Anterior Skull Base Tumors and Approaches: Transtuberculum, Transplanum, and Transcribriform
Summary
The anterior skull base (ASB) can be affected by primary tumors or by various lesions of sinonasal or intracranial origin. During the past decade, the expanded endoscopic endonasal approach (EEA) has gained in popularity and recognition as a safe and effective alternative route for the removal of ASB lesions. Most importantly, similar oncologic results to those obtained with traditional open approaches have been documented for sinonasal, sellar, and skull base lesions. In this chapter, we review, step-by-step, the transtubercular, transplanum, and transcribriform approaches for ABS lesions and address potential challenges.
Introduction
Traditionally, ASB lesions have been approached by transcranial and transfacial routes.1–3 These approaches, however, result in significant morbidity (35–40%) and mortality (5%).4–6 During the past decade, the expanded EEA has gained popularity and recognition as an alternative route for ASB lesions. Growing experience in centers around the world has demonstrated and reproduced the feasibility, safety, and effectiveness of these approaches. EEAs have shown many surgical advantages for the treatment of ASB lesions in comparison to the traditional transcranial and transfacial approaches. These include a more direct median exposure, superior visualization (angled lenses), decreased manipulation and injury of the brain parenchyma, no need for neurovascular manipulation, prompt decompression of visual apparatus (when indicated), and early tumor devascularization.7–12 Furthermore, similar oncologic results to those obtained with traditional open approaches have been documented for sinonasal, sellar, and skull base lesions.8,10,12–16 Piecemeal resection of malignant tumors does not compromise oncologic results as long as clear margins are obtained (confirmed intraoperatively with frozen section analysis).17 From the patient′s perspective, decreased surgery time, decreased length of stay, increased patient comfort, and lack of external incisions (lack of scars) are significant advantages of the EEA.18,19
Indications/Patient Selection
Pathologies involving the ASB can be categorized as benign or malignant lesions ( Table 40.1 ). Benign conditions may be developmental or neoplastic. Characteristics of the lesion, such as histopathology, fibrosity, primary location, size, and extension, all contribute to choosing the surgical route. In general, the transtuberculum/transplanum approach is indicated for lesions involving both the posterior aspect of the ASB and the suprasellar region. The transcribriform approach may be either added to the transtuberculum/transplanum approach or performed individually for skull base or sinonasal pathologies that involve the most anterior part of the median skull base ( Fig. 40.1 ). As a general rule, the EEA is not indicated if the ASB lesions extend lateral to the internal carotid artery (ICA), encase the ICA or anterior cerebral arteries (ACA), or extend laterally to the optic nerve8,10,20,21 ( Fig. 40.2 ). More specifically, an EEA is contraindicated for ASB malignancies in the presence of lateral intracranial extension (lateral to the midline of the orbit anteriorly or lateral to the optic nerve posteriorly) or skin and/or orbital extension requiring gross facial/scalp skin excision or orbital exenteration4,22 ( Fig. 40.3 ). Tumors that cannot be completely excised using an endonasal approach require an open approach or a combination of approaches11,22 ( Fig. 40.4 ).
Preoperative Planning
Standard metabolic and endocrine tests are performed as appropriate for each individual case. A preoperative visual assessment should include visual acuity and if possible baseline standardized visual fields. Patients customarily have preoperative computed tomography (CT) angiography and magnetic resonance imaging (MRI), which are fused and used for intraoperative image guidance. Preoperative imaging must be reviewed in detail with special attention to sinus anatomy.23
Tips and Tricks
Aggressive tumors may destroy the ethmoid sinuses and cribriform plate, thus rendering anatomical landmarks ambiguous. Preoperative assessment of each patient′s sinus anatomy and skull base, knowledge and recognition of anatomical variations, and intraoperative use of surgical navigation may prevent intraoperative complications.
Patient Counseling, Informed Consent
Preoperative consultation is a privileged time for the patient and surgeons. The patient is allowed to meet with each physician taking part in the surgery. Ideally, a multi-disciplinary clinic is organized during which the neurosurgeon and the head and neck surgeon explain to the patient the goals of surgery, the surgical technique, the role of both surgeons, the foreseen benefits, and the potential risks and sequelae. Use of intraoperative surgical navigation, as well as neuromonitoring (if required), is explained.
Details regarding the reconstruction are also essential to the preoperative consent. The patient is informed of the possible need for soft tissue grafts and/or flaps or the use of alloplastic materials. The possibility of using a lumbar spinal drain must also be mentioned, although it is rarely needed.
