38 Application of Endoscopy in the Management of Meningiomas
Meningioma surgery often places a great emphasis upon achieving a complete resection due to the potential curability or long-term survival advantage associated with the degree of resection. In deep-seated lesions within the cranial base, posterior fossa, or ventricles, the balance between degree of resection and risk of surgical access to such areas often means that traditional microscopic skull base approaches do not always achieve a complete or near-complete degree of resection. This may result from a calculated risk assessment of potential damage to neurovascular structures but may also be due to lack of visual access in terms of restricted line of sight and lack of illumination of areas where brain retraction is not an option.
The advantage that endoscopy brings to neurosurgery, and in particular meningioma resection, is that it allows a surgeon to perform minimally invasive neurosurgery in areas that are risky and difficult to access, such as the ventricles and cranial base, without necessarily compromising the degree of resection. The natural advantages that endoscopy brings to surgical approach to a deeply situated meningioma include the ability to angulate the field of view within a narrow line of approach, improved illumination, and superior up-close visualization of surrounding neurovascular structures.
There are certain limitations to the use of the endo-scope. The lack of a true three-dimensional image with endoscopy and the absence of any visualization of structures “behind” the endoscope lens make navigation in tight spaces and passing of instruments difficult. Furthermore, using an endoscope sometimes limits the surgeon in utilizing both hands for operating, thus requiring either reliance on an assistant or employment of specialized instrumentation.
A steady growth of interest in neuroendoscopy is currently producing an exhaustive series of operative atlases, and new courses are available worldwide to teach the operative art of neuroendoscopy. Industrial support is also producing innovative new equipment to overcome the natural disadvantages of endoscopy. Therefore, despite the steep learning curve and relative infancy of this technique, all neurosurgical trainees are strongly encouraged to train in the optimization of surgical resection of meningiomas by employment of all available technological aids.
Instrumentation in Neuroendoscopy
The surgeon’s intention to perform minimally invasive surgery, as a general rule, will benefit from the availability of frameless stereotactic technology by allowing a surgeon to plan the least traumatic approach. Apart from planning the ideal trajectory to a lesion, it further allows a surgeon to identify and preserve vital structures without having to increase surgical exposure.
Figure 38.1A,B shows two variants of scopes commonly used within neuroendoscopy. Larger rigid scopes measuring between 2 and 6 mm in diameter are typically employed for either endoscopically assisted or endoscopically controlled surgery. In endoscopically assisted surgery, the surgeon utilizes traditional microsurgical techniques but uses the endoscope to assist in the operation. In endoscopically controlled surgery, the endoscope is the sole means of visualization but the surgeon places the instruments down the surgical corridor, not down a sheath. Pure endoscopic surgery refers to burr hole access with a sheath passed into the cranium through which the surgeon passes both the scope and the instruments. These scopes are smaller, between 0.9 and 2 mm in diameter. They are more fragile, and care must be taken by operating staff, surgical attendants, and sterilizing units to prevent fracturing the internal glass construction. Instruments vary by the technique employed. For pure endoscopy, the instruments need to fit down narrow working channels and therefore are quite restricted in their size and versatility. This technique suffers from an inability of the surgeon to operate using two instruments at angles to each other because the channels in most sheaths run parallel. Endoscopically assisted surgery requires instruments that are angled at the ends to fully utilize the scope’s ability to look around corners. It is pointless to be able to see behind an immovable structure unless one can also operate around that structure. Many instrument companies have identified this impediment and are now producing angled and flexible tools. The most common example of endoscopically controlled surgery is the endonasal approach to the skull base. With the popularity of this technique has come a plethora of instrument sets designed specifically for this application. The general principles behind these instruments are that the tools should not be bayoneted and that hybrid tools should be encouraged. The bayonet handle on micro-surgical instruments was designed to keep the surgeon’s hand out of the line of sight. This is not a problem when the surgeon is viewing the operation on a monitor. Furthermore, rotation of an instrument with a bayonet handle results in clashing of the tool against the endoscope when placed down the nostril. During many portions of the endo-nasal approach, the surgeon is operating with the scope in one hand and a tool in the other. If the tool can have more than one function, then it compensates for the negative impact of being a one-handed surgeon. Such “hybrid” instruments that are currently available include suction/bipolar, suction/monopolar, suction/curettes, and suction/endo-scopes. We believe that the development of the videoscope (miniaturized cameras) will inspire a tremendous evolution of instrumentation.
The use of an endoscope through a burr hole opening as the sole means for resection of a meningioma would be extremely rare. There would be very few applications for this technique apart from small and symptomatic intraventricular tumors. Intraventricular tumors that are situated away from the cerebral aqueduct and foramen of Monro are often large and are fed by choroidal vessels that are medial and consequently “hidden” by the tumor mass. Due to the typically high vascularity of a meningioma, it is essential that the vessels of the stalk are readily and initially accessible. However, a meningioma of any significant size would likely restrict or preclude access to this pedicle, and render hemostasis impossible. Therefore we do not recommend removing these large meningiomas using pure endoscopic techniques.
If the tumor is small and blocking the cerebrospinal fluid (CSF) pathways at the level of the foramen of Monro, resulting in ventriculomegaly, then the technique by which one would remove such a lesion is very similar to removal of a colloid cyst. The burr hole is placed according to the desired trajectory. If the tumor is within the lateral ventricle, then its blood supply is from the choroid plexus, and for early devascularization it should be approached posterosuperiorly. In this case the burr hole is made ~13 cm behind the nasion and 3 to 5 cm from the midline. If the tumor is arising from the third ventricle, then its blood supply is coming from the roof and the trajectory is therefore more anterolateral. In this case the burr hole is made ~8 cm behind the nasion and 7 cm from the midline through the nondominant hemisphere. Once the tumor is devascularized, it can be removed either in piecemeal fashion or, if small enough, en bloc.
The nature of meningiomas to grow at the skull base and their relative biological inertia allow them to attain considerable size. This also causes vessels and nerves to be intimately involved in their capsules. It also means that many occupy more than one intracranial compartment. All these features make total extirpation of these tumors difficult through a standard craniotomy. Hence complex skull base techniques evolved in the 1980s to allow the surgeon to remove skull base meningiomas totally and with minimal collateral damage and morbidity. However, these techniques require fellowship training, are time consuming, and may create morbidity from the approach itself, and, in reality, most nonuniversity-based neurosurgeons perform these cases infrequently. Endoscopically assisted surgery allows most cranial surgeons to optimize removal of many skull base meningiomas without the need to perform extensive bony removal or dual approaches. It does so by helping the surgeon to see important neurovascular structures before, during, and after tumor removal. It also allows visualization through the tentorial incisura for petroclival tumors or over the sphenoid wing for all tumors of the anterior cranial base that extend into the middle cranial fossa. Tumors that extend into foramina, such as those in the cerebellopontine angle that may grow into the internal auditory meatus and those of the anterior cranial fossa that grow into the optic canal, may be more completely and safely removed using the endoscope to look around corners ( Figs. 38.2A–C and 38.3A–C ). Of course, there will be tumors that cannot be removed through standard or minicraniotomies, even with the use of endoscopes. Some of these patients will need more extensive bony removal for complete resection. When the scope is used to assist in the removal of a meningioma, the majority of the mass is removed using a microsurgical technique, and the scope is placed into the cavity at any stage during the operation when enhanced visualization is considered necessary. Although scopes come in 30, 45, and 70 degree angles, in reality, operating at angles greater than 30 degrees is awkward and dangerous.