Grand Rounds: Retromastoid Craniotomy and Expanding the Operative Corridor
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- Hello, ladies and gentlemen and my name is Aaron Cohen, thank you for joining us. During the following session, I would like to talk about technical nuances for retromastoid craniotomy. This is a basic approach familiar to all of us. However, this technically a skull base approach is very flexible. And we're gonna talk about today, how we can use this simple basic approach to expand our operative corridor within the posterior fossa and even within the supratentorial space. The first slide describes my disclosure, which does not interfere with the presentation today. Let's go ahead and start with a setup of the operating room for a retromastoid craniotomy. As you can see in this illustration, the patient is placed the lateral on the operating table, especially for heavy-set patient, this facilitates venous drainage and avoids the extreme turning of the head during placement of the patient in a supine position. The scrub nurse or a surgical assistant is standing across the table from the surgeon and this allows easy transfer of instruments without any device blocking the space. The surgical assistant or the resident fellow is sitting or standing at the head of the patient. The anesthesiologist may be located here or if possible, moved and located at the foot of the table. If the anesthesiologist is placed at the foot of the table, often this creates additional space for the surgeon and the assistant. Now that we discussed the operatory setup, let's move on to a patient positioning. As we discuss briefly, the patient is positioned in a ladder position, all the pressure points are well padded. The patient's head is fixed . One of the important points in patient positioning is trying to create extra working zone for the surgeon during posterior fossa operations, especially with patients with large and a fatty shoulders, their shoulders may interfere with transfer of instruments or working zone of the surgeon and therefore having a shoulder to fall away from the surgeon and be taped toward the foot of the table would be very effective to increase the working zone. Obviously roll is necessary. I would like to discuss the advantage of doing a lumbar puncture at the beginning of the procedure. I do that very frequently for most of my posterior fossa operations and the reason is decreasing tension in the posterior fossa. One of the most difficult steps and a risky steps in posterior fossa operations is to go around the cerebellum and enter the cerebellopontine angle to release CSF. By releasing CSF at the beginning of the operation while the surgeon is washing his hands or her hands, we release about 30 to 40 cc of CSF. And that decreases the tension in the posterior fossa and allows the surgeon to go around the cerebellum and easily enter the cerebellopontine angle cisterns without fighting the tension in the posterior fossa potentially injuring the cerebellum or tearing a bridging veins. So how about patient head positioning? In a skull clamp, I place one of the single-pin arm in the frontal area and the two-pin arm in the contralateral. So the head is turned slightly toward the floor and that helps when you go around the cerebellum to be able to have a better angle, I guess the petrous bone. Of course, the shoulder, as we talked about is allowed to fall away from the surgeon and taped toward the foot of the patient. You can see the skull clamps placed on both side of the head. In this situation, the skull clamp is placed above an anterior to the pinna. And you can see, again, the head of the patient turns slightly toward the floor. Now that we have positioned the head, let's talk about the incision. Here is the Walter Dandy incision that I have come to use, especially for the past few years and I think has certain advantages over the curvilinear incision that I'll review momentarily. So how do we use this curvilinear incision? And what are the landmarks? And what are the advantages? So the first most important landmark is inion and the root of the zygoma. And we coordinate the line between these two points and that line typically defines the transverse sinus. Then we find the tip of the mastoid, as you can see marked here and find the mastered groove and draw a line parallel to the mastoid groove and the point where the first line and the second line cross typically defines the transverse signal junction. And that's the incision. As this sort of incision is defined, we'll go ahead and draw a curvilinear incision starting just anterior to the mastoid groove with a long base and that would prepare our skull flap. The advantage of this flap is the fact that it may sometimes avoid the neurovascular bundle that you can see here and potentially decrease the numbness after the surgery. So this is the typical linear incision in cross section. And the disadvantages of a linear incision in my opinion, is that the scalp flap gets very much bunched up under your cerebellar retractor and increases the working zone of the surgeon. And a posterior fossa, where our surgeries typically have long working distances, adding extra working distance typically is undesirable. And so when I talked about the