18 Endoscopic Endonasal Surgery for the Craniovertebral Junction
Origins and Evolution of the Technique
Endoscopic sinus surgery was developed by Alfred Hirschmann1 and Etienne Escat in the early 1900s.2 However, only after a better understanding of the physiology and pathology of the nose and sinuses provided by Walter Messerklinger3 and the development of appropriate instruments and the rod-lens systems by Harold Hopkins in the 1960s did endoscopes establish their position in sinonasal surgery.1 David Kennedy4,5 coined the term functional endoscopic sinus surgery (FESS) in 1985 and, along with Heinz Stammberger6 and Wolfgang Draf,7 popularized the use of modern endoscopy for the paranasal sinuses in the 1980s. The use of endoscopes in neurosurgery had a different evolution. In an attempt to improve visualization, Gerald Guiot in 1963 was the first neurosurgeon to use the endoscope in transsphenoidal surgery.1,8,9 The endoscopes at this time, however, were still inadequate, and when Jules Hardy finally established the transsphenoidal route in 1967, he incorporated the operating microscope for demonstrating the superiority of accessing the pituitary fossa transsphenoidally as opposed to transcranially.1,8,10,11
In the late 1970s, Apuzzo and colleagues,12 as well as Bushe and Halves,13,14 reintroduced the endoscope as an adjunct to the microscope for resection of pituitary lesions with extrasellar extension. Endoscopically “assisted” transsphenoidal microsurgery has since been reported by various authors, stressing the advantages of visualization around corners, particularly for tumors that extend beyond the sella.15–20 The endoscope, however, often limits the working space, and there is limitation of maneuverability of the endoscope within the speculum when the microscope is the primary form of visualization.15–20 This limitation led to the development of a fully endoscopic, non-speculum-assisted endonasal approach to the ventral skull base resulting from collaboration between neurological and otolaryngology–head and neck surgeons.
The endoscope was first used as the only visualizing tool in skull base surgery for pituitary lesions in the early 1990s.1 In 1992, Jankowski and colleagues reported their experience with three cases using a pure endoscopic transsphenoidal approach to the sella.21 Others subsequently confirmed the feasibility of pure endoscopic approaches to the sella.22–25 Paolo Cappabianca and Enrico de Divitiis were extremely important in fostering and disseminating the technique.26–36
As a natural progression, various centers around the world began to perform pure endoscopic approaches to the sella.1 With the panoramic view offered by the endo-scope allowing visualization beyond the sella, combined with otolaryngologic and neurological surgeons working together, endoscopic skull base surgery was born.
In a similar evolution to the transsphenoidal approach, the anterior approach to the rostral cervical spine for treatment of the craniovertebral junction was first explored in 1935 by German.37 Since then, the transoral approach using microscopic visualization has been used extensively for a wide range of pathological entities, including basilar invagination,38–44 rheumatoid arthritis with craniocervical settling and/or pannus,45–48 odontoid fractures,49–52 tumor (both extradural and intradural),53–56 and odontoid hypoplasia.37 Only recently was the endo-scope introduced to transoral surgery as a secondary tool to the microscope,57,58 and the feasibility of endoscopic endonasal approaches to the cervicomedullary junction initially demonstrated in cadavers by Alfieri et al.59
In late 1998 at the University of Pittsburgh, we began to pursue the expanded fully endoscopic endonasal approach to the ventral skull base in a systematic fashion. Working as a team of otolaryngologic and neurological surgeons over the past 10 years, we have performed over 1000 endoscopic endonasal skull base procedures. As part of this, the first endoscopic endonasal approach for the cervicomedullary junction was reported in 2004.60
Indications (Patient Selection)
Clivectomy
A transclival approach is used for endonasal resection of extra- and intradural lesions, such as chordomas, chondrosar comas, meningiomas, and even some craniopharyngiomas.
