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.
About Alan Matsumoto, M.D.
Part-time Faculty, Division of Rheumatology
Johns Hopkins University