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Myositis refers to any condition causing inflammation in muscles. Weakness, swelling, and pain are the most common myositis symptoms. Myositis causes include infection, injury, autoimmune conditions, and drug side effects. Treatment of myositis varies according to the cause.

Causes of Myositis

Myositis is caused by any condition that leads to inflammation in the muscles. Myositis causes can be divided into several categories:

Inflammatory conditions. Conditions causing inflammation throughout the body may affect the muscles, causing myositis. Many of these causes are autoimmune conditions, in which the body attacks its own tissues. Inflammatory conditions causing potentially severe myositis include:

Other inflammatory conditions tend to cause milder forms of myositis, including:

Inflammatory conditions are often the most serious myositis causes, requiring long-term treatment.

Infection. Viral infections are the most common infections causing myositis. Rarely, bacteria, fungi, or other organisms can cause myositis as well. Viruses or bacteria may invade muscle tissue directly, or release substances that damage muscle fibers. Common cold and flu viruses, as well as HIV, are just a few of the viruses that can cause myositis.

Drugs. Many different medications and drugs can cause temporary muscle damage. Because inflammation in the muscles is often not identified, the muscle problem may be called myopathy rather than myositis. Drugs causing myositis or myopathy include:

Myopathy may occur right after starting a medication, or may occur after taking a drug for months or years. Sometimes it is caused by an interaction between two different medications. Severe myositis caused by medications is rare.

Injury. Vigorous exercise can lead to muscle pain, swelling, and weakness for hours or days after a workout. Inflammation contributes to these symptoms, technically making this a form of myositis. Myositis symptoms after exercise or injury nearly always resolve completely with rest and recovery.

Rhabdomyolysis. Rhabdomyolysis occurs when muscles break down quickly. Muscle pain, weakness, and swelling are symptoms of rhabdomyolysis. Urine may also turn a dark brown or red color.

Symptoms of Myositis

The main symptom of myositis is muscle weakness. The weakness may be noticeable or may only be found with testing. Muscle pain (myalgias) may or may not be present.

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