Myositis: Causes, Symptoms, and Classification
If you or someone you know has been diagnosed with myositis, it actually may be one of five different subtypes of myositis that can affect skin, muscles, lungs, and joints. In the past, the only type of myositis that was clearly different was dermatomyositis, because people usually experience a skin rash, along with muscle problems.
The more recent discovery of specific antibodies helped specialists better understand the disease, and separate myositis into 5 types, which have different symptoms and may respond to differing treatments.
Myositis is an autoimmune disease
Myositis is known as inflammatory myopathies; however, people may confuse it with other diseases like arthritis, which has inflammation as a symptom. These rare conditions are caused by the immune system mistakenly attacking the body’s own cells, as it would attack a virus, with antibodies. This action triggers an internal inflammatory process that can destroy healthy, normal cells, and cause muscle weakness. As a result, you may have heard your doctor refer to your condition as autoimmune myositis.
Five types of Inflammatory Myopathies (Myositis)
The five main types of inflammatory myopathies that have specific antibodies to identify them are:
Dermatomyositis
Immune-mediated necrotizing myopathy (IMNM)
Sporadic inclusion-body myositis
Overlap myositis (including antisynthetase syndrome) - anti-ARS
Polymyositis
Symptoms vary for the different types of myositis
Dermatomyositis – the most common and distinct symptom is a skin rash or redness, which commonly appears around the eyes. Muscle weakness usually affects the muscles close to the trunk of the body –like hips and shoulders– instead of arms and legs. Both the skin rash and muscle weakness can develop slowly, over a number of weeks or months. Some people may not have any muscle weakness, just the rash. This type of myositis is diagnosed after testing positive for the dermatomyositis-specific antibody.
Immune-mediated necrotizing myopathy (IMNM) - symptoms of extreme muscle weakness and inability to walk can come on suddenly, especially with people who have the SRP antibody. For those who have the HMGCR antibody, the symptoms can be more gradual and not as severe, and may be triggered by long-term cholesterol-lowering medications, known as ‘statins’.
Sporadic Inclusion Body Myositis (sIBM) - the most common type of myositis in the elderly, mostly in men. Symptoms of muscle weakness come on gradually as muscles deteriorate, and, commonly, when weakness in the knees causes a fall. A muscle biopsy can reveal the extent of the muscle damage. This type of condition is often misdiagnosed as polymyositis.
Polymyositis - usually found more frequently in women and people over the age of 20. It affects many different muscles, but in particular shoulders, hips, and thighs. It is associated with the use of certain medications, like statins (a common cholesterol medication). It can also be present along with other autoimmune diseases, such as Sjögren's syndrome or celiac disease, often known as Overlap Syndromes.
Anti-synthetase Syndrome - just like necrotizing myopathies and inclusion body myositis, it used to be considered part of polymyositis. Specialists recognize anti-synthetase syndrome as a separate condition that causes muscle weakness, affects the skin and lungs, and presents anti-synthetase antibodies.
If you or a loved one are affected by myositis and would like to learn more about the latest research and resources in myositis, connect with one of our patient advocates here.
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Sources:
The Myositis Association. https://www.myositis.org/about-myositis/types-of-myositis
Sarwar A. et al. Polymyositis. Continuing Education Activity. Stat Pearls Publication 2022.
Christopher-Stine, L. et al. Overview of and the approach to idiopathic inflammatory myopathies. Uptodate, 2022. www.uptodate.com. Accessed 8/20/2022.
