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

Colloid cysts of the third ventricle are non-neoplastic masses that typically arise from the roof of the third ventricle. They can occlude the foramen of Monro and cause headache, hydrocephalus, memory disturbances, and sudden death. A colloid cyst is a single layer of cuboidal to pseudostratified columnar epithelium encasing a collection of proteinaceous colloid material that ranges in consistency from that of mucus to hard cheese. Most cysts that are 1 cm or larger or those associated with symptoms or with signs of hydrocephalus are recommended for treatment.

Options for treatment include shunting (usually of both ventricles), stereotactic drainage, transcallosal or transcortical craniotomy, and endoscopic removal.⁵⁷ Shunting does not prevent the cyst from growing. In the event of shunt blockage, the risk for sudden death persists. Stereotactic drainage is associated with a high rate of failure or recurrence.^58,59 Microsurgical removal is effective, but more morbid than the endoscopic approach.^11,13,60 Therefore, endoscopic removal is recommended for most cases.^9,15,61,62

The approach is through a single bur hole located about 8 cm behind the nasion and as far lateral as the anatomy permits without transiting the caudate head, which is usually 5 to 7 cm from the midline.^10,63 A 1.5-cm incision and 8-mm bur hole usually suffice. Image guidance is recommended for the initial ventricular entry. A peel-away sheath is optional. The landmarks of the colloid cyst and foramen of Monro are identified. The overlying choroid plexus is coagulated while taking particular care to prevent heat damage or direct trauma to the fornix.

The face of the cyst is opened, and the contents are aspirated with suction or removed with forceps. The wall of the cyst is then dissected free of the roof of the third ventricle. The cyst seldom adheres to the fornix itself. The entire cyst wall can usually be removed in gross fashion. Occasionally, the cyst is so adherent to the walls of the third ventricle or internal cerebral veins that it must be truncated and a small fragment left. Under these circumstances, the rate of recurrence appears to be low,^12,64,65 and relief of obstructive hydrocephalus is the rule. Postoperatively, however, the decrease in size of the ventricles is sometimes modest.⁶⁴

In practical terms, colloid cysts larger than 2 cm are very tedious to remove and tend to be more adherent to the fornix. In such cases, the fornix is also stretched and therefore friable. These larger cysts should be considered for microsurgical removal.

Neuroendoscopic Management of Pineal Region Tumors

Neuroendoscopy has taken a prominent role in the management of pineal region tumors.⁶⁶ The pineal region is almost centrally located in the head. It is a common site for a wide range of pathologies, and a number of surgical corridors are available for access.^67–72 Frequent pineal tumors include germinoma, nongerminomatous germ cell tumors, pineal parenchymal tumors, glial tumors, and a wide range of miscellaneous tumors, including meningioma. Tumors in the pineal region frequently obstruct the aqueduct of Sylvius and create a setting of hydrocephalus that makes anterior transventricular endoscopic approaches even more attractive. Much pathology in this region responds to primary or initial adjunctive radiation therapy and chemotherapy. Consequently, there is a strong rationale for primary use of neuroendoscopy to obtain a biopsy specimen for histologic analysis in the most minimally invasive way possible.^73–76

In general, the initial work-up for a tumor in the pineal region involves serum and CSF analysis for markers (α-fetoprotein, β-human chorionic gonadotropin, lactate dehydrogenase, placental alkaline phosphatase) that could indicate a malignant tumor of germ cell lineage.⁷⁷ Germinomas, in particular, are highly responsive to radiation and may be treated by radiation therapy alone if the diagnosis can be made endoscopically. Other nongerminomatous germ cell tumors may respond well to preoperative chemotherapy and thereby decrease operative complexity and blood loss. Blind stereotactic needle biopsy may be considered as an alternative. A simple scheme for choosing among the alternatives is presented in Table 119-2.

TABLE 119-2 Approaches to Pineal Tumors

* Includes the supracerebellar infratentorial and occipital transtentorial approaches.

