Myositis, like many autoimmune diseases, is a complex illness that is often misunderstood. Each patient presents differently, making the diagnostic process that much harder. Learn more about the different types of myositis.
Myositis: The Basics
The primary focus of Myositis Support and Understanding is the heterogeneous group of rare autoimmune muscle diseases called the idiopathic inflammatory myopathies (IIM) or acquired myopathies, meaning muscle diseases which are typically not inherited and are caused by the immune system attacking our own muscles, connective tissue, blood vessels, lungs, skin, and/or other organs of the body.
Myopathy is a term used to describe muscle disorder. The inflammatory myopathies are a group of diseases that involve chronic muscle inflammation, accompanied by muscle weakness. Another word for chronic inflammation of muscle tissue is myositis.
Although we do provide information about other forms of myositis, our focus is polymyositis, dermatomyositis and a subtype called clinically amyopathic dermatomyositis, inclusion body myositis, immune-mediated necrotizing myopathy, antisynthetase syndrome, and juvenile myositis. All forms of idiopathic inflammatory myopathies are rare diseases.
Polymyositis (PM) presents with muscle weakness affecting the proximal muscles, those closest to the body’s core. These muscles can include the neck, hips, back, and shoulders. Muscle weakness may happen over days, weeks, or months. Myositis autoantibodies can aid in the proper diagnosis, and in some cases may aid in predicting disease course and potential treatment outcomes. Lung disease, and to a lesser extent, cancer, may accompany PM. It is currently thought PM is overdiagnosed and patients often fit into one of the other forms of myositis.
Sporadic Inclusion Body Myositis (sIBM)
Sporadic inclusion body myositis (sIBM) is a rare autoimmune and degenerative muscle-wasting disease and is the most common myopathy in those over age 50. It is characterized by slowly progressive, asymmetric, distal more than proximal muscle atrophy and weakness. Patients often experience falls and injuries, gradually lose the ability to walk, experience weakening of hand and wrist muscles, and patients may have swallowing difficulties (dysphagia), which causes a severe risk of choking and aspiration pneumonia.
Dermatomyositis (DM) has recently been found to be more prevalent in African-American women, although this is poorly understood. Like with PM, muscle weakness affects the proximal muscles but DM also presents with skin rashes and other skin-related markers that may make diagnosis easier. Myositis autoantibodies can aid in the diagnosis, and in some cases help predict disease course and potential treatment outcomes. Lung disease and/or cancer can accompany DM.
Clinically Amyopathic Dermatomyositis (CADM)
Amyopathic Dermatomyositis (ADM) and Hypomyopathic Dermatomyositis (HDM) together are referred to as Clinically Amyopathic Dermatomyositis (CADM), Like classic DM, CADM presents with similar skin manifestations and may be associated with lung disease or cancer, however, there is little-to-no evidence of muscle involvement on exam or testing. However, those without muscle weakness may develop muscle symptoms later in the disease course, while others do not.
Antisytnetase Syndrome (AS)
Antisynthetase syndrome is associated with a number of known autoantibodies called aminoacyl-tRNA synthase (ARS) autoantibodies with the most common being anti-Jo1. AS is a group of symptoms that include interstitial lung disease (ILD), polymyositis (PM) or dermatomyositis (DM), polyarthritis (inflammation of many joints), Raynaud’s Phenomenon, and Mechanic’s Hands. Not all of these symptoms have to be present in patients at the same time for a diagnosis. For some, the lungs may be the primary and only organ affected while other have lung disease and myopathy.
Immune-Mediated Necrotizing Myopathy (IMNM)
Immune-Mediated Necrotizing Myopathy (IMNM), also called necrotizing autoimmune myopathy (NAM), is the newest classification under the inflammatory myopathies and is a rare autoimmune disease with symptoms similar to polymyositis. There are two known myositis autoantibodies; anti-SRP and anti-HMCGR, along with the third category of patients who are autoantibody negative. Without positive autoantibodies, a muscle biopsy will help to make the diagnosis. CPK levels with IMNM are often extremely high and muscle involvement can be severe.
Juvenile Myositis (JM)
Juvenile Myositis (JM), including Juvenile Dermatomyositis (JDM) and Juvenile Polymyositis (JPM), are rare autoimmune conditions that cause inflammation. The inflammation for JPM primarily occurs in the muscles, while in JDM the inflammation is in the blood vessels that lie under the skin and in the muscle tissue. This inflammation causes weak muscles and – in the case of JDM – skin rashes.
Orbital/Ocular Myositis (OM)
Orbital or Ocular Myositis (OM) is an extremely rare autoimmune disorder that affects the extraocular muscles of the eye, which are muscles that control eye movement. Orbital Myositis frequently manifests with orbital pain and diplopia (double vision). Usually, OM impacts young to middle-aged adult women. Treatment is often successful but there is no cure and it can be difficult to find a doctor with knowledge of OM.