Summary
This chapter focuses on a region having a history of innovation, from Oscar Batson’s description of a no man’s land in the 1920s to Dwight Parkinson’s description of an anatomical jewel box in the 1970s to Vinko Dolenc’s pioneering work that created the foundation of skull base surgery in the 1980s. Today’s neurosurgeons find a refined strategy for their cavernous sinus armamentarium using traditional surgical approaches with skull base modifications (when needed) as well as increasing roles for endoscopic surgery and radiosurgery. The authors provide a nuanced guide for evaluating the lesions of this region, performing clinical assessment and diagnostic imaging, and using a building block concept for treatment approach. Success in complex cranial surgery depends on factors such as individual/team competencies, tumor characteristics, and patient characteristics. This chapter highlights some basic strategies as a guide to avoid complications and failures that often result from noncompliance with these concepts. Given the breadth of this subject with its exhaustive list of anatomical structures, surgical steps, and tumor names, interspersed within this chapter are specific topics from the authors’ lectures and hands-on courses on the cavernous sinus and parasellar space that illuminate the most critical issues for the skull base surgeon.
23 Tumors of the Cavernous Sinus and Parasellar Space
23.1 History
In the late 1920s, anatomist Oscar Batson at the University of Cincinnati created plastic casts of the venous anatomy of the cavernous sinuses (similar to those of the lumbar plexus) that would eventually bear his name. Passed down from Batson to our neuroanatomical mentor and partner Dr. Jeffrey Keller, they are a physical reminder of Dr. Batson’s admonition that the cavernous sinus was truly a “no-man’s land” for neurosurgeons, with exsanguination the expected outcome of the folly of surgical exploration (Fig. 23.1).
In 1973, Dwight Parkinson described the surgical opening of the lateral wall of the cavernous sinus, including insertion of pieces of muscle to obliterate cavernous sinus fistulas. This seminal publication transformed what was fiction to become the reality of Dwight Parkinson’s “Anatomical Jewel Box,” the cavernous sinus.1 Fast-forward 20 years and 5,000 miles to Yugoslavia (now Slovenia), where Vinko Dolenc’s2 , 3 pioneering work defined a comprehensive surgical approach to the cavernous sinus. In proving its feasibility and utility in hundreds of cases worldwide, Dolenc established cavernous sinus surgery as the centerpiece for the development of skull base surgery and sparked a renaissance of descriptive neuroanatomy.
As with any new technique or novel idea, adoption is met with skepticism and then embraced with fanaticism before finally finding its proper place in the world—in this case, the neurosurgical armamentarium. After many years of pursuing the aggressive surgical cure of cavernous sinus meningiomas, we witnessed the collision of this concept with a counterrevolution fueled by the combination of failure of cure, persistent cranial nerve morbidities, and gradual acceptance of a rational role for radiosurgery in the treatment of these tumors. Eventually we moved toward a more balanced two-part approach that included, first, retention of portions of tumor that encased the cavernous carotid artery and, second, radiosurgery as an adjunctive treatment for select cases and circumstances.
In recent years, a new set of pioneers have defined and promoted a surgical approach to the medial cavernous sinus that uses a transnasal route and an endoscopic technique. Once again, the idea was met with skepticism and then embraced with fanaticism before only now beginning to find its role in the neurosurgical armamentarium.
23.2 Surgical Anatomy
Key anatomical structures for the surgical approach to the cavernous sinus are discussed in the appendix. Briefly, topics include the anterior clinoid process (ACP), whose removal typically unlocks the anterior compartment of the cavernous sinus, and the major landmarks of the foramen lacerum and lacerum segment of the internal carotid artery (ICA), greater superficial petrosal nerve (GSPN), and vidian nerve, among others.4 , 5 , 6
The ACP overlies the clinoid segment of the ICA and is almost invariably removed to unlock the anterior part of the cavernous sinus. The ACP has three points of insertion to the skull base: the optic strut (also known as lateral opticocarotid recess from a transnasal endoscopic view), roof of the optic canal, and lesser wing of the sphenoid (part of the superior orbital fissure). Removal of the ACP extradurally exposes the clinoidal space (or anteromedial triangle in Dolenc’s description) and the anterior loop of the ICA. Its removal is essential if the distal dural ring of the ICA is to be sectioned and/or mobilized.
The foramen lacerum and lacerum segment of the ICA are major landmarks used to navigate the exposure of the cavernous sinus from an endonasal approach. From a transcranial perspective, the foramen lacerum and lacerum segment of the ICA are never really exposed; they are situated just underneath (deep to) the root of the trigeminal nerve and covered by the petrolingual ligament.
