Posterior Neuroendoscopic Applications at the Craniovertebral Junction

10.1055/b-0034-84456

Posterior Neuroendoscopic Applications at the Craniovertebral Junction

Andrew S. Little, Pankaj A. Gore, Charles Teo, and Peter Nakaji

Neuroendoscopy has been applied to diverse pathologies at the craniovertebral junction (CVJ). The endoscope provides a view that is complementary and in some ways superior to that of a microscope. It provides a high-magnification, panoramic view of the operative field and the ability to look around neurovascular structures that obstruct the view through the microscope. Endoscopy can be applied to anterior approaches (e.g., transnasal and transoral), midline posterior approaches (e.g., suboccipital), and posterolateral approaches (e.g., far-lateral and retrosigmoid) either as the sole source of visualization or as an adjunct to standard microsurgical techniques. In this chapter we consider the CVJ as the region from the pineal gland caudally to the second cervical vertebrae. We discuss the basics of neuroendoscopy, review the instrumentation and setup, describe the advantages and limitations of endoscopy compared with standard microscopy, and present its application to lesions near the CVJ approached from posterior and posterolateral corridors. Anterior approaches to the skull base are addressed in a separate chapter.

Neuroendoscopy Basics and Terminology

The endoscope can be used at the CVJ in three different manners. The most common application is the use of the endoscope to assist microsurgical procedures as a supplemental method of visualization in the same operative corridor. This is called endoscopic-assisted neurosurgery.1 Using the endoscope in this way permits inspection of resection cavities to assess for tumor remnants or to identify compressive vessels in patients with trigeminal neuralgia.2 The second type of application of the endoscope at the CVJ is termed endoscopic neurosurgery. In endoscopic neurosurgery, the endoscope provides the only source of visualization, and the instruments are passed through the endoscope′s working ports. This technique is used to fenestrate arachnoid cysts and for third ventriculostomy. When the endoscope is used as the only source of visualization and the instruments are passed alongside the endoscope, the technique is called endoscopic-controlled. Endoscopic Chiari decompression and endonasal endoscopic surgery are performed in this manner.

A system for endoscopic-assisted work at the CVJ consists of a 0-degree and 30-degree rigid endoscope, a fiberoptic light source, a video camera, a digital recorder with video and still image–capture functionalities, and a video monitor. The 0-degree endoscope provides excellent in-line visualization and a panoramic view at high magnification. The 30-degree endoscope supplements the 0-degree scope by allowing the surgeon to look around obstructing neurovascular structures that might otherwise be hidden from the microscopic view. For example, a 30-degree endoscope is used to look at the shoulder and axilla of the trigeminal nerve to inspect for compressive vessels, to assess for tumor remnants, or to identify neurovascular structures before tumor removal. Endoscopes of 70 degrees are available, but we avoid their use because they provide limited straight-ahead visualization, which places neurovascular structures at risk during insertion and manipulation of the endoscope. We prefer to use pistol-grip style endoscopes because they are well balanced, the surgeon′s hand is removed from the operative field, and their image resolution is excellent. The digital recorder, light source, and monitor should be housed in a mobile tower that can be repositioned in the theater depending on the surgeon′s preference. The use of additional monitors, often attached to the ceiling on mobile booms, further improves the surgeon′s ability to maintain ergonomics. Standard microsurgical instruments can be employed along with the endoscope. A variety of angled dissecting instruments allows the surgeon maximum use of the view provided by the endoscope. Every attempt should be made to use usual microsurgical technique, including sharp bimanual dissection, while using the endoscope. Doing so may require the adjunctive use of a holding arm or a good assistant to free the surgeon′s hands.

Anatomical Considerations for Neuroendoscopy at the Craniovertebral Junction

The posterior fossa presents the surgeon with a variety of challenges because it is a relatively tight space, the operative targets are often deeply located, and the cranial nerves provide both tethers and barriers to dissection that must be manipulated with caution to prevent functional deficits. However, the anatomy of the posterior fossa also presents opportunities. A relatively small opening in the retrosigmoid or far-lateral region provides extensive access up and down the cerebellopontine region. Drainage of spinal fluid often creates adequate room. Furthermore, the pathology itself may create space, such as in the case of a large acoustic neuroma or epidermoid tumor. Space must be developed to pass the endoscope safely into the posterior fossa, and care must be taken not to allow the endoscope itself to compress the brain.

Avoidance of Complications in Neuroendoscopy

There are several important caveats to using endoscopy near the CVJ. Because of the tight working space with numerous critical neurovascular structures, familiarization with basic endoscopy technique is essential. Surgeons wishing to develop endoscopy skills are encouraged to practice in a laboratory setting because of the steep learning curve. Practice with a cadaver allows the surgeon to become familiar with the endoscopic equipment and to learn about the pitfalls and technical challenges of working with new instrumentation.

