36 Endoscopic Assisted Approach to the Cerebellopontine Angle
Abstract
Although used less frequently than in ventral skull base, the endoscope can be a powerful tool for skull base surgery in certain regions of the lateral skull base such as the cerebellopontine angle (CPA). This chapter outlines the main advantges and disadvantages of endoscopic assisted surgery in the CPA and gives an overview of the main indications and uses of this technique.
cerebellopontine angle, endoscopic assisted surgery, lateral skull base
36.1 History
In 1917, Doyen was the first surgeon to describe the use of endoscopy in the cerebellopontine angle (CPA) for a trigeminal neurectomy. However, the technology of the lenses and light source allied to the development of the microscope significantly limited the endoscope’s utility. Therefore, just in 1974 we had new publications, when Prost used endoscopy for diagnostic purposes in the CPA.1 Fukushima, in 1978, used the endoscope to explore the CPA from a small burr hole affirming that this kind of procedure could be useful for small tumors, vascular abnormalities, and neurovascular compression syndromes. In 1979, Oppel and Mulch published results of a study of selective trigeminal root section via an endoscopic transpyramidal retrolabyrinthine approach2; finally in 1993, O’Donoghue and O’Flynn divided the endoscopic anatomy of the CPA into four levels.3 Since then, the use of the endoscope alone or as an adjunct to the microscope for CPA pathologies has been increasing, with a growing number of publications.
36.2 CPA Anatomy
The CPA is the angular space located between the superior and inferior limbs of the cerebellopontine fissure formed by the petrosal cerebellar surface folding around the pons and the middle cerebellar peduncle. The cerebellopontine fissure opens medially and has superior and inferior limbs that meet at a lateral apex. Vessels of the vertebrobasilar system and cranial nerves (CNs IV to XII) are located near or within this angular space and could be accessed by some operative approaches utilizing standard microscopy view.
The majority of the authors, as Rhoton, rather divide the region of the CPA into three neurovascular compartments: upper, middle, and lower. By this division, each complex is related to a major artery that crosses it. Therefore, the upper complex is related to the superior cerebellar artery (SCA), the middle complex related to the anterior inferior cerebellar artery (AICA), and the lower complex related to the posterior inferior cerebellar artery (PICA) (Fig. 36.1).
Fig. 36.1 (a, b) Overview of the upper complex of the cerebellopontine angle (CPA). (a) The posterior trigeminal roots join the brainstem about halfway between the lower and upper borders of the pons. A small lip of cerebellum may obscure this joint (here, reflected posteriorly with a small dissector). At this point, the sensorial (lateral) and the motor (medial) rootlets are separated. Note that the view of the anterior surface of the cisternal part of the Vth nerve is blocked by the suprameatal tubercle. (b) The endocope was advanced over the trigeminal nerve exposing medially the superior cerebellar artery (SCA) and the trochlear nerve bypassing the mesencephalon until it reached the tentorial edge. (c, d) The middle complex. (c) The VII–VIII complex is the center of the figure. Above and medially, the Vth nerve and the superior petrosal vein. The IX nerve is below the complex and separated from it by the flocculus. The anterior inferior cerebellar artery (AICA) has an intrinsic relationship with the vestibular-facial complex, sometimes crossing between them. (d) The endoscope was inserted below the VII–VIII complex. The entire journey of the VIth nerve is well visualized, from its exit from the medial portion of the pontomedullary sulcus, until it enters at the dura in the upper part. The proximal part of the AICA has an intimate relationship with the proximal part of the VIth nerve. (e, f) The lower complex. (e) General view of the IX, X, XI, and XII nerves. The first three nerves arise posterior to the olive and go toward the jugular tubercle; XII exits anterior to the olive and goes toward the hypoglossal canal. (f) The endoscope was inserted near the rootlets of the XIth nerve to give a closer look at the exit of the rootlets of the XIIth nerve and their relation with the vertebral artery and posterior inferior cerebellar artery (PICA).
36.3 Advantages/Disadvantages
The use of the endoscope has revolutionized and is already well established in anterior or ventral skull base surgery through either standard transsphenoidal surgery or expanded endonasal approaches to the planum sphenoidale, tuberculum sellae, and clivus. However, use of the endoscope has not yet had a similarly transformative effect on lateral skull base surgery, such as for the CPA surgeries.4
Endoscopy offers several distinct advantages over the operating microscope that make it a potential adjunctive tool during microsurgery at the cranial base. The brighter illumination and high magnification provide excellent definition of small blood vessels, CNs, and neural structures, sometimes better than a microscope. Besides that, the use of endoscope allows “looking around corners” and behind obstructions, providing improved lesion removal while maximizing preservation of normal structures. On the other hand, the use of endoscope in CPA surgeries carries many challenges and disadvantages. First of all, as in all endoscopic techniques, there is a steep learning curve to acquire endoscopic dexterity and three-dimensional orientation; the delicacy of surrounding structures and the lack of a natural access corridor make it even harder. Furthermore, bimanual operating is frequently impossible because the endoscope must be held in one hand while the other hand is used to perform surgery; this is not always the case, however, because it can be attached to a fixation device or alternatively held by the assistant. Other problems associated with this technique include the blood soiling the endoscope making the visualization difficult, the lack of readily available instrumentation, and the poor overview of the operative field, which can be seriously damaging once it does not allow one to look backward or sideways to prevent injury to structures next to the shaft of the endoscope. Moreover, the surgeon must be alert of potential thermal injury to structures caused by the heat generated by the light source.3 , 4 , 5 , 6 , 7
36.4 Indications
The endoscope has been used in CPA surgery in two primary ways: (1) endoscopic controlled, when the endoscope is the only source of visualization and instruments are passed alongside the endoscope; and (2) endoscopic assisted, when the endoscope is used as a supplementary visualization tool to assist in primarily microscopic procedures.8
When used as the only source of visualization, the endoscope can be used for microvascular decompression (MVD) of CNs (from IV to XII) or even for small extra-axial tumors of the CPA (epidermoid tumors, arachnoid cysts, small acoustic schwannomas, and others). The main indications for an endoscopic assisted surgery are: (1) to expand the visualization to microscope blind spots and consequently visualize residual lesions (e.g., to examine the fundus of internal acoustic canal); (2) to extend the surgical field (e.g., from the CPA, the endoscope allows an extension toward the Meckel’s cave and the middle fossa, enabling tumor removal from these areas); (3) to carefully inspect the adjacent structures such as bone and dura mater, aiming to identify invasions not seen in the microscope; and (4) to give a reliable prediction of the extent of resection.