The term paraclival ICA is often used in the endoscopic literature but does not have an exact correlate in the currently accepted ICA nomenclature. The endoscopic paraclival ICA corresponds to the lacerum (C3) segment and vertical portion of the cavernous (C4) segment of the ICA (the C1–C7 segments per Bouthillier et al).4 , 5
The GSPN remains a classic landmark for transcranial surgery of the middle fossa and parasellar space. Fortuitously, then, its anatomical continuation, the vidian nerve, has become a classic landmark for the transnasal approach to the same region. Both the GSPN and vidian nerve serve as reliable landmarks for the location of the ICA. During a transcranial approach, the GSPN must be carefully dissected from the temporal lobe dura to provide the lateral limit of Kawase’s quadrangle. During anterior petrosectomy, the GSPN that overlies the petrous ICA defines the lateral extent of the drilling. This nerve also serves as one of the landmarks of the internal auditory canal (IAC) during the middle fossa approach to vestibular schwannomas. During an expanded transnasal approach, the vidian canal and vidian nerve are identified in the pterygopalatine fossa and carefully followed back along the floor of the sphenoid sinus to the anterior skull base for safe exposure of the lacerum segment of the ICA.5 , 6
23.3 Regional Pathology and Differential Diagnosis
Various lesions originate in, grow contiguously into, or metastasize to the middle cranial fossa and parasellar space. Meningiomas, schwannomas, and chondrosarcomas are the most frequently encountered tumors in the parasellar space. Pituitary adenomas, craniopharyngiomas, and chordomas are parasellar tumors that can extend into the cavernous sinus. Cholesterol granulomas are nonneoplastic lesions often found in the petrous apex that should also be included in the differential diagnosis of parasellar lesions. Rare are the tumors that can affect the middle cranial fossa and parasellar space, including metastases, epidermoid tumors, cavernous sinus hemangioma, lymphoma, osteosarcoma, angiofibroma, nasopharyngeal carcinoma, and rhabdomyosarcoma.
23.3.1 Meningioma
Meningiomas of the parasellar space also involve the sphenoid wing and have been classified historically among medial, middle, and lateral sphenoid wing types. Clinoidal meningiomas are medial sphenoid wing meningiomas that have been classified separately on the basis of their relationship to the ACP.7 Along with tuberculum meningiomas, clinoidal meningiomas tend to present early with ophthalmologic disturbances secondary to optic nerve involvement. In contrast, middle sphenoid wing meningiomas often grow slowly and silently from the temporal fossa floor. They tend to present later, sometimes growing large enough to cause seizures from mass effect on the temporal lobe. Sphenocavernous meningiomas may present with signs and symptoms referable to cavernous sinus involvement, including eye movement problems (III, IV, VI), facial sensory disturbance (V), headaches (dura), or orbital venous congestion (venous obstruction).
For purposes of surgical planning, we group the middle fossa and parasellar meningiomas into clinoidal, sphenocavernous, and sphenoid wing types (Fig. 23.2). Sphenoid wing meningiomas are converted to convexity meningiomas once the wing is removed, allowing for complete resection of the tumor and its dural attachment. Although this is possible for clinoidal meningiomas, opening of the optic canal for removal of en plaque canalicular extension is generally required. Depending on the tumor’s consistency and adherence at surgery, leaving residual tumor is sometimes necessary to protect the optic nerve or surrounding vasculature or both. Complete resection of sphenocavernous meningiomas is limited to a small subset of tumors that superficially involve the lateral wall without significant cavernous sinus invasion (e.g., carotid encasement). For those that truly invade the sinus and are deemed to need treatment, in light of unacceptably high rates of neurological morbidity associated with intracavernous sinus surgery, we routinely remove the extracavernous tumor if it is large and symptomatic and radiate the intracavernous portion.8 , 9 If the tumor’s size is acceptable and the extracavernous portion is asymptomatic, we opt to radiate the entire tumor when treatment is necessary. Given the classic appearance of cavernous sinus meningiomas on imaging, radiosurgery or radiotherapy can usually be undertaken without biopsy confirmation.
Petroclival meningiomas are posterior fossa tumors that can grow into the cavernous sinus or middle fossa. Because of their deep location and association with multiple critical neurovascular structures, they remain one of the most challenging skull base tumors to treat surgically and thus require a command of the gamut of skull base approaches.10 , 11 , 12 When petroclival meningiomas extend anteriorly to involve the cavernous sinus or superolaterally toward the middle fossa, an anterior petrosal approach can deal with those tumor components and provide a window from the middle to the posterior fossa. Conversely, a retrosigmoid intradural suprameatal approach13 provides a window from the posterior to the middle fossa, allowing for removal of the middle fossa extension of a petroclival meningioma from below. Over time, as with cavernous sinus meningiomas, our strategy for managing petroclival meningiomas has evolved, reflecting our greater willingness to leave residual tumor behind, the better to preserve neurologic function, and radiate the residual as needed.
