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Resection of Brainstem Cavernous Malformations

Michael Lawton

April 05, 2021

Transcript

- Hello, ladies and gentlemen, and thank you for joining us for another installment of the virtual operating room from The Neurosurgical Atlas. My name is Aaron Cohen. Our guest today is Dr. Michael Lawton from Barrow Neurological Institute. Michael is one of our very frequently requested speaker for a very good reason. He is internationally known for his superior techniques, in cerebrovascular surgery. Michael, thank you for joining us. I know you will be talking about resection of brainstem cavernous malformation, a very exciting topic. Obviously, you have had a number of innovations in that area, and I'm very much looking forward to learning from you today. So with that, please go ahead.

- Well, thanks, Aaron. Always great to be invited to your shows. I guess this is the virtual operating room, so hopefully, we'll do some virtual surgery here. So my topic for today, is the brainstem cavernous malformation. And by way of introduction, I wanted to just show these numbers and you can see for yourself, these represent roughly a quarter to a third of the overall cavernous malformation volume in my practice. And I've accumulated a sizable series. So hopefully I've learned a few things and I'd like to share those with you today. So this has been something that I've been, stewing over for the last year and a half or so, working on this book. It's a book in progress and some of the goals of the book, are to develop a taxonomy for the surgical approaches, to advance these tissue-sparing approaches through subarachnoid spaces, which I think allow us to really get to these surgical targets with very little injury or side effect. Thirdly, is to apply the deep learning and AI technology for decision-making and surgical guidance. And those are really the things that I've been working on, with this project. Like with all my other Seven Series books, I like a metaphor. And so the metaphor for this has been maps. Here's a picture of my beloved Marin Headlands. I spent 20 years in San Francisco and this is just a picture of the beautiful Marin Headlands, as you look south towards the city. And the reason I show you this, is because if I dropped you here and asked you to walk back to home, you'd have to find your way through this field of grass and trees and sometimes you get lost and it's easy to not make it to your destination well. One of the ways to think about what we do in the brain, is a lot like how we get through that wilderness with this map. The map shows these various trails, through the Marin Headlands and surgically speaking, what we do is find these trails these pathways, that get us from point A to point B in a very similar way. We're all used to this way of thinking. This is just a screenshot from my Tesla, from my home to work and we've gotten very accustomed to these blue lines taking us from point A to point B. And in surgery, it's really the same thing. It's just defining or designing an approach, that gets you from the outside to the inside, to your target, in the best possible way. These are the various tenets that I have been focused on, and I'm gonna just walk you through some of these and then we'll get to some cases. The first is the idea of segmental anatomy. I view the arteries as those red lines, those trails, and those arterial landmarks, are what get you from point A to point B. One of the privileges of my career, was to do this project with Al Rhoton. It's the only paper I ever published with him. We've worked together on many things, but this was the only paper that we published. He had done a nice job of labeling, the segments of the cerebral arteries alphanumerically. We just finished the job, by putting the nomenclature together for the cerebellar arteries. And when you have this completed work, you essentially have a way of thinking of every different piece of the arterial tree. And so the arterial tree, since it goes everywhere in the brain, then becomes a way to guide your route to your target. So these are just some illustrations, showing these segmental alphanumerics that label the carotid artery. That define the different segments of the middle cerebral artery. And between the segmental addresses, the alphanumerics and the names of these distal arteries, you can really use the arteries as your trail markers along the way. So very important to correlate your arterial anatomy, with your cavernous malformation anatomy. Next is triangles. When you look at a space like this, the carotid cistern, you see arteries, you see nerves and it can still be kind of confusing knowing where to go, but when you think about triangular spaces, these define routes of dissection, routes that we can take to work, and define spaces, get to your targets and identify the lesion. So these triangles are invaluable , you've got here in this illustration here. Precommunicating triangle between the A1 segments here. You've got your junctional triangle between the A1 and A2. Over here, in this illustration, you can see the triangles for aneurysm surgery in the posterior circulation, the carotid optic triangle here, the carotid-oculomotor triangle here, and the supracarotid triangle between A1 and M1. This over here is the oculomotor-tentorial triangle. And these are ones that we've taken a lot of pain to define, to label, to study morphometrically in cadavers and really get people to use this. More triangles. Here's the vagoaccessory triangle. This is PICA triangle, which shows you the space between the medulla, the vagus nerve and the accessory nerve. It's divided by the hypoglossal nerve fibers, into an infra and a suprahypoglossal triangle. And these define routes to the PICA, typically, but can also define routes to medullary cavernous malformations. Here is just another illustration of that. Same triangle, different artists. And over here, another triangle, the glossopharyngeal cochlear triangle, which is what we use as we climb up from the vagoaccessory triangle, between eight and nine into this space over here. And once again, useful for PICA aneurysms There are other triangles as well. I won't dwell on these, but everywhere you look, you find triangles. These are tables that lists them all, in the anterior cranial fossa and the posterior cranial fossa. And it's important to use those as trail markers, as we work our way through that forest. Now, once you approach your target, you get to these surfaces. And the surface anatomy is really essential, because it's the surface anatomy that we have to carefully inspect, to look for where the lesion presents. We also have to study the surface anatomy, to define our entry zones. And these are just some illustrations showing how the different exposures reveal, or bring into a