The approach is defined by partial (upper, middle, or lower third) or complete removal of the clivus (panclivectomy). The upper third consists of the dorsum sellae in the midline and the posterior clinoid processes paramedially. The posterior clinoid processes can be removed either intradurally or extradurally via a combined trans- and subsellar corridor by first performing a pituitary transposition.61 This consists of elevating the pituitary gland (either intra- or extradurally) superiorly to access the underlying bony structures. Removal of these structures can provide access to the basilar artery and interpeduncular cistern.
The middle clivus can be directly accessed at the posterior aspect of the sphenoid sinus. Its resection is limited laterally by both ascending paraclival internal carotid arteries (ICAs). This lateral access can be extended by exposing and retracting the vertical portion of the ICAs to some extent. If the bone drilling continues inferiorly, the lower third of the clivus is limited laterally by the fossae of Rosenmüller and the torus tubarius. However, by sacrificing the eustachian tubes, the bony resection is limited laterally by the hypoglossal canals. A panclivectomy can extend all the way from the dorsum sellae and posterior clinoids to the basion at the foramen magnum.
Transodontoid Approach
Lesions involving or adjacent to the clivus may extend caudally to the cervicomedullary junction. The anterior cervicomedullary junction provides a unique surgical challenge. Access is difficult from a posterior or lateral approach due to intervening neural structures and mechanical stability concerns. Anterior, transoral approaches have their own set of difficulties, including poor rostral access, potential for palatal insufficiency, delayed feeding, and oral bacterial contamination. The endoscopic endo-nasal route avoids these problems.60,62
Extension of the transclival approach can continue caudally to the anterior atlas, dens, and superior body of the second cervical vertebra. Caudal access via the nose is restricted by the angle created with the endo-scope and instruments pivoting on the hard palate. To determine caudal access preoperatively, the Kassam line63 can be drawn on a midsagittal computed tomography (CT) or magnetic resonance imaging (MRI) scan from the tip of the bony nasal bridge to the posterior hard palate and extended to the cervicomedullary junction ( Fig. 18.1 ).
This approach can be used for resection of the odontoid process in degenerative/inflammatory diseases, pannus formation, and basilar invagination or to allow for exposure of the ventral medulla and upper cervical spinal cord for tumor resection. Foramen magnum meningiomas and rheumatoid pannus are examples of lesions that can be treated using this approach.
The approach is defined by the removal of the odontoid process of the axis (second vertebra). This approach is an extension of the transclival approach and may require partial removal of the clivus, especially in the setting of basilar invagination. The lower third of the clivus is exposed, as well as the anterior arch of C1, after dissection of the nasopharyngeal mucosa and the longus capitis muscles. The anterior arch of C1 is removed with a high-speed drill and rongeurs to expose the articulation with the odontoid process. The odontoid process can then be resected in a fashion similar to that previously used with transoral approaches. Lateral access is limited by the eustachian tubes, although these can be retracted or even resected, at which point lateral access is limited by the hypoglossal canals in the condyle and the parapharyngeal ICAs.
Description of the Technique
Planning
Frameless stereotactic image guidance is used in all expanded (endoscopic) endonasal approaches to the skull base. Image guidance is of value in corroborating the visual impression of the surgical anatomy, especially critical neurovascular structures, and helps to define a targeted re-section and ensure adequate exposure. A high- resolution CT angiogram is used for most skull base surgeries, as it allows for the simultaneous visualization of osseous, vascular, and soft tissue anatomy. Increasingly, we use image fusion of CT and MRI scans to take advantage of the best features of each: CT for the bony anatomy of the cranial base and MRI for intracranial tumor visualization.
Operating Room Setup
The surgeons are positioned on the right side of the patient opposite the anesthesia team. The surgical technician or nurse is positioned toward the foot of the bed. This arrangement gives the surgeons unrestricted access to the nasal region. Electrical cords and suction tubing are directed away from the surgical field toward the head and foot of the bed to minimize interference with surgical instruments.
