Minimal Access Skull Base Approaches

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Chapter E44 Minimal Access Skull Base Approaches

The skull base can be explored through two opposing approaches, which represent the two sides of the same coin: the intracranial approach, more familiar to neurosurgeons, and the extracranial approach, more familiar to ear, nose, and throat (ENT) surgeons and head and neck surgeons.

This chapter aims to provide a comprehensive discussion of endoscopic approaches to skull base lesions and the basis for an integrated understanding. The text includes detailed discussion about operative approaches and complication avoidance. Recently, early outcome data concerning the management of meningiomas, craniopharyngiomas, and chordomas through the endonasal route became available to guide decision making, although a consensus is far from being achieved. However, the continuous structured evolution of these techniques anticipates wider surgical series and longer follow-up times. The cross-pollination between subspecialties encourages the use of the endoscope during transcranial approaches, and this chapter will also summarize the current status of “pure” endoscopic supraorbital and retrosigmoid approaches.

Endoscopic Endonasal Approaches

Historical Background

Since the 1980s, several ENT surgeons pioneered the functional endoscopic sinus surgery (FESS),15 thus encouraging the “pure” endoscopic endonasal approach to the sella,6 performed first by Jankowski;7 then Jho and Carrau extended this technique,810 followed by other teams.1114 These experiences supported a continued structured evolution of the endoscopic technique: new skills have been transferred across subspecialties, new instruments have been designed,1518 new image-guidance systems have been fashioned,1924 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.14,2528 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.14,2527,3336 The midline ventral skull base can be exposed from the frontal sinus to the body of C2 through specific modules: transcribriform, transtuberculum, transclival, and transodontoid25,37,38 (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,3942 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.27,30,4345

TABLE E44.1 Pure Endoscopic Approaches to the Skull Base

Endoscopic Approach Surgical Target
Endoscopic Endonasal Approaches
Midline Approaches
Transcribriform Cribriform plate
Transtuberculum transplanum Suprasellar area
Third ventricle
Standard transsphenoidal Sellar, suprasellar, and parasellar areas
Transclival Clivus
Interpeduncular and prepontine areas
Prepontine area
Transodontoid Craniovertebral junction
Odontoid
Superior cervical spine (C2 body)
Paramedian Approaches
Supraorbital and transorbital Orbital roof
Medial intraconal space
Transmaxillary transpterygoid Pterygopalatine fossa
Lateral recess of the sphenoid sinus
Lateral compartment of the cavernous sinus
Meckel’s cave
Infratemporal fossa
Medial aspect of the petrous apex
Jugular foramen
Endoscopic Transcranial Approaches
Supraorbital Anterior skull base
Middle skull base
Retrosigmoid Cerebellopontine angle

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 technique14,26,27,52,53 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.

Standard Approach

The surgical corridor between the middle turbinate and the nasal septum is enlarged by pushing the middle turbinate laterally, after topical decongestion with cotton pledgets soaked with proper anesthetics. The endoscope is moved, parallel to the nasal floor, toward the choana and angled up along the sphenoethmoid recess. After anterior sphenoidotomy, the identification of the anatomical landmarks54 over the posterior wall of the sphenoid guide the opening of the sellar floor. Dura opening is tailored to the lesion extension.

Microadenomas can be removed en bloc or piecemeal, depending on their consistency and the presence of a pseudocapsule.55 Intra- and suprasellar macroadenomas are removed systematically, starting from the bottom of the sella, because early descent of the stretched suprasellar cistern may hide tumor remnants.52 Tumor extension in the medial compartment of the cavernous sinus can be removed with atraumatic curets and suction cannulas. Cystic lesions as well as pure intrasellar craniopharyngiomas are well established indications for this technique.56

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:

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.6164 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).

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

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).

The main aspects of this module are as follows:

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

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.

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.7174

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.7476 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.

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:

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.26,27,7985 The superior corridor unlocks the retrosellar area and has been described in the previous module. The middle corridor is developed through the following steps:

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.

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

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:

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.