CHAPTER 116 Basic Principles of Skull Base Surgery

Madjid Samii, Marcos Tatagiba, Venelin Gerganov

Historical Landmarks

Until the beginning of the 20th century, lesions located at the base of the skull were largely inoperable. Pioneers of different surgical areas started to envision and perform many of the significant approaches to the skull at the end of 19th century, and success was achieved in single cases. Classic examples are the suboccipital approach by Krause,¹ transsphenoidal approaches to pituitary tumors introduced by Halstead,² and the translabyrinthine route described by Panse.³ These techniques were then improved and became part of the routine armamentarium of the next generation of surgeons, including Cushing, Dandy, Guiot, Dott, Wüllstein, and Conley, among others.⁴ However, it was not until the mid-1960s that efforts to overcome interdisciplinary barriers led to further breakthroughs in skull base surgery and close cooperation among neurosurgeons, otorhinolaryngologists, and maxillofacial surgeons. The introduction of microsurgical techniques, advances in neuroanesthesiology, and new diagnostic tools such as high-resolution computed tomography (CT), magnetic resonance imaging (MRI), and superselective angiography were also essential.

The first cornerstone in the establishment of modern skull base surgery was set in 1980 with founding of the International Skull Base Study Group; its first president was Schürmann. In 1988, the International Skull Base Society was founded under the presidency of Fisch, and in 1992, the First International Skull Base Congress was held in Hannover under the presidency of Samii, with more than 1000 participants from 55 different countries. Today, in countries all over the world, skull base surgery has been established as an essential addition to neurosurgery and other surgical fields involved in the treatment of cranial base lesions.

Overview of Skull Base Surgery

The development of a field to deal with problems of the cranial base had two major goals. Lesions of the cranial base pose particular problems with regard to surgical treatment. The base of the skull is a complex area that houses the main neural and vascular structures. Skull base lesions usually affect adjacent areas, both intracranial structures and extracranial spaces, thus making an interdisciplinary approach for surgical treatment mandatory. With destruction of the bony barrier of the skull base, the pathway is open for intracranial or extracranial extension of the disease. Lesions of the anterior skull base are frequently related to the paranasal sinuses, and lesions of the middle and posterior skull base are frequently related to the clivus and the petrous bone. Efforts to overcome these barriers resulted in the development of a systematic interdisciplinary strategy to treat these disorders. Once the principles of cooperation were established, it became possible to go beyond simple eradication of disease and strive to improve results by preserving function and reconstructing lost function.

The second goal of skull base surgery involves the principle of drilling the skull base while avoiding major trauma to the brain. Experience with surgery on the skull base has shown the benefits of bone resection in reducing the need for brain retraction. Its indications have been expanded over the years to treat not only skull base–destroying lesions but also all intracranial lesions that can best be reached through the skull base. For example, some aneurysms of the basilar artery, although not true skull base lesions, are better attacked through transzygomatic or transpetrosal approaches, which involve minimal brain retraction and afford an enhanced view. High-speed drill techniques developed rapidly, and the drill has become a precise microsurgical instrument. Based on these principles, several approaches through the skull base were established, such as transfacial approaches, transpetrosal approaches, transcondylar approaches, and many others. The main goal of these techniques is to reduce the amount of brain retraction by means of bone resection, thus avoiding problems related to postoperative brain contusion and edema. Furthermore, the approach in itself should not be associated with significant procedure-related morbidity. Developments in computer technology and navigation devices have allowed online control of bony structures during the drilling procedure and tumor resection.⁵^–⁹ Moreover, navigation may be used for localizing displaced or encased vessels, as well as for assessing tumor extension and its relationship to main landmarks (Fig. 116-1).

FIGURE 116-1 A, Trigeminal schwannoma located predominantly in Meckel’s cave with extension along the cavernous portion of the nerve. B, Screenshot from the neuronavigation monitor presenting the tumor in three planes and its three-dimensional reconstruction. C and D, Postoperative T1-weighted magnetic resonance images after the application of contrast material confirm complete tumor removal.

Over the past decade the endoscope has become a widespread supplement to traditional skull base techniques, whether used in addition to the microscope or as the only visualizing tool.¹⁰^–¹² It provides a panoramic multi-angled view of the entire operative field. Freehand use of the endoscope allows a close-up view of the target area, and angled endoscopes enable one to see “around the corner“ (Video 116-1).

