Historical Background

Since the 1980s, several ENT surgeons pioneered the functional endoscopic sinus surgery (FESS),^1–5 thus encouraging the “pure” endoscopic endonasal approach to the sella,⁶ performed first by Jankowski;⁷ then Jho and Carrau extended this technique,^8–10 followed by other teams.^11–14 These experiences supported a continued structured evolution of the endoscopic technique: new skills have been transferred across subspecialties, new instruments have been designed,^15–18 new image-guidance systems have been fashioned,^19–24 and new surgical corridors have been defined, thus disclosing the way to the pure endoscopic approach to lesions affecting the skull base, from the frontal sinus to the C2 body.^14,25–28 The shift toward minimally invasive procedures represents the evolution of previous views: Ketcham’s concept of craniofacial resection²⁹ evolved in the concept of “endoscopic craniofacial resection,”³⁰ and Apuzzo’s inspired guess of using the endoscope in transcranial surgery³¹ has been translated in Kassam’s paradigm of “360 degree surgery,” which suggests the use of an endoport to unlock intracranial lesions.³²

Classification

The sphenoid sinus is in the center of the skull base and the starting point for many extended endonasal approaches (Table E44.1) because critical anatomical structures, such as the internal carotid artery and the optic nerve, are identified here and then followed to other areas.^{14,25–27,33–36} The midline ventral skull base can be exposed from the frontal sinus to the body of C2 through specific modules: transcribriform, transtuberculum, transclival, and transodontoid^25,37,38 (Fig. E44.1). Depending on the lesion extension along the sagittal plane,^14,26,28 contiguous modules can be combined, and the lateral boundaries of each module are dictated by critical neurovascular structures. Lesions originating in or extending to paramedian areas, such as the cavernous sinus, the lateral recess of the sphenoid sinus, and the pterygopalatine fossa, can be approached through a transethmoid transantral transpterygoid route,^39–42 and several paramedian modules have been developed to improve the endonasal exposure of the orbit, the petrous apex, the infratemporal fossa, and the jugular foramen.^{27,30,43–45}

TABLE E44.1 Pure Endoscopic Approaches to the Skull Base

FIGURE E44.1 Schematic drawing in sagittal projection showing the different trajectories that are followed to expose the olfactory groove (red), the planum sphenoidale (turquoise), the sella (yellow), the clivus (blue), and the craniovertebral junction (purple).

(Reprinted with permission from Cappabianca P, Cavallo LM, de Divitiis E. Endoscopic Pituitary and Skull Base Surgery. Tuttlingen, Germany: Endo Press; 2008:25, Fig. 79.)

Imaging Techniques

Patients are studied with both magnetic resonance imaging (MRI) and computed tomography (CT), which are complementary in delineating the individual anatomy of patients harboring skull base lesions, thus optimizing patient selection and operative procedure.^35,46,47

High-resolution CT scan, with coronal and sagittal reconstruction, including both soft tissue and bone algorithms, is performed perpendicular to the hard palate. Depending on the specific module, the following points are assessed:

Preoperative CT angiography is used for intraoperative image guidance.

MRI is performed in the three planes with 3-mm sections and contrast enhancement and addresses the following points:

If sinusitis is identified, an intradural approach should not be undertaken until the infection has cleared.

Magnetic resonance angiography better defines the internal carotid artery (ICA) and other vascular structures, and digital subtraction angiography delineates the vascular supply and is not routinely performed. The latter is mainly used when an ICA occlusion test is deemed opportune.

Three-dimensional reconstruction techniques and measurement software are used routinely to estimate the osteodural window, trajectory, and reconstructive requirements and, therefore, to adapt the modular exposure and work out an optimal vascularized flap, when necessary.

Surgical Technique⁴⁹

Perioperative antibiotic prophylaxis with a broad-spectrum cephalosporin⁵⁰ is administered half an hour before surgery and continued for 5 days. We do not usually insert any lumbar drainage. Neurophysiological monitoring of brain and nerve function allows detection of cerebral ischemia and prevention of cranial nerve injury. An optimal anesthesia and analgesia combination⁵¹ contributes to a dry field, will improve surgical precision, and can reduce complications and frustration during the surgical procedure.

The head is fixed by mean of a circumferential band-aid in the standard approach to the sellar region. The head is fixed in a three-point Mayfield-Kees holder in a neutral position and slightly extended or flexed for transcribriform and transclival modules, respectively. The thorax is elevated about 10 degrees and the height of the bed is adapted to the surgeon’s height. The endoscopic equipment, the neuronavigator, and the Doppler probe are checked and the monitor is positioned along the line of vision of the surgeon, who stands at the patient’s right side; the surgeon’s assistant stands on the left side and the nurse is positioned at the patient’s legs.

Cotton pledgets soaked in diluted (1:1) povidone-iodine solution are gently slipped into the nasal cavities and the nose and upper lip are disinfected.

The procedure is performed with a rigid 0-degree endoscope, 18 cm in length, 4 mm in diameter (Karl Storz & Co., Tuttlingen, Germany), without any working channel and inserted in an irrigation shaft to keep the lens clean. A two-nostrils four-hands technique^{14,26,27,52,53} is recommended, with the endoscope used freehand during the whole procedure; it is held by the surgeon’s nondominant hand and, after the anterior sphenoidotomy, by the surgeon’s assistant. A 30-degree or 45-degree angled scope increases the angle of view when a more lateral view is needed, quite often in the intradural phase of the procedure.