Pineal region tumors manifested as hydrocephalus should be considered for both endoscopic tumor biopsy and simultaneous third ventriculostomy (Fig. 119-2).⁷⁸ They can be performed in either order. The rationale for performing third ventriculostomy first is to do so before potential bleeding and clouding of the ventricle occur as a result of the biopsy procedure. The rationale for performing a biopsy first is that it is the main goal and should be completed before potential bleeding occurs. If the tumor is small, it may be possible to open the CSF pathways endoscopically, thereby obviating the need for third ventriculostomy (Fig. 119-E1). In fact, there is little chance that either procedure will affect the other.

FIGURE 119-2 Access to the pineal region for tumor biopsy and simultaneous third ventriculostomy. To achieve an ideal trajectory for each procedure, two separate trajectories are needed, one for the pineal approach (A) and one for third ventriculostomy (B). An intermediate starting point is possible, but it will also increase the amount of brain through which the endoscope must arc to achieve both trajectories. Such a trajectory may place the fornix at more risk for retraction injury.

(Used with permission from Barrow Neurological Institute.)

FIGURE 119-E1 A, Sagittal magnetic resonance image of a 60-year-old woman with a tumor in the pineal region. B, At the time of endoscopic biopsy, the tumor was found to be located entirely within the aqueduct. C, Biopsy was performed with forceps and demonstrated a metastatic adenocarcinoma. D, A trimmed pediatric endotracheal tube suction catheter was used to suction much of the tumor. E, Gross total removal was possible, and the patient’s hydrocephalus resolved without endoscopic third ventriculostomy or shunting. The patient remained neurologically intact and underwent radiation therapy.

(Used with permission from Barrow Neurological Institute.)

Endoscopic pineal tumor biopsy and third ventriculostomy with a rigid endoscope have been performed successfully through a single bur hole.⁷⁹ Although technically possible, a single approach requires transit through more brain than occurs with two trajectories and may place the fornix at more risk. Therefore, it may not be truly minimally invasive (see Fig. 119-2). The need for simultaneous biopsy of a pineal region tumor and endoscopic third ventriculostomy is the most common application for flexible neuroendoscopy (Figs. 119-3).⁸⁰ More often the two parts of the operation are performed through separate bur holes and trajectories with the use of rigid endoscopes (see Fig. 119-2).^81,82

FIGURE 119-3 A flexible endoscope can accomplish both pineal tumor biopsy and third ventriculostomy from one starting point.

(Used with permission from Barrow Neurological Institute.)

An alternative endoscopic approach to the pineal region is via the supracerebellar infratentorial approach, in a manner analogous to the open approach.^45,83 The patient is placed in the sitting position with a lumbar drain to create a working space without retraction. The procedure is performed in air. A purely endoscopic or an endoscope-assisted approach can be used via this route. The microsurgical route to this region is well established; however, it is a long corridor that keeps the pathology at some distance from the surgeon. The walls of the third ventricle, seen from behind, form a narrow corridor where residual tumor is often perpendicular to the direction of view and hence evades direct inspection. The endoscope overcomes all these limitations and thereby allows effective endoscopically controlled surgery. With a primarily microsurgical approach to the pineal region, angled endoscopes should be used to check the walls of the resection cavity for tumor and to inspect the third ventricle for residual blood that might obstruct the aqueduct.

A discussion of the intraventricular extension of sellar tumors is provided in the online version of this chapter.

Endoscopically Assisted Microneurosurgery

The lack of ease for bimanual work has limited general use of the endoscope as a substitute for the operating microscope. However, the outstanding visualization offered by the endoscope in terms of illumination, magnification, and angled views can still be highly useful as an adjunct in many microsurgical cases. In these applications, primary exposure of the tumor is performed under the operating microscope, along with the initial dissection. When the microscope becomes limited, the endoscope is introduced. Additional inspection and dissection are performed under the endoscope. Work alternates between the microscope and endoscope as needed. The endoscope is routinely used at the end of surgery to check for residual tumor.