A typical postoperative period is also explained prior to surgery. The patient is informed that some pain and/or headache may be present after surgery. Furthermore, the sites of headpins and the scalp neuromonitoring leads used for somatosensory evoked potentials (SSEPs) may be tender in the postoperative period. If nasal packing or splints are a possibility, they are discussed with the patient. The patient must also give consent for possible transfusions of blood products; if the patient refuses blood transfusion, options must be discussed.
Anesthesia and Positioning
The patient is positioned supine, and after induction of general anesthesia, the head is immobilized in a Mayfield three-pin head holder. The patient′s head is turned to the right, while the neck is angled 20 to 30 degrees toward the left shoulder and placed on mild to moderate extension to facilitate the visualization and instrumentation of the nasal vault. Greater neck extension may be warranted for lesions that extend to the cribriform plate and/or the frontal sinus. All patients receive a prophylactic dose of broad-spectrum intravenous antibiotics at induction (e.g., third-generation cephalosporin or vancomycin if the patient has a known allergy to penicillin).
Tips and Tricks
For patients with compression of the optic apparatus, it is imperative that the mean arterial pressure (MAP) be maintained .60 to 70 mm Hg during induction and that no episode of hypotension occurs during the procedure. Systemic corticosteroids, administered at the beginning of the procedure, may be of benefit.
Baseline SSEPs are confirmed after patient positioning and insertion of neuromonitoring leads. Electromyographic (EMG) monitoring is advocated when dissection of motor cranial nerves is expected. The surgical navigation system is installed, and its accuracy is verified. Pledgets soaked in oxymetazoline 0.05% while the nursing staff prepares the perinasal area and lower abdomen help to decongest the nose and aid hemostasis. Appropriate positioning of the monitors is an important element as EEAs for ASB lesions are often lengthy; the ergonomics for surgeons must be a priority to minimize physical strain related to position.
Anatomy
Mastering the ASB anatomy as viewed from the endonasal approach is crucial to performing an EEA ( Table 40.2 ). The transtuberculum/transplanum approach is limited anteriorly by the posterior ethmoidal artery, posteriorly by the sella turcica, and laterally by the optic nerves.24 This part of the ASB corresponds intracranially to the gyri recti, orbitofrontal gyri, and suprasellar and prechiasmatic cistern. Vital structures related to this module are the optic nerves, ICA, and anterior cerebral arteries (A1, Huebner′s, anterior communicating, and perforators). The transcribriform approach is limited anteriorly by the frontal sinus and anterior ethmoidal artery, posteriorly by the posterior ethmoidal artery, and laterally by the lamina papyracea and medial orbit.24 Intracranially, this corresponds to the gyri recti, orbitofrontal gyri, olfactory nerves, and interhemispheric fissure. The most important vital structures related to this module are the orbits and the anterior cerebral arteries (second segment of the anterior cerebral artery, A2) and their branches (fronto-orbital, frontopolar) ( Fig. 40.5 ).
Identification of the anterior and posterior ethmoidal arteries and their cauterization and/or clipping under direct visualization prevents uncontrolled bleeding and contributes to early tumor devascularization.24,25 The anterior ethmoidal artery runs in an anteromedial direction through the anterior ethmoidal canal, which is usually located between the second and third lamella, posterior to the frontal recess.26,27 The posterior ethmoidal artery courses horizontally in the posterior ethmoidal canal, usually located ~7 to 8 mm from the optic foramen and 18 mm from the tuberculum sella26,27 ( Fig. 40.6 ).
Preoperative imaging (MRI, CT, and MR and CT angiography) should be studied in depth to evaluate the relation of the ASB pathology to surrounding neurovascular structures. Throughout the tumor′s dissection and resection, the surgeon must be aware of the location of these structures and seek to identify them early in this process.
Operative Steps
Generalities for Endoscopic Endonasal Approaches
Ideally, EEAs should be performed by a team comprising a neurosurgeon and otorhinolaryngologist–head and neck surgeon who are also versed in traditional skull base approaches. In our opinion, a team of surgeons with substantial experience in endoscopy and ventral skull base and brain anatomy increases surgical efficiency, improves problem solving, and appropriately modulates enthusiasm throughout the procedure.17
The endoscope is introduced at the 12 o′clock position of the right nostril and is used to retract the nasal vestibule superiorly, increasing the available space for other instruments. A suction tip is introduced at the 6 o′clock position on the ipsilateral side. Although the dissecting instruments are usually introduced through the left nasal cavity, the suction and dissecting instruments should be freely exchanged throughout the case, taking advantage of the best angle of approach. Widening of the nasal corridor is achieved initially by infracturing and then outfracturing the inferior turbinates bilaterally, followed by removal of the right middle turbinate to provide room for the endoscope. In cases in which a cerebrospinal fluid (CSF) leak is anticipated, a nasoseptal vascularized mucosal flap is elevated during the initial stage of the procedure.28–30 In cases in which a CSF leak is unlikely, a bilateral sphenoidotomy/sphenoidectomy is performed, preserving the vascular pedicle for a nasoseptal flap (i.e., preserving the mucosa between the natural sphenoid ostium and the posterior choana). At the end of the procedure, a “rescue flap” can still be elevated in case of CSF leak. The technical details of vascularized flaps and the rescue flap are described in depth in Chapter 41.
