Surgery of Mesencephalic Brainstem Tumors Free

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- Colleagues and friends, thank you for joining us for another session of the Virtual Operating Room from "The Neurosurgical Atlas." My name is Aaron Cohen. Today we have a special guest, a dear friend, Dr. Miguel A. Arraez from Carlos Haya Department of Neurosurgery at University of Malaga. He's the professor and chairman there. He's truly a gifted surgeon, someone that I have followed very closely, someone who has contributed significantly to neurosurgery. Today he's going to talk to us about a very important topic, really a challenging area to reach, and that's the surgery of mesencephalon. Miguel, it's an honor to have you, and I look forward to learning from you. Please go ahead.

- Thank you very much. Thank you very much. First of all, I'd like to thank Professor Cohen for his kind invitation to participate in maybe the most prestigious learning tool in neurosurgery nowadays. It's to me a truly honor to be here, and I will try to show my experience and point of view about the surgery of mesencephalic brain tumors. This is the outline of our presentation, first of all, some introduction about general consideration in regard to cavernomas because we will deal with adult patient. A main indication is cavernoma, of course not the only one. Second, we will mention something about relevant aspect of the mesencephalic anatomy. And finally, we will do something like a tour, 360 degree around mesencephalon just to see different approaches and different techniques. Talking about brainstem cavernoma, why we are worried about that pathology? First of all, we have to mention that this prevalent location for brain for cavernoma, just 30% of the cavernomas are located at the posterior fossa and brainstem. Rate of bleeding, something between 0.6 and 5% annual rate. But the issue is once the lesion has bled, the probability of rebleeding is no less than 30%. Reason why once we have diagnosed that condition, we have to do something. Our important issue is the content, the concept of hemorrhage. Sometimes we are seeing a cavernomas with some blood inside. That may vary, but this is not a real hemorrhage, hemorrhages insofar as we have evidence of not only intra- but also extra-lesional blood in MRI along with acute onset and neurological deficit. Of course, indication for surgery is of paramount importance in that condition, although this is not the only concern. Apart from surgical indication, there are issue related to the appropriate moment of the operation, the timing. Of course, we will mention something about particular nuance of the surgical technique at the brainstem. And finally, there's so many different possibilities for approaches are going to be considered. So talking about surgical indication, we have to counterbalance the risk for the patient in the natural history of the cavernoma, compare with the possibilities of influence in some beneficial way in the natural history of the cavernoma. Good indication for surgery, first or second symptomatic hemorrhages insofar as you are gaining confidence in the techniques and possibilities of brainstem surgery. In some young patient, first episode is a case maybe you have to do the operation. Recurrent hemorrhage is also another indication. Lesion at the medullary, symptomatic lesion at the medullary region, exophytic tumors because they are very easy to be removed, and of course lesion with mass effect or involvement of consciousness are indication for surgery, usually are big lesion or bigger lesion, and when the size is over 20 millimeter, very frequently there is a clear indication for surgery. Last good indication would be severe and progressive neurological dysfunction. And if talking about good indication for surgery is important, even more important is talk about the bad indication. Patient with few or no symptom, deep and/or small lesions, it means that you are going to have a lot of morbidity probably in the approach. Minor hemorrhage with rapid improvement and asymptomatic lesion and lesion located in the area below the fourth ventricle. Below the fourth ventricle, from time to time we do operate cases, but they have to be patient with symptomatic lesions of course. And once the patient has been operated on, you have to take into mind that if you do a partial resection, approximately 50% of the patient will have another hemorrhage. So main goal in the operation is avoid morbidity but at the same time to carry out a complete removal of the lesion. Talking about complete resection, 85% of the patient are going to be okay with the same or even improve neurological situation. About 14, 15%, they may impair their condition, and a little less than 2% of the patient may die due to the operation. These are statistics taken from almost 750 patient. So important, the surgical indication, important just try to carry out a complete resection of the lesion. Regarding some nuance for cavernoma surgery, there are several rules, one of them trying to remove the lesion irrespective of the location where the lesion is abutting the surface. This is a