Typical corridors that are particularly amenable to endoscopic assistance are the retrosigmoid, subfrontal/pterional, and interhemispheric transcallosal.^87–93 In fact, any surgical corridor in the subarachnoid, intraparenchymal, or intraventricular space that is accessed by the microscope can benefit from endoscopic assistance.

A discussion of the technique for trans-eyebrow supraorbital craniotomy is provided in the online version of this chapter.

Neuroendoscopy for Intra-Axial Tumors

The use of neuroendoscopy as an adjunct for the surgical removal of intra-axial tumors is still relatively uncommon. Selected examples of anecdotal use illustrate its possibilities, which still wait to be fully explored. The applications for which it has both rationale and practical advantage are those in which the endoscope can help limit retraction and improve an otherwise restricted view. For example, during removal of an insular glioma, where the overlying opercula are extremely eloquent and cannot and should not be retracted, the endoscope can improve visualization of residual tumor (Fig. 119-E2).⁹⁷ For microsurgical removal of subcortical intrinsic tumors, the endoscope can be used through a small cortical portal to view the walls of the tumor cavity and to inspect the overhanging areas that are out of the line of sight of the microscope. This technique can allow the surgeon to avoid expanding the corticotomy solely to improve microscopic visualization.

FIGURE 119-E2 A, Preoperative axial T1-weighted magnetic resonance image (MRI) with gadolinium enhancement in a female patient with a left insular low-grade glioma. Minimization of brain retraction is important for achieving complete removal with minimal morbidity. B, Postoperative T1-weighted MRI with gadolinium enhancement showing transsylvian endoscopically assisted resection with minimal brain retraction. The patient remained neurologically intact after surgery.

Experience with endoscopic removal is limited to small case series.^98,99 Most techniques involve the use of neuronavigation to direct the placement of a sheath onto the tumor. Removal then proceeds under endoscopic control. Therefore, the sheath must be wide enough to accommodate both the endoscope and microinstruments.

Endonasal Transsphenoidal and Extended Transsphenoidal Approaches

In recent years, no area of neuroendoscopy has attracted as much interest and controversy as endoscopic endonasal approaches to the anterior skull base. Introduced by otorhinolaryngologists to treat sinus disease,¹⁰⁰ endonasal endoscopy was initially applied in neurosurgery primarily as an adjunct to the microsurgical transsphenoidal approach to the sella turcica for pituitary pathology.^101–105 However, in the past 2 decades, entirely endoscopic techniques have been brought to bear on a wide range of pathologies involving structures from the frontal fossa to the second cervical vertebra, all through a nostril approach without external incisions. A wide range of pathologies have proved accessible from this corridor, including sinonasal tumors (esthesioneuroblastomas, juvenile nasal angiofibromas), sellar pathologies (pituitary tumors, craniopharyngiomas), clival tumors (chordomas, chondrosarcomas), anterior cranial fossa dural-based tumors (meningiomas), and selected intradural tumors.^106–112 Even vascular pathologies have been accessed and treated successfully. Most midline and paramedian masses are highly accessible through the endonasal route in both adults and children (Fig. 119-4).^{110,113–115}

FIGURE 119-4 A, Preoperative sagittal T1-weighted magnetic resonance image (MRI) with gadolinium enhancement showing an olfactory groove meningioma located in the midline and anterior to the neurovascular structures of the optic nerve and anterior cerebral artery complex. This tumor is ideal for an anterior endonasal endoscopic approach. B, Postoperative sagittal T1-weighted MRI with gadolinium enhancement showing complete removal of the tumor from below. The remaining bright signal is the fat graft, which has shrunk down on the frontal fossa floor.