view, the different surfaces of the brainstem. I think this audience is certainly familiar with all of the approaches here. And then the entry zones, you know, once you get to those surfaces have to study the different creases, the different folds, the different cell sides to find exactly where these safe entry zones are. And it's really important. The brainstem is so loaded with nuclei and tracts, that we really have to choose carefully where we enter. Now there are, I think, two critical questions with every brainstem cavernous malformation, that have to be answered. The first is whether or not to operate. The second is which surgical approach. So years ago, we put together this grading scale, to try and answer that first question, which one to operate. And this table here just summarizes the different criteria. Its size, whether it crosses the axial midpoint, presence or absence of developmental venous anomaly, patient age, and hemorrhage, whether it's acute, subacute or chronic. And you might think, "Well, with five criteria, it's kind of hard to remember this." But it's really analogous to AVMs and both the Spetzler-Martin grading system and our supplementary grading system. Size is the same in Spetzler-Martin. Crossing the axial midpoint, is our way of saying eloquence or non-eloquence. Anything that crosses the midline, is a much more eloquent lesion than one that doesn't. So this is our translation to the cavernoma. The DVA is like your deep venous drainage and the presence or absence of that, affects the surgical route, injury, or compromise of the venous outflow in that territory affects outcomes, so we found this to be significant. In the supplementary system, we have age, bleeding and compactness. Well, compactness isn't really an issue, with a cavernous malformation. So we're left with just age and the bleeding. And a similar dividing point at age 40. You'll notice that there's a zero for less than 40, and a two for greater than 40. So all of these others are zero to one. And for hemorrhage, we have a three-point, or a three-tiered grading, where we go from acute with no points, to chronic, where there are two points assigned. And the whole idea behind this is that, a malformation that is freshly bled, liquefies. It separates nicely from the brainstem and you're able to dissect this much more easily and safely, than one that's had the chance for the hematoma to completely go away, or scar tissue to form, for that malformation to start to adhere to the tissues. And so that's the basis, for how we think about the variable of hemorrhage. And when you put these together, you end up with a score from zero to seven. And how does this help you? Well, I think you can see it here. This was this original publication, which allowed us to kind of draw a line in the sand, here between grade five and grade six. So that appears to be the threshold. Well, this is a paper coming soon, where we looked at the distribution of grades, but more importantly, we looked at the outcomes associated with grade. And if we look here, particularly this graph to the right, this is.. and you can see the theme morbidity or the deterioration, is highest for the grade sixes and sevens. So these are the ones, that I would consider to be the inoperable ones, or the ones that you shouldn't operate on. These others, we divide them into low grade, zero, one or two, intermediate grade three, four, five. These are ones that are operable. The outcomes are quite good for the low grades. The intermediate grades, you can see that there's a proportionate decline in outcomes. So just as with AVMs, as the grade goes up, the risks and outcomes are affected. And you have to decide where on the continuum, you're willing to pull the trigger. But we find this to be very helpful. The accuracy data are shown here. And, I think it's a very valuable tool. It's a heuristic. It's a tool that you can just use, to check your decision-making and help guide the patient through that choice. Now these are the different corridors that we use, generally speaking, to get the cavernous malformations, either through gyri, sulci, or, well, generally, it's sulci or we go through fissures and there's the Sylvian and the interhemispheric, there's the ventricular system, which is a beautiful corridor to approach these. There's the tentorium These paratentorium routes are phenomenal. Petrous bone is also, while not subarachnoid, is a invaluable corridor that we use. And then finally the foramen magnum. So those are the corridors. And I'd like to just now turn our attention to these brainstem ones. These are the seven regions that we see these, but I'm gonna just focus on midbrain, pons and medulla and take you through some of these case examples, Because again, just seeing what these look like, I think, will be instructive. So starting with midbrain thalamus. Here is a case. It is a patient who presents with a progressive right homonymous hemianopsia. The lesion, you'll see in a second, is in the midbrain and thalamus. By our grading system, this is a grade three. So you can see the assignment of points here. And for this one, I chose a contralateral, supracerebellar-transtentorial approach. This was an approach that we had defined and it's become one of my real favorites. And you'll see why in a minute. Here's the lesion. If you study this lesion and you apply some of our guidelines, like the two-point method, or some of these other tools, you would say, "Well, this does not come, really, to any peel or appendable surface. If you look here, particularly on this image to the right, it doesn't come to the ventricle. Doesn't come to the third ventricle, doesn't come to the temporal horn. Doesn't come to the convexity. It's buried on the medial temporal side. And you can't really go transylvian, because it's around this corner. But if you study this really carefully, there is one spot and it's right here. It's right in the back portion of the ambient cistern, where the ambient cistern and the quadrigeminal cistern come together. And that becomes our touchpoint to access this. And so here is the strategy, behind the contralateral, supracerebellar-transtentorial approach. By going to the contralateral side and using this cross-court angle, you change your lateral reach. You go from something that's really limited in the midline, to something that gets you a good two centimeters off of the midline. And it's using your angles to achieve that. That gets you the exposure. So we do this in the sitting position. I find that cerebellar sag and gravity retraction on the cerebellum makes this a really lovely approach. And so you can get that panoramic view. The tentorium becomes an obstacle. The tentorium only gives you a few millimeters, here off the midline, but by cutting the free edge of the tentorium and creating a window for yourself, you