Hey there, Not sure if this helps you, but about a year ago, one of my kitties suddenly became weakened in the hind quarters and actually became parlaysed within about 3 days. I mean no ability to stand or sit on his own. My own vet who is very knowledgeable had no idea what the problem could be and sent him to be seen by a vet neuro. He was put through a full gamut of every test imaginable (most of which you already tried) and still the neuro had no idea what to diagnose him as having. He was put on a heavy dose of prednisone and was showing improvement very quickly - within about a week. He continued on the predisone for about 6 months, with weaning dosages and so far, after about an additional 6 months, thank goodness, has shown no relapse. I believe what they finally decided on was catch all "multiple neuropathy". Not very specific, but I guess when you have a bill as large as the specialist's was - including internal med, cardiology and neurology, they thought they should have some diagnosis.
I have to admit, I have no idea about the eating litter problem though. I have never heard of it before, but as a side note, just before this happened, I had changed to clumping kitty litter about 6 months prior.
I hope you come to some resolution. I know how hard this can be for you and your kitty. Good luck.
No Heaven can be Heaven if my cats are not there to welcome me
Yes, I believe in the Rainbow Bridge