An MRI shot of my brain before my posterior fossa decompression (brain surgery) for Arnold Chiari Malformation. See the white blob towards the bottom right? That's not supposed to be there - it's my cerebellum sinking down into my spinal column. That's what Chiari does. The brain drops down & blocks the flow of CSF (spinal fluid.) Bad news & I was sick for almost three years before getting diagnosed in the fall of 2005. Posterior fossa decompression is where they remove part of the back of your skull & create a patch that holds your cerebellum up.