keystone in the management of the operation of these tumors. Second, we have the rule of the two points. You can see that bottom right. You have to imagine a point in the center of the lesion, another point in the closest vicinity of the surface of the brainstem, and then just imagine one line. But as we will see, particularly in mesencephalic brain tumors, this rule is not very good because we have several anatomical structures. If we follow that rules, that may hamper very much the removal. So the third key point is the concept of safe entry zone, that is to say which areas of the brainstem can be open, just trying to avoid morbidity and of course mortality. Along with that, we nowadays understand that we don't have to remove associate venous anomalies, and very important, try to avoid the management around the lesion. Try to be very careful because morbidity and mortality in cavernoma surgery at the brainstem is related to, one, the entrance of the brainstem, and second, the management of the lesion. The key issue is just to keep a very clean plane around the tumor, preserving the normal anatomical structure. So another issue is the timing for surgery. Sometimes you have to do that at the acute stage, but you have to know that the risk is obvious because of sometimes the neurological situation of the patient. Second and very important, you have more chance to leave some part of the tumor behind and have a potential bleeding over time. Subacute stage is another possibility. Delayed surgery is another possibility. Delayed surgery is not very convenient in our experience, and sometimes due to the waiting list, the patient is creating something like a gliotic plane around the tumor that makes difficult a very clean dissection and provokes morbidity in the patient. What is the best moment? This is something just to be applied to any location and of course mesencephalic tumor. There are several publication, but in summary, it is understood that the subacute stage between six weeks could be an appropriate moment for the operation. Intraoperative monitoring is, of course, needless to say, how important is for mesencephalic lesion. Of course, somatosensory evoked potential and motor evoked potential are necessary. Mapping of the fourth ventricle of course is for the pontine region, but it's important to mention the role of electromyographic recording. Here you can see the way of inserting the needles just to monitor the sixth nerve. If we can monitor the sixth, the third nerve, we do not monitor the fourth nerve due to technical difficulty and also fortunately less relevance in the function of the vision. And well, a very important point is just to define the anatomy of the lesion at the MRI, just try to figure out where the safe entry zone will be in regard to that lesion, and then just making a plan for the surgical approach. We have several possibilities according to the location at the brainstem. And of course, talking about the surgical approach, we have to mention that this is very much related to the neuroanatomy, and well, we will mention something about midbrain anatomy, and we will use the preparation, beautiful preparation of Dr. Pablo Gonzalez who kindly just allow me to do so. Dr. Pablo Gonzalez is a neurosurgeon based in Alicante and leading the neuroanatomy lab at Miguel Hernandez University in Spain. Thank you, Pablo, and I will show some of your material. We can see just the bottom A has this sagittal view how the mesencephalon is connecting the metencephalon with diencephalon and upper telencephalon through this cerebral peduncles that you can see here. If we go to that beautiful anatomical preparation, just this is a front view, we can see the cerebral peduncles. We can see the interpeduncular fossa. We see the third nerves, and they are exit at the anterior mesencephalon and the mesencephalic sulcus. And this is something giving some idea about the anatomy, and we will mention later on something about how to get inside. If we go to B, we can see this posterolateral view, superior colliculi, inferior colliculi. This is the PCA over there, could be the SCA. This is the exit of the fourth nerve. We are, of course, at the right side. If we continue with B, we can take into account for laterally place how easy could be the subtemporal approach to reach those lesion at the lateral mesencephalon. We can see the correspondence with the MRI the limits in D of the mesencephalon. And this is E, that is bottom right. This is a very important, sorry, a axial section that is showing cerebral peduncles anterolaterally placed, posteriorly placed the tectal area and just in between the tegmental region. Tegmental region is a very sensitive area, not very easy to be accessed. And we will mention at the end of our presentation, the last part will be how to get inside such a difficult region. We have to know that mesencephalon is surrounded by water. We have several oceans or at least seas around. We can see the interpeduncular cistern in front of mesencephalon midline. Laterally, we have the parapeduncular cistern, crural anterolaterally placed, ambient posterolaterally placed. Posteriorly, we