Extended Approaches

The idea of an approach extending beyond the sella should be credited to Hardy,⁵⁷ and the routine use in clinical practice is due to Martin Weiss.⁵⁸ This first step of the surgical procedure can be considered the common denominator of all the extended approaches. The endoscope is inserted in the right nostril, which is stretched upward while instruments run below it. Essential aspects of this stage, according to Kassam and his group,²⁷ include the following:

Middle turbinectomy, usually unilateral, with lateralization of the middle turbinate in the other nostril, provides the adequate room to access and manage confortably the lesion in most cases.

Harvesting of the nasoseptal flap is done (see later discussion under “Reconstruction”).

A wide anterior sphenoidotomy is the next step.

A posterior septectomy permits instruments coming from the two nostrils to work without conflict.

The removal of posterior ethmoidal cells completes the exposure of the posterior wall of the sphenoid sinus with its anatomical landmarks.

Once the sphenoid sinus is entered, the orientation of the endoscopic view in respect to the landmarks over its posterior wall is checked and carefully maintained. The sellar floor is in the center, the planum sphenoidale is above it, and the clival indentation lies below. Lateral to the sellar floor, the bony prominences of the intracavernous carotid artery, the optic nerve, and between them, the medial and lateral opticocarotid recesses can be observed.

In the presence of a presellar or a conchal sphenoid sinus, there will be a paucity of anatomical landmarks and the use of a neuronavigation system and the microdoppler probe provides accurate information with regard to the midline and position of the parasellar internal carotid artery.

Transcribriform Approach

This approach is used for the management of cerebrospinal fluid (CSF) leaks,⁵⁹ meningoencephaloceles, esthesioneuroblastoma,^30,60 and meningiomas.^61–64 It is limited by the medial orbital wall laterally, but it can be expanded along the sagittal plane, depending on the lesion extension. However, to expose the cribriform plate, the posterior limit is marked by the posterior ethmoidal artery (Fig. E44.2A). The exposure can be further expanded anteriorly, toward the frontal sinus, through a transfrontal module, which provides access to its floor and the posterior wall³⁰ (Fig. E44.2B).

FIGURE E44.2 Transcribriform approach. A and B, Anatomical images showing the anatomical landmarks of the approach. AEA, anterior ethmoidal artery; CG, crista galli; dm, dura mater; dmASB, dura mater of the anterior skull base; FR, frontal recess; O, orbit; OP, optic protuberance; PEA, posterior ethmoidal artery.

The main steps to expose the midline anterior skull base are as follows:⁶⁵

Bilateral anterior and posterior ethmoidectomy: entering the ethmoid bulla inferomedially, the lamina papyracea comes into view and should be preserved during the ethmoidectomy.

Exposure of the frontal recess and of the anterior ethmoidal artery: opening the agger nasi air cell highlights the posterior wall of the frontal recess, which leads to the anterior ethmoid canal. The lamina papyracea is fractured with a blunt instrument and lateralization of the periorbit highlights the artery, where it leaves the orbit to enter its osseous canal. At this level, the anterior ethmoidal artery can be easily bipolar coagulated. The frontal recess guides toward the frontal sinus floor, which can be removed to enlarge the exposure anteriorly.

Identification and coagulation of the posterior ethmoidal artery: its bony canal runs in the ethmoidal roof horizontally and is located at an average of 8.3 mm (range 6-11 mm) anterior to the optic nerve exit from the optic canal.⁶⁵

Opening of the ethmoidal plate with bone punches or microdrill.

In olfactory groove meningiomas, the dura is coagulated and opened along the midline. In lesions extended to the crista galli, the dura is cauterized and incised lateral to the insertion of the falx; after coagulation of the anterior falcine artery and the inferior sagittal sinus, the falx is transected and the dura is mobilized anteriorly. Further anterior extension of the lesion may require drilling of the crista galli, which can be hyperostotic or pneumatized (10% of cases).⁶⁶ After internal debulking, extracapsular dissection proceeds in a lateral to medial direction; when anterior cerebral artery branches or the anterior communicating artery lie on the surface of the tumor, a piecemeal removal is performed and the last tumor remnants are carefully dissected.

Anterior skull base lesions extending over the midplane of the orbital roof can be accessed through removal of the medial wall of the orbit and lateral displacement of the orbital tissues.

In esthesioneuroblastomas extending intradurally, the olfactory bulbs are dissected from the surface of the brain and remain attached to the dura; focal areas of brain invasion are removed by suction and dissection. The olfactory nerves are then transected posteriorly and dura is removed completely; additional dural margins can be excised for frozen section analysis.⁶⁰

Transtuberculum-Transplanum Sphenoidale Approach

In 1980 Ed Laws, Jr., described the microscopic transsphenoidal approach to suprasellar lesions by taking down the planum sphenoidale,⁶⁷ and the same approach was adapted for endoscopy.^67,68 It is bordered by the optic nerve’s bony prominence, the opticocarotid recess, and the bony prominence of the parasellar segment of the internal carotid artery (Fig. E44.3).

