Diagnosis is impossible without seeing the patient. But I share your concerns about diagnoses other than polymyositis given the long history and the marginal response to high dose steroids. Other possibilities inlude muscular dystrophy as you mentioned and metabolic myopathies. Muscle biopsy with EM and special stains is very useful to make those diagnoses.
40 old man with weekness from 20 yr ago. his diagnosis is polymyositis and treatment not good. Now he have progimal and distal weekness.cpk=2450 ,LDH=900,.EMG faverable polymyositis .Biopsy have regernation$degernation of muscle fiber. gint cell no cellar infiltriation.but patient tretment with high dose corticosteriod,mtx.no provment in weekness but enzyme decrease tocpk=1450,LDH=600.PMH:negative what is your diagnosis? here is not possible for evalution muscle dystrophy.