Imaging

In most patients, magnetic resonance imaging (MRI) is performed preoperatively to determine the suspected diagnosis, extent of disease, and status of surrounding anatomical structures. Computed tomography (CT) with bone windows is often useful for visualizing the extent of bony skull base involvement by tumor. Major arterial involvement by tumor should prompt diagnostic cerebral angiography with possible test occlusion. For internal carotid artery (ICA) involvement, the contralateral ICA is injected, as are the vertebral arteries to assess collateral supply. For venous sinus involvement, the pattern of venous drainage and patency of venous structures needs to be determined, including any collateralization. If there is arterial involvement with poor collateral supply or a failed occlusion test, preoperative evaluation of donor vessels for likely bypass needs to be performed. Finally, especially for dura-based tumors, preoperative angiography can be performed to embolize feeding vessels.

Selection of Approach

The choice of approach is dependent on the goals of the operation (total versus subtotal resection), the areas of invasion, major arteries involved, and access to the tumor blood supply (tentorium, clinoid carotid, etc.). Previous treatments, including surgery and radiation therapy, can affect the choice of surgical approach from the perspective of wound healing and surgical scar from a certain trajectory. Finally, assistance from other specialties (neuro-otology, plastic surgery) may be required, as is the support of neurointensivists for the immediate postoperative care.

Anesthesia and Intraoperative Monitoring

For cranial base operations, the neuroanesthesiologist maintains blood pressure and oxygenation within normal limits. Arterial line, central venous catheter, and in some cases jugular bulb oximetry are utilized. Brain relaxation is achieved with modest hyperventilation, diuretics, and in some cases, lumbar cerebrospinal fluid (CSF) drainage or ventriculostomy. For prolonged skull base tumor operations, infusions of tranexamic or aminocaproic acid are run to promote hemostasis.

In most cases, transcranial cortically evoked motor evoked potentials (MEPs) and somatosensory evoked potentials (SSEPs) provide information regarding the integrity of the long tracts during the procedure. For this reason, total intravenous anesthetics are often required, consisting typically of propofol and remifentanil. When needed, electrodes are placed in the target muscles of cranial nerves V, VII, IX, X, XI, and XII for intraoperative mapping and assessment of functional continuity.

Transsphenoidal Approach

The primary indication for the transsphenoidal approach is midline sellar and suprasellar lesions, and it is the workhorse approach for most pituitary adenomas. Several authors have reported endonasal techniques of resection of other lesions, such as tuberculum sellae meningiomas and craniopharyngiomas.⁶ This approach is limited laterally by the internal carotid arteries; therefore, lesions extending laterally to this point cannot be resected completely. Either the microscope or the endoscope (either as the primary or assistive device) may be used for visualization. Current transsphenoidal instruments remain limited in the ability to apply standard microsurgical techniques of sharp dissection for intradural lesions, and intraoperative complications such as arterial injury are difficult to repair for this reason. Selected intradural lesions, such as tuberculum sellae meningiomas and craniopharyngiomas, may be removed via an extended transsphenoidal approach, provided an adequate arachnoid plane is visualized on preoperative imaging, and the tumor is primarily midline with no lateral extension beyond the carotid arteries or optic nerves. The caveat of an extended approach is an increased risk of postoperative CSF rhinorrhea, given the inherent difficulty of primarily repairing dural or bony openings in a watertight fashion. Dural substitutes, fibrin glue, postoperative lumbar CSF diversion, and nasoseptal mucosal flaps are useful adjuncts to prevent CSF leakage.

The patient is positioned supine with the head elevated above the body to reduce venous pressure and turned slightly to the left for a right-handed surgeon, who operates from the patient’s right side. Intraoperative neuronavigation is preferred over C-arm fluoroscopy to assist in localizing the anatomy, particularly the position of the cavernous carotid arteries. Either a sublabial or a purely endonasal approach may be used. For a sublabial approach, the upper gingival mucosa is infiltrated with local anesthetic and incised transversely. A periosteal elevator is used to expose the cartilaginous and then nasal septum, down to the vomer. The cartilaginous septum is fractured from the bony septum, exposing the anterior wall of the sphenoid sinus. The sinus is then opened using Kerrison rongeurs inserted into either sphenoid ostia, or using a microdrill or pituitary rongeurs. At this point, the pituitary fossa and optic and carotid prominences are identified. The bone overlying the pituitary fossa is removed, exposing the sellar dura. For more extensive lesions, bony resection is extended to incorporate the planum and tuberculum sellae.

For a purely endonasal approach, the nasal cavity is first treated with cocaine-soaked cottonoids. The nasal cavity is then entered, with or without the use of a nasal speculum, and the inferior and middle turbinates as well as the choana are used as landmarks until the ipsilateral sphenoid ostium is reached. The middle turbinate is either temporarily outfractured to increase the working space, or resected either with scissors or a microdébrider. If a mucosal flap is not harvested, a small incision is made in the septal mucosa at the junction of the bony and cartilaginous septum, which is outfractured. Submucosal dissection of the contralateral bony septum and vomer is performed, exposing the contralateral sphenoid ostium. Removal of the anterior wall of the sphenoid sinus proceeds as for the sublabial approach described earlier, and the remainder of the surgery proceeds in a similar fashion as well.

For closure, if no CSF leak is apparent, a small piece of Duragen or other dural substitute is tucked under the bony edges of the sellar opening and secured with fibrin glue. The closure may be augmented with Nasopore packing. In cases of intraoperative CSF leak, fat is harvested from the abdomen and placed within the sphenoid sinus to augment the closure, followed by placement of a lumbar drain in the operating room. Occasionally, a Foley catheter may be inflated to secure the nasal packing postoperatively. For larger osteodural defects, bone substitute such as Medpor (Porex Technologies, Fairburn, GA) may be used, or bony septum harvested from the initial exposure, as well as a mucosal flap, which is rotated on its pedicle to provide vascularized tissue coverage.

