Overview of Endoscopy in the Approach to Brain Tumors

As a tool for visualization, the endoscope offers neurosurgeons several advantages. It delivers illumination directly to the point of interest, offers an extremely high degree of magnification, and provides a wide field of view. With the use of angled endoscopes, surgeons can see around corners.¹⁶ As familiarity with these attributes has grown, neurosurgeons have developed ways to use the endoscope both as a substitute for the microscope and for altogether new procedures.

Strictly speaking, neuroendoscopy is simply a specialized mode of viewing, whose relationship to microsurgery is analogous to the relationship between microsurgery and open neurosurgery in general. In practice, however, the views, techniques, and instrumentation of neuroendoscopy are so particular to itself that it constitutes its own field and requires its own set of definitions.

There are two principal forms of endoscopy: coaxial and extra-axial. Coaxial endoscopic approaches, or “pure” endoscopy, are those in which the instruments, lighting, and camera are all aligned in parallel axes enclosed in a single sheath. The instruments are introduced through working channels and are aimed by redirecting the endoscope itself. The impact is minimized because the entire working and visualization area is within the endoscope. However, the ability to put structures under tension and to work with both hands is limited because all instruments enter in parallel fashion.

Extra-axial endoscopic approaches are those in which the endoscope is the mode of visualization and the instruments are introduced alongside the endoscope separately. In “endoscope-assisted” applications, the microscope or unaided eye is the primary mode of visualization. The endoscope is added to improve visualization, especially around corners. In “endoscope-controlled” surgery, the endoscope is the sole mode of visualization. Surgery is performed with the same techniques and instrumentation as microsurgery, with the addition of curved instruments and suction to allow surgeons to operate around corners. A substantial learning curve is associated with these forms of endoscopy. Transitioning from the microscope is challenging because of the distortion caused by the endoscope and because of the proximity of the point of illumination and visualization to the target. Once the learning curve is mastered, these same problems can become advantages that may be used to improve outcomes.

In selecting the endoscope for a given application, surgeons must consider both its advantages and limitations in relation to traditional open or microscope-based surgical approaches. Advantages include the ability to deliver the light directly to the working area, to magnify the working view by bringing the camera directly to the area of interest, to work through a small portal, and to see around corners. The endoscope’s limitations vary, depending on the type of endoscopy. Through a purely endoscopic, coaxial approach, the means of placing structures under tension are limited. Complex dissection is therefore relatively difficult. From the perspective of maintaining visualization and controlling vascular structures, the endoscope also presents some difficulty in managing bleeding. Finally, the endoscope has a small diameter, and the working channels are even smaller. Consequently, piecemeal removal of large solid tumors is relatively tedious.

For extraventricular approaches, the main disadvantages of the endoscope are related to the unfamiliar view that it provides and to the need to hold the endoscope itself like an instrument. The former problem is overcome by familiarization with the endoscope and experience. It is also being improved significantly by brighter and clearer endoscopes. The latter problem is addressed by the use of a second surgeon or assistant who can hold the endoscope or less optimally by use of one of the commercially available endoscope-holding arms.

Intraventricular Endoscopy for Tumors

Endoscopic applications for the management of intraventricular tumors include tumor biopsy, tumor resection, and treatment of tumor-associated hydrocephalus.^17–19 Endoscopic biopsy is relatively easy to perform and should be considered for any tumor whose treatment strategy does not begin primarily with resection. If complete tumor resection is the initial goal, microsurgical removal remains the first choice for most tumors. A select minority are amenable to endoscopic removal. When feasible, endoscopic resection is performed through a transcortical route, usually with low neurological impact. In general, neuroendoscopic treatment of tumors is a high-level endoscopic skill that requires considerable familiarity with the endoscope, skill in its use, and experience. Detailed knowledge of anatomy is essential.^20,21

As noted, the diameter of ventricular endoscopes is necessarily small and the ability to provide bimanual dissection and to handle highly vascular structures is relatively limited. Given these limitations, the ideal tumors for endoscopic resection of an intraventricular tumor are relatively small (the smaller the better), partially or totally cystic, relatively avascular (the less vascular the better), and associated with hydrocephalus.

