Techniques

All endoscopic procedures are performed under general anesthesia independent of patient age or disease. The anesthetic agents are chosen based on the presence or absence of increased intracranial pressure. Anticonvulsants are not routinely administered.

For procedures that require an anterior approach through a frontal burr hole, the patient is positioned supine and the head is immobilized in a rigid or semirigid fashion. For routine endoscopic third ventriculostomy (ETV), the patient's head is placed in a gel doughnut. The neck is slightly flexed to allow a direct trajectory into the third ventricle through the foramen of Monro and lateral ventricle. Modifications of this technique have been introduced so that one can also approach the posterior aspect of the third ventricle through an anterior burr hole.

For procedures requiring a posterior approach, the patient is positioned so that the head is lateral and the sagittal suture is parallel to the floor. This may require rigid fixation such as the Mayfield head holder or some other similar device or placement on a cerebellar head rest. This allows for ease of orientation and recognition of the normal anatomy, which is crucial in endoscopy. Approaches may be modified based on the patient's disease and medical condition.

All endoscopic procedures rely on the presence of fluid-filled spaces for adequate visualization ,and therefore irrigation is essential to endoscopy. A number of authors have advocated continuous irrigation throughout any endo-scopic procedure, but related complications have been described in the literature (12,14,15), and therefore one has to be cautious and judicious in the use of continuous irrigation. The choice of fluid has also been a matter of discussion; generally Ringer's lactate has been recommended, although normal saline is equally safe as long as there is constant irrigation of the fluid through the CSF spaces.

Preoperative Assessment and Evaluation

Preoperative evaluation and planning for endoscopy requires magnetic resonance imaging (MRI) of the brain. This information provides adequate visualization of the intraventricular spaces, allows assessment of the size of the ventricles, and gives information as to the possible location of obstruction in the CSF spaces or the location of the cyst or tumor to be treated. On occasion, a contrast-enhanced dye study with injection of contrast into the ventricular system may provide more information regarding the communication between the ventricles, which can then lead to the appropriate choice of endoscopic procedure and endoscope. Computed tomography (CT) imaging of the brain alone is not sufficient prior to performing an endoscopic procedure unless there is a previous MRI of the brain and the CT scan is obtained only to document an increase in the ventricles, such as in a case of shunt malfunction in which an ETV is being considered.

If endoscopy is being performed in an attempt to remove a brain tumor, provisions need to be made for conversion of the procedure to an open craniotomy prior to the beginning of the operation (choice of incision, position, and immobilization of the head and appropriate consent).

Entry Sites for Endoscopy

The placement of the burr hole is dependent on the disease and the goals of surgery: is the fenestration (e.g., ETV) going to be accompanied by biopsy of an intraventricular tumor in a different location, will there be placement of an intraventricular device, will there be intraoperative microsurgery in addition to the endoscopy, and so on?

The approaches are generally via a frontal burr hole, which allows access to the frontal horns of the lateral ventricles, the foramen of Monro, and the third ventricle, or through a posterior occipital burr hole, which is usually utilized for lesions in the posterior aspect of the lateral ventricles and in the quadrigeminal cistern area. Burr holes placed in the forehead have also been recommended for approaches to the posterior part of the third ventricle and pineal area, as well as for aqueductoplasties. Laterally placed burr holes in the frontal area are utilized for septostomies and approaches to the midportion of the lateral ventricles. In some cases, more than one burr hole may be necessary, with a second endoscope or ventricular catheter placed for localization or additional illumination. Colloid cysts generally require two burr holes for adequate visualization and illumination, and the recent development of endoscopic resection of hypothalamic lesions also requires two burr holes.

Flexible Endoscopy

Flexible endoscopy is utilized for fenestration of multiloculated ventricles for which navigation in more than one ventricular space will be required. The choice of anterior or posterior approach depends on the patient's condition and ventricular anatomy. It is often helpful to have a second ventricular catheter placed as a guide, as the ventricular anatomy is frequently very abnormal and some of the normal landmarks may be distorted. This is especially true in patients who have had prior intraventricular hemorrhage or infection or have had long-standing shunts with previous revisions. Intraoperative imaging with ultrasound or frameless navigation to correlate the position of the endoscope with the ventricular anatomy is extremely helpful to ensure that the goals of surgery have been met. Flexible endoscopy can also be used to biopsy tumors or lesions in the posterior part of the third ventricle when the third ventricle is small or the placement of the burr hole does not allow the angulation of the rigid endoscope. Biopsy specimens with the flexible instruments are very small, and one requires experienced neuropathologists in such cases.

Rigid Endoscopy

Rigid endoscopy is the preferred method for most primary intracranial endo-scopic procedures. The most common procedure is the ETV, which is performed via a frontal burr hole. As with all endoscopic procedures, the most important aspect is to identify normal anatomy and orient oneself. Once the normal anatomy is confirmed, the procedure is performed. Obscuration of the landmarks and lack of identification of the normal anatomy are contraindications to performing this procedure as the potential for serious neurovascular injury is great. Some authors have suggested utilization of additional imaging techniques to guide the endoscope accurately into the lateral ventricle or to predict the location of the basilar artery and the other vascular structures around the floor of the third ventricle (16,17). One such system incorporates a sono probe at the tip of a catheter that is placed in the rigid endoscope sheath. It requires experience by the user and facility with interpretation of both endoscopic and ultrasound images but has potential in the treatment of multicystic hydro-cephalus and may have more indications as future refinements to the technology are made. Contact ultrasonic probes have also been described as the method to make the perforation through the floor of the third ventricle (18). Long-term safety data on this procedure are not available yet, and it is not known whether there are any injuries to the vessels from this probe.

Contact lasers have been used but are not recommended because of the potential of injury to the basilar artery and perforating vessels in the vicinity.

Rigid endoscopes are also the preferred endoscope for the fenestration of suprasellar arachnoid cysts and for the fenestration of intraventricular arachnoid cysts. The same principles of orientation and confirmation of anatomic landmarks apply here as with any endoscopic procedure.

Tumor biopsies can be performed with sufficiently large samples for tissue analysis. However, apart from colloid cyst removals, there are no documented cases of complete tumor resection performed entirely endoscopically, and it is primarily used as an assistive device. Because of the small size of the tumor samples with the endoscopic biopsy forceps, multiple samples usually need to be obtained. Large samples cannot be removed in one piece because of the small diameter of the working channel of the sheath, and another option is to remove the entire endoscope and sheath while holding on to the specimen itself. This has the disadvantage of removing and replacing the endoscope and sheath and the potential for additional injury to the brain.

Endoscopic imaging is becoming incorporated in aneurysm clipping and tumor resection when an open craniotomy with microsurgical resection is being performed (19-22). A diagnostic endoscope with a sheath that has only one channel or no working channels is utilized, with different angles allowing for side views or a backward view. In these situations, it is very important that the endoscopist be familiar with the anatomy, orientation, and different images from the different angles of the endoscope since lack of familiarity will lead to misinterpretation of the images.

Endoscopes are investigated for use in the removal of intraparenchymal hemorrhage (13,23) and intraparenchymal tumors (Manwaring, 2002, personal communication). These applications are still in development, although with the integration of frameless neuronavigation into the endoscopy system, there appears to be some potential application of endoscopy in these cases. So far, the only tumors that have been biopsied and resected have been intraventricu-lar tumors or tumors in the thalamus or walls of the ventricles so that the endoscope can have access to the tumor itself.

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