curvilinear incision, often the scalp flap is reflected inferiorly away from your working area and your working distance is not increased as you can see here by bunching up the scalp and the muscle during a linear incision and this can be especially an issue with patients who have a very fatty and thick scalp flap. As you can see in this video, the cerebellar retractor causes increased working distance and somewhat blocks the operative corridor to reach the cerebellar hemisphere and go across through the cerebellopontine angle. So when we talk about retromastoid that craniotomy, we often wanna fashion and tailor our bony removal based on the operative corridors that we need to reach the lesion. Typically, there are two operative corridors, one is supracerebellar versus lateral inferior cerebellar. And those are the corridors. First one is the one we use to reach the upper cranial nerves or the lesions close to the upper cranial leions in the posterior fossa. And the latter is the operative corridors we used to reach lesions typically around the lower cranial nerves in the posterior fossa. Let's talk about a generic retromastoid craniotomy to reach the posterior fossa. In this case, after the scalp flap has been reflected here, we go ahead and use a fish hook in order to mobilize the scalp flap more toward the ear and increase the amount of bone that is exposed. This will also prevent later with a reflection of our dural flap. We use the same landmarks to place a burr hole, the strategic burr hole and careful placement of this burr hole is critical to expose the transverse sigmoid junction. The moment you expose the transverse ssigmoid junction, and identify that the rest of the craniotomy is much easier, because you know where the sinuses are and you can protect them. If however, you don't place the burr hole in the right location, then identifying the sinuses can be difficult and their protection can be also more problematic. So we use the same point from inion to the root of the zygoma, the line will define the lower part of the transverse sinus and the horizontal line from the mastoid groove, where these two lines meet would be typically where the burr hole should be placed and defines the inferior aspect of the transverse sinus and the posterior aspect of sigmoid sinus, that's where we start with our burr hole. And here is really an operative view of showing the mastoid groove and where we desired to place the burr hole. You can see in this illustration, the burr hole has been completed. The edge of the transverse sigmoid junction is clear and we use a number three Penfield to undermine the bone and strip away the dura and edges of the sinuses and based on what you would like to expose, the size of the bono flap can be different. In case of an acoustic neuroma, obviously the bone flap is a larger in case of trigeminal neuralgia, microvascular decompression, the bony exposure is smaller and more toward the transverse sinus. For hemifacial spasm case, typically you don't need to expose the lower edge of the transverse sinus and the burr hole maybe placed actually slightly inferior than what you see here. After the craniotomy is elevated. And as you can see, the first cut is here. We're gonna review that in the video momentarily. And after you have completed the first cut, we usually use the B1 bit with a footplate and come across just closer to the edge of the sigmoid sinus. We avoid drilling or using B1 with a footplate right over the sinus, especially in the older individuals. Since the sigmoid sinus often is very adherent and embedded in the bone, much more than the transverse sinus and using footplate right over that sigmoid sinus can potentially lead to injury to this important venous structure. So the first cut is away from the sinus, second cut was just posterior to the sigmoid sinus, obviously the last cut is closer to the sinus. So if there is ample amount of bleeding encountered, you can elevate that bone flap immediately and manage the bleeding. After the bone flap has been elevated, I often use a M3 bit in order there or a round bit in order to drill the part of the mastoid bone over the sigmoid sinus. I do believe that removal of the point over the sigmoid sinus and exposing at least 2/3 of the width of the sinus was expand your operative corridor and would minimize the amount of retraction of the cerebellum. Therefore when safe, I highly recommend to remove bone over the sigmoid sinus and that allows us to mobilize the sigmoid sinus as the dura is open and reflected anteriorly. And that really moves the sinus slightly out of our way and provides a more open trajectory toward the cerebellopontine angle and decrease the amount of cerebellar retraction. After the bone over the sigmoid sinus has been removed and fend out, I typically use a Kerrison rongeur to remove a very thin inner cortical part of the bone and avoid drilling all the way down onto the wall of the sinus to avoid any injury to the sinus. You can obviously use the mouth of the Kerrison and point the mouth up away from the sigmoid sinus to avoid any injury to this important venous structure. After the dura, after the bony in removal is completed and part of the transverse sinuses exposed orientation, as well as the sigmoid sinus is