Video 116-1

Trigeminal schwannoma—endoscopically assisted retrosigmoid suprameatal approach.

A 29-year-old man was evaluated for partial trigeminal sensory loss. Magnetic resonance imaging (MRI) revealed a right-sided trigeminal schwannoma located in Meckel’s cave and the posterior cranial fossa along with compression of the brainstem.

The retrosigmoid suprameatal approach was selected.

A retrosigmoid craniotomy was performed; the dura had been incised along the transverse and sigmoid sinuses and fixed with several sutures.

The suprameatal tubercle obstructed the approach to the tumor and is drilled off to obtain adequate exposure of the lesion.

The schwannoma is initially debulked and dissected from the neural branches.

Complete tumor removal is accomplished. The uninvolved trigeminal branches are preserved.

In an endoscopic view with a 0-degree optic, the entire Meckel cave is visualized. No tumor remnants are seen.

Postoperative MRI confirms complete tumor removal. The patient did not have any neurological deficits besides the preoperative partial trigeminal sensory loss.

Consequently, surgeons may navigate around the lesion and precisely distinguish its borders and relationships with surrounding structures.

It would be an error, however, to try to separate the essence of skull base surgery from the microsurgical concepts of modern neurosurgery. Arachnoid dissection and navigation through the cisternal and sulcal anatomy are important facets of this concept. It makes little sense to drill the skull base extensively to minimize brain trauma if the arachnoid planes are not respected and if gentle and patient microsurgical dissection of vessels and nerves is ignored. The main goal of skull base surgery remains to preserve function and quality of life while treating the pathology. The basic principles of skull base surgery are the following:

• Interdisciplinary cooperation among different surgical and nonsurgical fields

• Resection of appropriate amounts of bone in the cranial base to reduce brain trauma

• Avoidance of significant approach-related complications

• Preservation of function and reconstruction of lost function

Our years of experience in treating skull base lesions have allowed us to recognize a number of cases in which the use of extensive skull base procedures does not improve the surgical result and may in fact endanger it. In particular cases, extensive skull base approaches may significantly increase the risk for postoperative deficits. We are now past the era of enthusiastic resection of skull base lesions, and simple cranial approaches are again gaining popularity. Thus, some simple approaches, such as the retrosigmoid approach to the cerebellopontine angle, have proved to be most favorable for tumors in that location. In other cases, however, the approach has to be selected individually and always tailored to the characteristics of the particular tumor, its location, and the patient’s expectations.¹³

In this chapter the philosophy of the senior author is described, which evolved since 1968 from experience in treating more than 5000 skull base lesions. It is beyond the scope of this chapter to depict in detail single pathologies and their clinical aspects and treatment. These are covered elsewhere in this book. In this chapter, surgery on the skull base is divided into two main topics: surgery on the anterior skull base in relation to the paranasal sinuses and surgery on the middle and posterior skull base in relation to the petrous bone.

Surgery on the Anterior Skull Base

Different lesions may involve the anterior skull base, such as benign or malignant tumors, vascular lesions, maldevelopmental diseases, and trauma. Meningiomas of the olfactory groove and planum sphenoidale are the most frequent benign tumors encountered at the anterior skull base (Fig. 116-2).^4,¹⁴ Adenocarcinomas and esthesioneuroblastomas are typical examples of malignant tumors that arise from the paranasal sinuses and secondarily involve the anterior skull base (Fig. 116-3). Fibrous dysplasia develops very slowly but may achieve a large size before it becomes symptomatic (Fig. 116-4). Other non-neoplastic lesions of the anterior cranial base include frontal encephaloceles and skull base trauma. Each of these lesions needs a particular treatment strategy. The surgeon must be familiar with the normal surgical anatomy of the skull base to understand the changes caused by these lesions and to manage them properly.

FIGURE 116-2 Top, A sagittal T1-weighted magnetic resonance image after the intravenous injection of contrast material shows a large meningioma of the olfactory groove and planum sphenoidale. Bottom, Hyperostosis in the area of the cribriform plate (arrows) on a coronal bone window computed tomographic scan.