can then follow the quadrigeminal cistern, excuse me, around the corner, into the ambient cistern. And you can follow the PCA around the corner. You can get high into that little recess here and access to the lesions. So without further ado, here's our video. All right. So here's an animation that shows you the sitting position, midline incision torque at a craniotomy. This gets us right over the cerebellum and gives us that view under the tentorium upwards. That's really critical. By dropping the cerebellum and looking up, we get that upward view to where we want. So here's the view now, in the operating room, you're looking down at the cisterna magna. And I like to take CSF from down low, just to drain the brain and begin that process of relaxing the cerebellum. Here you see some of the small bridging vein and some arachnoidal adhesions, that can tether the cerebellum, but now we're all the way at the free edge of the tentorium. Here's a stealth image, that shows you the midline trajectory, but now we're gonna start to work our way to the sides. Here I'm just following the basal vein of Rosenthal, laterally into this gutter. You can see the posterior cerebral artery, and some of its branches there. I'm separating the medial occipital lobe from the quadrigeminal plate. You can see PCA coming into view. And the further I go laterally, the closer I get to the malformation, but you see now the free edge of the tentorium becomes a barrier. I can't get my view or my instruments to move any further to the side. So this is just a little relaxing incision. I'm creating a little window in the tent. And by creating that little window in the tent, you can see the shift now in our trajectory. You've got that cross-court trajectory. I'm starting off to the patient's right side, swinging over to the patient's left side, and it's widening or extending my lateral reach. And here now is our entry site, where we see staining on the lateral midbrain and the undersurface of the thalamus, and see right below that peel incision is clot. So our target was a direct hit, and we've got a tremendous view straight down the axis of that region. So the resection of cavernous malformations, I like to kind of get inside, take out whatever clot there is inside, that helps to debulk the capsule. And as the capsule debulks, it pulls into itself, centrally. And then you can start working the plane around it. I helped design these instruments, which are these small round knives that help to sharply dissect the plane, between the capsule and the eloquent tissue, in this case, the thalamus. Here's a nice pull of a malformation from there, and here comes the big majority of this, but you can see how by coring it out and then working the capsule, you can get this nice delivery of a lesion. And here, after it comes out, we're seeing clean gliotic margins, no evidence of remnant, and you'll see no retraction, It's just gravity and a straight shot. Here's a trajectory that shows us all the way through the lesion to the back wall. And again, that cross-court trajectory on through. So this is a way to get to a spot, that would otherwise have been inaccessible. And it's using your craniotomy. It's using your approach and getting it done. We can skip this part. You can go to the next slide or I guess I can control that. Here's the postoperative imaging. And there's been no cortical incision. There's been no violation of tissue, other than that thin layer on the under surface. And we have a nice clean, empty resection cavity. Another case. This is a similar location, but we're gonna go from a different trajectory. So this patient presented with hemiparesis. A left third nerve palsy. And was found to have, once again, a mid-brain and thalamic cavernous malformation. Our grading is here. It's a grade four. One for size, two for age, and one for subacute bleeding. With this one we're gonna go contralateral transcallosal-transchoroidal fissure. There's some alternatives, but this transventricular approach proved to be the best. So here's just a diagrammatic illustration of how we position. Once again, like to use gravity, I put the dependent hemisphere down, so gravity opens up the fissure. We go through the corpus callosum into the ventricle, through the choroidal fissure and on our way. You can either go transchoroidally, along the taenia fornicia, or you can go subchoroidally along the taenia thalami. Both of those options are available and it just depends on how the veins at the venous angle lay themselves out for you. So here's the case. You can see how this cavernous malformation really occupies this area at the very bottom of the third ventricle. Presents to the medial wall of the thalamus. and that trajectory down through the choroidal fissure, gives us a very direct shot. So here we are. The midline is horizontal. We're now down through the corpus callosum and into the ventricle, you're seeing the foramen of Monro. Here you're seeing the top of the choroid as it loops through the frame. The fornix is up here. This is the septal vein. This is the septal vein here. This is the thalamostriate vein and you can see how they've looped together, into the internal cerebral vein over here. Now as we widen that foramen of Monro, you can see now that we're all the way down, into the third ventricle, and we're now coming up on the malformation itself. So here's malformation in here. This is the internal cerebral vein running along and you can see how our foramen of Monro here, has been widened dramatically. Once again, the dissection technique is a matter of internally debulking. Working with planar dissection around it, being very protective of normal tissue outside the capsule. Hunting very carefully for that plane. And it's a long reach. This is probably about an eight or so centimeter reach. Lighted instruments are helpful. We have a lighted bipolar. And in any event, you can see here, this thing is working its way free. As you go around this 360, all of these adhesions to the malformation start to light themselves. And eventually, this thing can be wiggled free. And you can see how, by protecting the capsule, it helps to contain all of the pathology within. It doesn't always come out in one glorious pull, but it's nice to try for that, because the capsule can be a way of ensuring that you've gotten all of the malformation out. So here we are towards the end. You can see here a nice view down the aqueduct. See here in the resection cavity that clean gliotic tissue. It tells you you've gotten pathology out and you're at your limits. And here, just taking a look around, we have a view, through the floor of the third ventricle, down on top of the basilar trunk. So really very spectacular view. And you can see with these lighted bipolars, the anatomy is exquisite. And here's just the overview of the approach. So here's