What are Idiopathic Inflammatory Myopathies?

Simply Put

The primary focus of Myositis Support and Understanding is muscle diseases which are referred to as “Idiopathic Inflammatory Myopathies (IIM).” These are typically not inherited and are thought to be caused by the immune system attacking muscles, connective tissue, blood vessels, and/or the skin. Idiopathic means the cause is unknown. These are often referred to as simply Myositis.

The prefix ”Myo” means muscle. The suffix “itis” means inflammation. Therefore, myositis means inflammation of the muscle.

The term “Myopathy” indicates any disease, inflammation, or weakness of muscle, whereas “Myositis” is a specific group of muscle diseases, in our case, the inflammatory myopathies, which are most often both idiopathic (no known cause) and thought to be autoimmune in nature.

Idiopathic inflammatory myopathies (IIMs), except for sporadic inclusion body myositis (sIBM), present with subacute symmetrical weakness of the limb girdle muscles, an elevated serum creatine kinase activity, and inflammatory cells in the muscle biopsy (necrotizing autoimmune myopathy being an exception).

The subtypes of  IIM’s are:

  • Dermatomyositis (DM)
  • Polymyositis (PM)
  • Juvenile forms (JDM, and more rare, JPM)
  • Sporadic Inclusion Body myositis (sIBM)
  • Necrotizing Autoimmune Myopathy (NAM, newer subtype)

Some IIM’s are potentially treatable when properly diagnosed and with the initiation of therapy. Getting a proper diagnosis early on can be key in managing these diseases.

DM, PM, JDM, and NAM are usually responsive to immunotherapies (see Treating Myositis). sIBM is typically refractory to these therapies.

Inflammatory myopathies, sIBM excluded, are amenable to immunosuppressive and immunomodulation therapies. The prognosis of IIM is not well known since long-term outcome and prognostic factors vary widely. Disease-related mortality rates in PM and DM are at least 10%. In DM mortality is attributed to cancer and pulmonary complications. Juvenile dermatomyositis has a low mortality rate. Because chronic immunosuppressive therapy is associated with significant side-effects, and many patients remain (partially) refractory to treatment, novel therapeutic agents that are safe and effective are needed.

In DM mortality is usually attributed to cancer and pulmonary complications. Juvenile dermatomyositis has a low mortality rate. Because chronic immunosuppressive therapy is associated with significant side-effects, and many patients remain (partially) refractory to treatment, novel therapeutic agents that are safe and effective are needed.

Since some IIM’s are potentially treatable, proper diagnosis and early initiation of therapy are necessary. At this time there is no consensus among experts regarding therapy and management, meaning there is no “right” way to go about treating IIM’s. However, there are key therapies many experts do agree on. An example, as with many autoimmune diseases, starting the patient on corticosteroids, such as Prednisone or Medrol, may help to reduce inflammation quickly which may improve the patient’s symptoms. Usually, this is followed by or in combination with the addition of an immune modulating drug, of which there are several physicians may try.

As research is released, experts are learning more and more about ways to treat patients. One example is a blood test for Myositis-Specific and Myositis-Associated antibodies. A patient that tests positive for certain antibodies allows experts to customize treatments as each antibody indicates a potential complication during the course of the life-long illness.

Another example that research has shown is with necrotizing autoimmune myopathy (NAM). A few years ago, NAM was typically categorized with polymyositis. Now it is a subtype of IIM.

More about Myopathies

There are many different types of myopathies. Myopathies may be divided into two main categories: inherited and acquired. Lack of family history and patterns of weakness can help distinguish between the two.

Furthermore, an early age of onset with a relatively longer duration of disease suggests an inherited myopathy, and a sudden (subacute) presentation at a later age is more consistent with an acquired myopathy. Inherited myopathies can be further subclassified as muscular dystrophies, congenital myopathies, mitochondrial myopathies, and metabolic myopathies. Acquired myopathies can be subclassified as inflammatory myopathies, toxic myopathies, and myopathies associated with systemic conditions.

Read more about Myopathy

Simply Put

“Simply Put” is a service of Myositis Support and Understanding, to provide overviews of Myositis-related medical and scientific information in understandable language.

MSU volunteers, who have no medical background, read and analyze often-complicated medical information and present it in more simplified terms so that readers have a starting point for further investigation and consultation with healthcare providers. The information provided is not meant to be medical advice of any type.

The above information is provided as a basic overview of a complicated group of diseases. For more in-depth information, you are encouraged to explore the individual topics covered in the MSU website.

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