Endoscopic Endonasal Approach: the Preferred Approach to the Cavernous Sinus

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- Colleagues and friends, thank you for joining us for session of the Virtual OR, my name is Aaron Cohen. Our guest today is Dr. Paul Gardner from the UPMC. He's Peter Jannetta chair of neurosurgery there. He is the most pioneering, if not one of the most pioneering, endonasal skull base surgeons in the world, and it's truly an honor for me to have him with us today. He's going to talk about why the endonasal route is the route to the cavernous sinus, and what are the approaches and techniques to be able to get most out of the endonasal route. Obviously, cavernous sinus has been a no man's land for many years, and I'm really excited to learn from him in terms of how he has pushed the boundaries in that region of surgery. So Paul, thanks again. You've been an immense contributor to this series, so it's always a pleasure to work with you again. Thank you.

- Thank you, Aaron. Genuinely a pleasure to be here, and thank you for all your efforts on the Virtual OR, it's certainly a resource for all of us. So this talk is about how the endoscopic endonasal approach is perhaps the preferred approach for the cavernous sinus. This is a understandable and intentionally controversial title for this, but I'm gonna try to explain what I mean by that. These are my disclosures, not really relevant for this. So the cavernous sinus certainly has been known as the jewel box of the skull base, and this is the region we're talking about, and of course, it's critical because of the number of critical neurovascular structures there. When we look at this from an open transcranial approach, this is really what we're seeing. We see lots of nerves that we have to try to operate through, and to try to access this area, and it can really look quite challenging, kind of like trying to pet a porcupine perhaps. And so really trying to work between those nerves and arteries, and we have some certainly brilliant surgeons who tackle this, but sometimes this is how we feel after we've done a transcavernous case. But when we look at this from the inside, from the endonasal approach, we really have all the nerves really quite lateral to it, and we're largely dealing with the venous component and the venous portion of the cavernous sinus. And not only that, but we can access both at the same time. Now looking at it from an endonasally, this is sort of the view that we get, and perhaps that's not quite as formidable, and I kind of think of this as looking at the belly of that same porcupine where all the nerves and everything are really outside where we're dealing with. Now when we compare it, when we look at a craniotomy, we certainly have well-defined triangles, and those triangles are designed to go between the different cranial nerves that we're trying to access the cavernous sinus through. Whereas endonasally, we're looking more at the compartments that were described by Juan Fernandez-Miranda. And you can see how these different compartments give us great access to the cavernous sinus. Now, the endoscopic endonasal approach is a whole series of modules of access, and one of the real advances has been moving from microscope to an endoscopic view, and that allows us especially to see more lateral when we're looking at areas like the cavernous sinus. When we do this, there's really a very big difference in not only what we have to expose, but how we deal with this. So you know, the traditional sellar exposure, we won't wanna see the cavernous sinus. We barely need to see the edge of the intercavernous and the medial cavernous sinus. We don't have to mobilize the carotid, and either don't monitor at all, or perhaps just meta-sensory book potentials. But as soon as we're extending to the parasellar space, into the cavernous sinus, we have to be comfortable drilling and exposing over that parasellar carotid artery, and that same carotid mobilization is what allows us access to the tumor in that parasellar space. So for a traditional sellar exposure, this is the bone removal that we need, but as soon as we wanna extend more lateral, you really need to be comfortable and able to remove all of this bone over the parasellar carotid artery. So there are two different approaches that we can take into the cavernous sinus. One is purely transsellar, and the other one is a transpterygoid approach for the more lateral compartments, and I'll talk about how we get to each of those. A standard transsellar approach really is just going right at the medial portion around the genu of the carotid artery. And this is the region, this is a typical tumor that we might access here. This is an acromegalic, where the tumor's going into the medial cavernous sinus. And as mentioned before, we have to really completely expose that cavernous carotid artery to be able to work into the cavernous sinus. And so here's the video showing exactly that exposure. And we start... You know, this is not done mostly with a Kerrison, but this is much more perhaps like otology, where we very carefully blue line the bone and then peel it away from first the sella, and then here's the critical portion where that same exposure has to be done over the carotid artery. You have to become familiar and comfortable with the anatomy of the artery here, as well as very careful drilling and thinning of the bone, and blue lining it before we peel it off. I don't try to drill right to the carotid artery, neither do I try to bite it off with a Kerrison, and the Kerrison here rather is used as a dissector and a grasper, not as a true Rongeur where we're trying to bite. So here, we dissect it, and then we grasp it. And we can of course, a doppler to ensure we know exactly the course of the carotid artery, and this is a very important step to take. And then opening into the sella itself, we do that again with microsurgical technique using two hands, and that allows us to identify the medial compartment with the sella, and here's just identifying... And then we want to also open into the cavernous sinus. And this little hook blade, it's a feather blade that has a inner hook on it or inner knife on it. This really allows us to open safely in the cavernous because you can see the blade as it cuts. We can then dissect between the medial cavernous wall and the sella because we've defined the carotid artery, and you can... And so here's working in that superior compartment of the medial cavernous sinus, and I'll talk a little more about those compartments in a moment, and then just cutting the final carotico-clinoid ligaments to have complete access to it. And there's our final view, and if we can go to the next slide then, and able to get a complete