that widened foramen of the Monro, See a view back into the atrium of the lateral ventricle and good preservation of the venous anatomy. Okay. That's good for that video, I think we can now... Yes, thank you. So here's the postoperative view, showing the resection cavity and once again, no tissue transgression, other than that portion through the corpus callosum. All right. Now this is gonna demonstrate a midbrain cavernous malformation. The patient presented with hemiparesis, dysconjugate gaze. The grade of this one was a grade two. So a low grade, well within our resection range. For this one, we're gonna go retrosigmoid and lateral supracerebellar-transtentorial. Here's the lesion. You can see, it presents itself to the posterolateral surface, this tegmental surface of the midbrain and extends upward from there on this sagittal view. You can see how high we have to reach. You can also see on this coronal view how, by going interhemispheric transchoroidal fissure, like I just showed you, that may not be so good, because we've got this extension laterally. We'd have to work underneath this blind ledge of thalamus, which puts us at high risk of bleeding residual. So I did not feel that the transchoroidal fissure approach, was the best one for this case. So here's some nice illustrations from Ken Probst, showing this lateral, supracerebellar-transtentorial approach. You can see it's over the cerebellum. The working triangle, is what I call this infragalenic triangle, that's between the basal vein and the precentral cerebellar vein, right in this corner here. It gets you right to the posterolateral aspect of the midbrain. And in this case, we're going supratrochlear. And let's go ahead and start the video. This patient had had a prior central neurocytoma. Had surgery and had a shunt. Here's the view after opening. Again, sitting position. Gravity is retracting the cerebellum. So it's gonna nicely widen the plane. You can see these arachnoidal adhesions here in the corner. These are little granulations, that go into the transverse sinus. You can see some arachnoidal lesions here. Free edge of the tentorium is right along here. And the fourth nerve is gonna be in this space under that. But this shows you just how nicely it's opened by gravity. The instruments are long. Not always the most comfortable operating position for the surgeon, but what you gain in exposure is tremendous. Here's the fourth nerve. I'm going supratrochlear. Here it is coming along here. And I'm just freeing the nerve, so that it drops into that cerebello lissencephalic fissure and it's out of my way. Now this is a transtentorial approach, so I'm gonna incise the tent. You saw on the pre-op imaging, just how high this malformation reached, up above the tentorium. So it's necessary to make this incision and extend our reach. And you'll see that as I make this cut, a whole other piece of the operative field comes into view. And it's that piece of the operative field, that contains the highest density of this hemosiderin staining. And it actually brings into view, the portion of the malformation that's on the surface, which is right here. So right at my instrument tip, we see a little bleb of malformation here coming into view. And we would not have seen that, were it not for that tentorial incision. So I have a direct shot at this now. I enter the malformation directly. It's what I would call it an exophytic lesion. I really don't even need to incise a safe entry zone here. And once again, I'm just gonna work the planes. I'm gonna centrally debulk this thing. I'm gonna try and preserve that capsule and pull it into the center and I'm gonna pull this thing down. It's quite a reach upward. It goes all the way to the top of the midbrain and into the thalamus even. And so by doing this dissection, you see, I'm killing it off of the pia of the midbrain. You can see the perforators to the midbrain here and the arteries that are traveling out the sides, Those all have to be preserved. And once again, the crux of the operation is the removal of the lesion, but it's not the most exciting or interesting thing to see, from a technical standpoint. There's a big chunk that comes out. There's more to it than that, so I'm continuing to explore. I'm pulling the lesion down. Remember it's keyhole surgery. You're working through this small keyhole, but this lesion is, orders of magnitude larger than that keyhole, so you really have to work that opening and work your trajectory, to see all of the different portions of this. So I think you get the idea here. The takeaways are just that, again, using gravity, using little tricks like this tentorial relaxing incision, really make a difference and allow you to get this done. So here's a postoperative view. You can see a nice clean resection cavity. Here's the sagittal view, once again, showing a clean cavity and did quite well. Okay, so our next approach is this one. It's for the midbrain cavernous malformation, that comes to the surface on the peduncular surfaces, cerebral peduncle. And this approach is very much like a vascular aneurysm approach. You can see here, the illustration showing the skin incision, the orbitozygomatic craniotomy , the transsylvian dissection route over here and over here, you see how we end up in exactly the same place as we would for a vascular bifurcation aneurysm, except that instead of going medial, we tend to go lateral towards the peduncle, rather than to the vascular bifurcation, unless, of course, we're dealing with an interpeduncular cavernous malformation. So here we go. This is the lesion. You can see it here on CT scan and here on MRI, it really expands. The peduncle is exophytic into that interpeduncular fossa and the cistern there. Here's the animation showing the view and our dissection is transsylvian. This is just a standard transsylvian dissection, splitting the frontal and temporal lobes. Following the arteries down. Anytime you go to this region, you have to do a complete Sylvian fissure split. This takes us down here now to the carotid cistern, opening over the optic nerve here. This just allows us to peel back the frontal lobe. Here we're in that precommunicating interoptic triangle, just up, getting to the lamina terminalis, to release some CSF and slacken things. Here now back to the third nerve region, you've got the carotid-oculomotor triangle, that I'm in right now. And Liliequist of course, that's here. We can open up the Liliequist membrane, to get back into that interpenducular fossa. And now as we go deeper, you'll see how the dissection transitions. We want to be lateral to the third nerve. We want to be in the peduncle, so here I am switching lateral to the third nerve. You can see the superior cerebellar artery, below there, and as I follow the tentorium, laterally, into this oculomotor-tentorial triangle, you