Polymyositis (pol-e-my-o-SY-tis) is an uncommon inflammatory disease that causes muscle weakness affecting both sides of your body. Polymyositis can make it difficult to climb stairs, rise from a seated position, lift objects or reach overhead.
Polymyositis most commonly affects adults in their 30s, 40s or 50s. It's more common in blacks than in whites, and women are affected more often than men. Polymyositis signs and symptoms usually develop gradually, over weeks or months.
While there is no cure for polymyositis, treatment — ranging from medications to physical therapy — can improve your muscle strength and function.