resection of tumor, even a fibrous tumor like this within the cavernous sinus. And then our early series, looking at the pituitary, we had very rare cases of cranial nerve deficits. You can see here there's a some periorbital ecchymosis from monitoring. We use a cartouche dissector and monitor with EMG the cranial nerves to avoid injury. So the first compartment here really is the superior compartment of the cavernous sinus, and it's the name, so because it's superior to that posterior genu of the carotid artery. And this is really relatively safe area, and this whole medial cavernous area, really it can be accessed quite safely and did not limit our degree of resection for giant adenomas when we looked at that series. Extending more inferiorly, now we're looking at just posterior to this compartment and posterior to the genu rather. And that's when we start to have to worry about cranial nerves. So here, the six cranial nerve just runs from dural's canal into the posterior aspect of the cavernous sinus there, so that's its first entry point. In here, we have to worry about cranial nerves. So if we can go to this video now, this was a hemangioma purely within the cavernous sinus. And here again opening into the cavernous sinus itself, and again, the carotid is gonna be right out here. So we're dissecting free from the carotid, and it has a very typical C-shape. The cavernous carotid runs up in the periclival segment and then runs in a C-shape out like this. And once you understand that, it can safely dissect it. Once we've dissected free from the medial aspect of the carotid, we can then start to peel this tumor. Now at the time, we didn't know this was a hemangioma, so we're trying to debulk it here. Obviously, relatively bloody tumor but manageable despite that because of using two hands and dynamic endoscopy. Now once the tumor's dissected free, and you'll notice this is entirely within the medial cavernous sinus because the gland here is completely separate. So this is completely within the medial cavernous sinus. It's got feed off of the carotid, which we've sacrificed here. And you can see now we're able to peel the capsule of the hemangioma and often as you would expect for any hemangioma, enters that posterior compartment becomes relevant. And we're catching a couple of glimpses of it right here every once in a while coming into view. And sure enough, there's our sixth nerve as it goes right behind the carotid artery, and this is in that posterior compartment. So now, we've dissected it free completely and just peeling out from the posterior clinoid. And there's our final view, just clean it up, and we can go to the postoperative MR on this. And there's our post-option and complete resection. So medial cavernous wall resection has only improved over time as we've started to understand the anatomy. And Dr. Fernandez-Miranda described this beautifully when he was with us, when we were starting to really resect the cavernous walls quite thoroughly and understanding these various ligaments as they were named. The inferior parasellar, the most important perhaps the carotico-clinoid ligament. And then in the depth, the superior parasellar ligament. If we know to cut these and how to cut them away from the carotid artery, we can actually safely resect the medial cavernous wall. And here's an example of how that comes into play in the case of recurrent Cushing's disease. This is under a woman who underwent initial resection of a pituitary adenoma but had invaded the cavernous sinus and so never really crashed in the postoperative period. So initial workup, pretty straightforward, very obvious tumor there that was resected by a very expert surgeon in an outside facility and got a nice resection of that component but stopped understandably at the cavernous wall. And so this was corticotropin adenoma, but the patient never had an appropriate postoperative crash. There's no overt tumor here, but was perhaps a hint of tumor there in that cavernous wall or in the medial aspect of the cavernous sinus. So as this progressed, you can see the tumor starting to grow, and obviously, you could do something like radiosurgery here, but we felt this was still accessible because the medial cavernous wall had not yet been resected. And so you can see a typical redo where there is some flat pedicle left, and we're just peeling mucosa and prior fat graft away. And then as that same technique of carefully thinning and blue lining the bone over the carotid artery. So now you can see that carotid is completely exposed and that gives us the ability to safely resect the cavernous wall. So here's opening into the sella, and you can see the gland right here. This is the gland margin. We're very clearly between the gland and the medial cavernous wall. And there's no tumor even in the gland itself. The gland's actually completely clear, there's just a hint of some soft tumor there. But all of this right here is tumor actually in the medial cavernous wall itself. So all of this tumor growth is in the medial cavernous. So we can use that same hook blade, open up the cavernous sinus, pack it off of some Surgifoam, and now we've defined the cavernous wall immediately at the sella. We've defined it laterally by identifying the carotid artery. And then we can start to resect here. So here's just opening further so we can define the entire parasellar cavernous carotid artery and then dissecting posteriorly. Here's dissecting back to that genu of the carotid. And you can start to see this attachment of the inferior parasellar ligament. So one of the problems from these tumors, they become very adherent and can even involve the wall of the cavernous carotid artery. But in this case as we dissect it, we can sharply cut this ligament. And as we cut these ligaments, we can start to free up. Here is that carotico-clinoid ligament. We can cut right back to the interclinoidal ligament, which often is not involved but represents sort of the roof of the cavernous sinus. But here, once we cut these carotico-clinoidal ligaments, obviously this takes very careful control and very careful visualization. So requires really an expert team to be able to do this surgery together with expert dynamic endoscopy. But once we've cut those ligaments, the tumor really can be completely dissected free. And so here's that whole medial cavernous wall coming out, and it's completely resected. And there, we can see... And this patient eventually crashed after that. Can you go to the next slide then? So does this really necessary? Well, when we looked at medial cavernous wall histologically in these cases, not just ones like this where it was obviously in the cavernous wall, but actually only in cases where the tumor touched the cavernous