exposed for about 2/3 of it's width. Waxing the air cells is critical to avoid postoperative CSF leak. As Dr. Janetta has very well mentioned, wax in wax out. That means wax the air cells on the way going in and doing the exposure and wax air cells on the way out and after you have closed the dura. And we believe in that principle to avoid any postoperative complication. The dural opening, for this purpose, I usually open the dura along the sinuses, along the transverse sinus, leaving small sleeve of dura to avoid injury and his small venous-like that typically irrigate into the sinuses. And the reason opening the dura along the sinuses is more useful in my opinion is that it keeps the dural flap outside in a way from the intense light of the microscope. I see often my colleagues opened the dura and reflect the dura away. In other words, their dural opening is inside, incised across this line and the dura flap is reflected over the mastoid bone. Unfortunately, during the entire case the dura gets very desiccated, dried and it's closure can be difficult in a watertight fashion for posterior fossa case. So as much as of the dura, you can leave on the cerebellar hemisphere, I think you would protect it. And you can also put three stitches to the edge of the dura. And this really would mobilize the sigmoid sinus and rotate it away from your operative corridor and give you extra two or three millimeters of distance to go around the cerebellum. You often need three stitches, one at the junction, one over the area of the sigmoid dura and one along the transverse sinus dura. Of course, if your bony opening is in this area, you most likely need one over the sigmoid, one at the junction and one parallel the floor off the posterior fossa in the case of a, for example, allow microvascular decompression for hemifacial spasm. After the craniotomy is elevated, the basic concept is to be able to go around the cerebellum and enter the cerebellopontine angle cisterns to really CSF and achieve maximum decompression in the attention of the posterior fossa. One of the important landmarks is junction of the tentorium-petrous And as long as you can identify that it would make your job a lot easier. And you can go around the cerebellum. I use a piece of glove, as you can see, we call the rubber dam and this is usually cut to the size of a carotenoid patty. and you let the carotenoid collide over the rubber dam in order to go around the cerebellum and reach the cerebellopontine angle cisterns. This maneuver often avoids the injury from friction between the surface of the carotenoid and the surface of the cerebellar hemisphere. As you can see here, slight rotation of this sigmoid sinus by placing your dural tack up stitches closer to the sinus, mobilizes the sinus away from you and gives you a nice view around the cerebellum toward the cisterns. Opening arachnoid membranes sharply is critical to avoid any injury from blunt dissection to the surrounding cranial nerves. Obviously, you wanna dissect the arachnoid membranes for cutting, as some of the vessels can be hiding between these arachnoid membranes and protection of these arteries, of course is critical. For trigeminal neuralgia, microvascular decompression, I have hardly ever taken the superior petrosal vein and we do not use extra retractors as a slight mobilization of the sigmoid sinus out of our surgical field and using dynamic retraction of the suction often provides ample amount of working zone for the surgeon to work through. So let's go ahead and review our first long video that will take you through basic concepts for a retromastoid craniotomy step by step. So we're gonna go ahead and talk about extended retromastoid approach and why extended, we mean additional bony removal over the sigmoid sinus to increase our working zone. So here's the patient position as we talked momentarily, the patient's shoulder again is rotated away from surgeon and toward the foot of the table. We talked about identifying the tip of the mastoid, the mastoid groove and the first line from inion to the root of zygoma, as well as the horizontal line from their mastoid groove joining the first line to define the somewhat of the incision, as well as the transverse sigmoid junction. Here is a higher magnified view of the incision. I use a 15 blade knife and not a 10 blade knife to complete this curvilinear incision off and the 10 blade knife makes incision more irregular, potentially increases the risk of not adequate healing, Bovie electrocautery is obviously used to extend our incision to the floor towards the bone. Any intradiploic veins are coagulated and also plugged with on wax. We'll go ahead and try to expose the mastoid groove, the tip mastoid is not necessary to be exposed and here you can appreciate the mastoid groove. This part of the scalp is mobilized interiorly. It's important to do that and use fish hooks, as some patients harbor there sigmoid sinus more anteriorly than others. And you wanna make sure you have an ample amount of space and bony exposure, if you need to mobilize your, if you wanna increase your bony removal anteriorly. As you complete the burr hole, you would like to see the edge of the transverse sigmoid junction, as you can see here, you may use a blunt hook, right