The advantages of the endonasal route are the relatively low surgical impact of the route on the patient and the lack of brain retraction. The limitations of the endoscope are minimized during this approach. The inability to see behind the endoscope is mostly immaterial because the only structures affected are the walls of the nasal cavity. Knowledge of the anatomy and careful avoidance of larger vascular structures help avoid major bleeding. The two nostrils afford a surprising range of motion and, by switching between 0-degree and 30-degree endoscopes, a good range of viewing angles as well. Two surgeons working simultaneously, often a neurosurgeon and an otolaryngologist experienced in sinus endoscopy, enable a four-handed technique that not only overcomes the limitations of one-handed surgery that plague many other types of endoscopy but also actually exceeds it by allowing simultaneous use of an endoscope, a suction device, and two microsurgical instruments. In this manner, sharp dissection and bipolar electrocautery can be used on tumors just as they are under the microscope.

Thorough knowledge of the anatomy of the nasal cavity, paranasal sinuses, and anterior skull base is essential for success and avoidance of complications. The surgeon should be able to identify the superior, middle, and inferior turbinates and the openings of the ostia into the maxillary and sphenoid sinuses. The middle turbinate can often be lateralized or resected to improve access to deeper structures. The cartilaginous septum can be mobilized, but it must be preserved and restored at the end of surgery to prevent nasality and whistling in speech postoperatively. The more posteriorly placed bony septum can be removed along with the face of the vomer, sphenoid, and surrounding structures as distant as the cavernous sinuses bilaterally. Below this level, even wider exposure can be obtained. Having defined the normal anatomy around the tumor, its margin can be defined, and the tumor can be removed with the same techniques as used in microsurgery, including sharp dissection and electrocautery.

A maxillotomy can expand this field even more if lateral structures need to be accessed. Moving anteriorly and superiorly from the face of the sella, the tuberculum sellae and planum sphenoidale can be removed to provide access to the optic chiasm and anterior communicating artery complex. At the limit of this trajectory the ostium of the frontal sinus can be accessed, but the sinus itself is frequently too anterior to work inside. However, all structures posterior to this point, from the olfactory groove back to the cervical spine, are accessible. Inferior to the sella, the entire central part of the clivus can be removed with minimal morbidity. Bone is removed sequentially with a high-speed drill or biting instruments to expose the target area. Because exposure is from below, neurovascular structures can often be separated from dural-based structures with minimal impact.

Tumors ideally suited to the minimally invasive transsphenoidal or expanded transsphenoidal approach are those that are midline (e.g., pituitary adenomas, meningiomas, and craniopharyngiomas), those that are midline and extradural (e.g., chordomas, fibrous dysplasia, and bony metastases), and those that are paramedian (e.g., lesions of the petrous apex, schwannomas, and meningiomas of the cavernous sinus, tuberculum sellae, planum sphenoidale, and petroclival region). Once tumors have invaded the dura or the surgeon breaches the dura, there is the added burden of skull base reconstruction to prevent CSF fistula formation, as discussed later.

Endoscopic Endonasal Approach: Applications and Limitations

The major limitation to application of the endoscopic endonasal approach is imposed by the anatomic location of the tumor. The greatest advantage of the endonasal approach to the skull base is the ability to attack the tumor directly without facing intervening neurovascular structures. Meningiomas of the skull base illustrate how advantageous it is to attack a tumor directly from below. Much of the blood supply to the lesion is immediately destroyed in the exposure. Consequently, the tumor burden can be reduced with less blood loss, more rapid definition of tumor margins, and earlier definition of the interface of the tumor with normal structures. These advantages are lost if any nerve or major vessel lies between the surgeon and the tumor. Hence, tumors that are superior to the optic nerves are best approached transcranially, and tumors that are lateral to the carotid artery or cranial nerves V, VII, and VIII are best approached laterally.

Another limitation of the endonasal approach is based on tumor type. Those who believe that chordomas of the clivus should be removed en bloc would be better served by using more traditional craniofacial approaches to the skull base. Although en bloc removal is possible in highly skilled hands, most lesions are removed piecemeal when an endoscope is used. Highly vascular tumors such as hemangiopericytomas create significant bleeding problems. Their removal can be very time-consuming given the constraints imposed by the relatively primitive endoscopic bipolar forceps that are available commercially.