now see the malformation. And so the triangle of dissection, is between S2, which is the second segment of the superior cerebellar artery, and P2, which is the second segment of the posterior cerebral artery. And the peduncle is entered. You see the malformation here. This one, there's quite a bit of dissection, because there are all these perforators and a lot of adhesions. I'm particularly cautious around these peduncle lesions, because the corticospinal tract is right next to you. And you want to be really gentle with that. I try and enter medial to the corticospinal tract. That's where our safe entry zone is located. The frontal punching fibers are medial in the peduncles, so you wanna enter at that point. And the other thing I wanted to point out, was the lighted suckers. You can see here, this is basically, a lighted tip of the sucker, that allows us to get some illumination, within the resection cavity, not just with the bipolar, which we rarely use for these cases, but with the dissection. So here, another technique. I'm using some traction on the malformation, but you wanna be very gentle. I pull a little bit just to define the planes and then I stop. And then I know exactly where this thing is still adhesed. I can then go back with my sharp dissection here, a little round knife and then do a little bit more work. I can then pull a little more. And I realized that, maybe it's not quite free, maybe it's free, but I can then keep working that plane and eventually free it up. So I think you get the idea of that window. Here you see very clearly, it's transsylvian between S2 and P2. And here we are. See how the lesions finally starting to give. And this was an enormous one, but note these little perforators. We've gotta really work hard to peel those away, to preserve those perforators and just to focus our attention on the malformation. All right, Luke, let's go ahead to the next slide. And we'll go to this one. The pontine brainstem cavernous malformation, is the most common. I'm gonna show you this case, because it's a pretty classic representative case. It presents here laterally. Its brainstem grade is a three. You have a point for the DVA and you have two points for the patient age. The approach is retrosigmoid and the pathway is trans-middle cerebellar peduncle. And let me show you the images. Here you can see that, if we look at the pons here, the pons really is something like this, and you can see how this lesion comes right to the lateral surface of the pons, where the peduncle here, joins it. So this is, really, what I would call, a lateral pontine malformation. And the best way for these, is this trans-middle cerebellar peduncle approach. It really has become one of the real workhorses. The technique involves an extended retrosigmoid craniotomy, splitting of the petrosal fissure, using that safe entry zone, as lateral on the peduncle as possible and then altering the trajectory, to get you the angles that you need. So by extended retrosigmoid, and I mean, a retrosigmoid craniotomy that is not just your burr hole asterion in the small cranium, like many neurosurgeons do, but it's an actual skeletonisation in the ENT tradition, from sigmoid all the way up to transverse sigmoid junction. And by doing that skeletonisation and the craniotomy next, you really get the full exposure of this CP angle. And why is that important? Well, if you just have a small opening and it limits you to this blue arrow, then you're gonna come in and you're gonna miss the lesion. You're gonna be looking this way. So you've gotta be able to swing the trajectory up a bit and the only way to do that is to get right up against where the sigmoid allows you. You've gotta really maximize that forward push, because that's what tips the needle and gets you this kind of an angle to the lesion. And, and that's the real key. Now, the other point on this slide is this dot right here. This is the entry point, into the peduncle. You don't want to enter the peduncle up here. If you do, if you enter it where the seventh and eighth nerve are located and the fifth nerve is located, then it's gonna be a pretty impossible angle, to get to the lesion. You really have to steer hard to get back. Whereas if you start further laterally, out in here, then you don't have to steer quite as hard to get there. And so that's a really important point, toward making this approach work. Now the petrosal fissures is the key to unlocking that anatomy, The petrosal fissure is where the superior labial, here, intersects with the inferior labial, here. You got the flocculus right in between them, but it's this portion here, where they come together, that you've gotta split. And it's here that you're gonna find that entry point for the peduncles. So let's take a look at that. Here's the en face view, showing the flocculus here, the petrosal fissure here and our secret spot is gonna be out here. So unless you split that fissure, you'll never find it. So here, in this cadaver dissection, you can see the fissures being split. Our entry point is revealed, and we can finally get to that safe entry zone. So here you see, it's demonstrated right here on this drawing. I actually think it should be a little more like so, but you get the idea. You want to get as far lateral on the peduncle as possible. So here we go with the case. You've got the petrous bone up in here. You've got the transverse sinus up in here. So we're entering the CP angle in this direction and here's our petrosal fissure. You can see it's just like the Sylvian fissure, where the tissue planes come together. The arteries help define the arachnoidal cistern. And just by staying in that plane of dissection, we can get right onto the vein. We can get right on these arteries and here the peduncle is coming into view. So there's a view of the fifth nerve, I'm giving you your anatomical tour. You can see this vein that runs up to the superior petrosal sinuses. It's a Dandy's vein. And look at this arrow here. You can see how, if we just look in the CP angle, we're going in this direction here, but we've gotta turn our angle more back. So you can see that as I realized that, I'm gonna need to split some more fissure. I need to actually tilt the angle even further. And when I open up the fissure maximally, here's my secret spot right back here. So that's the peduncle. It's got that classic white color. And look now, the angle that we created. We created the proper angle here for our trajectory. The lesion is squarely in the line of the trajectory now and we would not have created that, were it not for this petrosal fissure split. So here's a little finger of that venous malformation. Everybody talks a lot about these DVAs and you've gotta preserve the trunk. It's okay to take a little finger like I've taken here, but you've gotta preserve the trunk. And just on the