 Well, our next case is a 30-year-old female who's got left-sided weakness after a Kiari decompression. Let's have a look at some of the images. The Sagittal T1 series shows a pretty round appearance of the cerebellar tonsils. In no way are they compressed or peg-like. And it looks like one of your colleagues, Dr. Schupeck, went in here and took out the occipital bone and made plenty of room for the cerebellar tonsils. There is a little bit of cerebrospinal fluid in the cisterna magna. It's not compressed. There's no fluid outside of this area. When we look at the axial T1 weighted image, let's focus on the cranioservical junction. It looks fantastic. It looks like the surgeon really made a lot of room for the patient, the cord, and the cranioservical junction is completely unencumbered. And the ventricular size is totally normal. So I am a little baffled. I'm wondering maybe if her symptoms aren't coming from the cervical spine. And I might look at the entire cervical spine as a neuro-imager. Let me ask you the first question. And perhaps you can enlighten me as to what is going on here, because I don't have a clue. What's the diagnosis? A, inadequate decompression. B, hydrocephalus with slip ventricles. C, pseudo meningocel. D, cerebellar tosis. E, dandy walker continuum. OK. I am going to go with cerebellar tosis. But let me take some of these other possibilities that you mentioned. Inadequate decompression. I think you dealt with that. There is a suboccipital craniectomy that goes all the way up pretty much to the sinus, really beyond the equator of the cerebellum. So you can't say that's inadequate. So I think that one is out. Pseudo meningocel, generally this procedure, or at least in some manifestations of it, will include a dural graft that can leak. Pseudo meningocel is a very common cause of failure. But I don't really see one here. So you would expect the CSF to be out in here somewhere. Right. In the soft tissues, evidence of a CSF leak. So we don't really see a pseudo meningocel. OK, what about dandy walker continuum? Dandy walker continuum, because it's a continuum, can have a lot of manifestations. But one of the ones that's going to be true of all of them is that the inferior cerebellar vermis is going to be involved in some way. It looks pretty healthy right there. Sure. In fact, it looks kind of chubby. Right. And then you also lose the bottom of the fourth ventricle, or at least the fourth ventricle, is very wide and gaping and communicates with this area, which isn't present here. So let me move on to the next question. And I just want you to think about one thing. What the heck is cerebellar ptosis? But we'll get to that in a second. Because I think most imagers have no idea of that condition. I've never heard of it. Question two, causes of posterior fossa decompression failure in Kiari may include which of the following? A, insufficient posterior fossa decompression? B, overvigorous compression? C, unrecognized hydrocephalus? D, pseudomonin-gaseal formation? Or E, all of the above? I am going to go with all of the above, but I'm going to focus on the one that you mentioned, which is cerebellar ptosis. That would be great, because I never heard of that. Right. And what can occur is the idea, of course, is you want to decompress the tonsils to reestablish flow in the CSF spaces at the base of the skull, because that's part of the problem. On the other hand, you can be too successful. That is, if that suboccipital craniectomy is taken above the equator of the cerebellum, as it is in this case, cerebellum can drop down. And I think you can see that there's a little bit of an impression on the cerebellum by the residual occipital bone right there. And in a sense, can reestablish some of this obstruction. And that is a poorly understood cause and not that uncommon cause of failure. So the bone is pressing on the cerebellum, and the cerebellum is kind of sagging down a little more. It's sagging down and can reproduce some of the obstruction at a different place in the CSF pathway. So unlike eyelid ptosis, where your eyelid droops, now the cerebellum is drooping. Cerebellum is drooping. So cerebellar ptosis is the diagnosis. I learned something new today from my esteemed neurosurgical colleague. We'll move on to the next case.