An anatomical state where the cerebellumThis word means the “little brain” and refers to a large part of the brain that sits in lowermost part of the skull, at the back of the head, immediately above the top of the spine. This part of the skull is known as the posterior fossa. appears to have fallen (or is being pushed) downwards, into the volume created by a surgical decompression at the craniovertebral junctionThat part of the body where the base of the skull is joined to the top of the spine. Inside these bony enclosures, the craniovertebral junction is also where the brain stem continues downwards as the spinal cord..