wall, it turns out that there's media cavernous wall invasion, about 85% of cases even higher in functional tumors. And so there's very real value in resecting the wall to not leave microscopic tumor behind, and that's really been proven, so that... You know, those are the approaches we can get to simply through a standard transsphenoidal or transcellular approach. But the transpterygoid approach, where we go through the pterygopalatine fossa is it gives us much more lateral access to the what are called the coronal plane. To do this, we need to do at least a maxillary antrostomy. Doing that maxillary antrostomy is you can see moves us beyond the boundaries of simply the paranasal sinuses in the lateral nasal wall to gain access to the base of the pterygoid. And here's looking out at the maxillary wall. And then on the opposite side, this is the right side, and we can see here's the whole pterygopalatine space with the posterior nasal artery coming off, which will feed our nasal septal flap. We can then dissect this. This, you can see that there's V2, and then of course, we have IMAX looping through here, and here is after it's dissected. And so understanding this anatomy is important, but what we're really looking to do here is to work around between the vidian and V2 to gain access more laterally. And doing that gives us access out into the cavernous sinus. And so this now gives us access into these anterior or inferior compartment name. So again, 'cause it's anterior to that anterior genu of the carotid artery, and we can see that very nicely. I recommend looking on the sagittals like this. If you look on the sagittal film, you can see the relationship, and this particular tumor has tumor in the anterior as well as the posterior compartment. But this is a great example of a type of tumor that we would access. And here, we can see that tumor in the anterior compartment. And if we can start this video please. And then just working with two suctions, typical relatively soft tumor. And you can see here sometimes when we start working these more lateral compartments, this now is when we start having to see the inferior, the meningohypophyseal trunk or the inferior hypophyseal artery, and that can be controlled bipolar cautery. But as we start working into this anterior compartment, now we need to start worrying again about cranial nerves and the one in the anterior compartment is gonna be the sixth nerve. Now here when we look work medial, there's gonna be tumor into the posterior compartment. But as we start working lateral, here's the genu of the carotid just like this. So we're here in this anterior inferior compartment, and you can see he's working with this cartouche dissector, and sure enough, there we start to see this sixth nerve. So seeing that sixth nerve, we can stimulate it, and usually that often is philosophically when we stop resecting. So you can go to the next slide please. So all of us know that these tumors are relatively slow growing. You can perhaps treat lateral cavernous disease with Gamma Knife. And this really is what defines the lateral cavernous sinus is this sixth nerve. So anything here medial and inferior to the sixth nerve is that anterior or inferior compartment. And anything above or lateral of it defines the true lateral compartment of the cavernous sinus. And this is where we're really starting to get close to the quills of the porcupine. And this is when you start losing some of the advantage of the approach unless the tumor's completely medial there. And the whole idea here is that we're trying to avoid damage to these nerves. And as soon as we get in this lateral compartment, that's really where we have third, fourth, and sixth nerves all running freely. And so now when we start dealing with very large tumors in this area, this is when we really start to try to limit our approach. And so here's a tumor that goes all the way out to the lateral cavernous sinus, and you can skip forward through this in the interest of time, but this is just showing here how we can dissect all the way lateral to the carotids. So here's our parasellar carotid. This is working out beyond the inferior compartment, out up into the lateral compartment of the cavernous sinus. And the whole point here working with the cartouche we'll see the sixth nerve come into view there is we can really start to work here but it's limited. The tumors tend to get more fibrotic here. We certainly can open and can work safely here, but you have to start asking yourself at what cost is that. So working in the cavernous sinus here can be done safely into the lateral compartment. We can avoid cranial nerve injury, but of course, the risk goes up. Avoidance of this compartment can help you keep your cranial nerve risk quite low. And he's working very carefully with that cartouche dissector the whole time. We have to... We're working lateral to the carotid artery here. Really, this is in the more danger area of the cavernous sinus, so. Next please. Again, we can maintain low cranial nerve deficits by generally avoiding that, and this shows this very well, and these are most represented by Knosp 4 tumors where the tumor really starts to go into this lateral compartment of the cavernous sinus. And what you'll notice is our gross total resection rate, which here's about 50% for Knosp 3, even in our early series, and I think it's gotten only higher over time. For Knosp 4, we essentially always kept less than gross total, largely philosophically to avoid injury in this area. And also frankly, we were trying to understand the cavernous sinus better. And when we looked at a series of cavernous sinus tumors up in the 2012, so more than 10 years old now at this point, these are mostly adenomas but also some non adenomas. You'll see a big difference between these. We're largely only going into the cavernous sinus for tumors either more aggressive like a chordoma or tumors like a meningioma that have already caused a cranial neuropathy. And so these are largely presenting already with cranial neuropathies. For the pituitary adenomas, many of these were non-functioning, and you can see here not getting gross total resection, many of them only about a third. And the functioning tumors though, especially with acromegaly, we're actually able to get quite good outcomes by being able to go into the cavernous sinus. So here's a case of a very fibrous adenoma, and this is the concern, of course. Once we start working in the cavernous sinuses, there's risk of injury to the carotid artery. And so you can see a very challenging tumor here. And we've ended up injuring the carotid artery here by dissecting sharply in the cavernous sinus trying to cut this very fibrous tumor free. And we have a carotid