Myositis is a rare disease that involves inflammation of the muscles. This can present with a variety of symptoms such as skin involvement (i.e., rashes), muscle weakness, and other organ involvement. Systemic symptoms such as weight loss, fatigue, and low fever can also present. Causes Injury, medicines, infection, or an autoimmune disorder can lead to myositis. It can also be idiopathic. Injury - A mild form of myositis can occur with hard exercise. A more severe form of muscle injury, called rhabdomyolysis, is also associated with myositis. This is a condition where injury to your muscles causes them to quickly break down. Medicines - A variety of different medicines can cause myositis. One of the most common drug types that can cause myositis is statins. Statins are drugs that are used to help lower high cholesterol. One of the most common side effects of statin therapy is muscle pain. Rarely, statin therapy can lead to myositis. Infection - The most common infectious cause of myositis is viral infections, such as the common cold. Viruses, such as COVID-19, are also shown to be a rare cause of myositis. Benign acute childhood myositis has been described in children after prodromal viral infections with different viral agents. Autoimmune - Autoimmune disease is an abnormal immune response to a functioning body part, in this case the muscles. The three main types of autoimmune myositis are dermatomyositis, polymyositis, and inclusion body myositis. Other autoimmune diseases, such as systemic lupus erythematosus, can also cause myositis-like symptoms. Diagnosis There are various tools that can be used to help diagnose myositis. The most common ones are: physical exam, electromyography, magnetic resonance imaging, muscle biopsy, and blood tests. The first thing your doctor will likely do is perform a physical exam. They will look for various things such as muscle weakness and rashes. Another possible test is an electromyography (EMG). This is a test that inserts tiny needles into your muscles. This allows a physician to look at your muscles’ response to various electrical nerve signals and evaluate which muscles potentially have myositis. Magnetic resonance imaging (MRI) is also useful. This is a test that uses a big magnet to create images on a computer. This allows a physician to examine your muscles. It is painless. Muscle biopsies are the most reliable tests for diagnosing myositis. It will tell your doctor for certain if you have myositis or not.