**5. Application of endoscopy to neuro-oncology**

Endoscopy can be applied to neuro-oncology as it provides an ideal venue for this. The advancement of visualization of intraventricular pathology, management of tumor-related hydrocephalus, safer techniques of biopsies, and minimally invasive surgery for removal of intraventricular tumors are very useful adjuncts to traditional tumor management [6].

Once a tumor is removed, the surgeon can use the endoscope to assess the degree of resection. With the advantage of looking at the remote corners, the same surgery can be accomplished through a smaller craniotomy with the help of the endoscopes, may be with different angles, in keeping with the concept of minimally invasive surgery with maximally effective results [44]. By removing more tumor intraoperatively, endoscopy may improve the rate of survival for patients with benign tumors by reducing the less chance of recurrence and less requirement of adjuvant treatment [45, 46]. Adjunctive procedures, like third ventriculostomy and septostomy, can easily be performed through the same access points to manage concomitant problems like secondary hydrocephalus, where shunt placement can be avoided [6].

Endoscopic visualization and removal of intraaxial brain tumors using stereotactic guidance was first described in 1980 [47, 48]. In addition, stereotactic endoscopy for tumor removal through a conduit created by a bullet shaped dilator was reported in 1990 [49]. Kassam et al. described the development of a completely endoscopic system for resection of intraaxial tumors through a dilatable conduit.

The channel is created by dilatation of white matter, minimizing the neural structures by attempting to create a parafascicular approach to the tumor. The port creates an air medium that allows bimanual dissection. The instruments work parallel to the endoscope and the technique is safe as this does not deviate from the proven microsurgical principles [50].

Dr. Kelly pioneered a 20 mm diameter stereotactic tubular retraction system for the microscopic resection of deep brain tumors. Based on his work, the concept of endoscopic resection was initially developed [51–53]. As the microscope delivers a cone of light, tapering from the source until it reaches the target, the conduit that is required to deliver microscopic visualization is larger than an endoscopic conduit. In contrast, the endoscope delivers light and magnification via an inverted cone of light. Therefore, a much smaller port (11.5 mm) or conduit can be used to deliver the endoscope only very few millimeters from the target to visualize the tumor, creating a "flashlight" effect to illuminate the tumor. This advantage of the endoscope can be exploited to perform intraaxial tumor resections [50].

Although the conventional endoscope does not provide binocular vision, it has not been a major issue. Bimanual dissection allows proprioceptive feedback by which the loss of binocular vision can easily be compensated. This is similar to working from the observer's perspective during microscopic neurosurgery. Once adequate experience is gained, the surgeon develops a good perception relying on tactile feedback from touch and movement. The development of 3D technology overcomes this limitation. Kassam et al. reported that endoscopic approach may prove to be superior to microscopic one for subcortical tumors, because the endoscope allows unrestricted and better illumination in deep regions with a closer view of the pathology. For cortical lesions, due to its location, microscope has got obvious advantages and it is preferred. But for deep-seated brain tumors, direct endoscopic view can add lot more details and significant advantages for removal. Many surgeons now prefer to have a final endoscopic visualization to confirm adequate resection for intraparenchymal tumors once the microscope has been taken out of the field [54].

Selected primary and metastatic brain tumors may be safely removed with this approach. Using dynamic retraction of the port and the method of piecemeal extirpation, a small conduit can be used for an effective removal of tumors that are much larger than the conduit itself can be effectively removed. Obviously, the longterm prognosis will ultimately be determined by the biology of the tumor. However, in the group of appropriately selected patients the port may offer a viable option to achieve the goals of surgery—that is, partial removal with a view of cytoreduction or complete removal of tumor with an acceptable level of morbidity, minimizing both corticectomy volume and white-matter dissection required for the resection of the tumor [8].

The burr hole is placed in an area of skull so that the scope should enter the ventricle from a furthest possible point towards the tumor so that the scope is directly viewing the tumor, not looking from one corner. The distal approach helps the surgeon to perceive the abnormal anatomy following an orientation of normal anatomical structures while passing the scope through non-pathological part of the brain. As most of the distal part of the scope is within the ventricle, the surgeon can move the scope in multiple directions with more flexibility without damaging the normal surrounding neural structures [6].

Endoscopic approach is not ideal for all intraventricular tumors. The suitability criteria for endoscopic removal include –


iii.Associated secondary hydrocephalus

iv.Histologically low grade [6].

Even with the advancement of the MRI, all these criteria may always not be possible to confirm preoperatively.