There is no absolute limit to the size of a tumor, but 2 cm is often cited.²² Cystic tumors can be larger because decompression of the cyst reduces the effective size of the tumor. High vascularity is a relative contraindication. Patience, irrigation, and cautery allow even bloody tumors to be treated, but as the length of surgery increases, the relative advantage of the minimally invasive approach may be lost. Hydrocephalus enlarges the working space, which is advantageous for an intraventricular approach. However, the tumor itself usually creates a working space, so hydrocephalus is not mandatory. The normal ventricle is large enough to provide access to a tumor for biopsy, and smaller tumors can even be safely resected.^23–25 A major key in the surgical approach to intraventricular lesions is choosing the appropriate working trajectory. The brain must be transited to reach the ventricles. Therefore, a single working angle that minimizes yaw and pitch movement of the endoscope should be chosen.

Most ventricular endoscopic procedures are performed through a single portal, although the use of two portals has been described.^26–29 Because normally only a single portal is used, the choice of trajectory is extremely important.

The selected approach trajectory (Fig. 119-1) should do the following: (1) enter the ventricle with some normal ventricle between the entry point and the mass; (2) allow access to the blood supply, if vascular; (3) allow access to the point of attachment to the ventricular wall or choroid plexus; (4) originate outside or avoid eloquent structures; and (5) when possible, allow any other necessary endoscopic procedures (e.g., septum pellucidotomy, endoscopic third ventriculostomy) to be performed through the same approach.

FIGURE 119-1 An ideal trajectory enters the ventricle into spinal fluid (A, upper inset) rather than directly into the mass (B, lower inset). If possible, the blood supply should also be in direct view of the endoscope, as in the upper inset as opposed to the lower inset. If the attachment point can be reached early in the operation, removal of the mass will be greatly facilitated, especially for pedunculated or cystic masses.

(Used with permission from Barrow Neurological Institute.)

The choice of trajectory should be considered carefully by using the aforementioned principles. Once surgery is under way, the trajectory cannot be altered easily. Having some normal ventricle and cerebrospinal fluid (CSF) between the entry point into the ventricle and the tumor allows the endoscopist to see the tumor margins, to distinguish between tumor and ependyma, and to identify important structures for preservation. Access to the blood supply and point of attachment greatly facilitates removal of larger masses because tedious piecemeal coagulation and removal can be foregone in favor of detachment of the mass and en bloc removal.

Image guidance is particularly valuable for this planning and for achieving a good approach. It is worthwhile even if it is used only for this step of the procedure.^30–32 The trajectory should allow the edges of the tumor to be reached with excursions of the endoscope that subtend the minimal amount of normal brain tissue (see Fig. 119-1). The elastic modulus of the brain does not allow much stretching. Rather, it should be assumed that retraction has some impact on the brain with every movement of the endoscope. By limiting the amount of movement necessary to reach the entire target, the effects of the endoscope can be mitigated.

As in all forms of endoscopy, the major risk for complications during intraventricular endoscopic approaches to brain tumors is disorientation. Disorientation can result from a poor entry point, inappropriate orientation of the camera, failure of the video chain, misunderstanding of the anatomy, altered anatomy, failure to recognize entry into the wrong ventricle, or combinations thereof. Accordingly, many complications can be avoided by several precautions: careful planning of the entry point and trajectory; orientation of the camera image on a scene of known orientation (such as writing) before it is introduced into the ventricle; meticulous checking of the equipment and video image before entry into the brain; thorough knowledge of normal ventricular anatomy; and constant self-inquiry about the location of the endoscope, what the structures seen represent, and how the endoscope’s intrinsic optical distortion affects the scene.