Head pin fixation is used to ensure the lack of intraoperative movement of the head, especially during drilling and neurovascular dissection. The head is fixed following endotracheal intubation, with the neck in slight flexion, vertex minimally tilted to the left, and the face turned to the right 10 to 15°. The patient should be as close to the right side of the surgical bed as possible to limit the surgeon’s reach. The bed can be angled in the room so that the foot of the bed goes away from the surgeons, allowing for even more space.
Neurophysiological monitoring of cortical function (somatosensory evoked potentials) with or without brainstem function (brainstem evoked responses) is routinely performed in all cases where dura is exposed or dissection near the carotid or vertebral arteries is performed. In addition, neurophysiological monitoring can identify changes in cerebral blood flow that may occur with blood loss or changes in blood pressure and alert the anesthesiologist to make adjustments. Cranial nerve electromyography is often performed, particularly to monitor cranial nerve (CN) VI during upper third clival resections and CN XII during condyle dissection. If a lateral extension of the approach is planned in the infrapetrous area, CN VII and VIII are monitored, as well as CN IX, X, and XI, when dissection is planned near the jugular foramen.
A nasal decongestant, such as oxymetazoline 0.5%, is applied topically to the nasal mucosa using cottonoids. The skin of the external nose and nasal vestibule, as well as the abdomen (fat graft donor site), is prepped with a povidone antiseptic solution. The patient is given a third- or fourth-generation cephalosporin for perioperative antibiotic prophylaxis.
Initial Approach
The endoscope is introduced at the “12 o’clock” position of the nostril (usually the right) and is used to retract the nasal vestibule superiorly. This elongates the nostril and increases the available space for other instruments. A suction tip is generally introduced at the “6 o’clock” position on the same side. Dissecting instruments are introduced through the left nasal cavity. A suction irrigation sheath or irrigation applied by an assistant or cosurgeon cleans the lens of the scope and preserves visualization without removing the scope for frequent cleaning. If for any reason a bimanual (preferably binarial) approach cannot be performed, then the surgery should be modified to allow bimanual dissection or aborted. Furthermore, we discourage the use of an endoscopic holder for all expanded endonasal approaches.
Widening of the nasal corridor is achieved initially by out-fracturing of the inferior and middle turbinates, followed by removal of the right middle turbinate to provide room for the endoscope. Injection of vasoconstrictors is optional and is performed according to the surgeon’s preference.
For the past 2 years we have been using a nasoseptal vascularized mucosal flap for reconstruction of the skull base defect.63 Because it has to be harvested during the exposure, surgical planning is crucial. For cases in which dura mater will be opened for tumor exposure, the decision is simple, and the flap is elevated immediately without hesitation. Recently, we have experienced less nasal morbidity during the postoperative period for patients who received a vascularized mucosa covering the exposed sphenoid sinus bone. Posterior crusting is converted to anterior crusting, which can be easier to access and treat.64,65
We use unipolar electrocautery with an insulated needle tip to incise the septal soft tissues. Two parallel incisions are performed following the sagittal plane of the septum. One follows the maxillary crest, and a parallel incision follows a line 1 cm below the most superior aspect of the septum to preserve the olfactory epithelium and function. These parallel incisions are joined anteriorly by a vertical incision usually placed just anterior to the anterior head of the inferior turbinate. Posteriorly, the superior incision is extended laterally inferior to the natural sphenoid ostium. The inferior incision extends laterally on the superior margin of the choana. These cuts are critical and must be done properly to preserve the vascular supply via the posterior nasal artery (ies). Elevation of the mucoperichondrium using a Cottle dissector proceeds from anterior to posterior after ascertaining that all incisions have been carried through the periosteum and perichondrium. Elevation of the flap from the anterior face of the sphenoid sinus is completed preserving the vascular pedicle between the sphenoid ostium sinus and choana.
