Instrumentation for Endoscopic Skull Base Surgery

Miguel M. Sanchez, Joao Paulo Almeida, Claire Karekezi, and Fred Gentili

Abstract

The advent of new technology during the last two decades has paved the way for the development of neuroendoscopy. As a result, endoscopic surgical approaches in patients suffering from skull base lesions, that extend from anterior cranial fossa to craniocervical junction, have undergone a significant and steady revolution.

In this chapter, the basis of the endoscopic technique is introduced which provides a historical perspective of important breakthroughs that have contributed to its development and current state.

Additionally, the different tools which compose the neuroendoscopy armamentarium are introduced and explained. This chapter is divided into topics that addresses the following subjects: Physics principles entailed in endoscopic systems, types of endoscopes, digital cameras, light sources and pumps, and neuromonitoring equipment and microendoscopic instrumentation.

Keywords: Keywords: introduction, instrumentation, equipment, endoscopy, skull base surgery

1.1 Introduction

The advent of important technological milestones during the second half of the last century paved the way for the development of neuroendoscopy.1 One of the most influential contributions to neuroendoscopy was made by the British physicist Harold Hopkins in 1959.^2,3 He contributed to the development of the rigid rod-lens endoscope whose basic concept is still in use in modern surgery. Apart from this initial revolutionary development, other subsequent advances which have contributed to the increasing popularity and use of endoscopy include: the invention in 1969 of the charged-coupling devices (CCD), which transform optical signals into electrical information allowing for advanced image processing; the cold lighting sources; and more recently the development of high-definition video-cameras and external monitors.^4,5,6

Ultimately these technological improvements as well as the increased experience in micro endoscopic surgery boosted neuroendoscopy and contributed to its use in the new field of “Endoscopic Skull Base Surgery,” expanding its indications from the field of rhinology. The subsequent application of endoscopic approaches for sellar tumors and cerebrospinal fluid (CSF) leak repair to the treatment of lesions located in different regions of the sagittal plane, from the frontal sinus to the occipital-cervical junction, significantly expanded its use. Indeed endoscopic endonasal approaches (EEAs) have undergone a significant and rapid evolution during the last two decades.^5,7 Endoscopic access to skull base lesions has been primarily adopted by teams of neurosurgeons and ear, nose, and throat (ENT)/head and neck surgeons and now represents a routine surgical approach in major skull base centers for select skull base lesions, including pituitary adenomas, craniopharyngiomas, anterior skull base meningiomas, chordomas, and chondrosarcomas. The popularity of skull base endoscopy is underscored by the increasing numbers of publications related with this technique.^8,9 The often-quoted main advantages of the technique include a closer and wide panoramic view and better illumination of deep-seated structures and minimal brain manipulation or retraction, when compared to transcranial routes.

1.2 Special Requirements for Endoscopic Approaches

One of the main benefits of the application of endoscopy in surgery is the reduced tissue retraction and/or manipulation of neurovascular structures with a direct exposure and better visualization of the region of interest. In addition, the approach provides a different perception of anatomy due to improved illumination and better definition of details in regions otherwise obscure and hidden to open microsurgical approaches, with the added possibility of expanding the angles of visualization.^7,9 As a result, endoscopic surgery has become an effective and safe alternative to some traditional transcranial approaches. In selected patients both faster postoperative recovery and fewer complications have been reported using EEA.