injury, and here I'm trying to control that I place clips. I have a nice view of the distal clip at the parasellar carotid, but obviously approximately, I don't have great control because that periclival cavernous carotid itself is even invaded with tumor. So obviously, a very fibrous tumor in tumor biology dramatically impacts the outcome of being able to dissect here. So softer tumors like adenomas or very carefully selected tumors that have really widened our trajectory into this region is what we're looking at. So this is the thing we always fear most. And this was able to be be managed with muscle packing, eventually required a stent, but this is the great fear. Next. So when we look at our cavernous sinus for adenomas, we had new cranial nerve palsies in only about four patients, so pretty low risk but two carotid injuries in this case, but no neurologic sequelae able to control these in all the cases. Now when we start going into the non adenomas, what you'll see here is the risk goes up a bit. So here's the recurrent meningioma and had a craniotomy, and you can see this is tumor really extending a lot inferiorly into that parasellar space on both sides of the carotid artery and down into the anterior inferior. And then leading us into the lateral compartment. If you could play the video please. And so here's working lateral to the carotid artery. So here's our sella is gonna be right over here and here's the carotid artery. And so we're working on the right side, and this is that same area you saw in the adenoma, but we're having to very carefully dissect through meningioma. And in this case, the only reason we're here is because this tumor has progressed despite radiation, and the patient now has partial cranial neuropathies. So here we can see the third nerve in view, and we can dissect the tumor safely from it all the way up to the roof of the cavernous sinus there, but we wouldn't go in this area unless this tumor were uncontrolled on a meningioma and unless the patient had symptoms because the risk of giving them symptoms or damaging the carotid artery is really very high, but you can see this can be safely dissected here. We've completely removed all the tumor coming down into Meckel's cave, and the tumor going down to the pterygopalatine space. Next. And so this gives us this kind of resection, believe it or not, this has actually been well controlled ever since then. Other types of tumors that we might access. Here's an example of a sphenopetroclival chondrosarcoma. Sometimes these can be entirely exclusively within the medial cavernous sinus like you see here in this case. And this is unusual because, usually, they're starting down here at the petroclival synchondrosis, but this is one that grew purely in the cavernous sinus. And like that hemangioma we saw earlier, this is a tumor that... Here's the gland itself. And this is a tumor that's purely growing in the medial cavernous sinus between the carotid artery here and the gland here. Now we say, well, sure you can resect soft tumors. Well, this is a completely calcified tumor, cartilage based bony tumor, chondrosarcoma, but completely in that soft underbelly of the porcupine of the cavernous sinus. So you can see as we start to decompress the cavernous, you get that nice healthy venous bleeding, easily controlled with a little bit of very gentle Surgifoam packing. You don't wanna be too aggressive, and you wanna be sure that you're in the cavernous sinus and not getting an arterial bleeding here. So now, you band it. And now looking up with an angled endoscope towards the diaphragma. We've dissected it free from the carotid. We've cut those, the ligaments in the parasellar ligaments, and we can peel it free from the dura of the posterior fossa and again, get a complete resection. In this case, resecting any involved dura and any involved structures whatsoever. Next slide please. So what we found in these non adenomas, his overall gross total resection rate, again about a third but and near total also, but many of these cases we're just trying to debulk in the cavernous. It doesn't make it not an option though, but what you'll see is again, good improvements in the cranial nerve palsies, not nearly as high as the adenomas but pretty good improvement. But a much higher rate of new cranial nerve palsies, only about two or 3% in adenomas in the cavernous sinus and a much higher rate in non adenomas. Also, higher rate of carotid artery injury in one of these even died, and that has to do with the nature of tumors like a meningioma in this case. The patient who died, where the tumor itself invades the carotid wall, and therefore puts the injury risk at much higher risk. When we're looking at any of these coronal plane approaches, of course, it's critical to understand the different carotid segments. Here, we have the parapharyngeal, the horizontal petrous, the periclival or parasellar, and the periclinoidal carotid. And these are the landmarks that I use for those and very important to always understand what those are. So I've memorized those and use those quite frequently. And this can obviously even be extended for things like ICA aneurysm. Here's an example of a patient who presented with bilateral carotid aneurysms. I mentioned earlier, we have great access to either cavernous sinus. So here's a patient with bilateral superior hypophyseal or paraclinoidal carotid aneurysms. And these can be challenging to tell even with FIESTA imaging whether or not they're intradural, extradural. And this is a young patient with a family history of a mother, who died of aneurysm subarachnoid hemorrhage, has a new onset headache and concern that this could be sentinel expansion of an aneurysm. And so here, you can see we're exposing again that parasellar carotid artery. Here's the optic canal being exposed to be able to remove this critical area of the medial OCR, which gives us beautiful access as our key hole to the parasellar and suprasellar space. And you can see how widely we've exposed the whole cavernous carotid artery there. Similarly, on the other side, here's our sella in the middle and our carotid on one side, carotid on the other. Here we can see the obvious aneurysm right there, sort of peeking down into the top of the sella, and then getting exposure of both cavernous carotid arteries. We can explore both of these. Of course, most aneurysms with truly within the cavernous sinus can be avoided treatment, and it can just be observed unless they're expanding or cause a CC fistula. You can see a little opening in the cavernous sinus as we expose it, and that's pretty easily controlled. And now, we have complete access. We're gonna start with the larger aneurysm that's more evidently crossing the distal depth. This