Some people with myositis also have another autoimmune disease. This is often called "overlap syndrome." Some of the illnesses also found in people with myositis are rheumatoid arthritis, lupus, scleroderma and Sjogren's syndrome.
Rheumatoid arthritis affects about 1% of the population, mainly with inflammation of the joints; lupus is less common, afflicting 0.1% of the population with joint, kidney, blood, and skin problems. Even rarer is scleroderma (0.01%), a connective tissue disease characterized by thickening and hardening of fibrous tissue. Patients with Sjogren's syndrome (often undiagnosed because it's masked by medication symptoms) suffer from very dry mucus membranes.
When an overlapping disease is present, the other disease is often the main focus of treatment, with myositis symptoms often being less severe than in cases where it is the only disease.
Updated March 2012