Extended Endonasal Approaches to the Craniovertebral Junction
The endoscopic endonasal approach to the ventral craniovertebral junction (CVJ) offers an alternative to the traditional transoral, retropharyngeal upper cervical, and lateral skull base approaches to the CVJ. The endoscopic endonasal approach can be utilized to address pathology along the entire rostrocaudal extent of the clivus and ventral foramen magnum and upper cervical spine ( Fig. 26.1 ).1–7 Pathologies such as degenerative basilar invagination, odontoid fractures, rheumatoid arthritis with pannus or craniocervical settling, metastatic lesions, chordomas, and chondrosarcomas can be addressed with an endonasal endoscopic approach.8,9 The dura can also be opened to address intradural lesions, such as foramen magnum and clival meningiomas, epidermoid tumors, and neurenteric cysts. The endoscopic endonasal approach has several benefits compared with the traditional transoral approach, including improved exposure of the upper clivus without the need for a hard or soft palate incision.3,10–13 Incisions of the soft palate increase the risk of postoperative velopharyngeal insufficiency and can require a tracheostomy. The nasopharyngeal incision, as opposed to the oropharyngeal mucosal incision, permits earlier extubation and is less likely to become contaminated by food, saliva, and oral flora, enabling earlier feeding of the patient after surgery with little risk of infection and postoperative swallowing dysfunction.5,8,9,14–18
The endonasal endoscopic approach to the CVJ has several limitations. The most caudal exposure possible is limited by the extent to which the nasal bones and cartilaginous soft tissue around the nose can be elevated to increase the downward angled view. Another significant limitation is difficulty with obtaining lateral exposure. The lateral exposure with an endonasal approach is limited by the position of the medial pterygoid plate and the eustachian tube ( Fig. 26.2 ). In contrast, with an open transoral approach, the parapharyngeal carotid artery can be easily controlled, and the wide exposure provides improved access to lateral tumor extensions.
Patient Selection
When evaluating a patient for an endonasal approach to a lesion in the CVJ, it is important to determine whether a suitable operative corridor is possible considering the individual patient′s anatomy ( Tables 26.1 and 26.2 ). For lesions in the clivus, such as chordomas or clival meningiomas, the most important limitation is the relationship between the carotid arteries and the tumor.19 Midline and paramedian tumor extension can be addressed utilizing an endonasal, transmaxillary transpterygoid approach directed superiorly to the pterygopalatine fossa, petrous apex, and Meckel′s cave.20 Inferiorly, the transmaxillary approach can provide access to the infratemporal fossa, hypoglossal canal, and medial occipital condyle.2,20–22 Although these paramedian endonasal approaches enable surgical access lateral to the carotid arteries through an endonasal approach, the risks of this maneuver must be identified before surgery and should be considered with other lateral skull base approaches, depending on the lateral extent of the lesion.21 Large lesions in particular (>4 cm diameter) that are both medial and lateral to the lower cranial nerves may be best addressed in a staged fashion, removing the midline tumor endonasally and the lateral tumor transcranially.9,17
The need for exposure along the rostrocaudal axis must also be evaluated. Rostral lesions of the clivus may require extradural elevation of the sellar dura and a sphenoidotomy, and further rostral exposure may require pituitary transposition with extracapsular mobilization of the pituitary gland to access tumor extension in the retrosellar space, interpeduncular cistern, and along the posterior clinoids and dorsum sella.4,21,23–25 Preoperative imaging must also be scrutinized to identify the presence of intradural tumor extension so that harvesting of appropriate grafts for successful closure, such as fat, fascia lata, or a vascularized nasoseptal flap, are performed during the procedure.
In addressing lesions along the caudal aspect of the CVJ, such as upper cervical spine tumors or rheumatoid arthritis with pannus surrounding the odontoid process, endonasal exposure is limited superiorly by the nasal bones and cartilaginous soft tissues of the nose and, inferiorly, the exposure is limited by the hard palate and soft palate.26 The line created by connecting the inferior edge of the nasal bone to the posterior edge of the hard palate on a midsagittal computed tomography (CT) scan represents the most caudal dissection possible with straight endoscopic instruments ( Fig. 26.3 ). The most inferior limit of dissection is generally 0.9 cm above the base of the C2 vertebral body and even lower in patients with basilar invagination.26 Patients with a high positioned odontoid and platybasia often require removal of the anterior margin of the foramen magnum and lower clivus to ex pose the upper part of the odontoid, which can often be performed endonasally without disruption of the anterior rim of the atlas or alar and transverse ligaments because the superior to inferior angle is advantageous.5,16 In contrast, with a transoral exposure, the body of C2 and anterior ring of C1 are exposed first, and it can be difficult to remove the odontoid without disruption of the C1 anterior arch.