Exposure
In all ASB approaches, wide sphenoidotomies are initially performed by enlarging the natural ostium of the sphenoid sinus or by direct removal of the sphenoid rostrum, in the midline (see Video 63, Pituitary Surgery, Nasal Phase, and Video 64, Pituitary Surgery, Sphenoid Phase ). Removal of the bony rostrum is completed using Kerrison rongeurs and/or a surgical high-speed drill. The posterior edge of the nasal septum (1–2 cm) is resected with either a back-biting forceps or a microdebrider. A posterior septectomy facilitates instrumentation through a binarial access, essential for a two surgeon, three- or four-hand technique. Widening of the sphenoidotomy enables exposure of the sphenoid lateral recesses, the posterior ethmoidal cells, the planum/tuberculum junction, and the opticocarotid recesses (OCRs). If necessary, the floor of the sphenoid is drilled to provide greater rostrocaudal trajectory into the suprasellar space. Intrasphenoidal septations are to be drilled down carefully, as they may lead directly toward the vertical canal of the ICA, and the sphenoid mucosa is removed. A series of key anatomical landmarks can be identified on the sphenoid′s face: the sellar prominence in the center, the bone strut covering the superior intercavernous sinus, the clival recess inferiorly, the carotid prominences lateral to the sella, and more superiorly the optic nerves, as well as the medial and lateral OCRs.24
The extent of additional bony exposure is dictated by the pathology, its location, and skull base extension. Bone over the sellar face should be removed if the pathology requires a transsellar approach. In such circumstances, bone should be removed laterally beyond the medial portions of the cavernous sinus and expose both the superior and inferior intercavernous sinuses. The medial OCR does not need to be opened unless suprasellar and lateral extensions toward the opticocarotid cistern are present24 ( Fig. 40.7 ).
For the transtuberculum/transplanum approaches, the bony exposure is extended rostrally by completing wide bilateral anterior and posterior ethmoidectomies. Ethmoidal septations are brought flush with the ASB and the lamina papyracea bilaterally using either the microdebrider or a hybrid drill. To prevent injury to the olfactory neuroepithelium, the anterior margin of the exposure should not extend anterior to the posterior ethmoidal artery, and the most rostral margin of the nasal septum is left attached to the skull base24 ( Fig. 40.8 ). The planum sphenoidale is drilled eggshell thin in a rostral-to- caudal direction. After opening the most rostral part of the sella and removing the bone strut over the superior intercavernous sinus, this venous structure is exposed, cauterized, and mobilized or divided. This enables direct access to the suprasellar extensions of tumors in the prechiasmatic cisterns. If the OCR must be accessed intradurally, complete removal of the optic strut (lateral OCR) and the medial clinoids is required, preventing traction on the tumor and risk of damaging the perforator arteries. The rest of the bone covering the paraclinoid segment of the ICA can also be opened using Kerrison rongeurs. Arterial feeders arising from the distal portion of the paraclinoid carotid artery at the level of the medial OCR as well as from the posterior ethmoidal artery may be identified and coagulated.
Bone exposure for a transcribriform approach extends to the level of the crista galli or even the frontal sinus24 ( Fig. 40.9 ). The attachment of the anterior portion of the nasal septum to the skull base is resected, olfaction is sacrificed, but lesions in this area are likely to have already destroyed olfaction. Complete ethmoidectomies are performed, exposing the medial orbital walls. The skull base is drilled rostrocaudally starting at the frontoethmoidal recess. The anterior and posterior ethmoidal arteries are identified, cauterized, and transected medially, contributing to tumor devascularization. The lamina papyracea can be removed to gain lateral exposure, although the periorbita should not be disrupted. Prior to drilling the cribriform plate, soft tissue including olfactory filaments and branches of the ethmoidal arteries must be coagulated. This step further contributes to tumor devascularization. After bilateral removal of the cribriform plate, the crista galli is drilled eggshell thin and fractured. Select sinonasal pathologies require that the transethmoid transcribriform be complemented with another approach along the coronal plane, such as the transorbital and transpterygoid modules.31
Tips and Tricks
Bilateral nasal access and a posterior septectomy are essential to allow for a two-surgeon, three- or four-hand technique, with maximal ease of work within the sinonasal corridor (to prevent the “hand-cuffing” effect).
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