this surgical approach on a patient, for the first time in 1907. Over the years the technique was continuously improved through the contributions of giants such as Theodor Kocher, Albert Halstead, Harvey Williams Cushing, Norman McOmish Dott, and Jules Hardy, who favored the flourishing of the transsphenoidal route for the treatment of sellar and parasellar diseases.

Gerard Guiot was the first, in 1963, to adopt the endoscope during a transnasorhinoseptal microsurgical approach: he performed an endoscopic exploration of the sellar content, after the removal of a pituitary macroadenoma by means of a microsurgical technique. In the late 1970s, Michael L.J. Apuzzo captured this concept: he added safety and precision to microscopic transsphenoidal surgery, using the endoscope to highlight dark and deep anatomic corners during surgery.

After sporadic reports about the endoscopic technique—the most important of which was described by Bushe and Halves —in the early 1990s, with the contribution of ear, nose, and throat (ENT) surgeons in functional endoscopic sinus surgery (FESS), the use of endoscopic endonasal transsphenoidal surgery became widespread. Many techniques have been employed in endoscopic transsphenoidal surgery, leading to the definition of the “pure” endoscopic endonasal approach to the sella fixed by the Pittsburgh duo made up of otorhinolaryngologist Ricardo Carrau and neurosurgeon Hae-Dong Jho ; immediately thereafter, our group in Naples was the first in Europe to adopt the technique, leading to a revolutionary process in the pituitary and skull base lesions management field.

The evolution of the transsphenoidal technique resulting from the intrinsic technical advances of endoscopic equipment (computer chips, TV video cameras, xenon cold light sources) and of support instrumentation (neuronavigation systems, Doppler ultrasonography) has extended the approach beyond the sellar area, mainly targeted to the entire midline of the skull base from the anterior skull base to the craniovertebral junction and adjacent areas. The first to define an extended transsphenoidal approach —that is, the additional bone removal by the tuberculum sellae and the posterior planum sphenoidale between the optic canals, with opening of the dura mater above the diaphragma sellae—was Martin Weiss in 1987. Nevertheless, Amin Kassam's group in Pittsburgh popularized this route and, thanks to the use of the endoscope, categorized the main approaches to the skull base on the sagittal and coronal planes with a rigorously anatomic method. Endoscopic transsphenoidal surgery has been extended for the management of lesions involving regions surrounding the sella, such as CSF leaks, craniopharyngiomas, tuberculum sellae meningiomas, and upper clival chordomas. Our group in Naples adopted this technique for the treatment of various midline skull base lesions, providing a further boost to the refinement of the principles and strategies of the surgical technique as well as to the reconstruction strategies, complication, and instruments.

Skull Base Anatomy: The Endonasal Perspective

As explored in the endoscopic endonasal corridor, the skull base can be divided in three main areas:

1.
The anterior midline skull base (from the frontal sinus to the posterior ethmoidal artery), accessed through an endoscopic endonasal transcribriform approach
2.
The middle skull base (from the planum sphenoidale to the sellar floor), revealed via different corridors: the endoscopic endonasal transplanum-transtuberculum approach to the suprasellar area or the standard endoscopic endonasal transsphenoidal approach to the sellar region
3.
The posterior midline skull base (from the dorsum sellae to the craniovertebral junction), exposed by means of the endoscopic endonasal transclival approach

Anterior Midline Skull Base

From the endonasal point of view, the anterior midline skull base corresponds to the roof of the nasal cavities. After removal of the anterior and posterior ethmoidal cells' complex along with the posterior part of the nasal septum, the anterior skull base is identified as a rectangular area limited laterally by the medial orbital walls, posteriorly by the planum sphenoidale, and anteriorly by the recesses of the two frontal sinuses. This area could be equally split into two compartments by the lamina perpendicularis of the ethmoid; on both sides the ethmoidal labyrinth occupies laterally the nasal cavities, whereas in each aspect the lamina cribrosa, a thin bony layer pierced by the small olfactory phila lies medially.

The arterial supply of the ethmoidal dura mater is ensured by the anterior ethmoidal arteries (AEAs) and the posterior ethmoidal arteries (PEAs), branches of the ophthalmic artery. When approaching the anterior skull base via the endoscopic endonasal corridor, it is recommended to remove the superior portion of the lamina papyracea and to isolate, on both sides, the anterior and posterior ethmoidal arteries. Upon the opening of the anterior cranial fossa dura, the olfactory nerves and the basal surface of the frontal lobes are displayed.