Surgical Technique
A surgical team approach that includes an otolaryngologist and a neurosurgeon is used for all extended endoscopic skull base cases. Procedures are routinely performed under general anesthesia; antibiotics, glucocorticoids, and antihistamines are administered. Prior to the operation, a lumbar puncture is performed, and 0.2 mL of 10% fluorescein is injected in 10 mL of cerebrospinal fluid (CSF) to help visualize CSF leaks. A lumbar drain is placed prior to surgery in cases where a large skull base dural defect is anticipated. Topical 4% cocaine is used over the nasal mucosa for vasoconstriction, and then a mixture of lidocaine 1% and epinephrine (1:100,000) is injected. The patient′s head is placed in three-point fixation, slightly elevated and slightly turned to the right, and intraoperative neuronavigation with magnetic resonance imaging (MRI) and/or CT angiography is used for all procedures. The patient′s head is fixed in a slightly flexed position to improve the view to the clivus and the axial cervical spine. The lateral thigh is prepared for autologous fat and fascia lata grafts.
Under 0-degree endoscopic view (Karl Storz, Tuttlingen, Germany), the inferior, middle, and superior turbinates as well as the sphenoid ostia are identified bilaterally. The middle and superior turbinates are most often retracted laterally; however, in some patients the right middle turbinate is removed to optimize the exposure and allow easy passage of instruments and the endoscope.
Since 2008, we have used a nasoseptal flap as part of a multilayered closure of the skull base. The use of a pedicled nasoseptal vascular flap of the nasal septum mucoperiosteum based on the nasoseptal artery has become an important adjunct in the multilayered endoscopic reconstruction of large skull base defects.27 The nasoseptal flap is generally harvested at the beginning of the operation—before tumor removal and before the posterior septectomy is performed. The vascu lar supply of the nasoseptal flap is derived from the posterior septal artery, a terminal branch of the internal maxillary artery.27,28 The flap is raised by placing two parallel incisions in the septal mucosa, one along the nasal floor and the other just inferior to the most superior aspect of the septum. These incisions are joined anteriorly to create the flap and, posteriorly, these incisions are extended over the rostrum of the sphenoid superiorly and to the choana inferiorly. The flap is elevated anterior to posterior and lateral with a dissector and held out of the surgeon′s way during the operation. For CVJ approaches, the flap can be stored in the sphenoid sinus or maxillary sinus so as not to interfere with the surgical corridor.
The endonasal approach to lesions of the clivus can require a sphenoidotomy, depending on the rostral caudal extent of the pathology.19 For lesions of the upper third of the clivus located behind the posterior wall of the sphenoid sinus, a bilateral transsphenoidal opening and removal of the posterior third of the septum is performed. The ostium of the sphenoid sinus is first enlarged to expose the sphenoid sinus, and the posterior third of the nasal septum adjacent to the vomer and maxillary crest is resected with a tissue shaver. At this point, a panoramic view is achieved and bimanual surgery with four separate instruments is possible. A graft of vomer or cartilage can be harvested to reconstruct the floor of the sella at the end of the operation. The anterior wall of the sphenoid sinus is drilled flush with the floor of the sinus. It is important to remove the entire anterior wall of the sphenoid sinus to provide enough room for the endoscope and instruments to sit in the sphenoid during the procedure. All sphenoid septae are removed with a drill, and the mucosa of the sphenoid sinus is completely removed so that a mucocele does not form under the nasoseptal flap. The carotid protuberance, optic protuberance, and medial and lateral opticocarotid recesses are identified. The lateral extent of the sphenoidotomy is verified using intraoperative neuronavigation, ensuring that optimal exposure is obtained in all dimensions before proceeding with progressively deeper exposure.