23.3.2 Trigeminal Schwannoma
Trigeminal schwannomas are benign tumors that arise from Schwann cells of the trigeminal nerve. Although rare—accounting for only < 0.4% of all intracranial tumors—they are the second most common intracranial schwannoma, after vestibular schwannoma. Trigeminal schwannomas may be situated anywhere along the course of the trigeminal nerve, from its root in the prepontine cistern and ganglion in Meckel’s cave to major divisions in the middle fossa interdural space and extracranial locations (e.g., infratemporal fossa, pterygopalatine fossa, orbit). The numerous classifications with which to characterize trigeminal schwannomas anatomically all reflect the potential for involvement in the posterior or middle fossa, or both to varying extent.14 , 15 , 16 Common presenting symptoms include facial pain and numbness, headache, diplopia (cavernous sinus involvement), and signs of brainstem compression.
Typically, small or asymptomatic lesions are observed or radiated, whereas larger, symptomatic lesions undergo surgery, especially in young patients.17 At surgery, trigeminal schwannomas are typically soft and avascular, displacing (rather than encasing) surrounding nerves and vessels. Cystic changes, found in up to 7% of cases,18 can make the tumor more adherent to surrounding neurovascular structures and thus more difficult to resect.
Any approach should account for the almost universal finding that trigeminal schwannomas have a Meckel’s cave component. Accordingly, even with significant encroachment into the posterior fossa, our approach typically begins in the middle cranial fossa and includes opening Meckel’s cave. Overall, the surgical strategy should focus on radical resection of the tumor with preservation of neurological function. More recently, transnasal endoscopic approaches have achieved success in resecting these tumors and are becoming an important strategy in the management of trigeminal schwannomas.19
23.4 Clinical Assessment
Signs and symptoms of parasellar tumors depend on the lesion’s type, site of origin, size, morphology, growth rate, and involvement of surrounding anatomical structures. Benign, slow-growing lesions tend to gradually develop an onset of symptoms. Malignant lesions or infectious processes manifest a more rapid clinical course. Furthermore, a lesion that arises from or in proximity to a cranial nerve may produce symptoms referable to the nerve, facilitating diagnosis earlier than for a similar lesion located elsewhere (e.g., clinoidal versus middle sphenoid wing meningioma).
23.5 Diagnostic Imaging
Although the imaging characteristics of common skull base lesions are well described in an earlier chapter, certain observations deserve emphasis. For example, cavernous sinus tumors that track along nerves are more likely malignant than benign. Carotid encasement is a typical feature of meningiomas. Bony lesions, such as chondrosarcoma, will instead displace the vascular structures. Most of the parasellar cavernous sinus tumors are vascularized by branches of the ICA or ophthalmic artery that cannot be embolized or by branches of the middle meningeal artery that can be controlled early in the operation. Catheter angiography is not routinely necessary when managing these tumors; it should only be requested in specific situations, such as for the study of collaterals when carotid sacrifice is contemplated.
23.6 Preoperative Preparation
Success in complex cranial surgery depends on many factors, including individual and team competencies, tumor characteristics (e.g., softness, “suckability,” adherence), and patient characteristics. Complications and failures often result from noncompliance with just a handful of simple strategic concepts. The night before a complex surgery, we review our checklist, such as this example for cavernous sinus/parasellar and middle fossa pathologies for goal of surgery, extent of resection, neurological deficits, relation of tumor to ICA, anatomical variations, prevention of cerebrospinal fluid (CSF) leak, imaging, special equipment, and communications with the anesthesiologist.
Our checklist guides us in defining the goal of the surgery and implications if we fail to reach that goal. For example, will the surgical strategy change significantly based on intraoperative biopsy, and if so, can that change be accommodated in the same setting? Other important questions include the following: Will we conduct a complete resection, or a partial one? How aggressive should resection be? Should we check the extent of resection using intraoperative MRI or rely on intraoperative visual assessment and MRI on postoperative day 1? What are the implications if the goal is not achieved?
Any neurological deficit and deficits that we or the patients are willing to accept are carefully evaluated. For instance, damaging the oculomotor nerve in a blind eye is probably acceptable, but causing a permanent ptosis in a seeing eye will make that eye functionally blind.
