Transoral Approaches to Midline Pathology of the Ventral Skull Base, Craniovertebral Junction, and Upper Cervical Spine
The maxim “ventral pathology should be approached ventrally” is usually adhered to by most spinal surgeons throughout the spine, but is frequently forgotten when considering the craniovertebral junction (CVJ). Here, differing opinions are put forward, sometimes with little evidence to support them. There is no doubt that all surgical approaches to this region are technically demanding, require much practice and skill, and are best performed within a suitably specialized multidisciplinary framework. There are particular situations where a lateral approach would be ideal, and equally it is our opinion that there are specific occasions when a direct midline anterior approach through the mouth offers considerable benefits for extradural midline pathologies. The family of transoral procedures should be added to the surgical armamentarium of those who operate at this junctional level.
History
Surgery in the mouth—including dental treatment, nasal passages, and sinuses—has been performed for centuries without significant mortality of infection. Surgery through the nose for pituitary tumors was used by Cushing, but only came into widespread use with the development by Jules Hardy, of Montreal, of dedicated retraction instruments, forceps, and rongeurs and is now considered a standard neurosurgical procedure.1 Fear of infection is still one of the reasons given for avoiding transoral surgery,2 but use of specific retraction instrumentation (Crockard instruments; Codman, Raynham, MA) and attention to good technique will minimize this hazard.3
The first transoral removal of a bullet lodged in the clivus was described by Kanaval in 1917.4 It was not until the advent of computed tomography (CT) and magnetic resonance imaging (MRI) in the 1980s that the anatomy of the CVJ was better understood in regular clinical practice, allowing the development of appropriate retractors and, more importantly, the results of surgery could be seen by all and not just the operating surgeon.
In this chapter we lay out the indications and contraindications for ventral approaches as well as describe the variations on the standard transoral approach that we have developed in almost 600 cases over a 30-year period.
Transoral Surgery
The standard or “simple” transoral approach is ideal for mid-line extradural pathology between the anterior rim of the foramen magnum and C2 vertebra, in particular congenital or acquired bony abnormalities without basilar invagination, infections, and extradural tumors. It is contraindicated for completely intradural tumors, intradural vascular abnormalities, and lateral extradural tumors.
In the 1980s, for improved superior exposure, division of the hard palate or LeFort I osteotomy was tried,5 but these did not improve visibility sufficiently to warrant the increased risk of complications, particularly nasal regurgitation and midface problems. However, if the LeFort I maxillotomy was combined with a midsagittal division of the hard palate, it was found that the resultant “halves” could be swung laterally, thereby providing a much better field of view.6
Superior extension by this so-called “open-door maxillotomy” allows surgery of the upper clivus to the sphenoid sinus and pituitary regions, whereas inferior extension by mandibulotomy and glossotomy permits visualization down to the C4 vertebra.
Table 22.1 shows the numbers of these operations performed over 30 years in our institute.
Further approaches have been developed via the midface degloving transmaxillary approach7 and the transnasal endoscopic approach.2 Over time, the choice of approach has often been influenced by changes in the pathologies that have presented to surgeons. For example, 25 years ago we were surrounded by patients with rheumatoid disease and myelopathy from extensive translocation, but now we only see one or two a year in our unit.8 In contrast, the referral rate for tumors of the CVJ has encouraged us to develop and modify our approach.3 Figure 22.1 shows the change in number of operations performed for different pathologies over the past three decades in our unit. The number of transoral operations for chordomas has remained fairly constant, whereas there has been a steady decline for many other pathologies, particularly rheumatoid disease. For all these complex procedures, a team approach in specialized units is essential.
Operation | Number |
Standard transoral | 317 |
Transoral with palate split | 103 |
Open-door maxillotomy | 58 |
Mandibulotomy | 8 |
Standard Transoral Surgery
Transoral surgery may be performed with or without division of the soft palate. The approach is suitable for pathology in the midline, between the foramen magnum and the C2-C3 disk space. Division of the soft palate improves superior exposure of the lower third of the clivus. For all transoral procedures, the key to orientation is to identify the midline: from the rostrum of the sphenoid, the anterior tubercle of the lower clivus and the tubercle of the C1 anterior arch, to the midline of C2, which also may have a midline crest or tubercle. If in doubt, lateral fluoroscopy or frameless stereo tactic equipment may be used, but is often unnecessary.
