12 Intraventricular and Paraventricular Tumors
12.1 Introduction
Intraventricular and paraventricular tumors represent a diverse class of neoplasms and present a unique challenge for neurosurgeons due to their location deep within the brain. Microsurgical approaches are effective methods for biopsy and resection. These procedures can be associated with significant morbidity due to retraction injury, dissection complications, and ventricular emptying. Over the past 40 years, advances in minimally invasive endoscopic approaches to these tumors have resulted in shorter hospital stays, lower postoperative morbidity, and better quality of life following surgery.1,2 Concomitant with these advances have been the development of rod-lens high-resolution endoscopes, fiberoptic light-transmission technology, and compatible instrumentation. For many intraventricular tumors, the neuroendoscopic approach has become the preferred surgical method, if not the standard of care, as more neurosurgeons gain expertise with the instruments involved and their maneuvering.
12.2 Pathophysiology
The heterogeneity of intraventricular and paraventricular tumors is daunting and makes any unified surgical approach impossible. The ventricles and paraventricular structures are composed of a multitude of cells, all of which have the potential for neoplastic transformation. A disciplined approach toward predicting the pathophysiology of the tumor will contribute greatly to selecting the best surgical management approach. The pathophysiology of these tumors is best estimated through a good understanding of the clinical history of the patient, anatomical tumor position, patient age, and imaging characteristics.
Importantly, any history of polyuria and polydipsia restricts the potential diagnoses to a few categories, most notably primary central nervous system (CNS) germ cell tumors and Langherhans cell histiocytosis. A history of systemic cancer should lead to a differential that includes metastatic disease. Any history of radiation therapy, especially as a child or young adult, predisposes the individual to meningiomas. The need for chronic immunosuppression should raise the possibility of primary CNS lymphoma. A known or suspected diagnosis of the neurocutaneous syndrome Sturge-Weber syndrome (encephalotrigeminal angiomatosis) nearly always means that a foraminal tumor will be a subependymal giant cell astrocytoma (SEGA). Of course, these key findings need to be considered in the context of other findings including tumor anatomy and the age of the patient.
The age of the patient plays a crucial role in clarifying the potential disease type and hence the surgical goal. Owing to the disproportionate representation of primary CNS germ cell tumors (pure germinoma and nongerminomatous germ cell tumors) in children and adolescents, endoscopic biopsy for intraventricular tumors is far more common for them than for adults. Other common histologic subtypes of pediatric tumors include choroid plexus tumors, ependymomas, pineoblastoma, and atypical teratoid rhabdoid tumors (AT/RT). In the adult population, meningiomas, pineal parenchymal tumors (pineocytoma, intermediate grade, papillary tumors of the pineal region), and glial tumors (astrocytoma, subependymoma, and SEGA) dominate. Considerations of metastatic disease or primary CNS lymphoma are common indications for endoscopic biopsy in the adult population. Colloid cysts of the third ventricle although rare, represent an ideal candidate tumor for endoscopic management, a topic discussed in Chapter 13.
From the standpoint of intraventricular endoscopic surgery, a distinction must be made between tumors arising from within the ventricular compartment (i.e., intraventricular) and those extending into the ventricle from an extraventricular source (i.e., paraventricular). This discrimination plays a critical role in defining the potential surgical goal. Simply stated, tumors that emanate into the ventricular compartment from a subependymal or white matter origin make total removal by a purely endoscopic approach unlikely. On the contrary, tumors that have limited penetration outside of the ependymal interface are more amenable to endoscopic removal. Both classes of tumor however are good candidates for endoscopic sampling if other clinical features support that goal.