Assessment includes the relationship of the carotid artery to the tumor and the extent of collateral circulation in case of carotid injury. If the cavernous ICA is narrowed (as is often the case in cavernous sinus meningiomas), one should expect that tumor is tightly adherent to the ICA and will be difficult to separate from the vessel, because the adventitia can be invaded by the tumor. What are the criteria for stopping? Are we prepared to do a bypass procedure if necessary? When the ICA is displaced laterally (as is often the case for chondrosarcomas), is an expanded transnasal approach preferred to a transcranial approach? Do we have the right team for such an approach?
Anatomical variations to consider include degree of pneumatization of the optic strut and the presence of a middle clinoid process, interosseous bridge, or bony carotid ring. An unrecognized pneumatization of the optic strut during a transcranial anterior clinoidectomy will result in a profuse CSF leak on the patient’s arrival in the recovery room. An unrecognized carotid ring could turn a routine clinoidectomy into a life-threatening vascular complication. With today’s availability of high-quality CTs and MRIs, basing our surgical strategy on the described frequency of such anatomical variations no longer makes sense. Rather, we ascertain the exact anatomical situation of each patient before surgery using dedicated imaging and design an “individualized” surgical plan accordingly.
The best time to think about the closure is before the opening. Preserving a vascularized pericranial flap is the most basic step in anterior cranial base surgery. However, in some instances, avoiding a postoperative CSF leak will require the planned involvement of a plastic surgery team to supplement the closure with a free tissue transfer graft.
Once again we cannot stress enough the importance of preoperative studies in complex skull base cases. Are all radiographic studies/ancillary investigations completed, reviewed, and, if necessary, uploaded into the operating room navigation system? Not only should they be ordered, but the surgeon is responsible to ensure their proper format. Aside from their anatomical accuracy, these studies must be compatible with the hospital’s navigation system and in the proper format for uploading to the navigation system. Finally, if fusion of other image sets or postprocessing sets (e.g., fiber tractography) is necessary, complete those steps before the day of surgery.
Complex skull base procedures often require special equipment. For example, have the pieces of equipment needed for the case arrived and been cleared by the hospital’s biomedical department? The surgeon is responsible to know what equipment is available “off the shelf” and what must be specifically ordered. It is far more efficient to notify the operating room well in advance and to follow up on the request before surgery than to either postpone the case or scramble to have a piece of equipment brought in emergently.
Keeping the anesthesiologist in the loop is mandatory. Aside from routine cases, close communication with the anesthesia team before a complex case significantly affects the flow of the case and shortens the time to incision on the morning of the surgery.
23.7 Surgical Technique
The workhorse approach for the transcranial exposure of the cavernous sinus and parasellar space is the FTOZ (frontotemporal orbitozygomatic osteotomy), sometimes referred to as the COZ (cranio-orbito-zygomatic). A full FTOZ with orbital and zygomatic osteotomy always achieves a wide, generous approach to this region. It should be the default approach for large tumors or when the exact degree of necessary exposure is unclear from preoperative imaging. With greater experience and detailed understanding of the exposure added by each of those osteotomies, a more limited and tailored approach can be designed.
Exposure of the middle fossa is typically via a standard temporal craniotomy. A zygomatic osteotomy can bring the exposure flush with or even through the middle fossa floor. Anterior petrosectomy will allow resection of tumors that extend deep into the petrous apex or into the posterior fossa.
23.7.1 Surface Landmarks
The McCarty bur hole, a key step in the one-piece FTOZ craniotomy, is drilled over the frontosphenoidal suture 1 cm behind the frontozygomatic junction, between the frontal process of the zygomatic bone and zygomatic process of the frontal bone; its upper half exposes the frontal lobe dura and its lower half the periorbita. Note that the McCarty bur hole is usually located 5 to 10 mm below the standard keyhole bur hole.
Two surface landmarks are important for surgical exposures of the middle fossa. First, the external auditory canal (EAC) nearly perfectly aligns with the IAC in both the coronal and the axial planes. The geniculate ganglion is also found in the same coronal plane as the EAC and IAC. Second, the root of the zygoma is the external landmark for the middle fossa floor. The root has both a vertically oriented component, which is lateral, and a horizontal component, which is medial and connected to the squamous temporal bone. A coronal cut through the center of this root also runs through the foramen ovale, whereas a coronal cut through the posterior aspect of the zygomatic root runs through the foramen spinosum. The depression that overlies the lesser wing of the sphenoid marks the sylvian fissure and the anterior wall of the middle cranial fossa. The root of the zygoma marks the middle fossa floor near its junction with the petrous bone. These external landmarks and their reference to internal structures facilitate proper positioning of the craniotomy and help orient the surgeon during extradural exposure of the middle fossa.
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