FIGURE E44.3 Transtuberculum transplanum approach. Anatomical image. Identification of the landmarks on the posterior wall of the sphenoid sinus. Arrowhead shows the medial opticocarotid recess and double asterisk indicates the lateral opticocarotid recess. C, clivus; CP, carotid protuberance; CPc, paraclival segment of the carotid protuberance; CPs, parasellar segment of the carotid protuberance; OP, optic protuberance; SF, sellar floor.

The main aspects of this module are as follows:

Opening of the sellar floor and drilling of the tuberculum are done in order to widen the caudal-cranial angle of exposure of the third ventricle and retrosellar area.

Management of the superior intercavernous sinus with its isolation is done with two horizontal incisions and bipolar coagulation, in case of craniopharyngiomas, and by coagulation in meningiomas.

Lateral extension of the bone window: the medial opticocarotid recess lies at the point of maximum contact between the internal carotid artery and the optic nerve and corresponds intracranially to the middle clinoid process.⁶⁹ Bone removal in this critical area may be refined with a Kerrison rongeur or an ultrasonic bone curet.⁷⁰

Removal of the sphenoid planum: in order to preserve olfaction, the posterior ethmoidal artery marks the anterior limit of the exposure and the rostral margin of the nasal septum (1 to 2 cm) is left intact.

Once the dura has been opened, the following factors are evaluated:

Consistency of the lesion

Relationship of the lesion with the infundibulum, chiasm, and optic nerves

In many cases, mainly cystic retrosellar lesions, removal of the whole sellar floor, flush to the clivus, provides a certain degree of pituitary mobility and the parapituitary stalk corridor could be enough to expose the prepontine cistern. When dealing with solid intrasellar lesions, a transpituitary route or additional pituitary mobilization is considered (Fig. E44.4). The pituitary gland can be mobilized on one side or reflected upward. Kassam has described an en bloc transposition.⁶⁹ In order to reduce stretching of the infundibulum and the length of this step, a selective dislocation upward of the central part of the gland, through a V-shaped cut of the gland, may be considered.

FIGURE E44.4 Anatomical images. After detachment of the gland from the pituitary fossa, the dorsum sella and the posterior clinoids have been removed to highlight the retrosellar area. III, oculomotor nerve; A1, precommunicating segment of the anterior cerebral artery; BA, basilar artery; Ch, chiasm; ddm, dura of the dorsum sellae; ICA, internal carotid artery; MB, mammillary bodies; OT, optic tract; P1, precommunicating segment of the posterior cerebral artery; Pg, pituitary gland; Ps, pituitary stalk; sca, superior cerebellar artery; sha, superior hypophyseal artery.

After mobilization of the gland, the dorsum sellae and the posterior clinoids are drilled. During drilling of the dorsum sellae, the surgeon must be aware that the working area is funnel-shaped because the intercarotid distance at the anterior wall of the sella is about 20 mm, but the posterior interclinoidal distance is about 15 mm; this configuration poses a risk of injury of the parasellar area.

Passing behind the pituitary gland, the endonasal view can be expanded up to the floor of the third ventricle, interpeduncolar fossa, and the mesial surface of the temporal lobe.

Tumor removal is tailored to the lesion, following the microsurgical principles.^71–74

In suprasellar supradiaphragmatic craniopharyngiomas (Fig. E44.5) the surgical strategy is guided by their relationship with the infundibulum, as has been proposed by Kassam and associates.^74–76 In prechiasmatic lesions the tumor is seen immediately after dural opening, with the optic nerve and chiasm stretched superiorly. During extracapsular dissection, the distal branches of the superior hypophyseal artery, which supply the inferior aspect of the chiasm and the pars tuberalis of the pituitary stalk, should be preserved. Retrochiasmatic craniopharyngiomas can be unlocked through intra- or parapituitary stalk routes, depending on their pattern of growth. When the lesion expands in the infundibulum, the intrapituitary stalk corridor leads toward the third ventricle chamber.

FIGURE E44.5 Suprasellar craniopharyngiomas. Surgical corridors used in different patients. A and B, In these two cases the pituitary stalk was not involved by the tumor and the parapituitary stalk corridor was used. C and D, The infiltrated pituitary stalk (C) was used as a gate to enter through the infundibulum of the third ventricle (D). Ch, chiasm; dotted line, surgical corridor; ftV, floor of the third ventricle; itc, interthalamic commissure; Pg, pituitary gland; ps, pituitary stalk; SF, sellar floor.

Intraventricular craniopharyngiomas can be managed through the endonasal route when the floor of the third ventricle is displaced inferiorly by the lesion and the infundibulum is enlarged. When dealing with such lesions some points should be kept in mind:

The chiasm may lie in a prefixed position, just behind the dura, which should be opened carefully above the sella without transgressing the arachnoid.

During tumor dissection the ventricular floor should never be violated at the level of the mammillary bodies, or even more posteriorly, to avoid brainstem injury.

During tumor dissection along the lateral walls, adherent gliotic or invasive finger-like tumoral tissue at the level of the hypothalamus should not be violated.