Viewed from the anterior, the vertebral arteries circumscribe a hexagonal shape: 9 to 11 mm from the midline they penetrate the dura, 22 to 23 mm from the midline at the C1 foramina transversaria, and 9 to 10 mm at the body of C2. However, within the upper C2 vertebral body, the artery may loop medially before exiting the bone and passing superiorly to the C1 vertebra, and care should therefore be taken not to catch the vertebral artery when drilling in the C2 body. Careful preoperative evaluation with CT scan is important.
Indications for Transoral Surgery
In the past, the procedure was used extensively for odontoidectomy and removal of pannus in late-stage rheumatoid disease and neurological symptoms. However this is now much less common largely due to improvements in the medical management of the disease. The operation is now reserved for patients with acutely deteriorating neurology from bone or degenerative soft tissue compression of the cervical cord and in inflammatory or degenerative arthritides.
Predominantly midline extradural tumors of the clivus and C1-C2 region, usually chordomas or primary neoplasms.
Diagnostic biopsy for midline C1-C2 infections.
Although some pathologies such as chordomas may be followed intradurally, we do not recommend this approach for tumors that are fundamentally intradural (but often a far-lateral procedure should be performed in preference9).
Contraindications for Transoral Surgery
Oral infections.
Limited mouth opening (less than 25 mm between the upper and lower teeth in the midline). In the presence of limited opening, extension of the transoral approach can be performed with an “open-door” maxillotomy, or mandibulotomy.
If there is a fixed flexion deformity of the cervicothoracic spine, surgical access can be limited by the close proximity of the manubrium and sternum.
Tumors extending laterally more than 15 mm from the midline, unless supplementary surgery is planned by an additional lateral approach.
Preoperative Assessment and Anesthesiology
Flexion-extension X-rays are useful for assessing the degree of normal and abnormal movement at the craniovertebral junction
CT scans and CT angiography are useful for assessing vertebral anatomy and position of the vertebral artery, particularly when involved or displaced by tumor, or in cases of congenital variations in bone anatomy.
MRIs define the relationship of the pathology and vertebral arteries to the brainstem and cervical spinal cord.
Digital subtractive angiography should be considered for tumors and embolization performed if the feeding vessels are accessible or acquired anomaly.
Bacteriological swabs of the nasal and oropharyngeal cavity should be taken. If methicillin-resistant Staphylococcus aureus is detected, then preoperative eradication therapy should be given. If another unusual commensal organism is identified, then an appropriate antibiotic may be prescribed later if a postoperative infection develops subsequently. Usually antibiotics (cefuroxime and metronidazole) are administered with induction of anesthesia and continued during the first 24 hours from the start of surgery. Long-term antibiotics should be avoided unless there is a clinically significant infection, as they may encourage a postoperative infection with resistant bacteria or fungi.
Somatosensory evoked potentials (SSEPs) and motor evoked potentials (MEPs) should be recorded preand perioperatively, providing real-time information during the operation to alert the surgeon if certain maneuvers are compromising cord function.10
Fiberoptic nasotracheal intubation is performed in the awake patient or tracheostomy if there is significant preexisting neurological or bulbar dysfunction.11
A nasogastric tube is used for perioperative gastric emptying and postoperative feeding, which is initiated 4 hours postoperatively.
Pulmonary function (vital capacity, arterial blood gases, and oxygen saturation) is assessed preoperatively and followed intraoperatively. A vital capacity of less than 1.2 L is associated with a greater risk of postoperative complications.
When there is a high chance of intraoperative cerebrospinal fluid (CSF) leakage, then a lumbar drain may be inserted after the induction of anesthesia and prior to surgery. If the dura is not breached during the operation, then the drain should be removed at the completion of surgery.