other side of that little finger, we start to see the malformation in here. So you can see how, by going through the middle cerebellar peduncle, we've hit our target. There's this certain amount of corridor here, that we've had to create through parenchyma, but again, this is a safe entry zone. This is a tolerable pathway, and it takes us in the optimal trajectory, to the malformation. So here, clot's out, we've got some working space. I'm now transitioning to my instruments and I'm working in the planes. And, you know, again, this part of the operation, isn't quite as visually interesting, because it's hard to see. You're working through a keyhole and you're trying to develop these planes, that are concentric, they're outside the keyhole. And the real challenge is to try and do all of this work, using the keyhole technique. So you get this much larger lesion that comes out, so that we don't even see the lower cranial nerves here, 'Cause we're so far lateral, but here, I'm just showing you on the other side of the vein, where they're located, you don't even see our entry site in here. It's buried in the petrosal fissure. And that's really how you want to make this approach, work for you. All right, we can go to the next slide. This shows you the postoperative view and important to notice the trajectory. So we've entered very lateral, out here. We've got a route in that really avoids all of the critical nuclei and tracts, in the pons and is perfect for this. Yeah, this is another good one. This should be a quick resection of inferior cerebellar peduncle. And actually Luke, if you could just slide this forward, to the MRI. Yeah, there we go. So here's the lesion. This thing is sitting right at the inferior cerebellar peduncles and the approach for this is a telovelar approach. Here's our brainstem grading. You can see we've got a point, sorry. The red numbers are point assignments and the total here is a four. So an intermediate grade lesion. Still a good one for resection. And we can go ahead and slide forward, Luke, to the video. So here's the view. The feet are at the top of your screen. The top of the head is at the bottom of your screen. You can see the tonsils here and here. The approach is between the tonsils. The caudal loop and PICA is shown here. You can see the ventricle over here, and we're just gonna take down the inferior medullary velum. We're following the T3 segment of PICA superiorly. And as we do so, we can now mobilize the tonsil. We can take down some more of the roof of the fourth ventricle, this velum. And as we do, what we see here, we get to the inferior cerebellar peduncle, as it comes up. So this is the peduncle coming in this way. The lesion sits right in that peduncle. And as we enter this, we hit our target. So this is just a nice example, showing how this telovelar approach, can be very useful for the low-lying, more laterally-placed hunting lesions, those that come to the floor of the fourth ventricle, but in the lateral recess. So here you can see, I had to work a little bit to finally find the lesion. You can see this hemosiderin right in that little corner. That's the actual lesion itself. And once we identify the capsule, we can do our dissection. We can free that up. And this was not a big one, small lesion, but very symptomatic in this guy. And you can see how, you know, just once again, we get into the malformation, we get into the interior. Sometimes that comes with a little venous bleeding, as we see here, but when you see venous bleeding, it tells you that you've got malformation that's still left to identify and resect. And so it's an important clue. It tells me not to coagulate, but to keep that second, so that I get around this thing and find it fully. So here we are at the end. You can see this nice clean gliotic tissue, at the depths. You can see here, just an overview of where we're located. Here's our resection cavity, over there. Here's the whole tour at the end. You can see the vertebral artery running up here. This is our PICA coming off there. Here's the contralateral anatomy. You can see the vertebral artery and PICA on the opposite side. Beautiful view. Here's the obex down in here. And Luke, we can go to the next slide. So we're winding down on our tour. I'm just gonna show you this last case. I think it's the last case. It's a medullary brainstem cav mal. Three prior hemorrhages. A brainstem cav mal grade of four, so intermediate grade, We're gonna do a far lateral and we're gonna do this approach through the cistern. Now, this is an interesting one, because you look at this really carefully, you see that it's at the top of the medulla here. Really where it comes to the surface, is here on this lateral aspect. So it's a little tricky to expose, but to go from the back, through the ventricle, I think is a mistake, because there's a little too much tissue on this, for the fourth ventricle. And the more I've learned, the more I've done these cases, the more I've learned that you wanna not violate the flow of the fourth, whenever possible. So for this one, I chose a far lateral. The trajectory is a little bit awkward, because we're coming in an angle this way, with the lesion axis perpendicular. And we published a paper on this, called the right-angle method. When you have this right-angle intersection, between your trajectory and the lesion axis, you're very prone to leave residual, the further you get away from the axis of your trajectory. And so this right-angle rule, is a good trick to keep in mind, just to help you make sure you track down all of the lesion. So here's our video. Standard far lateral craniotomy, like you would do for a PICA aneurysm. Here is the dissection into that lateral medullary cistern. You can see nicely the eleventh nerve coming out here, The nerve rootlets of eleven from the medulla coming in. Here's a blown out portion of the olive. You can see it discolored in yellow. The ventricle is over here. And we're gonna get into this thing through the olive. We're in the vagoaccessory triangle. You can see these rootlets of eleven coming up. And as you can see here, we really have no trouble getting to the lesion. The lesion comes into view very quickly, just through this thin layer of tissue, but remember, this lesion is sort of projecting like this. So we've gotta work our way in this direction. So we're having to move to your left, into the medulla, in order to get around this, which means, in fact, that we're having to pull this lesion, into our field, this way. And this is one of the challenges of keyhole surgery. You've gotta constantly work the lesion into that small keyhole window, that you have for your visibility. You can't see the whole thing. It's like whale watching. You're seeing just pieces of the beast and not the entire thing. And you've gotta to find a way, to pull it all