I have tried to keep myself fit by swimming 3 times a week, but it exhausts me now, and I notice that my muscles are beginning to bulk. I guess i should also tell you I am a carrier of Fragile X Syndrome. I would be alittle unusual for the MRI to be perfeetly normal with only cerebellar atrophy and have brisk reflexes, as we think of cerebellar disease to give decreased reflexes and the stretch reflexes are monitored by the ceebellum. There are some of the spinocerebellar ataxias that might fit your condition. There are some commerical testing available for most of these entities.
Mitochondrial disease can also mask itself to look like a spinocerebellar ataxia.
I think I might purse these lines. I am sorry that I am not much help. I do hope you find the etiology to your problem. This is a completely new concept to us and we are quite frightened about the prognosis. The Dr explained what has happened to her brain, but we still do not know what the long term prognosis is.