The flap is usually stored in the nasopharynx; however, for exposure of the cervicomedullary junction, we prefer to tuck it in the maxillary sinus after an antrostomy is performed, which facilitates dissection by moving the flap out of the way.
The nasoseptal flap is pedicled on the posteronasoseptal artery. The mucosal pedicle goes from the roof of the choana to the sphenoid ostium. Inferiorly, it can be extended laterally in the floor of the nasal cavity if needed; or more often, it is harvested up to the transition between the septum and the floor.
Rostral Clivus
Clival approaches can be broken down based on division of the clivus into thirds. The rostral third includes the dorsum sellae and posterior clinoids. The middle third consists primarily of the clival recess of the sphenoid sinus and Dorello’s canal. The caudal third is the remainder located inferior to the level of Dorello’s canal at the petroclival junction down to the foramen magnum.
Access to the rostral clivus begins with access to the sella. The dorsum sellae and posterior clinoids help create the boundaries of the rostral clivus. These structures can be accessed via either an intra- or extradural approach.
The intradural approach essentially involves the en bloc elevation of the contents of the sella, providing access to the interpeduncular cisterns and basilar artery through a pituitary transposition.66 This approach is useful for lesions with retrosellar or retroinfundibular extension.
This approach begins with the complete exposure of the sella, planum, and tuberculum sellae and the junction of the sella with the clivus, followed by complete removal of the bone overlying the pituitary gland and inferior and superior intercavernous sinuses. These and the following steps are done essentially to allow safe mobilization of the pituitary gland and prevent its compression against a bony margin. The superior intercavernous sinus (SIS) is cauterized using bipolar cautery and transected. This allows opening of the diaphragma and subsequent stalk mobilization. The entire pituitary gland is exposed and the diaphragma is cut along the midline at its point of attachment to the sella to expose the stalk. The diaphragma is then cut in a paramedian direction to release the stalk circumferentially. Care must be taken not to damage the superior hypophyseal arteries. Next, the lateral connective tissue between the gland and the cavernous sinus is carefully dissected, freeing the gland while carefully preserving vital blood supply via superior hypophyseal branches. The gland can now be safely elevated, exposing the dura overlying the dorsum sellae and posterior clinoid. This dura harbors the posterior intercavernous sinus and is coagulated (bipolar cautery) and dissected, with further hemostasis achieved using fibrillar collagen/cottonoid or morcellized Gelfoam. The posterior clinoid is then drilled using a 1- or 2-mm diamond bit until eggshell thin, dissected free from its dural attachments, and then removed carefully to avoid injury to the carotid and CN III located laterally.
For petroclival lesions without retrosellar or more rostral extension, the dorsum sellae can be removed via a completely extradural approach. This is obviously preferred for extradural lesions and retroclival intradural lesions that do not reach the interpeduncular cistern superiorly. In this approach, the planum is left intact, but the sella and SIS are exposed. The clival recess and upper clivus are drilled with a 3- or 4-mm coarse hybrid diamond bit between the paraclival carotid protuberances. Next, the gland and its overlying dura are elevated together (it is at this point that the removal of bone overlying the SIS is critical, as it allows the free elevation of the gland). Constant awareness of the carotid is critical throughout any and all expanded endonasal approaches. The dorsum sellae and occasionally the posterior clinoids can be reached and dissected, exposing the posterior clival dura that harbors the basilar plexus. Once more, careful hemostasis is necessarily observed using morcellized Gelfoam, fibrillar collagen, and bone wax. Overpacking should be avoided, given the proximity of critical neurovascular structures.
Related posts:
9 Congenital and Developmental Anomalies of the Craniovertebral Junction
11 Atlantoaxial Transarticular Fixation Techniques
16 Transoral Surgery
20 Intraoperative Neurophysiological Monitoring in Surgery for the Craniovertebral Junction
19 Image-guided Surgery in the Craniovertebral Junction and Upper Cervical Spine
22 Techniques of Occipitocervical Fixation
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