40.1 Introduction

Since the late 1800s, skull base surgeons have been pioneering the use of endoscopes in order to enhance visualization and reduce operative morbidity. The first use of the endoscope in the cerebellopontine angle (CPA) was in 1917 by Doyen in London.s. Literatur Early neuroendoscopy was limited to the ventricular system, treating hydrocephalus and small tumors.s. Literatur Early endoscopes had a limited depth of field, poor optic quality, and low illumination from flames or small electric bulbs.s. Literatur A great advantage of the modern endoscope is the wide field of view—4-mm straight and angled endoscopes allow the surgeon to see beyond or around obstructing anatomical structures, thus limiting the degree of surgical exposure required. This is particularly useful in posterior fossa surgery by improving visualization around the cerebellum without the use of retractors or visualizing the lateral internal auditory canal (IAC), for example. The xenon light source is now standard within the industry, with peak wavelengths in the 800- to 1,000-nm range, improving visualization by increasing light output while minimizing heat production compared to a halogen light source.s. Literatur

Endoscopic sinus and anterior skull base surgery began as a result of the work of Messerklinger and Stammberger in Europe in the 1970s and then Kennedy in the United States, who described “functional endoscopic sinus surgery” in 1985.s. Literatur ^,​ s. Literatur Many of the advancements seen in surgical technique and instrumentation in sinus and anterior skull base surgery were subsequently adopted by lateral skull base surgeons. Initially used as an adjunct to the operating microscope, it was not until 2001 that fully endoscopic approaches to the CPA were reported.s. Literatur ^,​ s. Literatur Although the majority of the publications in the current literature regarding the endoscopic approach to the CPA pertain to microvascular decompression, several groups have published cases of arachnoid cyst, epidermoid, vestibular schwannoma (VS), and meningioma resection.s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur The endoscope is particularly helpful for those tumors or cysts that cross the midline, or extend above the incisura or below the foramen magnum.s. Literatur Marchioni et al have also recently described a fully endoscopic transcanal transpromontorial approach for resection of VSs that are limited to the IAC (refer to Chapter 39).s. Literatur

40.2 Advantages and Limitations

The endoscopic approach to the lateral skull base offers the obvious possible advantages of a smaller incision and smaller craniotomy, in addition to less soft-tissue dissection and cerebellar retraction. These factors may contribute to a shorter hospital stay and quicker recovery.s. Literatur ^,​ s. Literatur

Illumination with the endoscope can be problematic. The light is not dispersed as it is with the microscope, creating a focused intensity with the possibility of overheating critical structures within the CPA. This issue has been addressed with the design of a water-cooled sleeve to line the outside of the endoscope. The use of a foot pedal allows this device to periodically cool the scope and clean the distal end (Endoscrub).s. Literatur Furthermore, when a fixed endoscope holder is not used, the endoscope is frequently moved around the field, thereby limiting focal or prolonged heating of important structures.

Surgical instrumentation for neuroendoscopy has evolved to overcome early limitations, such as a limited trajectory for opposable grip instruments. Straight, “pistol-grip” retractable or guarded blades, instruments with 360-degree rotational capabilities, dual functioning suction curettes, and stimulating dissectors are now available. A great technical obstacle is the need for bimanual surgical technique and scope stabilization. A rigid endoscope holder is often used for endoscopic ventricular cases. However, visualization of the CPA requires constant changes to the field of view, making the rigid system less practical. In addition, the confined working space makes the use of an assistant scope holder challenging. A pneumatic endoscope holder with a multi-jointed, polyaxial arm (UniARM, Mitaka Kohki Co., Tokyo) has been utilized to overcome these obstacles.s. Literatur The possibility of head movement while a rigid endoscope is fixed in position is of utmost concern for potential injury to critical structures. For this reason, rigid fixation of the patient’s head in a neurosurgical head holder is recommended.

Lastly, for most lateral skull base surgeons who are mainly familiar with microscope use, endoscopic depth of field requires practice to become accustomed to. In contrast to surgical microscopes, most endoscopic systems only utilize a single optic and therefore three-dimensional images are not possible. Similar to endoscopic sinus surgery, movement of the instruments in and out of the field helps develop a sense of working depth.

40.3 Patient Selection

Patient selection is evolving as lateral skull base endoscopic techniques improve. Tumor size is one important consideration when choosing surgical approach. In our series of endoscopic VS cases, tumors ranged from 1 to 2 cm in size.s. Literatur Initially, patients with serviceable hearing and small tumors (<1 cm) with limited IAC involvement were considered ideal candidates. As experience has grown, criteria for patient selection have expanded to include cases with larger sized tumors (<2 cm). In our opinion, for patients with poor preoperative hearing, the translabyrinthine approach should be considered, as it provides direct tumor access, low risk of facial nerve injury, and short convalescence. Currently, IAC tumor extension is still a challenge in cases where hearing preservation is attempted, although meatal drilling can be performed and visualization with an angled endoscope can be helpful. In our opinion, a relative contraindication to endoscopic CPA surgery would be a tumor that results in significant mass effect and associated hydrocephalus, or one that is very vascular. A microscopic approach should be considered in these cases, as it allows for early access to the cisterna magna in order to relax the brain and provides more working room.s. Literatur

40.4 Relevant Anatomy

The endoscope provides a wide view of the CPA, with the tentorium as the superior limit, the pons and medulla medially, the temporal bone with the porus acusticus laterally, and the foramen magnum inferiorly (Fig. 40‑1 ; see Chapter 30 for further anatomical descriptions).