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The ulcers vs. vestibular issues may or maynot be related to one another. If there is an underlying infectious agent...that would explain why it is getting worse with depo injections.
Has your vet suggested a neurologist? A CT scan of your cats head may be needed.
Cryptococcosis is a good rule out...and diagnostic rule out is with a spinal tap...which is the direction you should be headed into anyway. That is where the neurologist comes in, definately the next step. He/She will perform extensive neuro exam and pinpoint probable location...then perform the correct diagnostics. Spinal taps are part of "their" thing...they may take the spinal fluid and run a FIV/FIP test on it as well for that rare type that does not show up in circulating blood.
I wouldn't wait too long to seek out the neurologist...crypto is fungal so you need an antifungal.
Let me know how it goes!
Good luck, and please let me know how it goes, it is a very interesting presentation...and I hope that your cat responds quickly to treatment!!
Thank you for letting me know. I will keep you in my prayers during this difficult time.