What Is It?

Myositis is the medical term for muscle inflammation. In myositis, inflammation damages the fibers of a muscle. This causes muscles to be weak by interfering with the ability of the muscles to contract. Myositis also can cause muscle aches and muscle tenderness.

In some cases, myositis is a short-term problem that goes away after a few days or weeks. In other cases, it is part of a chronic (long-term) condition. Chronic forms of myositis can lead to muscle atrophy (wasting and shrinking) and severe disability.

There are many different types of myositis, including:

  • Idiopathic inflammatory myopathies — In this group of muscle diseases, the cause of the muscle inflammation is unknown (idiopathic). There are three major types: dermatomyositis, polymyositis, and inclusion body myositis. All three illnesses cause muscle weakness, but the pattern of weakness varies.

    In the United States, idiopathic inflammatory myopathies affect about one out of every 100,000 people. Polymyositis and dermatomyositis are most common in women, whereas inclusion body myositis affects men more often. Symptoms can start at any age, but the average age of first symptoms is a bit younger in dermatomyositis and polymyositis (age 50) than in inclusion body myositis (age 60).

    So far, most of the evidence suggests that polymyositis and dermatomyositis are autoimmune disorders, illnesses in which the immune system mistakenly attacks the body’s own cells. The muscles of people with inclusion body myositis contain an abnormal protein called amyloid, but the reason it forms is unknown. This amyloid protein is similar to the protein deposited in the brains of people with Alzheimer’s disease, and some experts believe that the two diseases may develop the same way.

    In dermatomyositis, cancer is found in approximately 10 percent to 20 percent of cases. Sometimes, the muscle problem develops first, whereas in others, the cancer is detected before the myositis.

  • Infectious myositis — Myositis sometimes occurs as part of a systemic (whole body) infection, especially a viral infection. It is especially common in people who have the flu (influenza). Myositis also can be caused by trichinosis, an infection in which tiny parasites invade the muscles. People can develop this infection by eating meat that has not been cooked enough. One type of infectious myositis is called pyomyositis, a bacterial infection that causes one or more pockets of pus (abscesses) inside a muscle. It usually is caused by Staphylococcus (“staph”) bacteria. Pyomyositis is a relatively common infection in developing countries, where sanitation and health care are poor. However, it sometimes occurs in the United States, primarily in people who inject illegal drugs and in people infected with HIV.

  • Benign acute myositis — In benign acute myositis, a young child suddenly develops severe leg pain and cannot walk normally. These symptoms are dramatic and frightening, but they disappear within a few days. Benign acute myositis usually occurs in children who are recovering from the flu or some other respiratory infection caused by a virus. Doctors are not sure whether the child’s muscle symptoms are caused by the virus itself or by the body’s immune reaction to the virus.

  • Myositis ossificans — In myositis ossificans, a lump of bony material forms inside a muscle. This usually happens after a muscle injury, especially a bruise.

  • Drug-induced myositis — In drug-induced myositis, muscle inflammation occurs as a side effect of a medication or a combination of medicines. Although this is rare, the most common medications that can cause myositis are the cholesterol-lowering drugs called statins — including atorvastatin (Lipitor), lovastatin (Mevacor) and simvastatin (Zocor) — especially when combined with other similar drugs, and zidovudine (Retrovir), also called AZT, a drug used to treat HIV/AIDS.


Symptoms of myositis can include muscle weakness, muscle pain, and muscle tenderness. Other symptoms vary, depending on the specific cause of myositis:

  • Idiopathic inflammatory myopathies — These conditions typically cause painless muscle weakness that develops slowly over weeks, months or years. Although up to 40 percent of people with one of these conditions has muscle pain, the weakness is usually worse than the pain.

    In polymyositis and dermatomyositis, weakness usually affects the muscles near the center of the body, including the muscles of the neck, shoulders and hips. Some patients also have difficulty swallowing.