into your view. And so I'm gonna leave that arrow up. And you see that a lot of this dissection, is meant to pull that pathology to the right and bring it into view, because otherwise you miss it. And this is the challenge of the right-angle rule. When you have that perpendicular relationship, between the way that you view things and the lesion axis, it means that you have to work harder, to bring everything into view and to do a complete resection. So here you see it coming out piece by piece, but with slow steady dissection, I progressively work that malformation, from left to right, into my keyhole and I'm drawing it out. So here we are at the end. Last few pieces here. We're starting to see this clean gliotic tissue and I like what I see here. Not seeing any more mulberry. I'm just seeing the gliotic tissue, but once again, you gotta be meticulous. There's the last little bit of it, and you gotta hunt really aggressively, into those blind spots, to make sure you've got it all. So there you have it. You see that transolivary approach. You can see that a good far lateral is necessary, to open up that space and we can move on to the next slide. So here it is. There's our approach, right there. A nice clean cavity over here that you can see and she did quite well. Here's just a little illustration, that we created for that paper I mentioned, about the right-angle rule. Particularly when you're working through these long corridors that go through the craniotomy, through the subarachnoid space, through parenchyma, and down into a cavity, it's very easy to miss these portions, that are at right angles to the field of view. You lose perspective and so you've gotta think, "But what does the right angle rule tell me about where that remnant is likely to be?" And then have to, really aggressively, look in these corners here. This corner here. It's much easier to see down here because it's in line, but these blind corners, up here, can be a challenge. And here's an example of the right-angle rule. If everything's in alignment, the approach trajectory and the lesion axis, it's relatively straightforward, but when the axes are at odds with one another, you can see that you have a higher risk of residual. Well, I guess there is one more case. So let me just quickly show you this one. This will just show you the suboccipital-transventricular approach, for this grade three lesion. Midline approach, standard suboccipital. You can see here, this is a very large one that consumes the medulla. We're just gonna do a suboccipital craniotomy. This is as straightforward, as brainstem cavernous malformations get. It's right there on the surface. It's exophytic. But what you'll see with this one is that it's so big, that you really have to be meticulous about getting all of it. So first, I'm gonna do a dissection, along the inferior medullary velum. I'm gonna open up the vallecula between the tonsils. I'm gonna visualize my caudal loop of PICA. And only when I got everything nice and loosened up, do I enter the malformation. Here, the pia is incised. Luckily there's some fresh blood within it, that evacuate that creates our working space. And once again, this is one of the advantages of operating in the acute to subacute window. You have this blood. It helps to create some working space. It helps to, what I call, lubricate the malformation, so that it slips out. And I think it's just makes it a better, lower morbidity operation. And our results have borne that out. So once again, just working the plane of the malformation. Then a lot of debulking internally, but here you can see, the capsule comes into view and here, the hematoma is in those spaces around it. And once it's all worked out, you can start to feel this way. Now, if you look carefully, what you notice is that we're in this keyhole here, but we still have lesion, over here. And we have another lesion out of view that's up in here. But so the point... And here is yet another exophytic portion, accessed from another incision. And between these three openings, I'm able to get pretty much all the way around this thing and to remove all of the various pieces of it, trying my best to preserve everything around it. You go down in the region, of the gracile and cuneate nuclei, on the dorsal surface of the medulla. So we want to be very protective. And you can see, you've got three opens here. We've got one here. We've got another keyhole above that and then our main one below. So three discrete openings. You can see here at the depths, I'm all the way down to the pia on the front surface. This is pia on the front surface of the medulla, but that indicates that we're clean. Here I'm just making a little snip, just to make sure that that's not malformation. You can see that that's pia. And as we take a look through that, we see clival dura on the other side, but once again, we've got one, two and three. Three little openings that give us our exposure, to all the various components of this lesion. All right, Luke, I think you can move ahead. Here is the postoperative view. You can see a nice clean cavity here. Good, healthy medulla there. And everything went well with that. So just to summarize, as we finish up, these are just tables that show, by segment of the midbrain, the various approaches. The key is to choose the right approach. The workhorses are the orbitozygomatic transsylvian, the bifrontal transcallosal-transchoroidal fissure, both of which I showed you. The lateral supracerebellar and the midline supracerebellar. These are the workhorses for the midbrain, usually. For the pons, we also have four workhorses. The retrosigmoid transcerebellopontine angle cistern approach, the retrosigmoid trans-MCP approach, the suboccipital transventricular, and finally, what I didn't show you, was this transpontomedullary sulcus approach. A real great approach, which we'll have to come to another time. And then finally, for the medulla, these are generally either transventricular or far lateral. So to wrap it up, I think these are challenging lesions. The secret is patient selection. I look for patients that have had hemorrhage and have deficits. You don't want to operate on someone who's perfect. You'll only make them worse. You want to look for lesions that are on the surface, at the pia or the ependyma. And use the brainstem grading system, that will help you with these decisions. Approach and enter safely using those safe entry zones. I've demonstrated this intracapsular dissection technique. And I think that the final thought is just this. There's a very fine line between complete resection, getting all of the malformation out, and going too far and causing morbidity. So there's always that very fine line that you have to walk, but anyway, that's what makes this a real challenge. So Aaron, I hope that was helpful. Thank you for having me.