Retroinfundibular craniopharyngiomas can be unlocked, preserving the pituitary stalk, using the parapituitary stalk corridors and by mobilizing the gland on one side or upward, as proposed by Kassam; the critical point of this step is to avoid stretching of the pituitary stalk and retrograde cell death in the hypothalamus. When required, the surgical window can be further enlarged by drilling the dorsum sellae and removing the posterior clinoids.⁶⁹

Tuberculum sellae meningiomas can be devascularized early and completely removed, together with the dura and bone involved.^64,77,78 Internal debulking can be performed with radiofrequency monopolar wire electrodes (SurgiMax; Elliquence International, Hewlett, NY) and Cavitron ultrasonic aspirator.^37,61 In the extracapsular dissection the pituitary gland, the pituitary stalk, and the chiasm are identified and protected with a cottonoid, with sparing of the perforators to the visual apparatus. The optic chiasm can present a neurovascular conflict with the precommunicating segment of the anterior cerebral artery.⁷⁸

Transclival Approach

The clivus extends from the posterior clinoids to the craniovertebral junction (Fig. E44.6). Depending on the lesion extension along the cranial-caudal axis, the transclival approach is carried out across three main corridors—superior, middle, and inferior—which can be plainly combined as well.^{26,27,79–85} The superior corridor unlocks the retrosellar area and has been described in the previous module. The middle corridor is developed through the following steps:

Lateral dissection of the mucosa covering the vomer: in a medial to lateral direction, the vomer-sphenoid junction, the ventral end of the palatovaginal canal, and the pterygoid canal are identified.

Removal of the vomer.

Drilling of the sphenoid floor: the lateral limit is the pterygoid canal, which points toward the anterior genu of the intrapetrous carotid artery.⁸⁴

Drilling of the clival bone.

FIGURE E44.6 Transclival approach, intradural phase. A, Anatomical image. B, Surgical intradural exploration after removal of a small clival meningioma. III, oculomotor nerve; VI, abducens nerve; aica, anterior-inferior cerebellar artery; BA, basilar artery; ICA, internal carotid artery; sca, superior cerebellar artery.

In step 3 the sphenoidal and rhinopharyngeal segments of the clivus are thus exposed at once and the corridor is bordered by the paraclival segment of the internal carotid artery and the pterygoid canal. The identification of this latter structure is not always essential and sometimes is not easy, mainly in few pneumatized sphenoid sinus; in such cases, intraoperative neuronavigation and microdoppler guide the exposure.

Extradural lesions, such as chordomas, have a propensity for insinuation into bone along lines of least resistance, and careful exploration at the lateral borders of the clivectomy is mandatory. At this level the abducens nerve is at risk at the level of its dural porus (Dorello’s point), which is on average situated 1 cm from the midline and 20 mm below the posterior clinoid process.⁸⁶ At this level the nerve is in close relationship with the dorsal meningeal artery. Bleeding from the basilar venous plexus is managed.⁸⁷

The intradural extension of chordomas and meningiomas is unlocked through a midline dura opening.⁸¹

After tumor removal, intradural exploration highlights, on the midline, the pons, the medulla oblongata, and the basilar artery (Fig. E44.7). Running the endoscope above the cisternal segment of the abducens nerve, the cisternal segment of the trigeminal nerve can be followed toward its dural porus, which lies about 5 mm above and lateral to Dorello’s point. Running the endoscope below the abducens nerve, the anteroinferior cerebellar artery can be followed toward the acoustic-facial bundle and, just below it, the lower cranial nerves entering the jugular foramen, which lies about 14 mm lateral to Dorello’s point. The posteroinferior cerebellar artery or its branches can be found anterior to the lower cranial nerves.

FIGURE E44.7 Transclival approach, intradural phase. A and B, Anatomical images. Intradural exploration of the prepontine area. C and D, Anatomical images. Intradural exploration of the craniovertebral junction. Asterisk (∗) indicates Dorello’s point. III, oculomotor nerve; V, trigeminal nerve; VI, abducens nerve; aica, anterior-inferior cerebellar artery; asa, anterior spinal artery; BA, basilar artery; HC, hypoglossal canal; JF, jugular foramen; sca, superior cerebellar artery; VA, vertebral artery.

The inferior corridor unlocks the craniovertebral junction through the following steps:

The vomer is drilled; it should be flush with the hard palate.

C-shaped incision of the mucosa of the rhinopharynx: its blood supply from the ascending and greater palatine and the ascending pharyngeal arteries should be preserved.

Lateralization of longus colli and longus capitis muscles.

Identification of caudal edge of the clivus.

Sharp disinsertion of the atlanto-occipital membrane.

Drilling of the anterior edge of the foramen magnum.

Drilling of the occipital condyles: their medial one third is bounded by the hypoglossal canal. It is about 15 mm from the midline and the change from cancellous to cortical bone should be carefully appreciated during drilling. Furthermore, its opening will cause bleeding from the venous plexus surrounding the nerve.

Dura opening: the tectorial membrane, attached dura, and the longitudinal part of the cruciform ligament are incised en bloc along the midline. The tectorial membrane represents the superior extension of the posterior longitudinal ligament. The basilar venous sinus should be managed when opening the dura.

After tumor removal, the vertebral artery and the origin of the posteroinferior cerebellar artery are identified. The rootlets of the hypoglossal nerve converge toward the hypoglossal canal and can be stretched by the vertebral and posteroinferior cerebellar arteries. By extending bone removal below the hypoglossal canal and angling the endoscope laterally, the intradural segment of the vertebral artery can be followed toward its dural entry.