    In inclusion body myositis, weakness typically starts in the upper legs and later affects the upper arms and muscles farther away from the center of the body, including muscles of the hand and wrist and lower legs. Up to one-half of these patients has trouble swallowing.

    In dermatomyositis, skin symptoms accompany muscle problems. These can include purple discoloration of the eyelids, a red rash on the face and neck, or scaly patches, usually on the knuckles.

  • Infectious myositis — When infectious myositis is caused by the flu, symptoms include not only muscle aches and muscle weakness, but high fever, chills, sore throat, cough, fatigue and runny nose. When caused by trichinosis, symptoms in the early stages include diarrhea and vomiting. Later, as the parasites invade the muscles, symptoms can include fever, eye redness with swelling in the lids, and muscle pain. Patients with pyomyositis usually have a fever, and the abscessed muscle is painful, tender, and slightly swollen. Skin over the muscle may be red and hot.

  • Benign acute myositis — A child suddenly has trouble walking and complains of severe leg pain. This pain is often worst in the calf muscles. In most cases, the child also has a history of recent fever, runny nose, sore throat, and other upper respiratory symptoms.

  • Myositis ossificans — A lump appears in the affected muscle, and this lump may hurt when you press it. These symptoms usually begin several weeks after a muscle injury, especially a bruise.

  • Drug-induced myositis — Symptoms include muscle soreness, pain and weakness, beginning after a person starts taking a new drug or a combination of drugs.


Your doctor will ask you to describe your symptoms. It is especially important to describe the exact location of any weakness or muscle pain and how long you have had it. The doctor also will review your medical history and current medications.

Next, your doctor will examine you, paying special attention to your muscles and nerves. Depending on what your doctor finds, he or she may order diagnostic tests. Common tests used to diagnose muscle problems include:

  • Blood tests to measure levels of muscle enzymes or auto-antibodies (antibodies directed against one’s own cells or organs)

  • An electromyogram, a test that measures the electrical activity of muscles

  • A muscle biopsy — In this test, the doctor takes a sample of muscle tissue to be checked under the microscope. This test does not always confirm the diagnosis.

  • Magnetic resonance imaging (MRI) — This painless scan can identify abnormal muscle and can be used to locate the best site for a biopsy or to monitor the progress of a known type of myositis

  • A standard X-ray or a bone scan if your doctor suspects you have myositis ossificans

  • A bone biopsy, in which a sample of bone tissue is removed and examined in a laboratory

  • A computed tomography (CT) scan or MRI if pyomyositis is suspected

Expected Duration

How long myositis lasts varies depending on the cause:

  • Idiopathic inflammatory myopathies — Polymyositis and dermatomyositis usually improve after one to two months of treatment. There is no treatment for inclusion body myositis that is always effective. In most cases, symptoms worsen gradually over a period of years. If cancer is associated with the myositis, improvement may occur if the cancer is treated effectively.

  • Infectious myositis — Flu symptoms usually last three to seven days. In trichinosis, symptoms peak in about three weeks, then gradually subside. For pyomyositis, your doctor will drain the abscess and you will be given antibiotics. Once the infection is gone, healing can start. Recovery can take weeks or even months in people with chronic illnesses.

  • Benign acute myositis — Patients usually recover within three to seven days.

  • Myositis ossificans — In some cases, the bony lump disappears on its own. However, this can take a few months. If the lump does not go away, it may remain indefinitely or your doctor can remove it surgically.

  • Drug-induced myositis — Symptoms usually disappear once you stop taking the drug. However, this can take weeks or months.


Scientists do not know the cause of most forms of myositis. For this reason, there are no official guidelines for preventing myositis.

Only infectious myositis can be influenced by a person’s lifestyle. To help prevent these illnesses:

  • Get a flu shot each year.
  • Thoroughly cook pork and other meats.
  • Never inject illegal drugs under your skin or into your muscles. With prescription drugs that are injected, care should be taken to be sure the injection site is as clean as possible.
  • Keep your skin clean.
  • Take the lowest number and lowest doses of medications that are necessary. Routine blood tests may be helpful as well.

