Polymyositis

Polymyositis is a generalized systemic chronic inflammation of the muscles that causes muscle pain and wasting of the affected muscles. As a result, the patient loses strength in the muscles, particularly those in the pelvis.
The cause of polymyositis is unknown. It's currently believed to be the result of genetics, an autoimmune response and a viral infection. It's also associated with Lyme disease and toxoplasmosis.
In order to establish a definitive diagnosis, a patient must meet four criteria:
*Elevated creatine kinase levels
*A complete medical history and physical examination to rule out other causes
*An EMG
*A muscle biopsy
Once confirmed, steroids are indicated. Patients are generally started on prednisone and are moved to more powerful immunosuppressants if they do not improve. Immunoglobulin and physiotherapy can also be helpful.
Polymyositis at Wikipedia
Polymyositis at Mayo Clinic