There are three lateral boundaries of the surgical corridor:

Medial pterygoid plate and eustachian tube

Paraclival segment of the ICA and foramen lacerum

Hypoglossal canal

The foramen lacerum is formed by the body of the sphenoid bone anteromedially, by the petrous apex posteriorly, and the border of the great sphenoidal wing laterally. Medially it is plugged by fibrous tissue and cartilage, being a point of least resistance, and care should be taken especially when exposure of the petrous apex is required. Below the foramen lacerum the surgical window enlarges, being bordered by the eustachian tube, which guards the parapharyngeal carotid artery: the eustachian tube lies parallel and lateral to the petrous ICA and their relationship is critical when unlocking the infratemporal fossa and the jugular foramen.

Transodontoid Approach

The endoscopic endonasal exposure of the craniovertebral junction and cervical spine is limited inferiorly by the projection of the nasopalatine line; it is a sagittal line tangential to the inferior edge of the nasal bones and the posterior edge of the hard palate.^{25,26,79,88–91} Lateral exposure is limited by the eustachian tube, which guards the parapharyngeal ICA, and by the vertebral artery (VA) and hypoglossal nerve.²⁸

Extradural craniocervical decompression, mainly related to rheumatoid arthritis, has been performed with interesting results.^28,92–95 Inherent instability of the craniovertebral junction may require posterior fusion to the occiput. Intradural extension of chordomas as well as meningiomas and even aneurysms can be treated.^96,97

The main phases of the extradural exposure are similar to those used in the exposure of the inferior clivus, adding the following steps:

Identification of the anterior arch of C1

Removal of C1 with drill and Kerrison rongeur

Drilling of the dens after image guidance confirmation

Cut of the apical and alar ligaments

Removal of the dens

In patients with extradural cervicomedullary compression, an ultrasonic aspirator can be used to partially resect the pannus until pulsation transmitted by the brainstem is visible; the defect is covered with fibrin glue.

Intradural exposure of craniocervical meningiomas and chordomas requires packing of the circular sinus before midline dura opening; tumor debulking should be carried out, avoiding compression of the brainstem and strain of the hypoglossal rootlets and the perforators; the suction technique can be applied to chordomas and soft meningiomas, while fibrous meningiomas may require a low-profile Cavitron ultrasonic aspirator. Dissection of the lateral poles proceeds in a lateral to medial direction while the endoscopic closeup view highlights the intradural segment of the vertebral artery, the posteroinferior cerebellar artery, the lower cranial nerves, the ventral and dorsal rootlets of C1 and C2, and the dentate ligament between them.

Transmaxillary Transpterygoid Approach

This approach provides direct access to lesions located or extending in the lateral compartment of the cavernous sinus, lateral recess of the sphenoid sinus, anteromedial region of Meckel’s cave, or pterygopalatine and infratemporal fossae.^{27,39,41,42,98–101} These compartments can represent the surgical target as well as part of the operative corridor (Fig. E44.8).

FIGURE E44.8 Schematic drawings showing the anatomical landmarks that guide the transethmoidal transmaxillary approaches. A, Transcranial lateral view; B and C, endonasal view. V, trigeminal nerve; EAM, external acoustic meatus; ET, eustachian tube; FO, foramen ovale; GWE, great wing of the sphenoid; ICA, internal carotid artery; IJV, internal jugular vein; IMA, internal maxillary artery; ION, infraorbital nerve; M, mandible; MMA, middle meningeal artery; PP, pterygoid process; Ppf, pterygopalatine fossa; TM, temporal muscle; vn, vidian nerve.

Surgical Procedure

The procedure starts with the aforesaid basic module because a wide endonasal corridor is needed to work properly and the wide anterior sphenoidotomy highlights reference landmarks, which make lateral exposure easier, such as the sphenoid floor and the paraclival segment of the ICA. The width of the nasomaxillary window and the drilling of the pterygoid process are tailored to the target area.⁹⁸

The lateral recess of the sphenoid sinus is exposed through the following steps:^98,102

Retrograde uncinectomy: the free edge of the uncinate process is identified and the back-biting forceps points downward, away from the medial wall of the orbit.

Wide maxillary antrostomy: the nasolacrimal duct, posteriorly, and the descending palatine artery, anteriorly, must be preserved. The nasolacrimal duct is surrounded by denser bone of the lacrimal, maxillary, and inferior turbinate bones; thence if one feels increased resistance with the back-biting forceps, the bite should not be completed.

Opening of the sphenopalatine foramen.

Isolation and bipolar coagulation of the sphenopalatine and posterior nasal arteries: the descending palatine artery can be dissected from its bony canal and preserved to nourish a palatal flap.¹⁰³

Identification of the ventral exit of the pterygoid canal: it is located just posterior to the sphenopalatine foramen, at the cross point between the sphenoid floor and the medial pterygoid process, which constitutes the lateral wall of the choana.⁸⁴ This canal lies about 7 mm lateral to the vomerosphenoid junction, just posterior to the sphenopalatine foramen, at the junction between the sphenoid floor and the medial plate of the pterygoid process, which is the lateral wall of the choana.

Drilling of the sphenoid floor.

Removal of the posterior wall of the maxillary sinus up to the infraorbital nerve.

Identification and bipolar coagulation of the terminal vessels of the maxillary artery.

Identification of the base of the pterygoid process (Fig. E44.9).