Middle Skull Base

As seen from the nasal cavity, the middle skull base corresponds to the posterior and lateral walls of the sphenoid sinus. The sellar floor lies at the center on the posterior sphenoid sinus wall and continues above with the planum sphenoidale and below with the clivus. Several anatomic landmarks surround this area: the optic nerve prominences, above, formed by the bony covering the optic nerves; the carotid prominences, below, covering the C-shaped intracavernous internal carotid artery; and, between these two prominences, the optocarotid recess on each side. Once opened, the sellar floor and its dura, the anterior lobe of the pituitary gland, will come into view. Posteriorly, the neurohypophyseal part of the pituitary gland can be observed, which is softer and more densely adherent to the sellar wall. The diaphragma sellae above and, laterally, the internal carotid arteries within the cavernous sinus, can be appreciated.

Between the superior aspect of the sella turcica and the declining part of the planum sphenoidale lies a wide, shallow indentation; when this structure is viewed from the transcranial corridor it is called the tuberculum sellae, whereas when it viewed from the endonasal perspective it is called the suprasellar notch , due to its peculiar feature from this standpoint. The removal of tuberculum sellae and the posterior portion of the planum sphenoidale allows for the exposure and exploration of the suprasellar region. We divided this region into four areas using two ideal planes, one passing through the inferior surface of the chiasm and the mammillary bodies, and one passing through the posterior edge of chiasm and the dorsum sellae: suprachiasmatic , subchiasmatic , retrosellar, and ventricular.

The suprachiasmatic region is defined by the chiasmatic and the lamina terminalis cisterns along with their contents; the chiasm and the optic nerves, the anterior cerebral arteries, the anterior communicating artery, and the recurrent Heubner arteries, together with the gyri recti of the frontal lobes, are the main structures.

In the subchiasmatic space, the pituitary stalk is at the center with the superior hypophyseal artery and its perforating branches are seen, bordered by the superior aspect of the pituitary gland, and the dorsum sellae is also visible ( Fig. 48.1 ). The retrosellar area, which can be explored by passing the endoscope between the pituitary stalk and the internal carotid artery, above the dorsum sellae, encloses the upper third of the basilar artery, the pons, the superior cerebellar arteries, the oculomotor nerves, the posterior cerebral arteries, and, lastly, the mammillary bodies and the floor of the third ventricle.

Figure 48.1, Image of a specimen showing the anatomy of the intra- and suprasellar area. Ch, optic chiasm; FL, frontal lobe; ICA, internal carotid artery; ON, optic nerve; P, pituitary gland; pS, pituitary stalk; sha, superior hypophyseal artery.

Finally, the third ventricle could be explored, especially in the anterior areas, allowing an excellent surgical maneuverability and a panoramic view. In the infundibular and foraminal areas the surgical maneuverability seems to be better as compared with that obtained inside the mesencephalic region, whereas the tectal area could not be reached via the endonasal route.

Posterior Midline Skull Base

The endonasal corridor allows unlocking of this area of the skull base through the ventral surface of the clival aspect of the sphenoid bone in its whole extension from the dorsum sellae to the craniovertebral junction. The clivus can be divided, by the floor of the sphenoid sinus, into an upper part or superior third (sphenoidal portion) and a lower part, which includes the middle and inferior thirds (the rhinopharyngeal portion).

Laterally, on the sphenoidal portion of the clivus, the paraclival tract of the carotid arteries represents the lateral limit of the access; the bone removal can be extended to the lower third of the clivus to access to the rhinopharyngeal portion of the clivus. Once the clival bone and the dura layer are removed, the interpeduncular cistern can be exposed; the basilar artery, its branches (posterior cerebral artery, superior cerebellar artery, and anterior inferior cerebellar artery), and the upper cranial nerves (mainly the III and the VI cranial nerves) can be seen. It is possible to further enlarge this opening by removing the anterior third of the occipital condyles. Once the mucosa of the rhinopharynx is stripped off, the atlantooccipital membrane, the longus capitis and longus colli muscles, the atlas, and the axis are seen. After dissection of the muscular structures, the anterior arch of the atlas could be removed to expose the dens, which can be separated from the ligaments.

