What is juvenile myositis (JM)? What causes it?

Your doctor may have told you that you have juvenile dermatomyositis (JDM) or juvenile polymyositis (JPM). So what does this really mean?

Every person who has JM feels a little different than others who have it. You may

• have trouble standing up after sitting;
• find it harder to go up stairs;
• have problems lifting your arms; or
• feel more tired than usual.

JM affects people 18 years old or younger and is somewhat different than the adult forms of myositis.

Here's how one girl remembers her doctor explaining JM to her:

"He explained that my immune system, which was supposed to keep me healthy by fighting germs, was working too hard. My immune system thought I was sick and tried to fight off an infection, but there was no infection for it to fight, so instead it attacked my muscles. That was why I felt so weak and stiff. The redness on my knees, knuckles, elbows and eyelids was caused by my swelled blood vessels."

When your immune system is working against your own healthy tissues, it's known as autoimmunity. Your immune system is misdirected, fighting your healthy muscles and blood vessels as if they were germs or other invaders that are not supposed to be in your body.

Doctors don't know what causes JM. Many think there is something in the environment around you that triggers it, like a virus, vaccine or even sunlight, but you also have to have a certain genetic make-up that makes it more likely you will develop JM in the first place. Even though you share many of the same genes as others in your family, JM is not thought to be hereditary, or passed on from one family member to another. JM is NOT contagious-you did not "catch" it from someone and you cannot pass it on to someone else.

What are some things I might notice with JM?

• Sore, tired muscles
• Skin rash, especially over the eyes or over knuckles and joints
• General feeling of tiredness

The muscles most often affected by JM are those closest to the middle of your body-your stomach, arm, back and thigh muscles, for example. Depending on which muscles are weak, you may have trouble reaching up, bending down or climbing.

You may or may not have a rash that goes along with your achy, tired muscles. This rash is usually reddish-purple and can itch or hurt. If you have JPM, you will not have the rash that comes with JDM. Your rash may be very light, or it can be scaly and dry. Sunlight can make your rash worse.

Some, but not all people, have other symptoms as well, including:

• Calcinosis, or hard and sometimes painful lumps or sheets of calcium found under your skin;
• Joint pain and/or tenderness;
• Stomach pain;
• Trouble swallowing;
• Hoarse-sounding or weakened voice.

Updated March 2012