Sometimes with PD, the contracting & rigid muscles in the back conspire to compress the spine! Which in turn may squeeze and cause unwanted pinching upon the Sciatic nerves.
My experience : After a MRI scan to investigate and find compressed discs, it was recommended I should have surgery but I decided first to visit a Sports injury Physiotherapist and requested that he stretched my spine after a massage and to my relief the problem eased.
I then designed floor exercises based on the Physiotherapists advice to help stop a return of the problem…I now do the exercises on a regular basis and ensure I drink enough water to help the Discs rejuvenate from daily pressure etc.
That was at least three plus years ago and I have not looked back.
My exercises are described here [https://sites.google.com/site/beauxreflets/keep-fit]
I hope you can find relief through correct diagnosis and the help via a good Physiotherapist