I am sorry you have Interstitial Cystitis. Is that on top of Parkinson's?
I don't think the two things are connected, at least I hope not, as one of my daughters, aged 36, has had Interstitial Cystitis for 3 years now and I would not want her to develop Parkinson's as well. Also, I have not heard of any cases of Parkinson's patients with Interstitial Cystitis.
My daughter was offered a cystoscopy, but she refused when she heard that they would dilate her bladder and thought that would damage her bladder further. She controls her symptoms simply with a very strict diet. There is a diet sheet you can download from the Interstitial Cystitis Association, which is American.
In this country there is C.O.B., which is also for sufferers of bladder problems, including Interstitial Cystitis.
I do hope you find some respite from that very painful condition. All the very best