FIGURE 116-3 Large adenocarcinoma of the paranasal sinuses with intracranial invasion (top) and destruction of the medial orbital wall (bottom).

FIGURE 116-4 Fibrous dysplasia with extensive involvement of the anterior skull base on the left side.

Operative Anatomy of the Anterior Skull Base

From the endocranial view, the anterior cranial base has a flat surface that comprises the anterior border of the sphenoid wings and the roof of the orbita laterally and the planum sphenoidale medially (Fig. 116-5). In the middle, in varying prominence and height are the crista galli and the ethmoid plate. The dura in the medial portion at the area of the cribriform plate is more closely adherent to the skull base than in the lateral position. Depending on the degree of pneumatization of the paranasal sinuses, the size of the contact area between the paranasal sinuses and the anterior skull base may vary.^15,¹⁶

FIGURE 116-5 Anatomy of the anterior fossa of the skull.

The ethmoid cells form the lateral boundary of the contents of the orbita at the level of the skull base (Fig. 116-6). Medially, the lamina cribrosa constitutes the upper limit of the paranasal sinuses, and it is divided by the nasal septum. Behind are the two portions of the sphenoidal sinus. These portions vary in size, as do the other paranasal sinuses. Figure 116-7 shows the relationship of the sphenoidal sinus to the sella and its contents, along with the surrounding structures.

FIGURE 116-6 Axial section through the anterior skull base at the level of the orbits and ethmoidal sinus.

FIGURE 116-7 Sagittal section at the level of the pituitary gland (p.g.). Note the close relationship of the sphenoidal sinus (SS) to the sella. b.a., basilar artery; cl., clivus; F.L., frontal lobe; op.ch., optic chiasm; op.n., optic nerve; p.s., pituitary stalk; 3v, third ventricle.

A physiologic bacterial flora always populates the paranasal sinuses. Consequently, there is danger of an ascendant infection through the direct connection between the paranasal sinuses and the endocranium. The close relationship between the paranasal sinuses and the draining lacrimal ducts has to be considered.

Clinical Symptoms and Diagnostic Management

Tumors of the anterior skull base can originate from the paranasal sinuses (i.e., the frontal, ethmoidal, sphenoidal, and maxillary sinuses) and advance toward the endocranium.

Intracranial tumors may have their origin in the posterior frontal sinus wall, frontal sinus floor, ethmoidal roof, cribriform plate, planum sphenoidale, or tuberculum sellae. Tumors that originate in the paranasal sinus system can expand considerably before producing clinical signs. If they erode through the anterior skull cavity, they soon become symptomatic. Decreased olfaction, obstruction of nasal respiration, headaches that are dull in nature and deeply situated in the facial area, nasal hemorrhage, irritation of the trigeminal nerve, and orbital symptoms should evoke suspicion of a tumor in the anterior skull base area and paranasal sinuses.

Significant progress has been made in treating these lesions as a result of earlier diagnosis and the ability to determine their extent and configuration by endoscopy of the upper respiratory tract with modern optical instruments. For instance, a tumor in the area of the medial nasal duct, even in an early stage, may cause blockage of paranasal sinus drainage with consequent swelling of the mucosa. With serial x-ray examinations and CT, it is not always possible to distinguish between polyp-like mucosal swelling and actual tumor growth. MRI may be helpful in distinguishing between these processes, but endoscopic biopsies of the nose, paranasal sinuses, and nasopharyngeal roof are used to establish the diagnosis and determine the operative strategy. Both CT and MRI are indispensable for determining the extent of intracranial expansion and bone destruction at the skull base.

General Operative Principles

There is a basic difference in the treatment of benign versus malignant tumors of the skull base. In the case of benign tumors, the aim is complete resection with maximal preservation of function; the resection is frequently done piecemeal. In contrast, malignant growths have to be removed en bloc, along with resection of healthy tissue surrounding the tumor.

Operative treatment of a malignant process is indicated only if radical extirpation is possible. Undesirable results such as unilateral sacrifice of vision or sacrifice of the ligature of the internal carotid artery are only rarely necessary. The latter is done only after verification of sufficient collateral circulation and adequate cerebral blood flow by cerebral angiography with balloon occlusion and single-photon emission CT investigations. If a malignant tumor advances into the area of the cavernous sinus and infiltrates the sphenoid wing, an attempt at radical removal is no longer justified. The same is true when it is necessary to sacrifice vision bilaterally. For slowly growing tumors or extremely extended infiltrative processes, a palliative intervention with partial removal may be recommended to prolong life or at least temporarily maintain its quality.