Specific tumors have their own limitations for the endonasal approach. Anterior cranial fossa meningiomas can be removed extremely effectively both transcranially and endonasally. If midsagittal MRI shows an anatomic gap between the posterior margin of the tumor and the anterior circulation vascular complex (i.e., the origin of the tumor is primarily from the olfactory groove), the endonasal approach is ideal (see Fig. 119-4). Conversely, if there is anatomic juxtaposition of the posterior margin of the tumor and large vessels or if the vessels are involuted into the tumor (as is often the case with tumors that are primarily of tuberculum sellae origin), a transcranial approach is preferred (Fig. 119-5).¹¹⁶

FIGURE 119-5 A, Preoperative sagittal T1-weighted magnetic resonance image (MRI) with gadolinium enhancement showing a tuberculum sellae meningioma involving the anterior cerebral artery A1/A2 complex and impinging on the optic nerve from above. This tumor is poorly suited for an endonasal approach. It should be approached from above. B, Postoperative sagittal T1-weighted MRI with gadolinium enhancement showing complete removal of the meningioma through an eyebrow craniotomy with endoscopic assistance.

Craniopharyngiomas represent a challenging problem for neurosurgeons in many ways, particularly when a large suprasellar component adheres to the walls of the hypothalamus (Fig. 119-6A).⁸⁴ If the optic chiasm is prefixed, the subfrontal approach is relatively contraindicated. However, this anatomic variation creates an excellent corridor to these tumors when operating from below (Fig. 119-6B). Expanded endonasal endoscopic techniques have been used to aid the surgeon in seeing these attachments of the tumor, thereby allowing it to be mobilized under direct vision.^117–119 The pituitary gland can be mobilized and transposed to allow retrosellar exposure when needed.¹²⁰ A second endoscopic option for craniopharyngiomas with third ventricular extension is to combine a craniotomy with a transforaminal endoscopic approach from above through a precoronal bur hole. This strategy allows the ventricular extension of the tumor to be freed before it is resected from below.

FIGURE 119-6 A, Preoperative sagittal T1-weighted magnetic resonance image (MRI) with gadolinium enhancement showing a craniopharyngioma with relatively restricted space between the pituitary gland and planum sphenoidale and a normal optic chiasm. For this sort of tumor, a subfrontal or eyebrow approach is preferred. B, Preoperative sagittal T1-weighted MRI with gadolinium enhancement showing a craniopharyngioma with a large window between the pituitary gland and planum sphenoidale and a prefixed chiasm. This lesion would be difficult to remove through a subfrontal/prechiasmatic approach. An endonasal endoscopic approach is preferred.

The main difficulty posed by more extensive skull base work is achieving appropriate closure. In most cases, closure presents few problems. Even extensive removal of bone and mucosa alone usually has few serious consequences. With appropriate management, the sinuses normally mucosalize rapidly after surgery. If CSF is encountered, however, the dural defect must be repaired in watertight fashion. Primary suture closure can seldom be achieved. Therefore, other forms of closure must be used. Unlike convexity cranial approaches in which a tightly secured scalp can hold the spinal fluid and allow healing, there is no equivalent layer in the sinuses. Consequently, reconstruction is required to prevent a persistent leak with the attendant risks of CSF rhinorrhea, meningitis, herniation, or death.

The principles of reconstruction are to make the smallest dural defect possible, open the dura sharply, preserve the arachnoid, leave a shelf of bone on which bone or a bone substitute can rest, and cover the defect with a watertight layer, of which a vascularized mucosal flap is the best option. The addition of fat grafts, fascia lata grafts, dural substitutes, dural sealants, acellular dermis, and other adjuncts is at the discretion and experience of the treating surgeon.^121–123 Many applications require no nasal packing. In other cases, an inflated Foley catheter may be used to provide a bolster for the tissue repair.¹²² A lumbar drain is sometimes used to further improve the chance of healing.

