Endoscopes can be divided into two groups: flexible/steerable endoscopes or ventriculoscopes and rigid endoscopes. A number of systems are on the market, and this review is not intended to serve as an endorsement of any specific system. In addition to the actual endoscope, light sources, cameras and monitors for viewing while the procedure is performed, instruments for use with the endoscope, and a number of holding devices to immobilize the endoscopes are integral parts of the system. The procedures can be recorded with some of the newer digital imaging technologies available for neurosurgical procedures, as well as with VHS tapes; recording is advisable for most procedures.

Flexible Endoscopes

These endoscopes are similar to the standard bronchoscopes and rely on flexible fiberoptic illumination. The endoscopes are comprised of a number of glass fibers that are incorporated into a plastic sheath. The size of the fiber bundle determines the resolution of images. The endoscopes have up to 180° of freedom of movement at the distal tip of the endoscope, depending on the manufacturer. This occurs at the last 3-5 cm of length of the endoscope. The endoscopes are fitted with at least one working channel, which can also serve as a site for fluid egress and irrigation throughout the procedure. Working instruments, consisting of biopsy forceps, graspers, and scissors, are available and are also flexible in their design and construction. The available steer-able endoscopes range in size from 1 to 15 mm outer diameter depending on the number of fibers within them. The main disadvantage of flexible endoscopes is that their optics are worse than those of the rigid endoscopes. They cannot be autoclaved and must be gas-sterilized, which limits their longevity. In addition, frequent use can damage the fiber bundle, which further decreases image resolution. Their advantage is that they can be used to navigate in the ventricular system and around corners when used as an assist-device during microsurgical operations.

Rigid Endoscopes

Rigid endoscopes utilize rod lens telescopes and therefore have superior optics in comparison with the flexible endoscopes; as a result, they have become the most frequently utilized endoscopes in neurosurgery. The endoscopes consist of a lens, an illumination glass fiber, and a metal shaft that houses the lens and light fiber. There are two main designs: a sheath with a single channel for the endoscope and an endoscopic sheath with multiple separate channels for instruments in addition to the lens and light fiber. The rod lens system that is utilized in neuroendoscopy was invented by Hopkins and therefore is also referred to as the Hopkins system.

Rigid endoscopes come in a variety of sizes and lengths of shafts. The design of the lens can also allow for viewing angles of 0°, 30°, 70°, or 110°, thus allowing one to look straight forward, to the side, and also to the back. Rigid endoscopes can be autoclaved, are reusable, and are less fragile than flexible endoscopes. Their disadvantage is their rigidity; one cannot maneuver them in the intraventricular or intracranial spaces as freely as the flexible endoscopes. Therefore, with rigid endoscopy, it is crucial to plan the entry burr hole in such a location as to allow for the greatest freedom of movement without endangering any neurovascular structures.

Both flexible and rigid systems require immobilization of the endoscope so that the surgeons' hands are free to use instruments throughout the procedure. There are systems that allow fixation of any endoscope to the operating room table via mechanical or pneumatic devices. Some may be fitted or attached to the head holder or the stereotactic head frame with specially designed adapters. In addition, with the newly developed frameless navigation systems, the rigid endoscopes can also be registered and the images can be merged (5-8). These fixation devices are of crucial importance: the endoscope must be rigidly immobilized throughout the procedure without endangering the patient. They also need to be minimally intrusive and not restrict the surgeon's ability to manipulate instruments and the actual endoscope.

Flexible endoscopes require an introducer/trocar with a shaft, and a number of sizes are available for the different sizes of endoscopes. Usually, one utilizes a shaft that is slightly larger than the outer diameter of the endoscope so that there is some cerebrospinal fluid (CSF) egress around the endoscope, providing an additional outlet in case the working channel is occupied by an instrument. Frequently, these introducers have peel-away shafts that allow endoscopically guided placement of catheters, in either the ventricles or the cysts.

Rigid endoscopes may have a trocar as part of their system or are designed with a blunt tip to the shaft to allow for introduction without a trocar. Again, their sizes vary to allow for the different sizes of endoscopes. Most rigid endoscopes have at least two working channels, and some of the newer designs incorporate up to four channels (both for rigid instruments and for irrigation). The choice of an endoscope with one common channel or multiple channels depends to some extent on the procedure to be performed; a recent review by Kehler et al. (9) summarizes the advantages and disadvantages of both designs.

A number of instruments have been developed for use in endoscopy and consist of trocars, biopsy forceps (different sizes), grasping forceps, scissors, and both monopolar and bipolar coagulation tips. These instruments also have different angled tips allowing for more flexibility and for specialized applications. Balloon catheters are also utilized for dilation of the various fenestrations performed endoscopically; the most commonly used catheters are the Fogarty balloon catheters. Some lasers are also used with endoscopes (Nd-Yag), although with limited application (10,11).

Visualization with an endoscope requires fluid-filled spaces; any bleeding or spillage of tumor or cyst content may obscure the view. Therefore, irrigation is very important for clearing of the CSF spaces and can also be used to dilate the ventricular spaces so as to allow more movement of the endoscope within them.

Irrigation can be performed either continuously or via pulse/bolus injection throughout the procedure. This is the most helpful maneuver if bleeding is encountered during endoscopy; usually irrigation is continued until the bleeding is stopped and the CSF is clear. The solution utilized can be either lactated Ringer's solution or normal saline. It is important to monitor the amount of fluid injected/irrigated and the amount of fluid that has drained out so as to avoid a potential increase in intracranial pressure or pressure on the floor of the third ventricle. In addition, continuous confirmation of the anatomy and position by visualization of the endoscope and the landmarks can be accomplished in this manner and can avoid potentially serious complications (12). New methods of orientation within the ventricular system are being developed in case bleeding is encountered and the normal anatomic landmarks are lost. One new approach is connection of an optical position measurement system to the endoscope that allows for coupling of digitized endoscopic images to the accurate endoscopic position. In cases of bleeding, the previously set landmarks and the overlay of the images allows the surgeon to navigate within the operative field based on virtual images and to perform a procedure. This is still an experimental technique but clearly has the potential to be a helpful tool in neuroen-doscopy (13).

All endoscopes require a light source, and these are generally universally available but require filters to avoid the hot infrared spectrum. Observation of the procedure requires a camera, monitor, and video system, which often incorporates recording systems. Monitors need to be positioned so that the surgeon, the assistant, and the operating room personnel can all view them. The cameras require gas sterilization and therefore can be either sterile or draped in a special plastic sheath so that their longevity is increased. Documentation of all endo-scopic procedures is preferable and should be performed either on VHS tapes or digitally, as with the newer imaging technologies.

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