have the quadrigeminal cistern, and also we have to mention the velum interpositum, although usually we don't have many approaches to the mesencephalon through this way. Basically we have to cross some water before getting inside mesencephalon. How to get inside the mesencephalon, let's see what is around. This is from view of the mesencephalon. This is showing the interpeduncular fossa. And we see how the access, the pure anterior access is very much hampered, first, by the sella turcica structure, and second, because of this vascularization, our perforator. So in my particular experience, very seldom I try to approach mesencephalon through this way. In C, we are seeing the relationship of the mesencephalon with the temporal lobe. This is the cerebral peduncle, lateral mesencephalic sulcus. We will mention something more about this important structure as landmark to get inside mesencephalon. Also in this posterolateral view, we can see cerebral peduncles. We are at the left side, lateral mesencephalic sulcus. This is a small part of the surface of the mesencephalon that can be seen in between cerebral peduncles and tectal plate. And finally, this posterior view is showing the inferior and superior colliculi. This is something important to keep in mind, those structure. And I think this is a beautiful slide that is summarizing the safe entry zone. How can we get inside mesencephalon, try to avoid damage of what we can say the internal architecture of the mesencephalon. Anterior aspect, we see here the exit of the third nerve. Ideally it would be a safe entry zone medial to the third nerve. I have never actually done that because of the difficulty to go for an anterior. You can do through endoscopy, but I do endoscopy for many skull base lesion, but I try to avoid that approach through this number one safe entry zone. And at the lateral and posterior aspect, we have to mention the lateral mesencephalic sulcus. We have here some projection or the frontal pontine fibers. We have see the medial lemniscus, lateral lemniscus. And in between we have the mesencephalic safe entry zone that is usually pointed out anatomically thanks to the mesencephalic vein, the lateral mesencephalic vein. And also a very important safe entry zone is superior to the superior colliculi. That is the supracollicular safe entry zone. And opposite, we have inferiorly the infracollicular safe entry zone. So as I mentioned before, the safe entry zone are trying to take you inside, trying to respect the internal architecture of the mesencephalon. We have seen just in the previous slide the anatomical distribution of tectal area, cerebral peduncles, tegmental area. We have here what we have inside, the corticopontine fibers, corticobulbar fiber, corticospinal, substantia nigra. At the tegmentum, we have the brachium conjunctivum, that is, the superior cerebellar peduncle, ruber nucleus. At the tectal plate, we have the colliculi and some other fibers. We will just go to some detail in regard to the cerebral peduncle. We have to focus our attention in the preservation of the corticospinal, the pyramidal tract that you can see in this beautiful tractography in pink. This is the corticospinal or pyramidal tract. And well, we have to know that there are several others. The parietotemporal pontine tract can be projected in the way you can see in this animation. Pyramidal tract immediately anterior. You can see the frontopontine projection. And this is the way you have to have some idea about the anatomy of the cerebral peduncles. That is a very risky area. So now let's move to the third part of my presentation in regard to the different approaches according to the location of the tumor. We are just about to start focusing in the cerebral peduncle. As you can see here, according to the location, more anterior or more posterior, a subtemporal approach could be used, or for anterolaterally placed lesion, the COZ approach. This is where we have to go, and for that approach, the COZ has been devised, and there are many publication. In our experience, we think that only doing a very limited osteotomy of the zygomatic bone, you can, let's say, achieve your goals. This is a very easy procedure that is, you can see here the part of the zygoma that is removed and after the operation is put back in place with microplate. This is something that is going to take maybe 10 or 15 minutes more and allows you to reflect downwards the temporal muscle, because to my mind, what is important in this approach is just avoid the retraction of the temporal lobe. And as a matter of fact, what you are doing is not compressing, reflecting, but just lifting up and working underneath the temporal lobe. See this case, this is a 16-years-old patient, a piano player, with this lesion and progressive contralateral, left contralateral hemiparesis. You can see the lesion mainly at the mesencephalon also with some the diencephalic growing, and the approach in the way we have been mentioning. We are at the right side, and we take advantage of the release of the CSF. And this is the way you have to work underneath the temporal lobe. We are seeing the structure