CP Angle Epidermoid: Dissecting Neurovascular Structures

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CP angle epidermoid cysts can be at times challenging since this tumor grows around the cranial nerves and can remain very adherent to important neurovascular structures. This is a 32 year old female who presented with gait and balance, and imaging demonstrated an epidermoid cyst compressing the lateral pontine region. You can see the diffusion sequences confirm the identity of this tumor. One of the challenges in the removal of this tumor is really its medial extension all the way to the level of the fourth ventricle. I approached this lesion through the retro master approach. And as you can see, quite a medial operative trajectory is necessary to remove this tumor without aggressive retraction of the lateral cerebellum. Also this tumor extents somewhat superiorly around the tentorium and in accessorius and various operative tricks are necessary to remove this tumor without placing the neurovascular structures at risk. The patient underwent the procedure through retromastoid craniotomy. A lumbar puncture was performed early on to achieve brain relaxation. Standard curvilinear retromastoid incision was used. Brainstem auditory-evoked responses were monitored. You can see the incision around the dural venous sinuses, the mastoid air cells, the tentorium, the petrous bone and their junction. The arachnoid bands were exposed, the meningeal branches to the dural were sacrificed. Here you can see the pearly tumor, very characteristic of epidermoid cyst. Some of the more superficial arterio vessels were mobilized. The superior petrosal sinus was sacrificed. The next step involves debulking of the tumor. Suction can be quite effective in removing and enucleating the inside part of the cyst. Here is the fourth cranial nerve that has been sharply dissected. The nerve is not directly manipulated. High magnification and sharp dissection are key principles in protecting neurovascular structures. Here's the anterolateral aspect of brain stem. Again the fourth cranial nerve, branches of the superior cerebellar artery. Angle dissectors are used to work around the corners of the tumor. The tumor flakes are delivered into the resection cavity. Some of the capsule can be quite fibrous. Here's that portion of the tumor that is extending toward the fourth ventricle. A very acute angle is necessary to be able to remove the tumor. A piece of gel foam was used to cover the seventh and eighth cranial nerves and protect them from the intense light of the microscope. Here you can see the tumor is adherents to the eighth cranial nerve. I gently mobilized the tumor while paying special attention to the amplitude of our brainstem auditory-evoked responses and their duration. It's important to minimize dissection directly over the nerve using the intense slide of the microscope to avoid heat injury and vasospasm to the small anterior branches on the walls of the nerves. You can see the round arachnoid knife used to dissect the tumor from the cranial nerves. Here is another portion of the tumor that is directed more medially. The lateral component of the tumor is being dissected while carefully watching for the fourth cranial nerve and minimizing any undue traction on the nerve. Here's the fifth cranial nerve and a portion of the tumor that is being mobilized away from this nerve. The fifth cranial nerve is obviously most forgiving. This is the portion of the tumor invading the Meckel's cave that is being delivered into our resection cavity. I'm satisfied with removal of the tumor around the fifth cranial nerve and it's distal cisternal section. Now is the challenging part of the operation where the medially pointing the tumor has to be delivered. Here's the portion of the nerve affecting the proximal parts of the cranial nerves. You can see debulking is an important principle so that the medial capsule of the tumor can be mobilized into the resection cavity. Embryologically these tumors are ectodermal elements related to the skin that are trapped near the neural structures. The initial attachment involves the area around the fifth cranial nerve and as you can see, just above the fifth cranial nerve this tumor is very adherent to the brainstem. This is the part of the cyst that invades the pia and the brain stem. Here's the portion of the tumor. However, we're still far away from that medial tumor capsule. I continue to enucleate tumor. There's still plenty of tumor left. Dynamic retraction of that cerebellum is specially important. Mobilization of the sigmoid sinus is also important to provide a lateral to medial trajectory to the medial part of the tumor. You can see that the capsule of the tumor is invading the pia of the brain stem and I use arachnoid knife in order to avoid any clear pia invasion. Small part of the capsule has to be left behind at the exact point of attachment of the tumor to the brain stem. I do tolerate some slight invasion of the pia along the lateral aspect of the pons at the expense of aggressive resection of this tumor that has a higher likelihood of recurrence. This is an excellent demonstration of mobilizing the brain stem away from the capsule of the tumor with only mild invasion of the pia. Here again is mobilization of the decompressed tumor into our resection cavity. The nodules of the tumor can be quite adherent and embedded within the parenchyma of the tentorial surface or P3 surface of the cerebellum. Still ample amount of tumor is left medially. You can see these embedded nodules of the tumor into the brain stem. Very characteristic of medial CP angle dermoid cysts as well as epidermoid cyst. I continue to remain very close to the surface of the tumor. Aggressive coagulation is avoided. Irrigation is the best method of clearing the field. The suction device is not used directly on the surface of the brain stem. Here's portion of the capsule very adherent to the brain stem that is being transected. Tumor is removed piecemeal. Here's additional piece of the capsule. Now I'm able to again mobilize more of the tumor using angled dissectors into the resection cavity. Endoscopy can be quite effective to provide visualization around the cerebellum. Here's the more inferior pole of the tumor. Again, very adherent to the fifth cranial nerve as expected in this medial epidermoid cysts. Here's the inferior surface of the trigeminal nerve. You can see the capsule, the tumor that adherent to the brain stem was left behind in this location. Additional pieces of the tumor embedded within the root entry zone of the nerve are mobilized. I continue to mobilize the medial pole of the tumor into our resection cavity. Sharp dissection is most ideal. The tumor is mobilized away from the neurovascular structures rather than vice versa. Counter traction is an important principle. Here you can see a very large portion of the tumor that could have been easily missed if one does not remain persistent in debulking the tumor. Now I'm able to follow the capsule of the tumor. This is really important so that I do not lose the capsule of the tumor, which is the most important guiding factor for me to assure gross total resection of the medial pole of the tumor. Again, some of the very adherent portion of the tumor have to be left behind to avoid neurological morbidity. This does not mean avoiding safe, aggressive removal of the tumor when possible. Here you can see the medial pole of the tumor facing the lateral pons. Piecemeal removal continues. The lateral to medial operative view specially important for mobilizing these large nodules of the tumor adhering to the middle cerebellar peduncle. The suction is on a very low amplitude. Essentially acting more as a dissector rather than a suction device. When the capsule is very adherent, it is transected. Here is that very adherent portion of the capsule to the lateral pons that is left intact. Additional pieces of tumor are delivered into our resection cavity. Here's the capsule that is quite adherent and is not further manipulated with. Transected at its root. Careful inspection reveals small pieces of the capsule that can be reduced. I do not see any additional tumor flakes hiding within the operative blind spots. You can see the operative view of the very medial part of the resection cavity. I gently peel off some of the capsule that can be potentially resected that contains flakes of tumor. It is a very fine line between being safely aggressive versus overly aggressive causing neurological morbidity. I continue to carefully and meticulously inspect the resection cavity. Here is the root entry zone of the fifth cranial nerve. Some of the small flakes of the tumor are potentially still resectable. The nerve remains intact at its entry zone. Here is the final view of the resection cavity. amplifier irrigation is used to displace some of the flakes that could be still hidden. Here is the magnified view of the operative cavity. Post operative MRI reveals good resection of the tumor without any complicating feature. This patient did have some double vision after her surgery that since has resolved. You can also see the extent of the resection more immediately that is pretty reasonable when the aggressive lateral to medial approach was used. Thank you.

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