However, endoscopic skull base surgery has its unique perspectives that should be considered. While conventional open microneurosurgical instruments require wide working spaces for adequate illumination, stereoscopic vision, and manipulation under the microscope, endoscopes employ narrower entry portals to reach the targeted area which is wider in the depth than in the entry access.⁹ Therefore, the surgeon’s armamentarium also needs to be adapted by modifying regular microinstruments in order to fulfill the requirements of endoscopic skull base surgery. These specifically adapted instruments include: long rigid endoscopes, lens irrigation systems, holding arms for the two-hands approach, adapted microdrills, bipolars, and microdissectors, high-definition cameras with recorders as well as other specially designed microinstruments.¹⁰

In addition, despite the fact that a more widely illuminated exposure and close revaluation can be obtained with endoscopic approaches, one of the major limitations currently present in endoscopic procedures is the lack of stereoscopic vision. This important surgical point must be addressed with appropriate training and the progressive learning curve that allows the endoscopic surgeon to deal with distorted depth perceptions and understand light and shadow landmarks around the surgical cavity.¹¹

1.3 Patient Positioning and Operating Room Setup

1.3.1 Positioning

The patient is positioned supine on the operation table with the head of bed raised above the heart level and the head slightly extended, fixed in a Mayfield head holder to avoid potential movement during drilling or microsurgical dissection. The endotracheal tube and anesthesia machine are positioned on the left side so that the surgeon may operate without interference from the patient’s right side. In this position the surgeon can operate in the natural axis along the skull base thanks to the appropriate instruments described in more detail in this chapter.

1.3.2 Operating Room Setup

Once the patient is anesthetized and correctly positioned, two video monitors are placed in front of the surgeon and the assistant respectively. In the binostril bimanual technique, the camera holder or “driver” is situated on the surgeon’s left side while the surgeon is working on both nostrils. The endoscopic tower is situated behind the patient’s head in direct line of vision with the operating surgeon who stands on the patient’s right side. A proper alignment of the monitors and tower is essential to keep the surgeon oriented in a surgical plane perpendicular to the sphenoid rostrum. A summary of the operating room configuration is depicted in Fig. 1.1.

Fig. 1.1 (a) The configuration in the operating room is depicted in this image with the surgeon in front of the high-definition monitors on the patient’s right side. (b) Details of the perpendicular angle needed between the endoscope and the sphenoid in order to facilitate the orientation of the surgeon in a direct angle of work with the monitors.

1.4 Endoscopes and Video Systems

1.4.1 Types of Endoscopes: Function and Management

Endoscopes for neurosurgery may be divided into four categories: rigid-lens endoscopes which are the workhorse for skull base surgery, semi-rigid mini-fiberscopes, flexible endoscopes, and video-endoscopes also known as “distal chip” or “chip-in-the-tip” devices ((Table 1.1)). For this chapter, we will focus on the rigid endoscopes, the main tool for endoscopic skull base surgery. Initially developed by Harold Hopkins in 1959, this endoscope was specifically designed to overcome the conventional systems which contained larger gaps of air space in between each of the small lens accommodated along the endoscope’s tube and which provided poor quality images.² Unlike the former device, the new endoscope contained large glass rods in between small air spaces similar to thin lens. This solution provided higher refractive index allowing higher image quality and wider vision angles.^1,2,4,6,12 Hopkins rod-lens rigid endoscopes constitute the present gold standard for image quality and light transmission. When these endoscopes are combined with a xenon light source and full high-definition (HD) video resolution, they are able to focus with clarity on any deep-seated lesion along the skull base.^6,13 Currently, in terms of clarity and anatomic information, the gold standard in imaging definition is represented by the Hopkins II rod-lens endoscopes with the outer diameter > 3 mm and integrated light fibers which allow visualization of a wide area of high-definition quality.⁶ The standard length of the endoscope usually employed is 18 cm. Several angles of view by prisms in the tip are available: 0 degree (provides with direct view and it is the most frequently employed), 30 and 45 degrees (forward-oblique direction of view), 70 degrees (lateral view), and finally 120 degrees (backward perspective). Some of the newest rod-lens systems incorporate variable adjustments of direction from 0 to up to 120 degrees.^9,10,13

Although the 0-degree endoscopes employ a straightforward perspective, the variable angle of view obtained with 30 to 70 degrees can be useful for surgical manipulation in hidden angles or cavities far from the more direct perspective.