36.5 Vestibular Schwannoma
In 1993, McKennan9 was the first to describe the use of an endoscope to inspect the internal auditory canal (IAC) fundus after the removal of vestibular schwannomas. Since then, with the advance of the lens and materials, the endoscope has become an important adjunct to the operative microscope for this kind of tumor, aiming to achieve maximal resection with hearing preservation and maintenance of facial nerve function.
The transmeatal approach is an essential step in retrosigmoid surgery of vestibular schwannomas with intracanalicular extensions.10 This technique consists of removing the posterior lip of the IAC to expose the intracanalicular portion of the tumor as well the CNs VII and VIII. At the same time, its opening is limited by the labyrinth block, located just posterior to the fundus of the IAC. Injury to this block may result in significant postoperative hearing loss. Domb and Chole11 demonstrated in their laboratory dissections that it is impossible to expose the whole IAC without entering the labyrinth. Therefore, to avoid the risk of injury to the labyrinth, 2 to 3 mm of bone from the IAC should be left intact at the fundus. However, leaving this portion of bone results in hiding the lateral recess of the IAC and any piece of tumor that might be there.
In the past, to prevent labyrinth injury, vestibular schwannomas with a far lateral extension were often blindly rolled off the facial and cochlear nerves or even dissected with the aid of otologic mirrors. These techniques increased the risks of injuring the nerves and leaving residual tumor. The use of an endoscope added better visualization of this part of the IAC allowing safe dissection and maximal resection.
Another important applicability to the endoscope in the vestibular schwannomas surgery is to identify violated air cells within the IAC, which may be difficult to be viewed from microscope. This has an important value once these open air cells may be related to cerebrospinal fluid (CSF) rhinorrhea. Therefore, adequately sealing of those cells with bone wax reduces the CSF leakage rate.12
36.5.1 Vestibular Schwannoma Endoscopic Assisted Approach Technique
After placing the patient in a lateral position and monitoring the CNs, especially the VII, a retrosigmoid suboccipital craniotomy is made exposing the sinuses, transverse, sigmoid, and their junction. The dura is opened in a C-shaped fashion parallel to the medial border of sigmoid sinus and inferior border of transverse sinus. In sequence, the cerebellomedullary cistern is opened to drain CSF, leading to relaxation of the cerebellum. Often retractors are not necessary in the cerebellum for vestibular schwannoma surgeries. Usually dynamic retraction with the aspirator is enough to expose the tumor and the contents of the CPA. The next step is the openning of the IAC to expose intracanalicular portion of the tumor. The dura over the posterior wall of the canal is coagulated and opened in C shape with the base toward to the porus. A high-speed diamond burr is used to drill the exposed bone, under continuous irrigation, until the visualization of the dura mater of the canal. The length of bone drilled depends on the patient’s hearing status, the tumor’s extension into the IAC, and the individual relation between IAC and inner ear structures evaluated preoperatively on thin-slice CT scans.13 In large volume tumors, the opening of the IAC should be preceded by initial debulking of the tumor to create enough space for visualization and approach to the posterior wall of IAC.
Thereafter, the cisternal portion of the tumor is assessed with nerve stimulator looking for the facial nerve fibers that could, eventually, be displaced posterior to the tumor. If nothing is found, the tumor capsule is opened, followed by intratumoral debulking and subcapsular dissection, preserving the arachnoidal plane.
After tumor removal was completely achieved by microscopy, the endoscope is used. Neurosurgical cotton should be placed in the operative field, especially in the exposed cerebellum area, avoiding lesions to it. Moreover, copious irrigation should be done before inserting the endoscope to improve the visualization. We use a 30 degrees lens endoscope to inspect the whole surgical field and the nondrilled portion of the IAC, looking for residual tumors. The endoscope tip is moved at different proximities to achieve the sharpest images of the fundus contents. If residual tumor is found in the intrameatal portion, it may be removed by blunt dissection and careful aspiration under direct view of an endoscope. Besides that, the endoscope is used to evaluate the presence of air cells opened during the drilling process, which, once identified, should be closed with bone wax. The drilled posterior lip of IAC should be sealed with a fascia flap harvested from the wound margin and the dura mater is closed in a watertight fashion with running suture (Fig. 36.2).
