Polymyositis/Dermatomyositis and other inflammatory myopathies.

Inflammatory muscle diseases are a group of disorders where there is an autoimmune process directed against the muscle cells. Polymyositis features inflammation of the muscle fibers, resulting muscle weakness. The cause of the disease is not known, though it begins when white blood cells, the immune cells of inflammation, spontaneously invade muscles.  It is rarely seen in persons under age 18; most cases are seen in adults between the ages of 31 and 60, and primarily women twice as more.  It primarily affects the skeletal muscles  and is a slow progressive weakness which leads to difficulties climbing stairs, rising from a sitting position, lifting objects, or reaching overhead.

Polymyositis can be associated with skin rash and is then referred to as "dermatomyositis."   Dermatomyositis is characterized by a rash that precedes or accompanies muscle weakness. The cause is unknown. The rash that accompanies the symptoms of muscle weakness looks like patchy, bluish-purple discolorations on the face with violet discoloration and swelling around the eyes, scaly reddish discoloration over the neck, shoulders, upper chest, elbows, knees, knuckles, or back. Some people, especially children and young adults may also develop calcium deposits, which appear as hard bumps under the skin.  However, the most common symptom of dermatomyositis is muscle weakness affecting the muscles that closest to the trunk or torso. In addition to the muscle weakness, patients can also feel fatigue, have weight loss and/or low-grade fever.

Inclusion body myositis is a type of inflammtory muscle disease that affects older adults. It has a very slow onset and rate of progression and distal muscles are more involved compared to muscles closer to the trunk.