aneurysm crosses the distal dural ring. So there indeed is subarachnoid aneurysm in this case. Now, the dome of the aneurysm obviously is not pointing into the subarachnoid space, but there is aneurysm crossing there. So here, now we'll pack off the cavernous sinus on the other side. We feel pretty comfortable with the right side having gone well. And so we can start to look at this left side aneurysm. And again, the same kind of location where the aneurysm is, the neck of the aneurysm is right at the distal dural rings, so you have to cut down on these dural rings to have access to the aneurysm and really sometimes even place two clips because of the thickness of the distal dural ring on the aneurysm. So now when we look at that right side, because it's crossing the distal dural ring and we hadn't opened it quite well enough, the left one's can is clipped completely. The right one is not because we haven't completely opened. And sure enough, you can see that neck of the aneurysm, the clip has sort of slipped off, and we have neck of aneurysm here that the distal dural ring is protecting us from accessing. And so we need to cut that distal dural ring and then replace our clip and sure enough, here's our final view of the complete clipping on both sides, and we can see how we left some of that neck of the aneurysm. Now of course, the clips are an issue, so we place some fat to bolster around them and then place fascia and then a nasal septal flap on top of that. The nasal septal flap has to cover all of that fat and tissue to make sure we have a vascularized closure. Next slide. Really important to understand when we're accessing the cavernous sinus that there's a very significant learning curve. Certainly addressing aneurysms like this one could argue is something that perhaps nobody should be doing endonasally. But I think with significant experience, both of the vascular surgeon and an endoscopic surgeon, these perhaps can be done. And I think that endoscopic endonasal surgery has been shown to be safe and effective for many cavernous sinus pathology with a key advantage of providing a much better access to the larger medial compartment, that underbelly of the porcupine. But also very critical to understand that tumors like adenomas are far superior outcomes than non adenomas, which really should have much stricter criteria for accessing. So what do I mean that this is the preferred approach to the cavernous sinus? Well, there's this inherent anatomic advantage for pathologies in that medial three quarters of the cavernous sinus, the larger compartment, and we can access both cavernous sinus simultaneously. But you know, understanding the carotid anatomy, the involvement and having control are really critical factors, and there's a very significant learning curve with cavernous sinus approaches, whether open or endonasal. And the reason I say that this is preferred is because if you look at the different pathologies that can be approached to endonasal approach, this is things like invasive adenomas, medial sphenocavernous meningiomas that are growing into the sella, medially originating or projecting aneurysms. This includes carotid cave, superior hypophyseal aneurysms, and then cavernous sinus biopsy for lesions medial to the carotid. This is a completely extradural surgery, don't have to ever touch the brain. And then finally, lower or anterior Meckel's cave lesions. Here's an example of a very invasive adenoma that extends quite laterally, but by following the tumor through that belly of the cavernous sinus through the ocular motor triangle, we can get a complete resection. Medial sphenocavernous meningiomas like this one, not a complete resection, but only a small residual left here on the tentorium, right where the cranial nerves are entering, where you would want to leave that anyway. Here's another example of a medial sphenocavernous causing vision loss by compression of the optic nerve. And again, accessing it endonasally has an obvious advantage. So these are all the kinds of lesions we might access. Of course, craniotomy still plays a critical role, and you have to, as a skull-based surgeon, be able to provide any approach for patient. And tumors that involve a lateral cavernous sinus obviously should be approached from a lateral approach. Medial sphenoid wing tumors with cavernous sinus extension, most ophthalmic artery aneurysm point up or laterally. And then finally, if there's anterior clinoid involvement, lateral to optic nerve, this is an important open indication. So tumors like this require classic cavernous sinus peeling, menial sphenoid orbital meningiomas with cavernous extension like this, or he has a tumor, really not even the cavernous but the anterior cavernous, lateral cavernous sinus in the superior orbital fissure, again done very nicely through a lateral orbitotomy in this case. So of course, craniotomy has a role and being able to do both of these is critical. But if you look purely at incidents, again, why is endonasal approach preferred in general for the cavernous sinus? Look at pure incidents of tumors. Pituitary adenomas are far more common than tumors like cavernous meningiomas. So when you look at how many tumors you can treat through each of these, the endonasal does have some superiority there as well. So in the end, I think this larger portion of the cavernous sinus and understanding that is accessible via endonasal, making it a preferred route for many tumors depending on their anatomy, especially if they're medial to the cavernous sinus, rather than trying to work through those nerves, just go directly at the tumor because this minimizes cranial nerve manipulation when trying to work through those nerves. But of course, a modern skull-based surgeon has to provide all corridors so that you really can access this tumor based on its anatomic relationship. This, of course, takes a dramatic amount of experience with both approaches to be able to use them equally, but this is the ultimate goal that we're all trying to get to. So this is my perspective on the cavernous sinus. It will always be challenging, but I think it's important to think about it in the many ways as possible. This is a recent text we put out, which looks at really all vascular possible complications and associations in the skull base. And this includes everything from flap vascularity to venous injury to even aneurysm management. And like the Virtual OR, I invite you to join another free educational website, The Skull-Based Congress, which is used for all of us as sort of a community online. And again, inviting everyone to Pittsburgh, of course, for the Pittsburgh Course. And Aaron, thank you as always for this invaluable resource and for allowing me to take part.

