Prednisone 1mg/kg/day for 4-8 weeks until CK returns to reference range
Followed by prednisone taper
Other treatments that rheum may prescribe:
Methotrexate as second line for poor response to corticosteroids
Other agents with less evidence: IVIG, TNF Inhibitors
Followed by prednisone taper
Methotrexate as second line for poor response to corticosteroids
Other agents with less evidence: IVIG, TNF Inhibitors
Polymyositis
Infection:
Dermatomyositis
Viral infection 
Lambert-Eaton myasthenic syndrome
Bacterial infection
Amyotrophic lateral sclerosis
Spirochete infection 