of the tentorial incisura. And this is a very relevant anatomy. This is right carotid artery. This is become by definition, this is the superior, the posterior cerebral artery. This is the nerve. So that artery by definition, too, is superior cerebral artery. Just after examining, we just went, I'd like to go a little back, because just we moved to the area where the lesion is at the closest vicinity of the sort of face of the brainstem. And you can see how the lesion is right underneath the small opening. We have to do some opening and elongating this opening, unlike the incision at the supratentorial central nervous structures. We have seen the removal, and we have seen this clean cavity of the cyst. This is the postoperative MRI, satisfactory, but the patient became immediately after the operation completely hemiplegic. But this is four months after the operation with a good recovery. And this is ninth month after the operation, the patient is again playing piano. We were very happy when we received this video from the patient. And why the explanation about that is related to the anatomy. You can see the anatomy, and you can see the loosened distribution of the corticospinal tract. This is another beautiful preparation of Dr. Pablo Gonzalez and his team. And you can see the way the corticospinal tract has this loosened distribution, unlike the compact fibers of the cerebral peduncle anterolateral mesencephalon. So fortunately, at the pontine region, we don't have much problem with the corticospinal pathway, unlike the situation at the mesencephalon. Surgery in acute stage, this is another case at the anterolateral mesencephalon. You can see the lesion, and you can see the bleeding around. The patient was in a comatose situation. You have to do the operation. This is not the ideal scenario, but you have to do that. Same approach, and you can see, this is, again, right side, and you can see where to get inside mesencephalon where the lesion is in the closest vicinity of the surface. See the staining of the blood? And see how easily you can make some small opening and then elongating the opening, trying to, let's say, displace but not cut the fibers, the removal of the cavernoma. Cavernoma in subacute or acute stage is easy to be removed because you have blood around, and it's just leading to the good plane. And this is what we like to see after removal of a cavernoma. That is a very clean surface of the anatomy, unlike the situation of the gliotic plane. Just indicating that you will have some problems at the postoperative stage. This is the MRI. You can see how satisfactory the image is, and the patient also came back from, came round after this comatose situation. The patient, due to the hemorrhage, is still having some extrapyramidal signs. Just for the youngest colleagues in the audience, you can see this tremor that has some component of the Holmes tremor, very typical of the situation of the location of lesion at the mesencephalon. If we have a perfect lateral lesion, we have to keep in mind the subtemporal approach. This is a patient, 30 years old, with this acute hemorrhage with a slight hemiparesis and contralateral dysesthesia. See the lesion at the CT scan. See the lesion at the MRI, some venous anomaly over here, and very important to keep in mind anatomy. This is a very straightforward decision in regard to the surgical approach. We are at the right side. This is tentorium. You can see the fourth nerve. You can see the venous anomaly. This is a bridging vein we just wanted to preserve, and venous anomaly untouch. See just some moment of the easy removal of the cavernoma at the subacute stage. And this is the final cavity, a very clean cavity. You can see the postoperative MRI, very satisfactory by means of this temporal approach. And this is the patient still admitted with no, fortunately, no major neurological deficit and without any problem with vision and gaze. So subtemporal approach for lesion that are perfectly laterally placed at the mesencephalon. This is another situation at the mesencephalon. This is a young patient with a progressive clinical picture of lack of balance and also some tetraparesis with impairment. The patient was admitted, and we just use a course of corticosteroid. The patient did improve. We had that lesion at the mesencephalon. According to our theories, we should go from anterior, but in spite of the insinuating, let's say, clean area, we know that at least in our experience, this is a risky area because of vascularization, perforator, and the difficult access through the sellae or clivus. In any way, we manage conservatively the patient. See images of tractography. And the patient came back two months after with another abrupt onset of neurological deficit and clear impairment, so we decide to go ahead with the operation. Just the presumption of histology was cavernoma, according to the radiology report. And if we look at this picture, we can see the distortion of the tectal plate. You can see the lesion that is transecting the mesencephalon, and for that lesion, we just use the infracollicular approach. Infracollicular approach is very