The introduction of the endoscopic endonasal approach for the management of lesions of the skull base has produced a paradigm shift in the way complicated lesions of this complex anatomic location are managed. The endonasal approach provides the most direct route to the anterior cranial base and adjacent skull base locations. This article presents the 10 most important tenets that have proved useful to the authors regarding postoperative patient management and surgical practice.
The skull base is one of the most complex anatomic locations in the human body. The introduction of the endoscopic endonasal approach for the management of lesions of the skull base has produced a paradigm shift in the way these complicated lesions are managed. The endonasal approach provides the most direct route to the anterior cranial base (including sella, cribriform plate, planum sphenoidale, and suprasellar cistern) as well as the clivus, pterygopalatine fossa, and adjacent parasagittal skull base locations. The advantages over the traditional transcranial or transfacial approaches include decreased retraction of the brain and cranial nerves and improved visualization of not only the tumor but also the surrounding neurovasculature, which heralds the potential for improved surgical outcomes. All new techniques require a learning curve and this article describes the most important tenets that have proved useful to the authors regarding postoperative patient management as well as surgical practice.
A team approach between otolaryngology and neurosurgery
A cohesive collaboration between otolaryngology and neurosurgery is critical to the success of the development and performance of these surgical procedures. The most fundamental advances in open and endonasal cranial base surgery have been realized as a result of this collaboration. The skull base is the frontier that bridges these two specialties and the combination of knowledge is essential for the advancement of endoscopic skull base surgery. Otolaryngologists have an intimate knowledge of nasal and paranasal sinus anatomy and its complex array of variations whereas neurosurgeons have an unparalleled knowledge of neurovascular anatomy. In tandem, these two specialties can work together, otolaryngologists navigating the pathway to tumors and neurosurgeons removing tumors intracranially. The closure of the resulting defect in the skull base is aided by the thought processes of two different surgical mentalities, both of which emphasize different aspects of the repair, one the watertight separation of sterile CSF from the bacteria-laden nasopharynx and the other the maintenance of mucosal integrity, ciliary transport, olfaction, and air flow.
Meticulous review of preoperative CT and MRI; the use of image guidance intraoperatively
Systematic preoperative planning is essential in tackling lesions of the skull base. The first prerequisite for a successful approach is an understanding of the location of the lesion in 3-D.
CT provides critical information about the bony anatomic landmarks. CTs should be scrutinized for anatomic variations. The lamina papyracea should be examined for dehiscence. The degree of pneumatization of the sphenoid sinus should be evaluated to determine the amount of bone that must be removed to reach the pathology. The existence of any Onodi cells should be noted; these occur when the most posterior ethmoid cells are highly pneumatized, extending to the anterior wall of the sphenoid sinus and containing the optic nerve or carotid artery. This is not a contraindication to surgery but must be appreciated prior to surgery to avoid neurovascular injury.
The intersinus septae of the sphenoid sinus must also be carefully examined. There is generally one septum separating the right and left sphenoid, but these can be multiple or asymmetric. The location of parasagittal septations with respect to the carotid artery and optic nerve are useful in identifying the location of these structures. The height and trajectory of the skull base should be noted; this is an often-discussed entity in safe sinus surgery and has value in skull base surgery as well. Keros has described 3 types of skull base conformations. These categories relate to the risk of penetration of the skull base. In type 1, the olfactory sulcus is 1 to 3 mm deep, and the corresponding lateral lamella is short; this is the least hazardous configuration. In type 2, the olfactory sulcus is 3 to 7 mm deep, and in type 3 it is 7 to 16 mm deep. When the lateral lamella contributes significantly to the ethmoid roof, the risk of penetration of the skull base increases. By staying lateral to the insertion of the middle turbinate, a surgeon can avoid perforating the lamina cribosa inadvertently. The authors often use a CT angiogram to identify the precise course of the carotid artery with respect to the bony anatomy. The MRI, alternatively, is more sensitive to soft tissue and is invaluable in determining the location and extent of the pathology. The authors generally combine a CT angiogram and gadolinium-enhanced MRI for the purposes of image guidance to maximize the information available for surgical planning.
Image-guidance technology is a crucial adjunct when operating at the skull base. Contemporary image-guided surgery is not cumbersome and allows for rapid registration and calibration with accurate localization. Image guidance is thought to be a valuable adjunct by many investigators. The authors often obtain the CT angiography as an outpatient prior to surgery for coregistration with an MRI obtained the morning of surgery.
Meticulous review of preoperative CT and MRI; the use of image guidance intraoperatively
Systematic preoperative planning is essential in tackling lesions of the skull base. The first prerequisite for a successful approach is an understanding of the location of the lesion in 3-D.
CT provides critical information about the bony anatomic landmarks. CTs should be scrutinized for anatomic variations. The lamina papyracea should be examined for dehiscence. The degree of pneumatization of the sphenoid sinus should be evaluated to determine the amount of bone that must be removed to reach the pathology. The existence of any Onodi cells should be noted; these occur when the most posterior ethmoid cells are highly pneumatized, extending to the anterior wall of the sphenoid sinus and containing the optic nerve or carotid artery. This is not a contraindication to surgery but must be appreciated prior to surgery to avoid neurovascular injury.