Published rates of CSF leakage vary widely among series, depending largely on the pathology treated and the experience of the reporting group. With pituitary tumors, for instance, the typical leak rate is 2% to 10%.^116,124 With dural-based or intradural masses, it may range as high as 16% to 58%.^{117,118,125,126}

Future Directions

More than for most other neurosurgical techniques, endoscopic techniques depend completely on instrumentation. All available endoscopic systems for both coaxial endoscopy and endoscope-assisted surgery have limitations. For pure endoscopy, the major areas of tension involve visualization, illumination, and the size and configuration of the instrumentation. With current technology, for visualization to improve, the size of the endoscope must increase, which works against the goals of minimal invasiveness. Similarly, to increase illumination, the size of the illumination channel, temperature, or both must increase. The working channel for the endoscope limits the size and shape of instruments; such limitations are more constraining than with open microsurgical approaches. However, increases in size also increase the impact on the brain. More importantly, the need to work coaxially places restrictions on the ability to place tension on structures, thereby hampering the surgeon’s ability to perform speedy and precise microsurgical dissection. With rigid endoscopes, the more angles and trajectories used, the greater the effect on the brain.

Technologic advances are likely to create solutions for many of these problems. Flexible fiberoptic fiberscopes can overcome the limitations imposed by the rigidity of the endoscope but are plagued by poor image quality. Newer, high-resolution, chip-on-the-tip charge-coupled device cameras can move the camera to the tip of the endoscope, thus removing the need for a rod lens and bulky camera. This technology will also allow flexible endoscopes to offer the same picture as a rigid endoscope. The quality of the picture is improving rapidly as chip technology improves. Flexible and curved instruments and innovative new tools for coagulating and cutting are needed to bring the possibilities of pure endoscopy to their culmination. Improved instruments for morcellating and suctioning away tumors would greatly increase the efficiency of endoscopic tumor removal and increase the size of masses that can be removed. Innovative new instruments that allow true bimanual dissection through a single endoscope would increase the complexity of what can be assayed. The possibility of robotic instruments that could allow very fine work to be performed through a small portal has great appeal.

For endoscope-assisted and endoscope-controlled surgery, major advances will be directed at allowing neurosurgeons to do microsurgical work “around the corners” under endoscopic view only. Curved instruments that allow microsurgical work to be done in these recesses and instruments that combine the functions of multiple instruments (for instance, a suction and endoscope or suction and cautery) may facilitate working with the endoscope with a single hand. New tools for sharp endoscopic dissection, primary dural closure, and control of vascular structures are needed. Finally, every improvement in visualization is welcome. A sharper and brighter picture, the possibility of true three-dimensional endoscopy, flexible endoscopy with a picture equivalent to that of a rod lens endoscope, and incorporation of frameless stereotactic data onto the video image would all be welcome additions to the endoscopic armamentarium.

A reality behind innovation in neuroendoscopic instrumentation is the need for enough economy of scale for instrument companies to invest in potential new developments. As more and more neurosurgeons learn these techniques and apply them to neurosurgical pathologies, the market will expand and the attractiveness of investment in this area will increase.

Conclusion

The endoscope is a useful tool that can allow minimally invasive management of many neuro-oncologic problems. Its ability to illuminate and magnify the operative field and to see around corners can improve the neurosurgeon’s ability to visualize pathology and improve the completeness of tumor resection. The endoscope can be used to manage associated conditions such as hydrocephalus. In the anterior skull base, a wide variety of tumors can be managed via an endonasal approach, from the anterior fossa to the clivus to the top of the cervical spine. The skills and instrumentation of neuroendoscopy are highly specialized, and specific training is required. Increasingly, this training is becoming available in residency programs and in practical courses. All neurosurgeons should avail themselves of the opportunities offered by this versatile tool.