In addition to the ependymal relationship, the presumed anatomical site of origin is also a key feature in defining the surgical goal. Tumors of the atrium are most frequently meningiomas or choroid plexus tumors, both best managed with aggressive surgical removal and having little need for biopsy. On the contrary, endoscopic sampling of tumors having a pineal or parasellar origin might have a dramatic impact on clinical decision making. Foraminal tumors are limited to a few subtypes, most notably the SEGAs, subependymomas, and central neurocytomas, all of which are best handled via total tumor removal. Paraventricular and multicentric tumors, especially in the subependymal region, are more varied in histologic subtype and best managed with endoscopic biopsy prior to defining a clinical management scheme.
12.3 Clinical Features
The presenting symptoms of intraventricular tumors primarily depend on location. Many intraventricular tumors block the egress of cerebrospinal fluid (CSF) at the foramen of Monro (Fig. 12.1), cerebral aqueduct (Fig. 12.1, Fig. 12.2, Fig. 12.3, Fig. 12.4, and Fig. 12.5), or foramina of Magendie and Luschka, producing obstructive hydrocephalus and elevated intracranial pressure (ICP). Depending upon the age of the individual, these symptoms will vary. In infants and young children, divergent macrocephaly, poor feeding, irritability, and developmental delay are most prominent. Beyond childhood, headache, vomiting, confusion, and gait ataxia are most frequent.
The importance of a thorough review of magnetic resonance imaging (MRI) cannot be understated when considering endoscopic surgery for intraventricular tumors. High-resolution T2-weighted MRI sequences are most useful owing to the fact that they provide the best definition of cerebrospinal fluid (CSF) pathways and tumor interfaces (Fig. 12.1, Fig. 12.2, Fig. 12.3, Fig. 12.4, and Fig. 12.5).
12.4 Treatment with Endoscopic Surgery
Using imaging detail and clinical history to define a clear surgical objective is of the utmost importance. Notably, one needs to define whether the surgical intent is to obtain representative tissue samples or attempt total removal. Owing to the large size of many of intraventricular tumors, conventional microsurgery may be the best option if the surgical intent is total removal (Fig. 12.6). Defining the surgical goal and intent prior to surgery will impact surgical planning, trajectories, and requisite equipment. Similarly, identifying whether CSF diversion (third ventriculostomy or septostomy) is necessary is crucial to the success of the procedure.
12.4.1 Patient Positioning
Patient positioning and room orientation can greatly facilitate a smooth operative course. For most intraventricular tumors, the patient′s head can be placed on a padded horseshoe head holder, although pin fixation should be used when stereotactic navigation is being used. Body position will be dictated based upon the planned entry site, but for most indications a frontal bur hole and hence variations on a supine position will be used. Parietal or occipital entry sites with a requisite lateral or three-quarter prone position will rarely be used, usually only for a longitudinal exposure of the body or temporal horn of the lateral ventricle. Irrespective of the goal or position, a reverse Trendelenberg position with the entry site being the highest point of the surgical fields will help minimize CSF outflow and prevent ventricular collapse during cannulation. Positioning of monitors should be in such a way as to minimize the need for the surgeon to turn, which can result in inadvertent movement of the endoscope and instruments.
12.4.2 Equipment
Most neuorendoscopic procedures are performed via the use of a sheath that remains in the ventricular compartment and allows the insertion and removal of various endoscopes and instruments (e.g., scissors, cupped forceps, bipolar cautery, suction catheter). Ideally there should be two working portals for these purposes in addition to one or two extra portals for irrigation and fluid evacuation. Rod–lens systems are the preferred endoscope used for intraventricular tumor resection. These endoscopes have superior resolution compared with their fiberoptic counterparts, though they are more costly, fragile, and have no steering capabilities. The most common lens diameter of these rigid endoscopes used for intraventricular purposes range from 2 to 4 mm.3 In addition to the standard 0º endoscope, 30º lenses are frequently used, which affords a wider field of visualization without torqueing simply by rotating the device on its long axis.