The intersinus septae of the sphenoid sinus must also be carefully examined. There is generally one septum separating the right and left sphenoid, but these can be multiple or asymmetric. The location of parasagittal septations with respect to the carotid artery and optic nerve are useful in identifying the location of these structures. The height and trajectory of the skull base should be noted; this is an often-discussed entity in safe sinus surgery and has value in skull base surgery as well. Keros has described 3 types of skull base conformations. These categories relate to the risk of penetration of the skull base. In type 1, the olfactory sulcus is 1 to 3 mm deep, and the corresponding lateral lamella is short; this is the least hazardous configuration. In type 2, the olfactory sulcus is 3 to 7 mm deep, and in type 3 it is 7 to 16 mm deep. When the lateral lamella contributes significantly to the ethmoid roof, the risk of penetration of the skull base increases. By staying lateral to the insertion of the middle turbinate, a surgeon can avoid perforating the lamina cribosa inadvertently. The authors often use a CT angiogram to identify the precise course of the carotid artery with respect to the bony anatomy. The MRI, alternatively, is more sensitive to soft tissue and is invaluable in determining the location and extent of the pathology. The authors generally combine a CT angiogram and gadolinium-enhanced MRI for the purposes of image guidance to maximize the information available for surgical planning.
Image-guidance technology is a crucial adjunct when operating at the skull base. Contemporary image-guided surgery is not cumbersome and allows for rapid registration and calibration with accurate localization. Image guidance is thought to be a valuable adjunct by many investigators. The authors often obtain the CT angiography as an outpatient prior to surgery for coregistration with an MRI obtained the morning of surgery.
Mucosa-sparing surgical technique with preservation of the middle turbinate
The authors advocate a minimally invasive approach with maximal preservation of normal intranasal structure and function. Preserving as much normal mucosa as possible and only removing that which is necessary promotes healthy postoperative ciliary movement and an earlier re-establishment of mucociliary flow. In most surgeries it is not necessary to remove the middle turbinate. These can easily be preserved with simple lateralization. A meticulous mucosa-sparing technique is critical in the postoperative course of these patients. With this approach there is decreased crusting, synechiae, and obstruction of natural ostia. If a nasoseptal flap is harvested, alternatively, the mucosa from the sphenoid must be completely removed to assure that a mucocele does not form behind the flap. Likewise, in certain circumstances, the superior, middle, or inferior turbinate must be removed to expose the skull base adequately, as in the transpterygoidal approach.
The relationship of the sphenoid sinus septations with respect to the carotid artery and carotid localization with a Doppler ultrasound
As discussed previously, the exact insertion of the sphenoid sinus septae onto the posterior wall of the sphenoid sinus is an invaluable landmark for identifying the carotid artery intraoperatively. If the carotid artery is lateral to the insertion of the septum, surgeons can use as this a landmark intraoperatively and understand that as long as they are operating medial to the septum the carotid remains protected. The reverse is true if the carotid lies medial to the septum; if this is the case, surgeons must take special care to avoid injury because there is not an obvious septum protecting the carotid. It is also crucial to identify those septae that insert directly onto the carotid artery because removal of the septae in this case could lead to hemorrhage; thus, manipulation should be done with caution.
The use of intraoperative Doppler ultrasound is useful for carotid localization once the bone over the carotids has been removed and the anterior wall of the cavernous sinus is exposed. Although image guidance can also be used, the Doppler is superior because it measures blood flow in real time. This is useful if there is a question that the navigation may not be precise.
The use of intrathecal fluoreoscein for identification of intraoperative CSF leaks
Intrathecal fluorescein is a useful tool for the identification of intraoperative CSF leaks. Fluorescein is a green fluorescent dye that can be introduced intrathecally via lumbar puncture to alter the color of CSF. The dosage the authors recommend is 0.25 mL of injectable 10% solution mixed in 10 mL of CSF. Patients should first be premedicated with diphenhydramine (50 mg intravenously) and dexamethasone (10 mg intravenously) to reduce the risk of inflammatory or allergic reaction.
Clear CSF can often go unrecognized in a field of blood and secretions and this dye makes the identification and repair of subtle leaks possible, thus avoiding the risk of postoperative meningitis. The intrathecal application of fluorescein represents an off-label use of the product and requires informed consent discussion with the patient. There have been sporadic reports in the literature of adverse events associated with its use, including lower-extremity weakness and numbness, seizures, hemiparesis, cranial nerve palsies, and neuropathic pain. The reported incidence is exceptionally low and most reports occurred after the use of higher and more concentrated doses than that used at the authors’ institution without premedication. The efficacy and safety of intrathecal fluorescein has been demonstrated by the authors’ group.
Multilayered watertight seal for skullbase closure
The endoscopic closure of small defects has a greater than 90% success rate during primary endoscopic surgery, rising to 97% at revision.
Creating a watertight separation between the sinonasal and intracranial cavities is a challenge. Multilayered closure of the skull base defect remains the centerpiece of adequate reconstructive techniques, with or without the addition of a vascularized flap. The cornerstone of success in the closure of iatrogenic CSF leaks created in endoscopic skull base surgery is an impermeable seal. The consequences of an inadequate closure include the possibility for CSF leak, meningitis, pneumocephalus, and death.
The authors have previously described a technique for achieving a watertight skull base closure, called a gasket seal. This involves an onlay fascia lata graft over the bony defect. A rigid buttress (vomer or Medpor implant) is then countersunk over the fascia lata graft, which countersinks the center of the fascia lata, allowing the circumference of the fascia to close the bone edges ( Fig. 1 ). In addition, a nasoseptal flap can be placed over the gasket seal to achieve a vascularized flap closure at the skull base and a fat graft can be used intracranially to reduce pooling of CSF in the resection cavity. A final layer of tissue sealant, such as DuraSeal (Covidien, Hazelwood, Missouri), is then placed to keep the flap in place and maintain a watertight closure until the fibrous union of the graft materials occurs. Several other methods of closure, with varying success rates, have been described. The gasket seal reduced the CSF leak from approximately 25% to 0% in a preliminary small series of patients, and, currently, in the authors’ past 150 cases to 2.3% (Vijay K. Anand, MD and Theodore H. Schwartz, MD, unpublished data, 2009).
