May 09, 2016
Let's review another case of epidermoid cyst located within the CP angle. This is a 35-year-old female who presented with gait disturbance. MRI evaluation revealed a sizeable mass within the cerebellopontine angle indenting the brainstem. You can see the location of the fifth nerve on this axial T2 MRI. Diffusion-weighted images confirm the character of the pathology as an epidermoid cyst. The tumor extended all the way toward the lower cranial nerves and also extended superiorly toward the tentorial incisura. The most difficult part of this operation is removal of this very medially-located capsule of the tumor, significant part that is indenting the brainstem. As we come around the cerebellum, this is part of the tumor that is within the operative blind spot. Let's go ahead and review some of the tenants for maximizing tumor resection and going around the challenges in this operation. Here's a right sided retromastoid craniotomy, an extended one. You can see the bone over this sigmoid sinus is removed, and the sinus is slightly mobilized laterally. Retention sutures are placed along the transverse sinus as well so that the lateral to medial operative trajectory is maximized. Next, additional CSF is released and the superior petrosal sinus is sacrificed. The arachnoid bands over the tumor are heavily dissected so that the pearls of the tumor are readily evacuatable. I continue microdissection and remove the tumor using Pituitary Rongeurs. A suction device can be as effective at times. As a portion of the tumor is resected, the anatomy is more understandable. You can see the fifth cranial nerve that was evident there. You can see seventh and eighth cranial nerves and the lower cranial nerve apparent along the lower part of our operative space. It's important for all arachnoid bands to be widely dissected to minimize the amount of attraction on the cranial nerves. Brainstem auditory evoked responses are monitored during this procedure. Part of the tumor that is encased in the seventh and eighth cranial nerves is also being removed. If the capsule of the tumor is very adherent to the pia of the cranial nerves or brainstem, the capsule should be left behind to minimize the risk of operative neurological injury. Now we come to the challenging part of the operation. Here you can see the sigmoid sinus was unroofed so I can get a more lateral to medial trajectory to handle this piece of the tumor. And I gently continue to mobilize the tumor capsule without losing the sight of the capsule as much as possible. Obviously, there's plenty of tumor left, more medially. We're just at the beginning. There is additional tumor around the seven and eighth complex that are being evacuated. Microdissection continues to disconnect the tumor fragments, atraumatically from the cranial nerves, including the fifth cranial nerve. I look all the way around the interior brainstem and around the basilar artery to make sure the tumor is aggressively removed. Here's the root entry zone of the fifth cranial nerve. The capsule of the tumor somewhat adherent to the surrounding structures. Now I, again, focus my attention more medially, where I follow the contours of the tumor capsule. Here's the edge of the tentorium. I watch for the trochlear nerve and continue to dissect the tumor fragments from the surrounding cerebrovascular structures. The epidermoid cyst form as part of ectodermal elements that join the neural elements early during embryonic development, and I frequently find a significant point of attachment that invades the pia of the brainstem around the root entry zone of the fifth cranial nerve for these tumors. I suspect this is the area where during the embryonic development the skin elements adhered to the central nervous system and did not disconnect themselves. So, this is the center of where the tumor essentially originated from. I try to avoid any parenchymal injury, but I best remove most of the tumor as these tumors can readily recur, especially in a young patient. Here's primarily part of the middle cerebellar peduncle, lateral to the level of the root entry zone of the fifth cranial nerve. And some manipulation of the pia can be readily tolerated without any significant neurological sequelae. So I follow the contours of the capsule to be able to deliver the tumor more laterally, even though it's within my operative blind spot. Importantly, endoscope can be used to inspect the more medial aspects of the tumor to assure complete gross total removal of the tumor, again, in the operative blind spots. I inspect the operative cavity very carefully, assuring myself that as much of the tumor that can be safely removed is removed. Small piece of capsule was left over the seventh and eighth cranial nerve to minimize the risk of hearing loss. Post-operative MRI revealed nice resection of this mass without any complicating features. Thank you.
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