Image guidance is strongly recommended as an adjunct to neuroendoscopic approaches to overcome many of these obstacles.³³ It is useful for intraventricular, intraparenchymal, and extra-axial approaches. For intraventricular neuroendoscopy, image guidance plays a role in selecting the appropriate entry point and optimal trajectory. It is a useful source of reorientation, especially when blood clouds the view or the anatomy is altered. It can be used successfully in both children and adults.³⁴ Most rigid neuroendoscopes can support the mounting of a fiducial array for integration into a frameless stereotactic system. When such a system is used, the location of the tip of the endoscope can be updated continuously. With a rigid endoscope, the option of pulling the endoscope directly back out the way that it entered is almost always safe. With a flexible endoscope, the endoscope must be in its straight configuration to avoid hooking tissues as it is removed. Image guidance can also facilitate replacement of the endoscope should the ventricle be lost when the endoscope is removed.

Purely Endoscopic Approaches to Intraventricular Tumors

Details of the diagnosis, pathology, and nonsurgical treatment of intraventricular tumors are discussed in Chapter 138. As noted, certain tumors are more amenable than others to endoscopic removal.³⁵ Tumors with low vascularity are preferred for purely endoscopic removal. Examples are colloid cysts, subependymomas, some ependymomas, subependymal giant cell astrocytomas associated with tuberous sclerosis, selected neurocytomas, exophytic gliomas (primarily pilocytic or low grade), cavernous hemangiomas, and hypothalamic hamartomas.³⁶ Some choroid plexus tumors and pedunculated tumors can also be approached endoscopically because although they may be vascular, their blood supply is well defined.

Endoscopic tumor removal requires a high level of familiarity with the endoscope and its use. It represents one of the most technically complex skills in neuroendoscopy and often requires coordination of the efforts of two surgeons or a surgeon and an experienced assistant. The basic approach is straightforward. For almost all tumors, including colloid cysts, use of a peel-away sheath should strongly be considered. The sheath serves several purposes. It allows free egress of irrigant, thereby preventing increased intracranial pressure. It allows the endoscope to be removed and replaced with ease. It also permits the entire endoscope to be withdrawn from the ventricle with pieces of tumor too large to be removed though the working channel. The advantages of not using the peel-away sheath are a slight reduction in the size of the endoscope and the ability to keep some pressure in the ventricle, which may reduce venous bleeding and ventricular collapse. Most commercially available neuroendoscopes have a working channel with a maximum diameter of about 2 mm. The space within the jaws of a typical biopsy forceps is on the order of a cubic millimeter. Therefore, to remove a cubic centimeter of tumor requires 1000 separate bites if performed in piecemeal fashion. Piecemeal removal can also lead to spread of tumor if fragments are released and float free in the spinal fluid. Removal of larger chunks of tumor through the endoscope is more efficient and improves the quality of biopsy specimens.

Before the trocar or peel-away sheath is placed, the ventricle should be tapped with a brain needle or a ventriculostomy catheter. Both the trocar and peel-away sheath are quite large and blunt, and multiple passes to locate the ventricle can cause unacceptable damage. The bluntness also sometimes results in the tip being deflected from the ependyma, thereby making cannulation of the ventricle more difficult. Initial puncture with a smaller tool can ease subsequent introduction of the endoscope. Image guidance is also recommended for this step.

Most tumors are approached by taking initial biopsy specimens with a cup forceps. Coagulation must be minimized to maintain the quality of the tissue for histopathologic analysis.^35,37 For many tumors, this is sufficient management. If more extensive debulking and complete removal are intended, vessels on the surface of the tumor are coagulated with bipolar or monopolar electrocautery or a laser. Electrocautery (especially monopolar) is capable of generating high CSF temperatures and must therefore be used with caution. Copious irrigation with body-temperature lactated Ringer’s solution or a spinal fluid substitute solution is used to dissipate this heat.³⁸ Normal saline is not used because it lacks electrolytes and is significantly acidotic. Consequently, it can alter the electrolytic balance in the brain and lead to postoperative confusion.³⁹ When irrigating, it is essential to have a secure path of egress for the fluid to prevent trapping the fluid in the brain and resulting in increases in intracranial pressure.

