I have cerebellar degeneration of unknown cause that is most evident on a PET scan. This was not demonstrated by a brain MRI scan. If the MRI doesn't answer the question, additional tests would be a PET scan and possibly a spinal tap to analyze the cerebrospinal fluid. That is helpful for identifying immune system problems affecting the brain. I have found the National Ataxia Foundation helpful in learning more about disorders affecting the cerebellum. Many of them have a genetic cause. The website is: https://www.ataxia.org.
Also, to Helen, there is a specific cognitive syndrome related to the cerebellum: Schmahmann's syndrome.