easy to be done. This is just about a suboccipital transtentorial approach. And you can see here top left the exit. This is tentorial sectioning, some retraction of the occipital lobe. I like to do this operation in semi-sitting position. Maybe you do that in lateral position, is better because the occipital lobe is lying down, but I do prefer those, let's say, midline and semi-sitting, in semi-sitting position approaches. So this is one of the very seldom instances in which in neurosurgery we see the exit, really see the exit of the fourth nerve. And just you can see, very important, you will see some staining over there. We use neuronavigation, but see this staining just pointing out toward the approach. And you can see the way we do a small incision, that small entrance that we elongate little by little, try to elongate the fibers instead of cutting that after removal of the lesion. A very funny lesion just bleeding with some vessels coming from the anterior area, from the vascular artery area. And you can see this frightening view. This is from posterior, and we have completely removed the lesion, that the lesion itself has transected mesencephalon, and this is vascular bifurcation. After removal of the lesion, see the view, these are the inferior colliculi. This is tentorium. We have reflected the occipital lobe, and this is the way the infracollicular route is giving you access to that lesion. The patient for our, after the operation, unfortunately regret. We took the patient back to the operating room and just removed the clot. And this is the MRI. Mesencephalon seems to have disappeared. It's not like that if you look at this axial view. But in any way, satisfactory result, although the patient has some disability due to the lack of balance, still some residual tetraparesis, arm, unfortunately partial bilateral . Histology, melanocytic tumor. I think it's important to mention this particular case because once we are in front of a MRI picture, at the brainstem, we are always thinking in two possibilities, glioma or cavernoma. But actually this is not like that. This is a very good example. This is a melanocytic tumor that is a complete benign lesion. And important to go to this publication 2013, 10 years ago. This is about almost 1,500 cases of biopsies. And this is a meta-analysis, of course, but the important issue is that as far as almost 9% of the lesion are not even neoplastic lesion. You have a variety of miscellaneous lesion, infectious inflammatories and other lesion and so and so, and this is important to be kept in mind in regard to surgical removal and/or at least biopsy in dubious cases. And now let's go to the last part of our tour. This is the posterior mesencephalon, the tectum. This is a 62-years-old patient with episodes of diplopia, and the patient was admitted due to aggravation with limitation of the upward gaze. See the lesion, atypical cavernoma just laterally place at the left side. And see a very easy approach through suboccipital transtentorial approach. This is a very important image. This is an abnormal vessel feeding the cavernoma. This is not related to the anatomy of that area, and giving the explanation why sometimes the cavernoma is bleeding so frequently in spite of being considered a non, let's say, highly vascularized lesion and in spite of having a negative angiogram when done. Cerebello-mesencephalic sulcus, and this is the way we could remove in total the lesion. See the clean plane underneath. And this is the general view of classical suboccipital transtentorial approach. This is the incision of the tentorial region and the vermian aspect. And this is the MRI, satisfactory. And the patient during admittance, the patient have only limitation of the upward gaze as had before the operation. And talking about tectum, we have to mention that the mesencephalic posterior region is also a frequent area to harbor a brainstem glioma, and well, there are many publications. We all know that tectal plate gliomas, you can see one of these examples, and this is a very important review about the management of and clinical history of tectal glioma. We understand that usually are indolent, no biopsy unless atypical finding in MRI. Very frequently they produce hydrocephalus and diversionary procedure. Usually ETV plus biopsy can be done and only operation is advised if the lesion is growing or have atypical features. This is a young patient with a tectal glioma with some uptake of contrast and some growing of the lesion, big size. So you decide first to do ETV and biopsy, pilocytic astrocytoma, and then subtotal but gross removal of the lesion by means of suboccipital transtentorial approach. And last part of our tour that we can consider 360 degree around mesencephalon are lesion that are not anterolaterally placed, not at the tectal area or at the tegmentum. This is a patient, 42-years-old patient, and well, the wife of a very good friend of mine, orthopedic, surgeon with that lesion that deemed to be considered a glioma, a high-grade glioma according to a spectroscopy. See the location of the tumor at the tegmental area. So for lesion like that, there