- Great work, Paul, really a pleasure watching the work of you and Juan Carlos. And I think it's just spectacular to be able to see what you guys are doing, pushing the boundaries, being able to do more with less, which I think is so important. Skull-based surgery, traditionally, since late '80s till, you know, sometime in I would say 2010, '15, maybe it was all about these radical skull-based approaches having this big, you know, osteotomy and everybody thought that's the pathway to go for skull-based surgery. As we sort of analyzed it more and more, we found out that they come with a lot of morbidity, and in fact, these radical excisions and bypass of the carotid artery may not lead to ultimately great outcome long-term, but are associated with significant morbidity. So we sort of turned around and expanded our annual nasal reach, found out what really is the limits of our exposure and use, you know, radiosurgery as needed as an adjunct, and then push the boundaries of minimally invasive approaches through the nose. I believe that is the modern neurosurgery. The modern neurosurgery starts with endonasal route, expanding your armamentarium using radiosurgery and definitely being very facile in open osteotomies and retrosigmoid subtemporal pterional, not necessarily transpetrosal and you know, anti petrosal or far lateral for every case. So there used to be that everybody thought, "Oh, it's OZ for every pterional." There is, "Oh, we can use, you know, transpetrosal for a craniopharyngioma." But we have learned that those approaches may not be the best route going forward. So it's good to sort of see this evolution of techniques and we're doing more for the patient through less approaches. What are your thoughts about these evolution of techniques for the past 30 years?

