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Opening the Arachnoid Membranes during Retromastoid Craniotomy

January 16, 2015

Transcript

Going around the cerebellum during a retromastoid craniotomy, to reach CP angle cisterns can be quite tricky. And if not performed carefully can be one of the most dangerous parts of doing a posterior fossa operation, including a, a microvascular decompression surgery. Let's go ahead and review. Take a look on your analysis for going around the cerebellum. This is a left sided, retromastoid, craniotomy. This is the transfer sinus, transfer sigmoid junction. And this is the sigmoid dural sinus. As you can see, the dural has been opened along the dural sinuses, three tack of stitches, one at the sigmoid, one at the junction, and one at the transfer sinus level have been inserted. And these tack-up sutures are mobilizing the sinus. And more specifically, the sigmoid sinus laterally and outside of the working zone of the surgeon. This expands the operative corridor toward the CP angle. For this retromastoid craniotomy, as you can see, the landmark is the tentorium, the tentorial petrous junction, and the petrous bone. If there is any confusion of which element the surgeon is looking at, it is obvious that the tentorium is soft and the petrous bone is hard. And by just slight palpation with a tip of bipolar electrocautery, you will be able to identify the difference between the two. The most important landmark is the petrous tentorial junction right here. For lesions that are located at or superior to level of the trigeminal nerve entry zone, this is the best landmark to go around the cerebellum. Since the anatomy of the posterior fossa can be quite variable, the surgeon may be disoriented if an adequate landmark or a constant landmark is not used. If you're too superiorly, you may go over the cerebellum and tear some of the bridging veins and obviously cause complicating features and add complicating factors to your operation. Or if you're too inferior, you may end up being parallel to the cranial nerve seven and eight and retraction vectors parallel to such cranial nerves can lead to hearing loss. Now that we have identified the petrous tentorial junction, we'll go ahead and go around the cerebellum. Certain principles are very important. Number one is the rubber dam or a piece of glove inserted underneath the cottonoid is crucial for allowing the cottonoid to slide over the cerebellum, and the cerebellum this way will be protected from the rough or the coarse surface of the cottonoid. This maneuver also is very efficient as you do not have to take out your cottonoid every time to advance it. The cottonoid just very nicely slides over the rubber dam, as you will see momentarily. Let's go ahead and focus to the, on the next step now. So you can see the cottonoid is going to be sliding. The direction of view is now moved more along the lateral aspect of the hemisphere, to where the cerebellopontine angle cisterns. You can see how the cottonoid was sliding smoothly. The arachnoid membranes are exposed. You can see the cottonoid is more toward the petrous bone the tentorium. First the tip of the micro scissors carefully inspect the arachnoid membranes before any cutting has been done in order to avoid injuring any vessels that could be entangled within the arachnoid membranes. After the arachnoid memories are open, you can see, we stay very superficial. We only cut where we can see in order to avoid cutting the veins or arteries that are crucial in this region. As you can see, the superior petrosal vein is evident. There are two branches for the superior petrosal vein. The fifth cranial nerve is also demonstrated at the depth of our dissection and the seven and eight cranial nerve would be more superiorly at the tip of the arrow. A gentle manipulation of the cerebellum without the use of fixed retractors, as you can see, allows gentle amount of space to conduct micro surgery under direct vision without necessarily cutting blindly and injuring any of their vital cerebrovascular structures in the area. Also performance of the lumbar puncture at the beginning of the procedure tremendously relaxes the posterior fossa and significantly facilitates my movement going around the cerebellum without necessarily working against a very tense cerebellar hemisphere, requiring significant amount of retraction, potentially placing the veins at risk of traction injury and unfortunate trend short bleeding within the angle without adequate visualization for control of the bleeding. These are the principles that I think are tremendously important for going around the cerebellum. As again, doing this part of the operation could be potentially the most difficult part, and has to be done most safely and efficiently. Thank you.

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