Once the bleeding is controlled, cautery and blunt dissection are used to separate the tumor from normal tissue. The best tumors for neuroendoscopy have a distinct margin and can be retracted gently from the surrounding tissue. Ideally, a perimeter can be created, the tumor can be isolated as a mass, and it can be removed in one or more large pieces. If the tumor is soft, a stainless steel cannula or a hand-trimmed pediatric endotracheal suction catheter placed down the working channel can be used to remove significant portions of the tumor by suction.⁴⁰ The gelatinous contents of colloid cysts and some other cystic tumors respond particularly well to this technique. An attempt is made to not spread tumor tissue around the ventricle. If care is not taken, tumor may fall with gravity toward dependent areas of the ventricles.

At the conclusion of tumor removal, attention is paid to obtaining good hemostasis, usually by irrigation alone. The ventricle is inspected for residual tumor and blood clots, particularly over the foramen of Monro and the aqueduct of Sylvius, where obstruction may occur. Whether to leave an external ventricular drain is controversial and should depend primarily on the risk for obstruction. When the working trajectory permits, the addition of septum pellucidotomy or third ventriculostomy can decrease the chance of postoperative symptomatic hydrocephalus.

Location plays an important role in choice of trajectory and whether to approach a lesion endoscopically at all. A list of standard approaches and entry sites is presented in Table 119-1. Tumors of the third ventricle are generally good targets for biopsy and reasonable for resection, depending on their size and vascularity. Almost all approaches to the third ventricle are performed through the foramen of Monro. The entry points on the surface are chosen with the foramen as the pivot point. The fornix, which constitutes the anterior-superior margin of the foramen, must be respected in both choice and execution of approach to prevent potentially catastrophic memory loss.

TABLE 119-1 Entry Points by Lesion Location

Tumors of the fourth ventricle can be accessed endoscopically from above. However, doing so usually entails dilating the aqueduct, with consequent diplopia. Hence, these tumors are usually best approached microsurgically.

Tumors in the lateral ventricles are best chosen for endoscopy based on their vascularity, which can be extremely variable. Those of low vascularity are readily approached with a pure endoscopic technique. Ideal tumors are subependymal giant cell astrocytomas, central neurocytomas, and other low-grade gliomas. More vascular lesions such as thalamic glioblastomas that are exophytic into the ventricle should not be treated endoscopically. One needs to choose a trajectory that enters the lateral ventricle as far from the tumor as possible and maintains visualization of its margins.

Hemorrhage is frequently the rate-limiting step in the removal of these tumors. Several accepted techniques are available to address hemorrhage with the endoscope. The first maneuver is to irrigate generously; with patience, most bleeding in the ventricle stops with irrigation alone. The second maneuver is to attempt coagulating the bleeding source with bipolar or monopolar cautery. Cauterization should be done with caution, however, and only if the actual source of the bleeding can be directly visualized. A third maneuver is to tamponade the bleeding point gently with the endoscope itself. This strategy can be particularly effective for larger veins such as the thalamostriate vein. Finally, draining the ventricle of CSF and replacing it with air negates the visual obscuration created by bloody CSF and improves the ability to coagulate by cautery.

Leaving an external ventricular drain in place postoperatively is a matter of personal choice. It is often tempting to leave a drain as a “safety valve” and to allow drainage of blood products. Arguments against leaving a drain include increasing the risk for infection and possibly increasing the incidence of postoperative hydrocephalus by discouraging the flow of CSF along normal pathways. We do not routinely leave drains in place after endoscopic tumor surgery.

Complications of tumor biopsy and removal include intraventricular hemorrhage, neurological deficit, tension pneumocephalus, hydrocephalus, and injury to the basilar artery.^41–43 In one series the hemorrhage rate was 3.5%.⁴⁴ Intraventricular hemorrhage is avoided by respect for vascular structures and prophylactic coagulation. Neurological deficit is avoided by choosing an ideal working trajectory, respecting key areas of anatomy, and not working where visualization is poor. Tension pneumocephalus is caused by air entering the ventricles during the procedure. The ventricles should be refilled with lactated Ringer’s solution as much as possible. If large quantities of air are left, high inspired oxygen tension helps speed its absorption.⁴⁵

