World Myositis Day Date in the current year: September 21, 2025

Myositis is an umbrella term for a group of conditions involving inflammation of the skeletal muscles and potentially other organs, such as the skin, heart, and lungs. The most common symptoms are progressive muscle weakness, extreme fatigue, low-grade fever, and weight loss due to loss of muscle mass.

Myositis can have various causes and can be primary or secondary to another disease. One of the most common causes is an autoimmune reaction, in which the immune system attacks the skeletal muscles. This type of myositis is also known as idiopathic inflammatory myopathy, where “idiopathic” means the underlying cause is unknown. Its three main subtypes are dermatomyositis, polymyositis, and inclusion body myositis.

Many inherited myopathies (muscle diseases) are accompanied by myositis. These include calpainopathy, dystrophinopathy, facioscapulohumeral muscular dystrophy, and limb-girdle muscular dystrophy. Myositis can also develop as a result of bacterial, fungal, parasitic, and viral infections, including the common cold and SARS-CoV-2 (the virus that causes COVID-19).

Some medications can cause myositis as a side effect. This is often the case with statins, a class of medications prescribed to lower cholesterol levels. About 10-15% of people who take statins experience muscle pain, which can lead to myositis. Finally, a mild form of myositis can result from muscle injury. However, injury-related myositis should not be confused with delayed onset muscle soreness (DOMS), which is caused by strenuous exercise and resolves spontaneously within several days. Unlike myositis, DOMS is not considered a disease.

The treatment for myositis depends on the underlying cause. For instance, if myositis is caused by an infection, it is treated by addressing the infection. If myositis is caused by a medication, it is treated by stopping use of the medication. Treatment options for autoimmune myositis include glucocorticoids, immunosuppressants, biopharmaceuticals, and intravenous immunoglobulins.

Like many other rare diseases, myositis can be difficult to diagnose, often requiring extensive testing and a muscle biopsy to confirm the suspected diagnosis. Many people live with undiagnosed myositis, especially those who develop it later in life and dismiss the symptoms as signs of aging. This is why raising awareness of myositis among medical professionals and the general public is so important: people need to know about it so that they can be diagnosed and receive the necessary treatment.

September 21 was first observed as Myositis Awareness Day in Massachusetts in 2001. it was organized by Janice Goodell, a member of The Myositis Association. In 2023, representatives of various international myositis groups and organizations from several countries established World Myositis Day to raise awareness of this rare disease. They chose September 21 to coincide with the already established National Myositis Awareness Day in the US.