Sounds like it could be any number of issues, it is also possible Eden could have more than 1 issue going on at the same time
Has the vet suggested seeing a neurologist or having an MRI?
Has hypothyroidism, pancreatitis and kidney failure been considered/ruled out?
What about a Homeopathic Vet consultation?
When looking for a homeopathic vet this has a list of homeopaths following world renowned Dr Pitcairns' teachings http://www.drpitcairn.com/referrals/anhc_referrals.html