Instruments and Tools

The transsphenoidal approach to the skull base has been developing in a simple, safe, and elegant manner thanks to the refinement of instrumentation. All of the instruments in the operating room (considered itself a basic equipment for this kind of surgery)—the endoscope, camera, monitor, light source, video-recording system, neuronavigator, coagulator, drill, micro-Doppler probe, and so on—form a sort of chain, where each link should work fine to corroborate the whole mechanism during the surgical procedure.

Dedicated Operating Room

An integrated operating room helps to optimize teamwork and improve patient care. All components of the endoscopic equipment (light source, video camera, monitor, neuronavigator, and video recording system) are placed ergonomically behind the patient's head, in front of the first surgeon, who is at the right side of the patient, regardless of whether he or she is right- or left-handed. The second surgeon is at the left side, and the scrub nurse is positioned at the level of the patient's legs.

Endoscopic Equipment

A rigid rod-lens endoscope—usually 18 cm in length and 4 mm in diameter—is adopted to run a whole endoscopic endonasal procedure targeted to the skull base. The angle of view of the lenses ranges from 0 to 120 degrees, according to the objective, but the scopes most frequently used are the 0-, 30-, and 45-degree lenses. A new device offering the possibility of shifting between 15 and 90 degrees with a fixed horizon has been developed (EndoCAMeleon, Karl Storz, Tuttlingen, Germany).

It is possible to use an endoscope sheath, which is connected to an irrigation system to permit cleaning and defogging of the distal lens without retracting the scope out of nostril. The sheath does not embed any working channel; main instruments are inserted into the same nostril, sliding alongside or inserted into the contralateral nostril. The endoscope in skull base surgery can be used free hand or fixed to a scope holder, namely a steerable or extendable arm or a jointed arm that can be straight, curved, or pneumatic.

The endoscope is connected to a high-definition (HD) camera,; the most common of these uses a 3-CCD (charge-coupled device) sensor, which has a separate chip per color (red, green, blue; RGB) resulting in a better color separation, more brilliant colors, a sharper image, and higher contrast. Another innovation is the Karl Storz Image1 Spies (Storz Professional Image Enhancement System) camera platform. It is combined with new full-HD three-chip camera heads, which feature a pioneering technology related to an improved image quality visualization system.

On the other hand, it should be said that the use of three-dimensional (3D) endoscopes is expanding every day; it avoids the main limit of lack of depth perception brought by conventional endoscopes, but it still is characterized by higher cost, larger size, and greater weight. Besides, symptoms such as diplopia and nausea have been described with prolonged use of 3D endoscopy systems.

Dedicated Surgical Instruments

Because of the unique setting created by the endoscopic endonasal technique, the instruments routinely employed in microscopical operative scenarios do not optimally fit the endoscopic endonasal surgery environment. Since the introduction of the endoscopic endonasal transsphenoidal approach, new instruments have been designed to meet these specific criteria : easy and safe maneuverability in a limited surgical corridor, well-balanced and ergonomic safe handling, streamlined movements, and the coordination of instruments and endoscope. These instruments need to be inserted along the same axis as the endoscope and maintain the same position in respect to the endoscope for their entire length. Furthermore, the widening of the indications for the endoscopic endonasal approach, mostly throughout the midline skull base, has boosted the definition of new surgical instruments in an attempt to increase maneuverability under such conditions.

Coagulators

The control of bleeding, above all arterial, is one of the major issues in endoscopic endonasal surgery. Monopolar coagulation can be easily used inside the nasal cavity with specific monopolar sticks, but its extensive use is not recommended in order to avoid damaging olfactory fibers and major vascular mucosa feeders. Furthermore, monopolar cautery does not prevent late rebleeding, and if the clot falls off, the bleeding could start again. For such reasons, bipolar coagulation should be preferred. The use of the classic microsurgical bipolar forceps through the nose is possible, but maneuverability is limited. Because different endonasal bipolar forceps have been designed, with various diameters and lengths, for the purposes of endonasal endoscopic surgery, the type of bipolar forceps used should be straight with pistol-grip ring handles, whose movement determines the tips' closure for coagulation.

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