The lateral margins of the floor of the sphenoid sinus are marked by the course of the vidian nerve, which runs posteriorly along the floor into the vertical segment of the carotid artery. The sella can also be opened to mobilize the pituitary gland laterally or rostrally to allow drilling of the posterior wall of the sella, which forms the upper extent of the clivus and extends up behind the pituitary gland. With this maneuver, the posterior clinoid processes and the dorsum sella can be thinned with a microdrill and removed with a Kerrison rongeur.
The bone of the upper clivus can be opened from carotid to carotid artery with a microdrill, and the venous plexus can be controlled with hemostatic agents. The amount of drilling required will depend on the aeration of the sinus, and the location of the carotid arteries is verified using a Doppler ultrasound probe. The inferior intercavernous sinus is cauterized and transected. The dura is then opened in the midline to expose the basilar tip, superior cerebellar and posterior cerebral arteries, and cranial nerve III. The cranial nerve VI runs at the lateral edge of the exposure as it enters Dorello′s canal.
The approach to the inferior two thirds of the clivus is also combined with a transsphenoidal exposure to facilitate instrument placement. The basipharyngeal fascia and prevertebral musculature are dissected free from the clivus, cauterized, and cut laterally to create a U-shaped flap that is reflected inferiorly ( Fig. 26.4 ). The lateral margins of the nasopharyngeal flap are the vidian nerves superiorly and eustachian tubes, which mark the location of the carotid arteries, laterally. The clivus is drilled back until flush with the dura. Extensive venous bleeding from the basilar plexus can be controlled with careful cautery, hemostatic agents, and gentle pressure. The use of cautery along the dural incision should only be performed after the position of the abducens nerves is determined. Opening the dura, when necessary, will expose the basilar trunk, anteroinferior cerebellar and vertebral arteries, and ventral pons. The dura is opened with an I shaped incision to avoid damaging the near midline abducens nerve. The average distance between the abducens nerves at the dural emergence is 19.8 mm.29
The transodontoid approach is a continuation of the inferior extent of the transclival approach. Often, it is not necessary to perform a sphenoidotomy at this level unless there is a significant degree of basilar invagination, requiring removal of the inferior aspects of the clivus. However, bimanual dissection is achieved by removing the inferior part of the vomer. The approach courses parallel to the hard palate, and an angled scope can be helpful to gain an inferiorly directed view. The mucosal flap is reflected starting at the base of the sphenoid sinus and limited laterally by the eustachian tubes, exposing the lower third of the clivus. The bone at the base of the clivus is removed from medial occipital condyle to medial occipital condyle. Below this, the atlanto occipital membrane, longus capitis, and longus colli muscles as well as the anterior aspects of C1 and C2 are exposed. If necessary, the anterior arch of C1 can be removed to expose the odontoid, which is then removed with a microdrill ( Fig. 26.5 ). Once the central portion of the dens has been removed, the lateral aspects can be mobilized medially with a small curette. The use of intraoperative tomography is useful to ensure complete removal of the dens.15 Retrodental inflammatory pannus can be resected with an ultrasonic aspirator until the dura is visualized. Stereotactic navigation is extremely useful for these maneuvers ( Fig. 26.6 ).
Tumors that have a significant component extending lateral to the paraclival carotid artery and are to be exposed through an endonasal as opposed to a lateral skull base corridor require a transpterygoid approach to identify the vidian nerve and the paraclival carotid at the level of foramen lacerum. This approach requires maxillary antrostomy and posterior maxillary wall resection to gain access to the pterygopalatine fossa. The transmaxillary transpterygoid approach can be used to reach a variety of targets in the paramedian skull base, including the pterygopalatine fossa, infratemporal fossa, petrous apex, and Meckel′s cave.22 The approach begins with an uncinectomy and opening of the maxillary ostium followed by elevation of the posterior maxillary sinus mucosa off the orbital process of the palatine bone and posterior wall of the maxillary sinus. The sphenopalatine artery is identified and ligated at the crista ethmoidalis, and the orbital process of the palatine bone and posteromedial wall of the maxillary sinus are removed to expose the pterygopalatine fossa. The anterior genu between the petrous carotid and the paraclival carotid at the foramen lacerum is then identified by following the vidian nerve in the pterygoid canal and drilling the medial pterygoid plate and sphenoid floor.