A wide range of compatible instrumentation is available to assist with neuroendoscopic procedures. Sharp and blunt dissection of intraventricular masses is accomplished with the use of microscissors, cupped forceps, and aspiration catheters. Variable speed aspiration catheters, which are clear, allow direct visualization of the tissue being removed to provide real-time feedback on the appropriate speed of suction and to prevent inadvertent suction of normal tissue or choroid plexus. For more mineralized tumors and avascular membranes, tissue shaving devices are becoming increasingly popular to allow for more efficient and expansive resections. Hemostasis is limited to relatively rudimentary methods of monopolar and bipolar cautery, but is supplemented with balloon catheter tamponade and irrigation.
12.4.3 Integrated Navigational Guidance
The goal in choosing the endoscopic approach for tumor biopsy or resection is to choose the trajectory that allows direct access to the intraventricular or paraventricular mass without applying excessive torque to surrounding tissue. To this end, stereotactic navigation is recommended for identifying the most appropriate approach. Establishing an ideal trajectory should always incorporate three elements: the entry site, the endoscopic corridor, and the target site. Intuitively, the entry site is an extension of a linear path from the intended goal, but modifications should be made using intraoperative navigational guidance. Placement of the bur hole with stereotaxy helps avoid sulcal anatomy and appreciable vascular tributaries, minimizing the potential for hemorrhage. The surgical path likewise should take into account all subcortical pathways, intervening sulci, and vascularity. Establishing an optimal target is self-explanatory and dependent on the surrounding anatomy and CSF conduits. As much as possible, a perpendicular trajectory to the target based on navigational guidance will optimize the function of most compatible instruments.
12.4.4 Procedures
Endoscopic Tumor Biopsy
Tumors situated in the third ventricle and pineal region present a diagnostic challenge due to their heterogeneity and accessibility. Tissue sampling is frequently needed and will impact management decisions. Neuroendoscopy has drastically altered the management of these tumors, as histopathologic diagnosis is obtainable in 90% of cases.4 Similarly, paraventricular parenchymal tumors (i.e., thalamomesencephalic or basal ganglia tumors) situated near critical structures can have extension into the intraventricular compartment and provide an alternative avenue for biopsy (Fig. 12.7 and Fig. 12.8, Video 12.1).
Using endoscopy for direct visualization of the tumor, identify the area that will maximize diagnostic efficacy. Avoid obvious vascular structures on the tumor surface to prevent intraventricular hemorrhage and subsequent reduction in visibility. Additional procedures for the treatment of hydrocephalus, such as septostomy (Fig. 12.8) or endoscopic third ventriculostomy (ETV), can be simultaneously performed at the time of biopsy. These scenarios may require alternate bur holes to be placed or may alter the location of the primary bur hole. In preparation, the incision and bur hole location are dictated by the trajectory to the tumor within the ventricular system. Tumors located at the foramen of Monro or arising from the septum pellicidum are best approached through a right-sided, precoronal entry point that is 3 to 4 cm from midline. For tumors that are eccentric within the lateral or third ventricles, a contralateral approach should be utilized, using stereotactic navigation when available (Fig. 12.8 and Fig. 12.9).
Endoscopic biopsy of third ventricular tumors requires careful review of preoperative imaging, since the location of the tumor relative to the massa inter-media will dictate the entry site. For lesions anterior to the massa intermedia, a standard precoronal bur hole ~ 3 cm from midline is used. In contrast, lesions located posterior to the massa intermedia require a more anterior approach to account for the linear trajectory necessary to safely access the tumor. In these cases, the bur hole should be placed further in the precoronal-frontal area, with final bur hole location predicated on the optimal navigation-guided trajectory.5 Rigid endoscopes are usually used for lateral and anterior third ventricular tumors, whereas flexible endoscopes are often used for posterior third ventricular tumors (Fig. 12.10, Video 12.2). Once the tumor is approached, an avascular region of the mass is identified and biopsied with cupped forceps. Samples should be taken from multiple sites to prevent indeterminate histopathologic diagnosis. It is imperative that coagulation of the tumor surface is avoided until after biopsies have been obtained to preclude inaccurate analysis due to artifacts.
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