In addition, altering the medications you take may help prevent drug-induced myositis. For example, myositis is more common when a person is taking a combination of lipid-lowering medications, such as gemfibrozil (Lopid) and lovastatin (Mevacor), than when a single medication is used.


Treatment varies, depending on the type of myositis.

  • Idiopathic inflammatory myopathies — For polymyositis and dermatomyositis, doctors begin treatment with a corticosteroid drug, usually prednisone (sold as a generic). If this fails, methotrexate (Rheumatrex) or azathioprine (Imuran) may be added. Intravenous immunoglobulin (an injection of antibodies collected from blood donors) may be effective in cases that fail these other treatments, particularly in children in whom this treatment may be safer than methotrexate or azathioprine. Right now, there is no reliably effective treatment for inclusion body myositis, although corticosteroid treatment and immunosuppressive therapies (as with polymyositis and dermatomyositis) are often tried for at least several months; if treatment is effective, ongoing therapy may improve strength and/or prevent worsening weakness.

  • Infectious myositis — If you have the flu, you should rest in bed and drink plenty of fluids. You also can take nonprescription medicines for fever and muscle aches. If you have trichinosis, your doctor may treat you with mebendazole (Vermox) or albendazole (Albenza), antibiotic drugs that kill the trichinosis parasites. In addition, you should rest in bed and take nonprescription drugs for pain. If your symptoms do not improve, your doctor also may prescribe prednisone. For pyomyositis, your doctor will make an incision or use a needle to drain the muscle abscess and will prescribe antibiotics to stop the infection.

  • Benign acute myositis — Your child’s doctor will prescribe medication for pain. No other treatment is necessary, because the illness should go away in a few days.

  • Myositis ossificans — Your doctor may wait to see whether the bony lump disappears on its own. If it doesn’t, the doctor may recommend surgery to remove the lump.

  • Drug-induced myositis — Your doctor will discontinue any medication that may be causing myositis. Medications called corticosteroids may help you to recover faster.

When To Call A Professional

Make an appointment to see your doctor if you have:

  • Muscle weakness that doesn’t go away
  • A red or purple rash on your face that doesn’t go away or scaly patches on your knuckles
  • Muscle aches that don’t go away with rest and nonprescription pain medication
  • A lump in any muscle, especially if you also have a fever or other symptoms
  • Muscle pain and weakness that begin after you start taking a new medication

Call your doctor immediately if:

  • You have a fever together with muscle pain and weakness
  • You have a muscle that becomes hot, painful, swollen and stiff
  • Your child complains of severe leg pain and has trouble walking


The prognosis (outlook) varies, depending on the type of myositis.

  • Idiopathic inflammatory myopathies — With proper treatment, most people with polymyositis or dermatomyositis eventually regain at least some muscle strength. Often, muscle strength returns to normal. Typically, inclusion body myositis does not improve with treatment. However, the illness usually progresses slowly and does not affect a person’s lifespan unless complications lead to serious illness, such as pneumonia caused by swallowing problems. Eventually, some patients need a cane to help them walk. Others require a wheelchair. If a person with dermatomyositis develops cancer, the prognosis may be worse.

  • Infectious myositis — Once the flu passes, muscle symptoms improve. Treatment usually is effective, although recovery can be slow in people with trichinosis. For pyomyositis, the prognosis is good if the infection is treated promptly. If not, the infection can pass into the bloodstream and spread throughout the body.

  • Benign acute myositis — Children typically walk normally again within a few days.

  • Myositis ossificans — The prognosis is very good. If the bony lump does not disappear on its own, your doctor may offer to remove it surgically.

  • Drug-induced myositis — The prognosis is very good. In most cases, symptoms subside when the drug is stopped.

Johns Hopkins patient information

Last revised:

Diseases and Conditions Center

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