Myositis is the general term used to describe inflammation of the muscles.
Dermatomyositis (DM), polymyositis (PM), inclusion-body myositis (IBM), and juvenile forms of myositis are all considered inflammatory myopathies.  Inflammatory myopathies are diseases where there is swelling and loss of muscle.  Inflammatory myopathies are thought to be autoimmune diseases, where the body’s immune system attacks normal, healthy tissue.
Inflammatory myopathies are considered rare and affect an estimated 50,000 people in the U.S.

There are four main types of Myositis:

Inclusion-body myositis

Inclusion-body myositis (IBM) is found in more men than women. Signs of IBM usually start after age 50. IBM progresses more slowly than the other types of myositis as weakness happens over months or years. Most muscles are affected, including neck, hip, thigh, back, shoulder, wrist and finger muscles. Many IBM patients notice shrinking, or atrophy, in the arms and thighs as the muscles become weaker. Dysphagia (weakened throat muscles causing difficulty with swallowing) is a common problem for IBM patients. Weakness of face muscles is sometimes seen.

Juvenile myositis

Juvenile forms of myositis (JM) are found in children younger than 18. Juvenile dermatomyositis (JDM) is the most common and affects 3,000 to 5,000 children in the United States. Polymyositis in children is very rare. Signs of JM may include the [DM] rash, falling, weak voice, or problems swallowing. About half of the children with JM have pain in their muscles. Some children may have calcinosis or contractures. Contractures happen when the muscle shortens and causes the joint to stay bent.

Dermatomyositis

Dermatomyositis (DM) affects people of any age or sex, but is found in more women than men. DM is the easiest type of myositis to diagnose because of its visible skin rash. The DM rash looks patchy, and reddish or purple. It is found on the eyelids, cheeks, nose, back, upper chest, elbows, knees and knuckles. Some people also have hardened bumps under the skin, called calcinosis. The rash is often seen before muscle weakness is felt.

Polymyositis

Polymyositis (PM) is found mostly in people over the age of 20. More women than men have PM. PM does not cause a rash. Muscle weakness usually happens over days, weeks or months. The weakness begins with muscles closest to and within the trunk of the body – neck, hip, thigh, back and shoulder muscles.   Some patients also have weakness in muscles farther from the trunk, like hands and fingers.  Many PM patients have trouble swallowing (dysphagia).  Some patients have trouble breathing and experience muscle pain.