Extracranial Approach to the Anterior Skull Base

Malignant tumors of the paranasal sinuses that do not cause any bone destruction or cause only limited osseous destruction are exposed by an extracranial, fronto-orbital approach with resection of the osseous skull base. If necessary, the underlying dura is resected with appropriate margins, and a dural graft is fashioned. Benign processes may be approached in the same way if it is probable that they have not penetrated the dural barrier. If they have invaded the intracranial spaces, endocranial exposure is necessary.

Operative Technique

The extracranial approach may be unilateral or bilateral, through an incision below the eyebrow caudally. Depending on extension of the tumor, the incision is continued paranasally to reach the upper lip. If maxillectomy is required, the exposure may be extended with a subciliary incision. The soft tissues and periosteum, including the periorbita, are stripped from the bone, and the lacrimal sac is freed. After removal of the lateral osseous nasal margin, the anterior ethmoid margin, and the anteroinferior wall and floor of the frontal sinus, the tumor is exposed step by step. The ethmoidal septa are removed, and the sphenoidal sinus is opened if required. If the tumor is benign, an attempt should be made to preserve as much mucosa as possible. In malignant cases, tumor removal must include sufficient mucosal resection in all directions. Usually, it is necessary to begin by removing part of the tumor to obtain sufficient visualization for further debulking of tumor adherent to the skull base. The osseous skull base is incised with a bur, with use of the microscope as necessary. The tumor is encircled and left free from the dura. If a malignant tumor has penetrated the bony skull base, the dura should be resected in the appropriate way. Any dural defect is closed with a dural transplant. Mucosal epithelialization of the wound cavity is assisted by lining it with a large piece of silicone film. Finally, a tamponade (gauze) is placed through the nose for 2 weeks as support for the plastic dura. Care must be taken to introduce Gelfoam between the dural graft and the tamponade; otherwise, the dural graft may be removed along with the tamponade when it is extracted.

If the lamina papyracea has been included in the tumor resection, care must be taken that the tamponade does not exert undue pressure on the eyeball. A modification of the surgical technique allows preservation of the lacrimal ducts; otherwise, patients complain of uncomfortable dacryorrhea postoperatively. In the case of highly placed benign tumors, the lacrimal sac and nasolacrimal duct are easily preserved by proper preparation. If the tumor resection includes a large portion of the nasal mucosa and superior maxilla, drainage of tear fluids may be ensured by the following technique. A funnel-shaped cut is made around the orifice of the nasolacrimal duct in the nasal mucosa so that the duct and lacrimal sac can be held to one side with a holding suture during the remaining part of the operation. At the end, the funnel is opened up and stitched into the soft tissues of the cheek. The functional results are favorable. If the nasal orifice cannot be preserved, the nasolacrimal duct is resected as far as possible to enable the lacrimal sac to drain directly into the operative cavity. It is beneficial to irrigate the ducts for several days postoperatively.

Extracranial Approach with Unilateral Orbital Exenteration

The skin incision is extended, in accordance with extension of the tumor, paranasally toward the upper lip. If necessary, the lip is split (Fig. 116-8). The stepped incision produces a favorable aesthetic result. With ethmoidal tumors, depending on the extent of involvement of the medial edge and orbit, a decision must be made whether the upper and lower lids have to be resected or only the skin near the edge needs to be sacrificed. In the latter case, the upper and lower lids can be used as tissue for epithelialization of the remaining orbita. The extent of freeing the soft cheek tissues is determined by the extension of the tumor.

FIGURE 116-8 A, Skin incision for wide exposure during a transfacial approach for unilateral orbital exenteration. B, Soft tissue has been deflected from the maxillary and frontal bones. The plane of dissection is subperiosteal. A bur is shown penetrating the frontoethmoidal sinus complex.