Surgical Procedure
Positioning
The patient is positioned supine, with the head fixed in a Mayfield headrest, extended at the CVJ. This extension allows better access to the lower clivus, particularly in the case of limited mouth opening ( Fig. 22.2 ). Tilting the table laterally may allow the surgeon to have a better view within the mouth, while keeping the orientation of the patient in a neutral midline sagittal plane. Head rotation by itself on the fully supine body is not advised because this may cause distortion of the anatomy and may also rotate the vertebral arteries into the operative field.
The surgeon stands on the right side of the patient if right-handed, and the opposite side if left-handed, with the anesthetic team at the feet of the patient and the scrub nurse at the head of the patient ( Fig. 22.3 ).
Equipment
Use of a dedicated transoral system (Crockard Transoral Instruments, Codman & Shurtleff, Raynham, MA) makes access easier. Newer retractor systems developed for the minimally invasive market have also been used with some success. The traditional equipment includes an oral retractor/tongue depressor, attachable retractors for the soft palate and tubes, a long monopolar diathermy cutting blade, and appropriately long bayoneted instruments. (Additional components are available for performing extended transoral surgery.) Additional necessary equipment includes an operating microscope, a high speed air drill with different burrs, and a knot pushing device for use during closure of the prevertebral muscles and pharyngeal mucosa.
Procedure
The oropharyngeal cavity is cleaned with an aqueous solution of 0.5% chlorhexidine, although many otolaryngologists do not use preoperative sterilizing solution prior to routine nose and throat surgery, and preoperative cleaning perhaps does not influence the usually low rate of infection. The mouth, tongue, and lips are coated with topical 1% hydrocortisone ointment to minimize postoperative inflammation. The dedicated transoral retractor system is inserted to depress the tongue and open the mouth, while retracting also the soft palate superiorly and the endotracheal and nasogastric tubes laterally. A small bolster, such as a rolled-up towel, is placed between the handle of the retractor and the sternum to allow further exposure below the arch of C1. One percent lidocaine with 1:200,000 adrenaline is injected submucosally at the back of the pharynx, prior to making a vertical incision through the mucosa. Box incisions in the mucosa may also be performed, but a linear incision is easier to close and provides good lateral exposure with the correct use of dedicated retractor systems. The mucosa is retracted with a pharyngeal self-retaining retractor, exposing the deeper muscle layer. These longus colli and longus capitis muscles are elevated from the underlying foramen magnum, C1, and C2 with a Howarth elevator and monopolar diathermy to expose the bone. To remove the odontoid process, the tough apical and alar ligaments must be divided by sharp dissection or diathermy after the drilling has been completed, with care to avoid creating a CSF leak.
For odontoid resection in rheumatoid and degenerative conditions ( Fig. 22.4 ), the peg is thinned and disconnected from the C2 body using a high-speed drill, and after division of the apical and alar ligaments, the remaining cortex of peg is removed using the odontoid grasping forceps of the transoral instrument set. In tumors, pituitary rongeurs and curettes may be used together with suction to remove the tumor, or an ultrasonic aspirator can be used if the tumor is firm.
Closure is performed in two layers: The longitudinal prevertebral muscles are closed with interrupted 2–0 Vicryl (Ethicon, Somerville, NJ) or similar absorbable sutures, and the mucosa is closed as a separate layer using inverted 3–0 Vicryl. If there is a small dural defect, it can be closed with DuraGen artificial dura (Integra, LifeSciences Corp., Plainsboro, NJ) and Tisseel fibrin glue (Baxter, Deerfield, IL). Larger dural defects may require fascia lata and fat grafts or flaps of nasopharyngeal mucosa.
Related posts:
Posterior Neuroendoscopic Applications at the Craniovertebral Junction
Stereotactic Methods of Localization and Image-Guided Surgery
Approaches to the Jugular Foramen
Transfacial Approaches to the Craniovertebral Junction
Bypass Options for the Posterior Fossa
Suboccipital and Retrosigmoid Approaches
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