The exposure lateral to the carotid artery in the middle third of the clivus can then be extended superiorly to the petrous apex and Meckel′s cave or inferiorly to the parapharyngeal carotid and occipital condyle, jugular foramen, and hypoglossal canal. The eustachian tube is the anatomical landmark to gain control of the parapharyngeal carotid artery during exposure of the occipital condyle, jugular foramen, and hypoglossal canal. Entry into Meckel′s cave requires removal of the pterygoid process along the vidian nerve and identification of the infraorbital nerve up to the foramen rotundum. The so-called “quadrangular space,” which is bounded medially by the vertical carotid artery and laterally by the maxillary nerve, can be drilled open to expose Meckel′s cave.
Large chordomas or meningiomas should be internally decompressed using either two suctions or suction and microscissor. If the tumor is firm, a cavitational ultrasonic surgical aspirator (Valleylab, Boulder, CO), Elliquence monopolar ball or ring cautery (Elliquence, Oceanside, NY), or NICO Myriad tissue resection device (NICO, Indianapolis, IN) can be used to debulk the tumor. Visualization is enhanced with a 30-degree, 30-cm rigid 4-mm endoscope (Karl Storz). In a subset of cases, we have used a three dimensional (3D) endoscope (Visionsense, Orangeburg, NY) for dissection to increase stereoscopic vision.30,31 Once decompressed, the tumor capsule can be mobilized and sharply dissected. Care must be taken to preserve all perforator vessels to the midbrain and pons. Avoidance of coagulation and “pulling” are critical to the preservation of vital neurovascular structures while the remaining capsule is removed. The resection bed is examined with a 45-degree, 18-cm rigid 4-mm endoscope to ensure the absence of any residual tumor. Curved suctions, angled micropituitary rongeurs, and dissectors can be used to reach residual pieces of tumor. Once the tumor and cyst wall are completely removed, the resection bed is examined with a 45 degree, 18 cm rigid 4 mm endoscope to ensure the absence of residual tumor ( Fig. 26.7 ).
Multilayered Closure
The use of intrathecal fluorescein helps to ensure adequate closure at two stages—identifying small CSF leaks and ensuring a watertight closure at the end of the case.32 First, any dead space left behind after the tumor resection is filled with an autologous fat graft. Then a “gasket seal” closure is performed, consisting of a fascia lata onlay placed over a rigid buttress of either vomer or Medpor (Porex Corporation, Newnan, GA) that is cut to the exact size of the defect and countersunk into the skull base defect.33 Small areas of defect between the vomeric bone or the Medpor plate are filled in with an autologous fat graft. Finally, a vascularized nasoseptal flap is placed directly over the gasket seal construct followed by a final layer of DuraSeal (Covidien, Norwalk, CT) to hold everything in place and ensure a watertight closure.27,28 No DuraSeal is placed between the flap and the gasket seal construct because it would prevent fibrosis and vascularization of the skull base. It is critical to assure that the nasoseptal flap is positioned over the entire defect, that it is not doubled over on itself, and that the mucosal surface is facing the nasal cavity. If fascia lata is used, it is critical that the nasoseptal flap lies beyond the edges of the fascia lata so that the flap is in contact with the sphenoid bone to fully cover the defect. For odontoid resection, merely replacing the fascial flap is adequate.
The nasal cavity is then filled with FloSeal (Baxter, Deerfield, IL) for hemostasis. A small piece of Telfa dressing (Covidien) is placed in each nostril overnight to absorb drainage and is removed in 1 to 2 days. If a lumbar drain is placed, it is typically drained at ~5 mL/hour for 1 to 2 days and then clamped and removed in the evening. The patient lies flat after its removal and sleeps to decrease the risk of spinal headache. Patients are placed on low doses of heparin to prevent deep venous thrombosis.
Related posts:
Posterior Neuroendoscopic Applications at the Craniovertebral Junction
Approaches to the Jugular Foramen
Transoral–Translabiomandibular Approach to the Craniovertebral Junction
Transfacial Approaches to the Craniovertebral Junction
Bypass Options for the Posterior Fossa
Suboccipital and Retrosigmoid Approaches
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