- Spectacular work, Michael. Nothing less than expected, obviously, from you. Fascinating, really, the videos, the technique, innovation, classification. Very consistent with your previous work. You have done both in aneurysms, AVMs, bypass, and now cavernous malformations. Look forward to getting your book, which I know coming out shortly. I want to talk to you about a couple of topics. Number one, easy one. I assume you monitor every patient and what kind of monitoring modalities do you use?

- Yeah, so you can't do the surgery, without neuromonitoring in standard care. I think that what I've learned over the years, is that monitoring does nothing but ruin your day, because you go from a really well-executed resection, to getting to the very last bit of the case, and then telling you that they've lost 50% of your somatosensory evoked potential. The way I interpret that, is that it just tells you you've reached the very edge of the lesion. You're right up against that eloquent tract. You're going to tickle it. You're gonna have a little bit of a change. And if you listen too hard to them and you stop your work, then you'll leave remnant and then there's really no point, to have been there. So I think there's some value to it. I don't ever operate on these without it, but I think you have to be a little bit, I don't know what the right word is, a little bit callous or a little bit focused on--

- Convolution, cavalier. Yes. Yeah, it's just to make sure, that you get to your objective and also not give up too early.

- I agree. I think you put it very well. It's just one piece of the puzzle, the monitoring, You may just use it as a method to relieve traction, increase blood pressure, and just to irrigate the field. Maybe use some papaverine, soak Gelfoam to bathe the vessels that could vasospasm, but by itself, you use the anatomy and also see how the monitoring acts, but you can't just use the results of the monitoring alone, to make decisions, unless obviously, you have transgressed vital tissue, which is a different story, but we're not talking about that. This is a good transition point, talking about recurrence. I think, especially in the periphery of cavernous malformations, there is tissue that can look very much gliotic, and similar to normal brain stem, but in fact, it's cavernous malformation. So the still jury is out, about the long-term recurrence of cavernous malformation surgery. I have seen postoperative MRIs, that looked beautifully clean, from very well-accomplished surgeons, that the patient come back to me with a recurrence. So are these dynamic lesions, especially in younger people and can recur, or is there a phenomenon of this normal-looking tissue around that malformation that can really trick the surgeon? I think we as surgeons, don't confess enough, to the fact that these lesions can recur and our techniques may have some fault. What are your thoughts about that?

- Well, yeah, it's the big problem with these. If you leave any malformation behind, you run the risk of it recurring, and it's true that that gliotic plane can be so stained, that it can really hide, what is in fact a residual lesion remnant, so I think it's really important to be meticulous, to try and preserve the capsule as you're dissecting, that will help you pull it all in. I think you have to really have a sense of how much volume of pathology you're expecting and how much you've actually removed. It can be a good sense, or a good guide, whether or not you've gotten it all. It's that fine line, though. If you push too hard and you go for the complete resection, at the expense of taking some normal tissue, you're gonna hurt people. And so that said, I think it's probably not unreasonable to expect a certain percentage of the brainstem lesions to have recurrence. It's kinda like the negative appendectomy, that we all learned about in general surgery. You want to operate on a few negative patients, so that you make sure that you get all of the ones that are hot abdomens, high appendices. And so it's the same situation here with the basilar. I mean, with the brainstem cavernous malformation. You wanna probably have a few cases of recurrence, just so that you're not overly aggressive and hurting people.

- No, that is well said. And in closure, you will compare aneurysm surgery to ballet. AVM surgery to the war. Bypass surgery to architecture. And now the cavernous malformation surgery to maps. I've used a slightly different metaphor, for cavernous malformation. That's expressway, because I feel there is very creative ways, to get to these malformations, that really tests our surgical skills. And you spend so much of your time on dissection of beautiful, pristine anatomy, that is truly enjoyable. So cavernous malformation surgery, is one of the most enjoyable operations I've ever done, just because it's so pristine, clean, enjoyable, and the patients do well. And you can really be innovative in your trajectories to these lesions and also use operative angles, rather than operative space, to maximize your reach. So with that, Michael, I want to thank you, for all you have done for neurosurgery. For so much. You have driven all of us to be better and hope to have you with us in the near future.

- Well Aaron, as always, great to see you and great work with the Atlas and with this offering. My hats off to you and great to see.

- Same here. Thank you, Michael, all the best wishes for you. Thank you.

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