The tumor is approached from below by removing the anteroinferior wall and floor of the frontal sinus, the lateral wall of the nose, and the required portion of the frontal process of the maxillary bone and the anterior wall of the maxillary sinus. The periosteum is then cut from the supraorbital rim. The optic nerve and the blood vessels entering the apex of the orbital funnel are cut with curved scissors and coagulated. This allows the orbital contents to be included in the surgical specimen.

Next, a partial or complete maxilloethmoidectomy is performed, depending on the circumstances. With maxillary and ethmoidal tumors that have penetrated the orbit, intraoperative histologic examination of the mucosa of the fovea ethmoidalis may be positive for tumor, even if there were no radiographic or clinical evidence of skull base infiltration before surgery. In such cases, it is possible to follow orbital exenteration with resection of the adjacent anterior skull base, possibly including the cribriform plate and contralateral fovea ethmoidalis, from the inferior part. After removal of the frontal sinus floor and lateral osseous nasal wall and excavation of the ethmoid, the anterior skull base can be readily evaluated. The lateral nasal wall is resected medially up to the nasal root. The next step is circumcision of the skull base with a diamond bur as appropriate, depending on extension of the tumor.

The anteroposterior limits of the resection are from the anterior limit of the crista galli up to the middle of the roof of the sphenoidal sinus. The resection is continued laterally to include the orbital roof as required. The circular incision of the anterior skull base is followed by sufficient mobilization of the dura in the area of the bone resection.

To guard against infection, the remainder of the field should be covered with cotton patties before the dura is incised. If the basal craniectomy extends far anteriorly, the superior sagittal sinus is ligated and cut. To continue, the dura is resected step by step while pulling it caudally to avoid injury to brain structures. The incision is started at the anterior limit of the craniectomy so that resection of the dura on one or both sides can proceed to provide a clear view of the base of the frontal lobe. The resected piece of dura is marked and sent for serial histologic examination. The resulting dural defect is patched with fascia lata tucked between the bone and dural margins. The graft is additionally secured with a few anchoring sutures and fibrin glue. After the graft has been covered with silicone film, epithelialization of the lyophilized dura or the fascia lata occurs relatively quickly from the remaining mucosal rim. In a few weeks, the initially loose or sagging dural transplant changes into a firm, flat sheet of scar tissue. If resection of the anterior skull base extends beyond the fovea ethmoidalis and cribriform plate, a horizontal forehead and scalp flap conforming to the size of the cranial defect should be folded over to provide stable coverage of the duraplasty and epithelialization of the remaining orbital roof and fovea. The donor area of the scalp flap can be covered either with a split-thickness skin graft or by advancing other scalp flaps (Fig. 116-9).

FIGURE 116-9 Epithelialization of the skull base and orbit is accomplished with the aid of a forehead scalp flap. A, After tumor removal and dissection of the forehead scalp flap. B, The scalp flap has been sutured, and the donor site is covered by advancing other scalp flaps.

Intracranial Approach to the Anterior Skull Base

Lesions of the anterior skull base that are predominantly extradural in origin can be exposed by elevating the dura. In doing so, the olfactory nerves have to be cut at their site of passage through the cribriform plate. Smaller openings of the dura require repair. Intradural lesions such as meningiomas should be exposed through a transfrontal intradural route. Basically, this can be done via a bifrontal approach or a small frontolateral approach.

Transfrontal Extradural Approach

Depending on the extent of the lesion, a bitemporal coronal incision is made, and a unilateral or bifrontal craniotomy is performed. The extent of the latter depends on the site of the tumor. With midline lesions, the dura is mobilized on both sides of the crista galli and in part is sharply elevated from the cribriform plate. If necessary, the dura can be detached bilaterally as far as the lower sphenoid wing and tuberculum sellae (Fig. 116-10). With this maneuver, clear delineation of the anterior skull base is possible. This approach also permits good exposure of the frontal sinus, ethmoid cells, and sphenoidal sinus, supplemented if necessary by removal of the crista galli itself. This method also offers good exposure of the optic canal and the superior aspect of the orbital contents. The optic chiasm with the intradural portion of the optic nerve is not visualized with this approach. Lesions extending to the clivus are accessible by dissecting along the posterior wall of the sphenoidal sinus or the anterior wall of the sella. The endocranium is sealed off from the paranasal sinus system with a dural or fascial transplant—in some cases with a galea-periosteal flap that has a basal pedicle (Fig. 116-11).