Most complications occur because of deviation from two principles. The greatest risk of endoscopy is the operator becoming disoriented. Because of endoscope construction, the camera can rotate separately from the endoscope. The operator must maintain orientation of the camera regardless of the orientation of the scope. Orientation of the image can be easily verified by viewing an object with writing on it and adjusting the camera as necessary. The second principle is that structures outside the field of view are at risk for injury by the endoscope′s shaft and instruments. As the endoscope is passed into the operative field, the neural and vascular structures that are within the visual field are easily avoided. However, after the endoscope passes these structures, they are no longer visible and lateral movements of the endoscope present risks to the cranial nerves. When instruments are inserted, they should be placed in a parallel trajectory along with the endoscope, trailing slightly to enable visualization of their tip. If the endoscopic view needs to be changed, the scope should be removed by withdrawing it straight backward. Its orientation is then confirmed, redirected, and reintroduced in a straight line toward the target. One way to limit complications resulting from the shaft of the endoscope is to have the assistant visualize the endoscope and adjacent critical structures through the operative microscope as the endoscope is being used.

Applications of Neuroendoscopy

Tumors

Acoustic Neuromas and Epidermoid Tumors

Neuroendoscopy is a powerful tool for tumor surgery near the CVJ because of its abilities to look around corners and to provide a panoramic operative view. Acoustic neuroma surgery is a good illustration of the application of neuroendoscopy to tumor surgery ( Fig. 25.1 ). We have found endoscopy useful for the early identification of the facial nerve before tumor removal and for detecting residual tumor in the internal auditory canal (IAC). Drilling of the internal acoustic meatus can open temporal bone air cells. If the air cells are not sealed adequately, patients may develop spinal fluid rhinorrhea. One disadvantage of microsurgery is that some air cell violations may not be detected because they are hidden from the line-of-sight. Valtonen and colleagues demonstrated that angled endoscopes provided superior visualization of the IAC and permitted better closures.3 Postoperative rhinorrhea declined to 0 of 24 patients with endoscopy compared with 7 of 38 patients without endoscopy. Endoscopy combined with neuronavigation also may allow a more complete and accurate drilling of the IAC. Pillai and colleagues performed a cadaveric study using the retrosigmoid exposure coupling these two techniques and were able to drill the posterior IAC completely without violating the bony labyrinth.4

Neuroendoscopy also may improve the extent of tumor resection. One common site of residual tumor is within the fundus of the IAC. Wackym and colleagues reviewed their experience with 68 patients treated primarily through a retrosigmoid approach.5 Their impression was that neuroendoscopy detected residual tumor in the IAC in 11 cases that were not detected with light microscopy. They also noted improved detection of open air cells.

Epidermoids are also good targets for endoscopy because they are soft and suctionable tumors that insinuate themselves in hard-to-visualize spaces. Angled endoscopy is useful for detecting tumor remnants and cystic contents outside the field of view. Because of their suctionable nature, the remnants can be removed safely with angled suction using a one-handed technique.

In our experience, the application of endoscopy to tumor surgery involves several key considerations. A thorough understanding of the limitations of endoscopy allows the surgeon to maximize its advantages safely. For example, although it may improve visualization, endoscopy does not abrogate the need for thorough drilling of the IAC in acoustic neuroma surgery. Without sufficient bony exposure, tumor in the fundus is not accessible even with angled instrumentation. Furthermore, direct tumor suctioning instead of careful microdissection can inadvertently traumatize the facial nerve by aspirating it into the suction device. In endoscopic-assisted work, the surgeon often holds the endoscope in one hand and a suction device or dissector in the other. Therefore, the surgeon is functionally one-handed, making it difficult to complete fine microdissection of tumor from the facial nerve. When used this way, the primary use of the endoscope is to identify tumor remnants, and then using standard microscopy with a two-handed technique, to remove the tumor. Where possible, a two-handed technique with the endoscope is preferred if the microscopic view is inadequate.

For any endoscope-assisted work in the posterior fossa, standard endoscope-handling principles should be followed to reduce risk maximally. The endoscope should be directed in and out in straight lines. When the direction of an angled endoscope is altered, it should be removed from the head, re-angled, and re-directed. Back-and-forth swinging movements within the operative field should be avoided because of the risk of injury to neurovascular structures. The shaft can conflict with critical anatomy any time the endoscope is beyond a nerve or vessel and the structure is therefore out of view. The cranial nerves (CNs) at risk primarily include CN V superiorly, the CN VII–VIII complex centrally, and the CN IX–XI complex inferiorly.