Development of polymyositis after long-standing rheumatoid arthritis.

Rheumatoid arthritis (RA) and polymyositis (PM) are distinct clinical syndromes. The concurrent diagnoses of RA and PM in the same patient are rare. We describe a patient who developed outright PM after 16 years of well established RA, review the literature, and highlight the need to consider a broad base of differentials including PM in the diagnosis of muscle weakness in RA.