are many publication. This is one publication from the group of Professor Spetzler, extreme lateral supracerebellar infratentorial approach for posterolateral mesencephalic lesions. Well, you can see here, the suboccipital craniotomy. You can see here some osteotomy about the transverse sinus and also the exposure of the sagittal sinus. I think that the conclusion of those anatomical works and our personal belief is that the suboccipital craniotomy is of course completely necessary. You have to do the osteotomy in order to be allowed to lift up tentorium, but you don't need to expose the sagittal sinus. So the rationale for that approach would be suboccipital craniotomy. You have to do the osteotomy about transverse sinus. You have to section the tentorium from below. We are seeing the exposure of the lesion, of our lesion at the right side. You have to cut the tentorium from below until tentorial incisura, and that way you can displace laterally the occipital lobe in order to get the proper lateral, ideally, almost perpendicular angle, the more perpendicular you can to the lateral mesencephalon. And you can see the vermian region occipital lobe. Sometime it is not necessary to be much displaced because of the anatomy and anatomy of the tentorium. You can see the lateral mesencephalic vein. This is a very important landmark, to find the lateral mesencephalic sulcus. That could be a good entry zone. These are some branches of the superior cerebellar artery after the lateral mesencephalic segment and before, just giving branches to the upper cerebellum and vermian area. And this is the lesion that was almost safe, exposed by means of this approach. You can see the relevant anatomy and the lesion, by the way, very easy to be removed. That was pilocytic astrocytoma. This is another case, another young patient with contralateral hemiparesis and hemidysesthesia. See this lesion. Again, a spectroscopy was malignant glioma. For unknown reason, a spectroscopy at the brainstem is not much reliable. In any way, it was a young patient, and the lesion was something intriguing, and another reason, because this vessel, and we decide to go through this lateral mesencephalic sulcus. The operation was a nightmare because the lesion was bleeding like a real AVM. As a matter of fact, you can see the postoperative MRI histology was vascular malformation. You got a lesion with many vessels. And this is our patient at the immediate postoperative period with some impairment of the previous slight hemiparesis due to the difficulty of the removal of the lesion in the context of the bleeding. This is something we see sometimes, impairment of the symptoms, but fortunately in many cases, young patient, we have a pretty good, satisfactory neurological recovery. Well, this is serials MRI without any recurrency of any lesion. And this is the patient walking months after the operation and improving the hemiparesis and fortunately without any problems in regard to the sight and to the gaze. He just underwent a very small ophthalmological repair, and the function is pretty satisfactory. And this is the final case I'd like to present, and this is a case for the youngest colleagues in the audience. This is a patient, 55 years old, with progressive dysphagia. Dysphagia was very much bothering and worrying the patient and the doctors that were looking after the patient, and the case was sent. See this enlargement. Intriguing change of sight signal at the MRI, The lesion here with some small spots of of blood. And this is the lesion at the mesencephalic area. The patient was sent just to be operated on and the cavernoma removed. But we have to do a very exhaustive examination in brainstem tumor cases. And in the context of this examination, this is what we saw. This is a palatal myoclonus. Palatal myoclonus is pathognomonic of this condition, that is, the generation in regard to the interruption of the dentato-rubro-olivary pathway that are, let's say, establishing the boundaries and the structure of the Guillain-Mollaret triangle. So the symptoms of the patient were related to that problem. That problem came up over the last years, not related to the location of the lesion, not related to any bleeding. So that way we avoid the operation of the patient due to the clinical picture and also due to the fact that the patient didn't have any evidence of cavernoma hemorrhage according to our understanding. Well, this is my last slide. This is clear that we are dealing with a lesion in the context of complex anatomy. Surgical indication has several consideration in regard to the lesion and also in regard to the difficulty of the tumor in the context of that anatomy. Anatomical definition of the lesion nowadays is very well provide by MRI, and important, of course, the concept of safe entry zone in the mesencephalon, and as otherwise can be microsurgical technique with a particular nuance for cavernoma surgery are also of paramount importance. I'd like to thank Professor Cohen again, the kind invitation to participate in this wonderful series of virtual anatomy cases. And well, I'm very happy and truly honored. Thank you very much.