- Yeah, it's very interesting because, you know, things like radiosurgery came up about the same time that endonasal approaches were just first being developed. And I think without one, you may not have the other because, you know, radiosurgery is the thing that pushed us toward perhaps trying to have lower morbidity. And the truth is that I think modern skull-based surgery, as you mentioned, is really corridor surgery. So we're trying to look at which corridor or corridors are appropriate to treat a particular tumor rather than saying, "Well, you know, I can do everything in the anterior middle fossa through an OZ." And you know, the same's not true of endonasal, you can't do everything that way. But by understanding these different corridors or approaches, rather than trying to do the biggest approach possible and get every last bit out and push that approach, no matter what the morbidity, we select corridors based on the anatomy and based on the patient. And hopefully over time, as you know, all approaches are refined and taught together, I think this multi-corridor approach really becomes the standard of care.

- I agree with you completely. And as a pioneer in this field, where do you think we'll be looking at, I'm sure you've been asked this question before, where do you think we'll be in 15, 20 years? Do you think we're gonna have more robotics through the nose, which would even expand our reach farther, we'll be able to use artificial intelligence with robots and advanced navigation to even do a lot more? Would there be a da Vinci of skull-based surgery that would expand our exposure? What are your thoughts?

- I sure hope so. You know, I think one of the issues is that we're so passionate about skull-based surgery, but it's such a niche, you know, and so I think it's likely to be a robot that's developed for something else that we apply to skull-based surgery. But the truth is that there's a huge... You know, there are a lot of things such as drilling or dissection that could be done as well or better by a robot with our guidance. And so that, you know, human machine interface would be ideal. I don't know how quickly that technology's going to move. I also think another thing that will dramatically change in skull-based surgery is just the molecular understanding of tumors and perhaps coming up with non-operative treatments. And in the end, if we... And this is not gonna happen, it hasn't happened for many specialties, but if we become surgeons who perhaps need a decent specimen to be able to molecularly classify a tumor to determine if or what other treatments needed, now more and more the less invasive your approach is to get tissue becomes even more important. So that will push a certain percentage of the tumors we treat in that direction. I don't believe it's gonna take over, but I think that's gonna be another key piece of this as the molecular understanding, and what's the safest and easiest way to get pieces of tumors to define their treatment.

