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Cerebellar Pilocytic Astrocytoma

December 17, 2015

Transcript

This video focuses on describing techniques for performance of a suboccipital craniotomy related to a midline pilocytic astrocytoma. This is a young patient who presented with obtundation and was diagnosed with hydrocephalus. On imaging, a midline cystic mass along the cerebellum was identified. The cystic mass contained a relatively heterogeneously enhancing nodule. The imaging characteristics are consistent with the pilocytic astrocytoma. Let's review the details of a midline suboccipital craniotomy. This lesion is quite at the level of midline therefore significant expansion of the craniotomy on both sides of the midline is non-necessary. I'll place the patient in the lateral position with a head slightly turned. The lateral position allows me to sit during the microsurgical part of the procedure and also the position of the head allows the gravity to clear the operative field from blood and other fluids. The shoulder is mobilized out of the working zone of the surgeon by gently taping it anteriorly and inferiorly. A midline incision is used. The transverse sinus is located somewhere here. The incision is extended slightly cranially to allow the exposure of the torcular and the transverse sinuses. The incision is completed at the level of the midline and subcutaneous dissection is maintained strictly also at the level the midline within the nuchal ligament and the avascular plane to avoid injury to the suboccipital muscles that may lead to postoperative pain. Here you can see that avascular plane, it's quite obvious. It's relatively easy to deviate from the midline and enter the muscles. Here you can see that on the muscles toward the left side, here's that avascular plane between the suboccipital muscles. Here's the area of the mid-inion. This avascular plane becomes more limited as the dissection is extended more deeply. Now, the suboccipital muscles are disconnected from the superior nuchal line and the inion. The suboccipital is exposed. Some of the intradiploic veins are exposed and controlled via a piece of bone wax. The soft tissues are the dissected away from these intradiploic veins effectively before the bone wax is applied, so that the backs can be inserted into their lumen without interference from the surrounding soft tissues. The exposure of the suboccipital is extended more laterally as much as possible. Based on the extent of the pathology at hand, the posterior rim of the foramen magnum is carefully avoided at this stage. Now that I'm getting close to the posterior rim of the foramen magnum, the C1 spinous process is palpated, and the dissection again continued just over the midline. Here's the spinous process of C1. There is an area of bone missing between the posterior cranial cervical junction, that one should not inadvertently enter using the Bovie electrocautery. Now that this C1 is exposed and the edge of the foramen magnum is identified, dissection is carefully conducted over the dura at the level of their posterior craniocervical junction. I avoid using the Bovie electrocautery very close to the dura, as you can see here. Again, I emphasize the importance of not using the sharp instruments, or Bovie electrocautery directly over the area that is missing bone between the edge of the foramen magnum and the lamina of C1. Sulcus arteriosus carries the vertebral artery laterally over the superior aspect of their C1. The edge of the foramen magnum was developed using an angled curette. Here's the area of the inion. A burr hole is placed just as lateral as possible. Here's the regional anatomy. Here's the inion. The craniocervical junction. Here is the midline. The retractors are repositioned for placement of the burr holes. Next I'm going to review the techniques for placement of a burr hole over the transverse sinuses. I avoid the perforator because the M3 bit, using the handheld drill, is more effective and provides more control in removing the bone in thin layers and protecting the sinus. The automatic stop mechanism of the perforator may not be reliably functional at all times based on the surface of the bone that can be uneven. So, the bone is removed from the outer cortex and the cancellous bone of the calvarium. Now the inner cortex of the skull is exposed. When I reach the inner table of the calvarium, I slow down and remove the bone in very thin layers while watching for any opening of the bone over the dura. The burr hole is generous, so I can visualize what I'm doing through the burr hole without blind drilling, and just using palpation to identify the soft areas where the bone has broken through. So here is a part of the dura exposed. This burr hole is slightly more inferior than desired. The transverse sinus is most likely superior to the burr hole, therefore, that bony exposure is extended superiorly. Neuro navigation may avoid misplacement of the burr hole slightly more inferiorly. An alternative option is to place tuber holes on both sides of the sinus, and then strip the sinus away. Some operators consider the maneuver of tuber holes more safe. I believe that the single burr hole over the sinus is quite effective and when performed under direct vision, the risk of injury to the sinus is quite limited. So here you can see the wall of the sinus is identified. Small bleeding from the wall of sinus is encountered, which is readily controlled using pieces of gelfoam soaked in thrombin. Now, I know where the transverse sinus is. The second burr hole can be placed more effectively just over this sinus. I'm going to review the technique for placement of this burr hole also. Again, you can see the diameter of the burr hole is quite generous, so I can work through the burr hole and see the layers of bone under direct vision without plunging into the dura inadvertently. The drill is mobilized circularly, so that the bone is removed evenly through the burr hole. Here you can see the transverse sinus, the dura right below it. And this is an ideal burr hole for performance of suboccipital craniotomy, where the supracerebellar quarter may be used, and therefore the transverse sinus should be exposed, so that they dura can be reflected over it, and the supracerebellar corridor is widened. And number three, Penfield dissector is used to dissect the dura away from the inner surface of the calvarium. Again, the posterior age of the foramen magnum is clearly identified and the bony cuts are completed between the burr holes and the edge of the foramen magnum. Again, the extended bony removal is customized and tailored based on the underlying pathology. Not every lesion requires opening of the foramen magnum. If the lesion is mostly vermin and more cranially located, a more limited suboccipital craniotomy without involvement of the foramen magnum is desired. Because the bone turns, sometimes drilling toward the foramen magnum and around the foramen magnum can be difficult. I don't drill directly over the torcular, I use a B1 without a foot plate to fin the bone over this very critical venous structure. And as the bone is very thinned out, the bone flap is gently fractured across this area. Now, the bone flap is elevated. Additional bone removal of the torcular is extended using and thinning the bone in layers. This very controlled bony removal is safe and prevents any injury to the torcular and the transverse sinuses. Here's the midline. Here's the dura over the tonsils and the upper cervical court. If necessary, additional bone over the foramen magnum can be removed. Again, you can see the lateral extend of bone removal is very limited for removal of this midline vermial lesion. The torcular and the transverse sinuses are exposed in a very limited fashion. But if a supracerebellar corridor is desired, the bone over the transverse sinuses are more thoroughly removed, so the sinuses can be normalized superiorly with retention sutures on the dura. I open the dura in a Y fashion. The midline cerebellar venous sinus can be a source of bleeding, and should be carefully controlled using or sutures. Here's the extent of opening for resection of this lesion. The latest steps of the operational relatively straightforward in terms of entering the cystic portion of the tumor, draining it, and then removing the nodule of the tumor using microsurgical techniques. This is the post-operative MRI demonstrating gross total resection of mass, small enhancement, known to be some scar tissue that has remained stable on subsequent MRI evaluations. Thank you.

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