and blunt hook in order to carefully dissect the sinus away from the inner aspect of the bone. Use Kerrison rongeurs to remove the bone over the sinus. Any bleeding, maybe easily controlled with Gelfoam soaked in thrombin. Now the edges of the normal dura is being exposed, bony removal over the sigmoid sinus is carefully done. You may use the typical the Kerrison rongeur to dissect the sinus away before cutting away at bone. Here is more exposure over this sigmoid sinus. We use a number three Penfield or cryer elavator to dissect the bone away from the sinuses and the dura. As we discussed, the first cut is just along the inferior edge of the transverse sinus and a toward the sigmoid sinus, but stopping just short of the posterior edge of the sigmoid sinus. Patients who have a very adherent dura and inner sinuses may benefit from a craniectomy in that case, then we back up the drill and complete the second cut, either using a footplate or in this case, would be one without a footplate in order to thin the dura and be able to elevate the craniotomy flap. If the bone is very thick, often a B1 with a footplate, maybe you without a footplate, may be used to thin the bone and then Kerrison rongeurs may be used to finish off the inner cortical bone. And here is the elevation of the bone flap, exposure of the dura, edges of the similar sinuses evident and would like to remove more bone in order to achieve a more extended approach and mobilize the sigmoid sinus out of our working area. So the bone may be carefully thinned out. You can see that the bone is being drilled and beveled out a way, because as you put your tack-up stitches over the sigmoid, sinus dura, you would like to mobilize the sinus away from you and therefore you have to bevel away their mastoid bone to make that maneuver possible. If you don't drill the edges of the mastoid bone away from your working area, as you open the dura, you will not achieve additional space by mobilizing the sigmoid sinus anteriorly. Here's the critical step for placement of the bone wax to avoid any formation of CSF fistula. Obviously, you do not want to have the bone wax interfere with your mobilization of the sigmoid sinus. Often you may have to bevel out the bone also along the posterior aspect of your craniotomy to be able to walk around the bony edges. You can appreciate the amount of bone removed over the sigmoid sinus. Dura opening leaves a sleeve of dura here along the edges of their sigmoid sinuses. You can see that performance of lumbar puncture and leads of CSF at the beginning of the case, achieves remarkable amount of posterior fossa decompression and prevents that herniation of the cerebellum into the surgical field. Now we use tack-up stitches, three of them to rotate the dura away from our working zone and leave as much of the dura as possible over the cerebellum. Here is that mobilization of the sigmoid sinus using the stitch. And here's the final product before starting our journey within the intradermal space. We use again, Gelfoam powder soaked in thrombin to avoid any minor epidural bleeding and achieve immaculate hemostasis. I would like to, again, pay we're asked for attention for mobilization of the sigmoid sinus here and that additional few millimeters you can see available to go around the cerebellum. Here is the rubber dam placed over cerebellar hemisphere and then the carotenoid patty. We can slide the carotenoid, as you can see over the rubber dam, identify the tentorial petrous junction and opening the arachnoid membranes sharply. Here is the seventh and eighth cranial nerves. Aggressive retractions away from these nerves should be avoided since the eighth cranial nerve is very sensitive and hearing can be placed at risk. Here is exposure of the lower cranial nerves have really a nice panoramic view of the posterior fossa in this case. I have used the interdural portions of this case, which is microvascular decompression for energy efficient spasm to illustrate some of the basic concepts regarding this section in the posterior fossa. Here, you can see the seventh and eighth cranial nerve, you can see the axilla of the seventh nerve and here is the vascular loop, the offending loop causing spasms. The lower cranial nerves are located here. We avoid using fixed retractors. You can see the suction is a dynamic retractor, and allows the brain to technically breathe during alternating motion of the suction. Now we use the intra-follicular approach to expose the exact exit of the seventh cranial nerve. In this case, we encountered some perforators and have them mobilize these perforators before further exposing the root exit of the seventh nerve. You're getting closer to the road exit zone of the nerve. And that probably is the exact area to take with the suction, where the seventh cranial exit zone is. Here is more magnified view of the exit zone of the seventh cranial nerve. We wanna make sure the artery and the perforators are perfectly mobilized before the Teflon implant is placed to avoid blunt the section with a Teflon implant and avulsion of the perforators. Piece of Teflon implant is placed. I use shredded Teflon. If the vessels are in any way in spasm, we use a papaverine soaked piece of cotton in order to bthe the vessels and relieve