Identification of the foramen rotundum.

Drilling of the medial pterygoid plate: it should be performed in a caudal-to-rostral direction and perpendicular to the clivus,^39,41 up to the vidian canal, which is drilled in its inferomedial aspect while the vidian artery is highlighted and can be followed toward its origin from the anterior genu of the ICA.^27,84

FIGURE E44.9 Identification of the base of the pterygoid process. A, Anatomical image. B, Surgical image. After middle antrostomy and coagulation of the terminal branches of the sphenopalatine artery (asterisk), the base of the pterygoid process is drilled. PP, base of the pterygoid process.

Meningoencephaloceles and CSF leaks can be reconstructed through this wide exposure.^98,101 Soft tissue lesions extended in the lateral compartment of the cavernous sinus, such as adenomas and chordomas, can be removed using atraumatic angled suction cannulas and curets through a quadrangular area bounded superiorly by V2, inferiorly by the vidian nerve, and posteriorly by the parasellar and paraclival carotid artery^{13,36,40,99,104,105} (Fig. E44.10). Maintaining the dura opening and dissection below the level of V2 avoids damaging the abducens nerve running toward the superior orbital fissure.¹⁰⁶ Furthermore, the inferolateral trunk and its branches to the cranial nerves lie in a deeper plane, which is preserved through the endonasal route.¹⁰⁷ Meckel’s cave can be reached through this route, thus providing another strategy to deal with schwannomas extending in this area^27,44,106 or to follow perineural tumor spreading when dealing with the adenoidocystic carcinoma or other sinonasal malignant tumors.

FIGURE E44.10 Exposure of the lateral compartment of the cavernous sinus. A, Anatomical image. The boundaries of the quadrangular safety area are underlined (dotted line). B, Surgical image. An invasive pituitary adenoma extending to the left cavernous sinus is removed through the quadrangular area to decompress the homolateral abducent and maxillary nerves. The dotted line borders the area of drilling of the pterygoid base. V2, maxillary nerve; VI, abducens nerve; C, clivus; Co, choana; FR, foramen rotundum; ICA, paraclival segment of the internal carotid artery; IMA, internal maxillary artery; ion, infraorbital nerve; ITF, level of the infratemporal fossa behind the posterior wall of the maxillary sinus; lwSS, lateral wall of the sphenoid sinus after bone removal; ON, optic nerve; PP, pterygoid process; Ppf, pterygopalatine fossa; S, sella; SphA, sphenopalatine artery; vn, vidian nerve.

The petrous apex is limited by the internal carotid artery and the bony labyrinth anteriorly, Dorello’s point (dural porus of the abducent nerve) and the posterior cranial fossa posteriorly, Meckel’s cave and the middle cranial fossa superiorly, and the jugular bulb and inferior petrosal sinus inferiorly. It is situated deep to the vertical and horizontal internal carotid artery and can be approached at the level of the clival recess between the midclival bone and the paraclival segment of the ICA.

Extradural lesions of the medial petrous apex can expand into the sphenoid sinus, thus creating by themselves a surgical window between the dura and the carotid artery at the level of the clival recess; in such cases, after evacuation, the tumor bed can be marsupialized in the sphenoid sinus.¹⁰⁸ If the lesion does not remodel the petrous apex, the endonasal exposure requires isolation and lateralization of the internal carotid artery. Carotid artery manipulation may be associated with postoperative unilateral rhinorrhea due to dissection of the sympathetic supply to the nose, which may be misinterpreted as a CSF leak.

The pterygopalatine fossa, lateralized to expose the lateral recess of the sphenoid sinus, can be unlocked through the same route. It is unlocked through the following steps:

Opening of the periosteum.

Fat dissection must proceed on a superficial plane in a lateral to medial direction, thus highlighting the blood vessels, which lie ventral to the nervous structures. Distal branches of the internal maxillary artery are often encountered before the main artery is identified and coagulated, namely, the descending palatine artery. Usually the posterior superior alveolar artery and infraorbital artery run proximally to the descending palatine, pharyngeal, vidian, and sphenopalatine arteries.

The neural structures lie deep to the vascular structures. The maxillary nerve can be easily identified at the infraorbital incisure, which borders the limit between the pterygopalatine and infratemporal fossa, pointing toward the foramen rotundum.

Access to the infratemporal fossa is gained by enlarging the nasomaxillary window: the medial and posterolateral walls of the antrum are removed;^27,44,45,109 lateral exposure may be further increased by removing the piriform aperture, and inferior extension can be obtained by removing the posterior one half of the inferior turbinate; depending on tumor extension, the pterygoid plates, the torus tubarius, and the middle one third of the eustachian tube can be removed. When dealing with sinonasal malignancies the maxillary, mandibular, and vidian nerves must be inspected for perineural tumor spreading and sacrified when necessary.

The eustachian tube is the key landmark to follow lesions extending toward the jugular foramen; the clinical application of this module must be reserved for highly experienced skull base surgeons.¹¹⁰ Proficiency with the three-dimensional relationship between the eustachian tube and the internal carotid is mandatory for control and injury avoidance. The eustachian tube is about 3 to 4 cm long (adult) and has an S-shaped configuration; its bony portion, which makes up about one third of the tube, is lateral to the carotid canal and superior to the jugular fossa. Its cartilaginous part is attached to the skull base at the level of the sulcus tubae, posteromedial to the foramen ovale and spinosum, and anterolateral to the foramen lacerum and the carotid canal.

