How can one deal with polyneuropathy and piriformis syndrome?

Double Crush. Definitely treat the piriformis with pelvic re-balancing, stretches or myofascial release therapy, because if the piriformis is compressing the sciatic nerve that's already injured it could significantly compound the problem.
It is very hard. The symptoms get mix up but the piriformis syndrome is one side and polyneuropathy are general, the treatment of both are similar as far medications.