their spasms. We leave the piece of cotton soaked in papaverine on the vessels to allow more relief of the spasms while the dissection is in the process. Here is the exact root exits of the facial nerve and enlarges shredded Teflon and mobilization of the arteries. I think there should have Teflon would allow more gentle mobilization step by step, rather than a larger piece of Teflon in one step mobilizing the offensive vascular loop After the procedure is completed, you can see the rubber dam protects the surface of the cerebellum very nicely. The dura is not dry or desiccated as it was protected over the cerebellum and underneath the carotenoid and avoidance, fixed retractors again, protects the cerebellum very effectively. Ample amount of irrigation is used to assure immaculate hemostasis. And again, the dura that is not desiccated can be easily approximated in a watertight fashion. And running or interrupted sort of sutures may be used. And the mastoid air cells are rewaxed before replacement of the bone flap. Irrigation also applied before final stitches placed. Now that we reviewed that via general principles for our retromastoid craniotomy, let's talk about a variation of craniotomy, just for exposing the lower cranial nerves, where exposure off the transverse sinus may not be necessary. In this case, a patient is undergoing microvascular decompression surgery for genicular neuralgia. The basic principles reviewed previously for position of the patient applies. And as you can see, the curvilinear incision is located slightly below the junction of the two lines, first line between the inion to the root of the zygoma and the second line parallel to the mastoid groove. Obviously we do not need to expose the junction and therefore the incision may be placed slightly infiriorly and therefore smaller. As the scalp flap is reflecting inferiorly you can see the mastoid groove and the amount of exposure. And again, the mobilization of the scalp flap anteriorly using facial retractors. The amount of bony opening is tailored, you can see the edge of the sigmoid sinus, however, the transverse sinus is not exposed. And the dural opening is very different since we really would like to expose the lower cranial nerves and not the area of the fifth cranial nerve. They dural opening is parallel to the sigmoid sinus and then parallel to the floor of the posterior fossa. And we're gonna use the intra-medial chordal, again, using micro scissors to open the right membranes over the lower cranial nerves. And in this case, identifying the offending vessel causing geniculate neuralgia, fixed retractors are not used. Another challenge that we often run into is when you expose, for example, the area of the fifth cranial nerve and there is an hypertrophied piece to piece of petrous wall interfering with the working zone of the surgeon. And this hypertrophied bone can be easily drilled away and expose more distal part of the trigeminal there, as you can see here. It's important during drilling to remember that often an artery or a vein can be lying just underneath or anterior to the piece of hypertrophied bone and if you're not careful, you may injure the artery or the important vessel. Let's review a brief video regarding removal of the hypertrophied bone, you can see a piece of hypertrophied bone in this case, very much overhanging the trigeminal nerve and the brain stem. We can use a in order to drill the bone away, which is often very soft. And that would really improve the workings on other surgeon and allow minimization of a cerebellar retraction and more direct trajectory to this area of interest. Okay, here you can see much better, the anterior aspect of the pons and brainstem and an offending vessel, which was hiding previously on their overhanging hypertrophied bone. Is subsequently mobilized for relief of preoperative trigeminal neuralgia. Let's talk about variations of the retromastoid or extended retromastoid craniotomy to reach even the basal aspects of the occipital lobe and the medial posterior temporal region in the supratentorial space. When one tries to remove tentorial meningioma extension into superatentorial space. They're most symptomatic part of the tumor is often in the posterior fossa compressing the brain stem. As you can see this left retromastoid exposure, the illustration is trying to convince you that the tumor obviously is originating from the medial aspect of the tentorium, it has mobilized the cranial nerves inferior and advanced immediately. The tumor can be vascularized from the tentorium initially. then debulked, then dissected away from the cranial nerves, very carefully by grabbing the arachnoid and mobilizing cranial nerves that way without their significant manipulation, After the portion of the tumor within the posterior fossa is removed and trochlear nerve is protected, if possible, you can use your retromastoid or partially supracerebellar approach and cut a piece of tentorium, obviously, identifying trochlear nerve and protecting it and remove the extension of the tumor along the edges of the tentorium, along the medial temporal region. And really cutting this window within their lateral aspect of the tentorium provides a working zone for the surgeon to work from the posterior fossa