I think you have recieved some bad information on this disorder.
Yes it is an inflamitory myopathy which causes chronic muscle inflamation.
It is not however a disorder which causes death within 5 years.
In approximately 15 % of cases there may be shortened life span, but those are the cases with other issues such as cancer, heart problems and other connective tissue disorders.
Most patients with polymyositis have great success with medications and go into remissions.
Some may only have a single flare up in thier lifetime, Others may have on again off again flare ups.
But the bottom line is it is treatable, though not cureable. The earier it is diagnosed the better as those diagnosed early respond more quikly to treatment.
I suggest you do a search on polymyositis and do some more research, there are many good sites.
My Best Wishes
My son has a cogenital myopathy, as yet undiagnosed.
Been through four years of testing with no answers. I have done alot of research!