Few principles should be followed during removal of the intraventricular tumor safely. The surgeon needs to choose a trajectory that avoids eloquent structures but allows a good view of the tumor. The outside of the tumor is coagulated with either monopolar electrocautery or a laser to ease the removal of the tumor. Profuse irrigation is needed for clearing the blood and debris and to prevent building up of too much heat inside the ventricle. Cysts need to be opened and the contents are either drained or sucked or removed as piecemeal. Remaining wall is removed piecemeal which allows removal of the whole pathology with a smaller access. With completion of the procedure, the scope is withdrawn while inspecting the tract for intraparenchymal bleeding [6]. Hemostasis is obtained with copious irrigation. If there is a clot noted, a cut-end foley catheter can be used with gentle aspiration to remove blood clot during intraventricular hemostasis [55].

Endoscopic tumor biopsy is a well-known procedure for the management of intraventricular tumors. It has a high diagnostic yield of more than 90% and low risk of less than 3.5%. Germ cell tumor, infiltrative hypothalamic/optic pathway glioma, and Langerhans cell histiocytosis can be addressed with endoscopic biopsy [2]. Endoscopic removal is well-established procedure for colloid cysts or tumors that are also pedunculated at the ependymal surface. Endoscopic excision of a colloid cyst is accessible and technically possible through the lateral ventricle in most cases unless the cyst is very large, which poses more risk of venous injury at the foramen of Monro [2, 56]. Transventricular endoscopic decompression of tumor cyst can temporarily or permanently treat the obstructive hydrocephalus or prevent the visual loss [7].

Hypothalamic hamartoms are rare non-neoplastic congenital malformations that arises from inferior hypothalamus and give rise to the symptoms of gelastic seizures, precocious puberty and cognitive problems. Surgical treatment is required for all patients except those with precocious puberty. Single or combination treatment are supposed to be used according to HH type as per the classification of Delalande and Fohlen [57] or Choi et al. [58] Endoscopic resection guided by stereotactic navigation has been attempted for surgical removal of small HHs, but parts of the tumors remained. Surgical resection of HHs is typically carried out in multiple steps. Despite that, recent reports indicate that endoscopic disconnection of HHs seems to be more effective and safer than other modalities [58, 59]. In spite of good visualization of the endoscopes, navigation assistance is recommended, in most cases, for obvious reason of precision in normal sized lateral and third ventricles [7].

### **6. Endoscope-assisted microsurgery**

Endoscope-assisted microsurgery is the most rapidly growing area in endoscopic neurosurgery. Use of endoscope is the next step for surpassing the limitations of traditional microsurgery and allows the neurosurgeon to view tumor remnants such as those hidden behind eloquent brain tissue, a cranial nerve, or the tentorial edge.

Microsurgery evolved to maximize visualization and minimize retraction. Endoscopy allows the neurosurgeon to move another step further towards achieving these goals. Rigid endoscopes with various angles and flexible endoscopes help the surgeon to look around the remote corners which can be very useful in the removal of tumors and the clipping of cerebral aneurysms. Several approaches to the

extra-axial structures of the skull base have been described to improve visualization with strict adherence to the principles of standard microsurgical techniques. The most commonly adopted method is to insert the scope down the same operative field. This, obviously, creates no further morbidity but tends to clutter the already limited surgical field, which can be avoided by inserting instruments through a contralateral burr hole. The subarachnoid space can be accessed through a small supra-orbital incision and then standard microsurgical dissection is carried out to identify the pathology. Once the pathology is identified under direct vision, the endoscope should be fixed in place and the attention is focused on the ipsilateral side. This technique offers excellent visualization of the tips of an aneurysm clip or the contralateral extent of a tumor. Endoscopes are increasingly used to inspect tumors, tumor beds following resection, aneurysms and other pathologies. Various authors have described the advantages of endoscopes for these purposes [25, 60, 61].

The endoscope offers a superior and often novel view of the anatomy, which can be extremely useful for the understanding of the operative approach for the trainee neurosurgeons. Furthermore, the trainer and the learner share the same view of the surgical field, which may not always be possible even with an operating microscope.

There are risks, the most problematic of which of using the scope is the risk of friction upon structures while introducing the scope. It is of crucial importance to guide the endoscope by visualizing it along the length of its shaft, rather than watching the images on the monitor. Following placement of the scope into the surgical field, if the scope is not fixed, then small, barely noticeable movements at the tip can be the result of larger excursions at the back of the scope. Obviously, this can have potentially disastrous consequences. Hence, the endoscopic field of work should be observed and the movement of the shaft of the endoscope has to be delicate at the same time. A fixed endoscope holder can stop the unnecessary movement of the endoscope and aid the surgeon to work with both hands and to use more complex instruments, and will also prevent the endoscope from drifting against vital structures located superficially along the operative corridor [6].