Endoscopic Supraorbital Approach

The microsurgical and the endoscope-assisted supraorbital approaches can be used for lesions in the anterior fossa, the sellar region, and the anterior circle of Willis. They seem advantageous in the following cases:

Surgical Technique

The Mayfield headholder should be positioned away from the operative field, with one pin at the level of the contralateral frontal area, behind the hairline. The patient is positioned supine and the head should be positioned as follows:

Elevated about 15 degrees above the thorax, to improve venous drainage

Hyperextended about 15 degrees to improve gravity-related slumping of the frontal lobe

Rotated contralateral to the lesion as follows:

q About 50 degrees to expose the olfactory groove

q About 30 degrees for the midline regions (suprasellar and interpeduncular)

q About 10 degrees for the sylvian fissure

The technique then proceeds along the following steps:

The eyebrow is not shaved but is carefully disinfected.

A small cross-hatch is made across the skin incision for realignment of the edges at the time of closure.

The scalpel runs parallel to the hair follicles, which are directed upward obliquely, to prevent their damage. Skin incision is made between the supraorbital incisure and the lateral end of the eyebrow.

Fibers of the frontalis and the orbicularis oculi muscles are longitudinally divided just above the orbital rim.

Subperiosteal dissection of the frontal muscle is performed upward.

Subperiosteal disinsertion of the temporal muscle.

Identification of the keyhole, the hole made posterior to the lateral rim of the orbit.

Lateral subperiosteal dissection of the temporal muscle should be limited to about 1.5 to 3 cm behind the keyhole, and the galeal-periosteal layer can be cut to increase bone exposure.

Positioning of the the frontobasal bur hole.

Craniotomy (about 1.5 × 3 cm) is performed.

The inner bone edge above the orbital rim is drilled while the frontopolar dura is protected, not retracted, with a spatula.

The dura is opened in a C-shaped fashion, reflected over the orbital rim, and covered with a wet cotton pledget.

Opening of the chiasmatic, carotid, and depending on the lesion extension, sylvian fissure to release CSF.

At the level of the anterior skull base, olfactory groove meningiomas with lateral extension can be debulked after bipolar coagulation of their basal attachment; the tumor can be followed toward the midline. During development of the surgical corridor and extracapsular tumor removal, meticulous arachnoid dissection with diamond microscalpel or tube-shaft scissors avoids traction of the optic nerve, the ophthalmic artery, the superior hypophyseal artery, and the perforators from the anterior communicating complex.

In patients with tuberculum sellae and diaphragma sellae meningiomas (Fig. E44.11) close inspection of the optic nerve when entering the canal is mandatory; optic canal decompression or intradural drilling of the anterior clinoid process can be performed with an ultrasonic bone curet.¹³⁶

FIGURE E44.11 A 51-year-old man with a recurrent craniopharyngioma presented with progressive visual deterioration associated with an increase in tumor cyst size. Previously, he had undergone multiple extended endonasal approaches for craniopharyngioma recurrences as well as intracavitary radiotherapy. A right supraorbital approach was performed to drain the suprachiasmatic cyst and decompress the optic apparatus (A). Intraoperative view obtained after translamina terminalis cyst decompression using the microscope (B) and the endoscope (C and D). Postoperative magnetic resonance imaging (MRI) confirmed satisfactory optic apparatus decompression (E) and improved visual status.

(Courtesy of Drs. Daniel Kelly, Amin Kassam, and Nancy McLaughlin.)

When dealing with craniopharyngiomas extending beyond the supraclinoidal ICA, the well-known multiple corridors can be used to approach the lesion¹³⁷ (Fig. E44.12).

FIGURE E44.12 A 57-year-old woman with a tuberculum sellae meningioma that had grown, causing chiasmal compression (A to C). A left supraorbital approach and gross total tumor resection was performed. Intraoperative view obtained using the microscope (D) and the endoscope (E and F) before and after tumor removal. Three-month postoperative magnetic resonance imaging (MRI) (G to I).

(Courtesy of Drs. Daniel Kelly, Amin Kassam, and Nancy McLaughlin.)

After tumor removal, the dura is closed in watertight fashion and the frontal sinus is properly managed, if opened; the bone flap should be carefully approximated medially while the bur hole can be covered with the titanium plates. Closure is accomplished in three layers: periosteal-galeal layer, subcutaneous tissue, and skin.

Endoscopic Retrosigmoid Approach

The microsurgical retrosigmoid approach is a well-established and highly effective technique for both lesion removal and microvascular decompression. During surgery for vestibular schwannomas, the need to improve exposure of the lateral internal acoustic canal has driven the introduction of the endoscope-assisted technique;^138,139 the endoscopic visualization has been advantageous in the following cases:

Recently, the pure endoscopic technique has been applied to well-selected avascular lesions such as epidermoids¹⁴² and arachnoid cysts and during microvascular decompression procedures.