Currently, our practice has been to use the endoscopic transsphenoidal approach for most pituitary tumors that appear to have reasonable dissection planes from surrounding neurovascular structures. More invasive tumors with irregular suprasellar involvement or with extensive clival involvement are removed with either a cranial or transmaxillary approach.

Extended Subfrontal Approach

This versatile approach is useful for intra- and extradural lesions of the anterior skull base, paranasal sinuses, as well as the sella, and midline clivus down to the foramen magnum. It is also better suited than the transmaxillary approach for intradural lesions in these areas. Furthermore, it can be adapted for more lateral exposure, including the cavernous sinus and frontotemporal area as needed. As such, it is useful for purely extradural lesions, such as for craniofacial resection of sinonasal malignancies, and for mixed intra- and extradural lesions such as esthesioneuroblastomas, chordomas, and chondrosarcomas (Fig. 43.1).

FIGURE 43.1 Extended subfrontal approach. A, Craniotomy and orbital osteotomy. B, Superior view of osteotomy and area of planum sphenoidale that is removed with a high-speed drill. C, View after planum drilled and optic nerves unroofed. Note cavernous carotid arteries bilaterally. D, Reconstruction of the anterior cranial base with autologous fat and pericranial grafts.

The patient is positioned supine with the head neutral and the body secured for tilting as required during the procedure for contralateral exposure. Brain relaxation and CSF diversion are facilitated by intraoperative placement of a lumbar drain. A bicoronal incision is performed behind the hairline and reflected anteriorly. A pericranial incision is made from behind the skin incision to maximize the available graft and the pericranium is reflected anteriorly as a separate layer to repair the frontal and ethmoid sinuses as well as the dura at the end of the surgery. Care is taken to preserve the supratrochlear and supraorbital bundles, which may emerge either from a notch or true foramen from above the orbital rim. In the case of a foramen, they are osteotomized with a small straight osteotome and outfractured with the scalp flap. The periorbita is dissected for a distance of approximately 3 cm to allow the orbitofrontal osteotomy to be sufficiently posterior, and the frontonasal suture is also exposed. At this point a low bifrontal craniotomy is performed, taking care not to lacerate the dura or the superior sagittal sinus. The subfrontal dura is mobilized off the orbital roofs bilaterally, sparing the cribriform plate region. A bilateral orbitofrontal osteotomy is then performed using the reciprocating saw, incorporating the orbital bar and through the nasoethmoidal complex in the midline, and circumventing the cribriform plate in order to spare the olfactory dura. The remaining cribriform plate is drilled until the olfactory dural sheath is exposed. Depending on the pathology and the preoperative state of the patient’s olfaction, the olfactory nerves can be preserved during this approach, particularly for when the pathology is primarily intradural. Following an osteotomy circumventing the cribriform plate, the dura is opened, and the olfactory tract and sulcus are identified. Arachnoid dissection is used to mobilize the olfactory tracts off their sulci, allowing the basal frontal lobes to relax without transmitting traction on the olfactory tracts. In more extensive primarily extradural lesions, particularly if the patient is anosmic, the entire complex can then be suture-ligated and divided from within the nasal cavity, allowing the frontal lobes to relax superiorly. The anterior and posterior ethmoidal arteries are divided.

At this point the optic canals are unroofed using an irrigating microdrilled or ultrasonic curet under microscope visualization. The planum down to the tuberculum sellae is drilled as well, leading into the sphenoid sinus, the pituitary fossa, and the medial cavernous sinus walls bilaterally. Extradural exposure of the cavernous carotid artery is performed if required. Venous bleeding from the cavernous sinus is controlled by filling of the bleeding point with oxidized cellulose followed by fibrin glue, which is very efficacious. From the sphenoid sinus, the sella and clivus are exposed extradurally, and the entire clivus can be drilled if needed down to the foramen magnum, achieving an exposure in this area similar to that obtained with the extended maxillotomy approach. The lateral limits of this clival exposure include the cavernous sinuses, petrous apices, and hypoglossal canals.

Reconstruction involves suturing the pericranial flap to repair all dura defects in a watertight fashion. Mucosal tissue is resected from the exposed sinus and on the craniotomy flap, and the sinus opening and any dead space are obliterated with fat harvested from the abdomen and the pericranial flap as a second layer. The olfactory dural sheaths are also sutured closed in watertight fashion. For larger bony defects along the planum or clivus, structural support is achieved with either a split-thickness bone graft or titanium plate, with screw fixation when possible. The fronto-orbital osteotomy is reaffixed over the pericranial repair and the entire closure is injected with fibrin glue to promote a watertight seal. In occasional cases, a lumbar drain is placed for postoperative CSF diversion. For significant splaying of the eyes secondary to the tumor, a medial orbital canthopexy is performed.

Transmaxillary and Extended Transmaxillary Approaches

This approach is best suited for midline extradural lesions centered on the midportion of the clivus, such as chordomas and chondrosarcomas (Fig. 43.2). The exposure is also well suited for lesions extended superiorly up to the sella turcica. The lateral limits of this exposure include the cavernous sinus and carotid arteries, and the pterygoid space. Depending on the extent of disease, however, the lower clivus, anterior cervical spine, or lateral infratemporal space may be accessed via extended maxillotomy approaches.

FIGURE 43.2 Transmaxillary and extended transmaxillary approaches. A and B, Schematic lateral and anterior views, respectively, of the transmaxillary approach, demonstrating the extent of exposure of the sella and mid and upper clivus. C and D, Exposure is extended caudally down to the upper cervical spine with the extended maxillotomy approach.