- I agree with you completely, and I tell you about the evolution of my own techniques since we talk about that in the first section, is that I think I'm doing less and less transpetrosal and more and more retrosigmoid than combined approaches. I think I used to be able to maybe do 15 transpetrosals a year, now it's three or four max. I used to maybe do 20 orbitozygomatic, maybe less than 10. I think we have learned that by dynamic retraction, by advancing our own skills that you don't always have to see everything to be able to operate. Because when you have that three dimensional understanding, and you have advanced your skills in surgery, the first thing is you can do more through less smaller approaches. So I think some of the younger neurosurgeons all say, "We did a big OZ. We did a big transpetrosal." And they feel very proud of that. They feel like that makes them to go to the next level. I do believe... I don't believe in that. At least that's my personal opinion. People may disagree. I think the more advanced surgery you become, the more you can do through smaller approaches and preserve more normal structures. What are your thoughts there?

- Yeah, I agree completely. I think the challenge is, you know, I think all of us go through that evolution that you spoke of. And the challenge is, you know, when you see a big open approach, when you see an OZ, and you skeletonize every single structure at the base of the skull, there's no better visual learning than that and understanding of that. Without doing that, how do you possibly learn how to do it through a smaller exposure and then recreate this image in your head, which is sort of what you're doing. And that's really the challenge and perhaps through some sort of VR we can start to create those kinds of images for younger surgeons without them having to do the open approaches that then get refined down. But truthfully, that's how you learn to be minimally invasive is by understanding what's well beyond where you're working and how to recreate that through a smaller quarter. So I think that's the real challenge is, you know, all of us went through that evolution. How do we teach it and have the same understanding without having to go through it.

- Right. How can we unrepeat or not have to repeat the history of skull-based surgery for every individual surgeon? And I think it would come down to immersive high fidelity simulation. I think this is where simulation would potentially play a huge role to be able to get the surgeon to just feel confident. You're not teaching them anything. Patient specific, immersive simulation I think would be the frontier next in skull-based surgery because it would allow you for that specific patient, you convince yourself that you can do it through the smaller approach. I think the first step is convincing the surgeon they can do it and creating that confidence will be a huge step forward, in my opinion, to advancing the care of every patient.

- Yeah, and watching surgical videos, you know, watching other videos. We don't watch other surgeons enough, you know, and again, that's why resources like the Virtual OR are so important because you get to see these approaches done. You know, see more than one way to do them and understand that anatomy. The more times you see it, the better.

- I agree. Paul, I wanna thank you. You've been a dear friend. Your contribution and your legacy is one of the most unparalleled. What you have done through this skull base, both open non-endonasal and endonasal, demonstrates really your incredible technical skills. I really want to congratulate you. I learned so much from you throughout my career and yeah, it's really... I'm so proud to have you as a friend and a great colleague and look forward to having you with us again.

- Absolutely. Thanks for having me, and thank you for the kind words, and hopefully we'll see you in person soon.

- You bet. God bless you and your family. Thank you.

- Same to you.

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