through and across the tentorium into the supratentorial space and avoid staging of tumors that may have two components. The approach suprameatal approach is one when some of the bone around the fifth cranial nerve can be removed for those tumors that extend along the fifth cranial nerve, such as meningiomas into the cavernous sinus. Let's go ahead and review a video of a multicompartmental epidermoid and see how we can use cutting the section of the tentorium in expanding our operative corridor into the supratentorial space. This patient did have a very large epidermoid and a stage approach was necessary. He was a 42-year-old male, who presented with left-sided hemifacial spasm. You can see the tumor that is patronymically very hyperintense on diffusion weighted images, typical of an epidermal tumor, extending from the posterior fossa across the tentorium into the medial temporal region. We can see there a sizable portion of this tumor in the medial temporal lobe anteriorly. And again, you can see the tumor tracking between the cranial nerves and cross the edges of the tentorium. Unfortunately, cutting the tantrum in this case will not be adequate to remove this portion of the tumor, but we'll expand our operative corridor to remove the tumor along the edges of the tentorium. Here is a left-sided retromastoid craniotomy as we just discussed. Cerebrallar hemisphere is gentle retracted inferiorly, medially. The spare petrosal vein in the case of large tumors such as this is sacrificed at the beginning of the operation, the tumor capsule is carefully opened and the pearly tumor is removed. Here, you can see the opening of arachnoid membranes along the inferior base of the tumor in order to facilitate exploration and identification of the lower cranial nerves. Pituitary rongeurs is used to debulk the tumor. Here's the mobilization of the tumor away from the lower cranial nerves. Sharp dissection should be used to avoid traction on these nerves. Here's further superior mobilization of the tumor away from the nerves. Mobilization of the tumor away from the brain stem. Sixth cranial nerve also has to be protected. Here's the sixth cranial nerve entering the dural's canal. They arachnoid membranes or open and removed. However, the tumor capsule that's very adherent to the cranial nerve is left behind to protect the nerves. You can appreciate the basilar artery and work in across the basilar artery to the other side to remove additional tumor in the capsule. Working between the fifth and seventh and eighth cranial nerve, we can expand our operative corridor. Obviously, the a seven and eighth cranial being untethered to allow working between the seventh and eighth cranial nerves. Seventh cranial nerve is often more forgiving. And so the fifth cranial nerve is more forgiving in terms of it mobilization. Brainstem auditory evoked potentials were used to monitor hearing in this case. You can see how expanding our operative corridor more medially along the midline would allow removal of the tumor, adjust in the clivus. Here's more tumor being mobilized away from the fifth cranial nerve. All the membranes are open to a low careful investigation and infiltration of tumor. You can appreciate the extent of the tumor that is going across the tentorium. And here is the transtentorial maneuver, where a piece of the tentorium is cut. Obviously, the edges are coagulated and the surgeon has to exercise caution to avoid cutting the fourth cranial nerve along the age of the tentorium. And you can see the suction is elevating the brain in the supratentorial space to protect the fourth cranial nerve, we'll continue cutting the tentorium more anteriorly. And then the tentorial cut is extended more medially and anteriorly until that piece of tentorium is completely disconnected. You can see that by cutting the edge of the tentorium. we now can mobilize the occipital lobe and work across into the supratentorial space in order to remove tumor above the trochlear nerve while protecting this cranial nerve. This piece of tuner is off and in accessible during the second stage of the operation through arteriolar approach. You can see the medial portion of the midbrain and the piece of the tumor that is delivered. Here is further inspecting the area, including visualization of third cranial nerve and mobilizing the tumor away from this nerve. You can see the ample amount of expansion of the working zone of the surgeon. And again, continuing to remove additional tumor and only needing a arteriolar approach for a second stage of the operation, rather than a more extensive skull base approach to remove residual supratentorial tumor. Here is reaching along the entry aspect of the midbrain to deliver additional tumor. And further inspection reveals, no obvious residual tumor in the posterior fossa. During stage two, we remove the medial temporal lobe portion of the tumor through a transsylvian approach. As you see in these post-operative images, we're able to achieve a gross-total resection of the mass and the patients spasm were relieved without any evidence of hearing loss, post-operatively. I would like to thank all of you guys for joining us and I hope you found this session helpful, thank you.
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