Many intraventricular tumors cannot be removed completely through a purely endoscopic approach, but endoscopy still plays an important role in their treatment. Diagnostic ventriculoscopy should be performed whenever an endoscope is placed in the ventricle for any reason. With a 30-degree endoscope, wide inspection can often be performed by rotating the endoscope with no additional brain retraction. For example, ventriculoscopy can identify ependymal tumor deposits that cannot be seen on magnetic resonance imaging (MRI). It can confirm whether the septum pellucidum is patent or perforated and whether the opening of the aqueduct is obstructed or open. In patients undergoing shunting, ventriculoscopy can be performed before the catheter is placed or through the endoscope with a fiberscope. In most cases, however, hydrocephalus associated with tumors can be managed with the endoscope, and this course is preferred.

Neuroendoscopic Management of Tumor-Associated Hydrocephalus

Obstructive hydrocephalus frequently complicates the course of patients with various intracranial tumors. Such hydrocephalus can be caused by direct blockage of the ventricle by tumor, by compression of the aqueduct or foramen of Monro, or by the formation of cysts or membranes. Endoscopic third ventriculostomy, septum pellucidotomy, or cyst fenestration should always be considered as the first-line management of this type of hydrocephalus.^46–51 Endoscopic procedures are often sufficient management of tumor-associated obstructive hydrocephalus. Doing so avoids the complications intrinsic to shunting: infection and blockage and the rare cases of abdominal spread.⁵² Success rates are frequently 80% or higher.⁴⁶ Common examples of tumor-associated obstruction are aqueductal obstruction by tectal gliomas, pineal tumors, and fourth ventricular tumors.^47,48,53 The techniques of endoscopic third ventriculostomy and septum pellucidotomy are discussed elsewhere. Many tumors, despite their daunting appearance on computed tomography (CT) or MRI, will present a smooth surface at the ventricle, thereby allowing successful cannulation and opening of alternative CSF pathways. Equalization of pressure between the two ventricles, as afforded by septum pellucidotomy, or between the ventricles and the extraventricular subarachnoid space can prevent shift, herniation, and extreme ventricular dilation.

Endoscopic aqueductoplasty and stenting should be considered in the context of an isolated fourth ventricle, for example, when tumor fills the basal cisterns of the posterior fossa and obstructs the foramina of Luschka and Magendie or when removal of a fourth ventricular tumor has scarred these avenues closed. The consequences of fourth ventricular dilation can be severe but can be relieved by aqueductoplasty when combined with endoscopic third ventriculostomy or ventricular shunting. The technique of aqueductoplasty is described elsewhere.⁵⁴ Aqueductoplasty may need to be combined with shunting or third ventriculostomy to be effective. Endoscopic third ventriculostomy has also been used successfully to manage hydrocephalus associated with tumors of the cerebellopontine angle and brainstem.^47,55

Endoscopic management of hydrocephalus is also frequently possible in combination with diagnostic tumor biopsy, as discussed in the next section.

Role of Endoscopic Biopsy in the Management of Intracranial Tumors

Endoscopic tumor biopsy solely for histopathologic diagnosis is an appropriate alternative to complete resection (whether endoscopic or microscopic) whenever a tumor appears to be amenable to adjuvant therapy. Indeed, a wide variety of tumors are accessible endoscopically with minimal morbidity. Issues of sampling error are the same with endoscopic biopsy as with other forms of biopsy.⁵⁶ Lesions considered for management with endoscopic biopsy include pineal region tumors, tumors with ependymal spread in which a diagnosis is unclear (such as metastatic tumors and lymphoma), and even otherwise unresectable intrinsic tumors with an exophytic portion in the ventricle. Intrinsic tumors that are primarily intrinsic but reach the ependymal surface must be approached with caution because the ependymal surface may appear normal and thus complicate biopsy. Furthermore, if hemorrhage occurs, it drains into the ventricle with little to resist it.

Endoscopic Removal of Colloid Cysts of the Third Ventricle