- Thank you so much, Miguel. A beautiful case series, enjoyable to watch. You know, as I've sort of evolved through my practice, it seems like the paramedian supracerebellar approach is the most flexible approach to the brainstem. It really provides a panoramic view of the posterolateral as well as the posterior part of the brainstem. It is minimally invasive. It is very efficient to perform, and it's really become my workhorse approach. I don't really do any more midline suboccipital approach anymore. I haven't done it in about five years. I wanna hear your thoughts about the difference between a midline classical suboccipital supracerebellar and the paramedian approach. Even if the lesion is very midline or large, it seems like the lateral trajectory is really efficient in order to remove the tumor, although the anatomy is a little bit distorted. What are your thoughts about that?

- Right, thank you, thank you very much for this question. Well, just actually, I have been moving more and more in favor of the paramedian and suboccipital approach. But I have to confess that insofar I see the possibility of doing that by means of a suboccipital transtentorial, I have the position that I get more possibilities to maneuver, dissect, and work around. And it also comes from, let's say, the practice in pineal region. I have done pineal tumor through suboccipital transtentorial, also through infratentorial approaches. Infratentorial approach is conceptually nice, but you have less maneuverability. We are now starting to use the exoscope, but before that, uncomfortable. And so to me, in midline semi-sitting position, I can do that, I go through this approach. But more and more, I've been using the paramedial supravermian approach, mainly in tegmental lesion. For tegmental lesions, no, no way. This is a very good approach.

- I agree with you 100%. It really provides a good sort of line of sight to the midline. It's like approaching a olfactory groove meningioma through a pterional approach. You can get early recognition of neurovascular structures, avoid a larger craniotomy, and really, it really is incredible, incredibly powerful. I agree with you that the interhemispheric transtentorial approach provides a more panoramic view of the pineal region. However, obviously it takes more effort to get there, and sometimes you don't need all that panoramic view in, you know, medium to even large cases. But I'm sure there are certain cases that you really need more of that approach to be able to get there. So I think that's one idea. The other idea that I think you well mentioned is if there's any tissue of brainstem or floor of the fourth ventricle over the lesion, I come through the middle cerebellar peduncle. I would rather, you know, take a lot more tissue and go through the middle cerebellar peduncle and have a more violation of normal brain in that area rather than go through the any minimal amount of floor of the fourth ventricle, because it turns out that anything, even the smallest amount of tissue over there, can be very, very eloquent. What are your thoughts about that philosophy?

- Well, I 100% concur. One issue at the pontine region, sometimes you have lesion that can be removed through several routes. Sometimes you can be tempt to go through the fourth ventricle. We have to avoid the fourth ventricle because it's something that is carrying a higher risk of morbidity. For laterally placed contained lesion, I very much use the safe entry zone in between the exit of the fifth nerve and the exit of the seven nerve. The tumor itself is enlarging that area. This is something maybe you have some difficulty, let's say, in a normal patient without any lesion to find that safe entry zone. But insofar as the lesion is big, once you open the dura, maybe you have the safe entry zone right in front of you, and this is my favorite. I have used for lesion at the pontine region. we have doing presigmoid approach. We have abandoned that. We have done many routes for lesion that are, of course, abutting that that area. I use the retrosigmoid approach by means of the safe entry zone. Of course, sometimes you have to do approaches through the fourth ventricle, and in cases of lesion that are growing or are symptomatic but are small, are not abutting the lateral aspect of the brainstem, you have to go through the fourth ventricle. We use nowadays, well, I'm old, I'm getting old, and we started to use many years ago at the beginning of my training program with my Master Professor Hona, to use the midline vermiotomy just to get inside the fourth ventricle. Nowadays, thanks to the works of Professor Matsushima, we just go laterally, and we don't need the, such a cutting of the cerebellum for the entrance inside the fourth ventricle. But, well, I concur with you. I try to avoid the fourth ventricle entrance to the brainstem.

- Yeah, well, thank you. Well, I really enjoyed the lecture. I wanna thank you for your time, Miguel. I look forward to having you with us in the near future.

- Thank you. It will be a great honor and great pleasure. Thank you very much again for the invitation. Thank you.

- You're welcome. Thank you.

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