Surgical Technique

Depending on the surgeon’s preference the patient can be positioned in supine, park-bench, or prone position, the latter being reserved for patients with very short thin neck or prominent shoulders. The optimal position should allow gravity-related self-retraction and comfort for the patient, for the surgeon, and the operative staff; it should also avoid venous and CSF congestion. In our opinion the supine position fulfills such requirements granted that the following is accomplished:

The ipsilateral shoulder of the patient is elevated with a pillow and should not be in the way of the surgeon’s dominant hand.

The patient is secured to the table with adhesive tape, in order to further rotate the operative field away from the surgeon when dealing with medial lesion extension.

Bone exposure is the most critical point in obtaining adequate exposure, and image guidance can aid even the experienced surgeon to pinpoint the skin incision and the first bur hole. However, precise orientation of the bony surface landmarks is mandatory once the desired trajectory has been confirmed by image guidance.

We use a 10-cm, 0-degree endoscope held by the co-surgeon free hand during the whole procedure in a dynamic fashion, which means following in and out movements of the instruments and updating the view (panoramic or closeup) depending on the need of the co-surgeon.

The main steps of the procedure are as follows:

Lateral suboccipital craniotomy and exposure of the borders of the transverse and sigmoid sinus

Dura opening in a “K” fashion

Introduction of the endoscope between the lateral surface of the cerebellar hemisphere and the petrous bone

After arachnoid dissection and CSF drainage, the endoscope highlights the acoustic facial bundle as the first surgical landmark.¹⁴³ The whole cerebellopontine angle can be unlocked through the following specific working corridors, above and below the acoustic-facial nerve bundle:

The superior corridor is bordered by the tentorium and the trochlear nerve superiorly and by the acoustic-facial bundle, inferiorly. The superior petrosal vein (Dandy’s vein) comes into view and can usually be preserved, aside from its configuration. Running the endoscope along this corridor, the trigeminal nerve is highlighted from the entry point to its dural porus. Running the endoscope between the tentorium and the trigeminal nerve allows a closeup view of the interpeduncular cistern, trochlear nerve, and superior cerebellar artery. Running the endoscope below the trigeminal nerve, the anteroinferior cerebellar artery, the sixth cranial nerve, and its dural porus (Dorello’s point) are visualized.

The inferior corridor is bordered by the acoustic-facial bundle superiorly and by the hypoglossal nerve inferiorly. The lower cranial nerves entering the jugular foramen and the posteroinferior cerebellar artery are highlighted. The latter can be followed toward its origin from the vertebral artery.

The endoscopic technique allows optimal removal of avascular lesions extending along several corridors such as epidermoids¹⁴² (Fig. E44.13) and several reports concerning endoscope-assisted microvascular decompression underline the ability of the endoscope to highlight nerve conflicts hidden with the microsurgical view. Furthermore, the endoscope may ease the treatment of complex neurovascular conflicts when transposition of the vessel is required.

FIGURE E44.13 Intraoperative pictures during the removal of an epidermoid in the left cerebellopontine angle. IV, trochlear nerve; V, trigeminal nerve; VI, abducent nerve entering its dural porus; VII/VIII, acoustic-facial nerve bundle; IX-X-XI, lower cranial nerves; DV, Dandy’s vein.

Current Status of the Endoscopic Technique

The endoscopic technique offers an optimal and minimal route to the whole skull base along natural surgical corridors, thus reducing manipulation of cranial nerves and main vessels. This technique also affords quicker postoperative recovery and better patient compliance. However, further assessment and refinement are required to determine its actual benefits.

Future Developments

Research in endoscopic surgery is expanding in three main directions:

The evolution of the optical devices has helped to overcome the limits of microsurgical visualization and has disclosed new horizons. The microscope is and will continue to be essential for many neurosurgical procedures. However, the endoscope is an integral component of the surgical armamentarium contributing to modern diagnostic and operative management of the patients which cannot be absent in any operating suite.

Further development of the endoscopic techniques will be driven by several technological advances such as high-definition intraoperative digital imaging, three-dimensional endoscopy, chip-stick technology, integrated operating room, dedicated instruments, virtual reality systems,¹⁴⁶ and nanotechnology.^147,148 Modern integrated operating rooms provide advanced capabilities in telemedicine and live telesurgery, which will further speed the use of the endoscopic techniques. Three-dimensional endoscopy^149,150 provides a sense of depth similar to that of the microscope that most neurosurgeons are familiar with. Virtual imaging will flank cadaver dissection in training laboratories and one day might guide a robot-like system in an up-to-date virtual space.^16,47 Furthermore, new instruments may use nanosensors for in vivo real-time histopathological examination of lesions and may permit implantation of antineoplastic agents tailored to the patient’s tumor. International congresses on endoscopy feature a chance for interactive dialogue among surgeons, researchers, and health care leaders, providing a base for active cooperation and further developments.¹⁴⁷

Training

Future skull base surgeons should be trained in both open and endoscopic techniques to offer the best surgical option on a case-specific basis. Proper training in endonasal surgical techniques is essential to improve patient care and long-term outcomes. An adequate training should cover several areas:

Concerning surgical experience, a gradual learning experience is mandatory to minimize complications. Kassam’s group developed a five-level training program¹⁵¹ to underline that surgeons should be adequately trained in basic procedures such as standard endoscopic approaches to pituitary adenomas and CSF leak reconstruction; furthermore, they should have adequate surgical volume and resources before undertaking extended approaches to the midline. In the same way, pure endoscopic transcranial approaches require specific training and careful patient selection.