Originally published by Harvard Health.

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. Although myositis can cause muscle aches and muscle tenderness, weakness is usually the dominant symptom.

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 rare group of muscle diseases, the cause of the muscle inflammation is unknown (idiopathic). There are three major types: dermatomyositis, polymyositis and inclusion body myositis.

  In the United States, idiopathic inflammatory myopathies affect about 1 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 tissues. 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 inclusion body myositis, the muscle also contains tiny structures that resemble viral particles (called inclusion bodies), although no viral infection has been consistently identified in association with this disease.

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

  Myositis quite similar to polymyositis or dermatomyositis may accompany other autoimmune disease such as systemic lupus erythematosus (SLE) or progressive systemic sclerosis (also called scleroderma ).

• 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 in tropical climates, especially 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 usually 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 deep 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) and zidovudine (Retrovir), also called AZT, a drug used to treat HIV/AIDS.

Symptoms

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% of people with 1 of these conditions have 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 (called proximal muscles), including the muscles of the neck, shoulders and hips. Some patients also have difficulty 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.

  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 (called distal muscles), including muscles of the hand and wrist and lower legs. Muscle wasting (atrophy) is often prominent. Up to one-half of these patients have trouble swallowing.

• Infectious myositis. When infectious myositis is caused by the flu, symptoms include not only muscle aches and muscle weakness, but also 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.  These symptoms usually begin soon after a person starts taking a new drug or a combination of drugs.   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.

Diagnosis

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, auto-antibodies (antibodies directed against one’s own cells or organs) and antibodies to infectious agents
• An electromyogram, a test that measures the electrical activity of muscles
• Magnetic resonance imaging (MRI), a painless scan that can identify abnormal muscle and can be used to locate the best site for a biopsy to establish the diagnosis 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 computed tomography (CT) scan or MRI if pyomyositis is suspected
• A muscle biopsy. With this test, the doctor takes a sample of muscle tissue to be checked under the microscope. Muscle biopsy is the best way to establish the diagnosis of myositis; it may be diagnostic if it shows clear muscle inflammation. However, the results are not always abnormal even in people who have myositis.

Expected Duration

How long myositis lasts varies depending on the cause:

• Idiopathic inflammatory myopathies. Polymyositis and dermatomyositis are usually chronic (long-lasting) but typically improve after one to two months of treatment. Inclusion body myositis is also chronic. Since there is no reliably effective treatment for inclusion body myositis, symptoms usually 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, and 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.

Prevention

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 and drug-induced myositis are potentially preventable. 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, the injection site should be as clean as possible.
• Keep your skin clean.
• Take the lowest number and lowest doses of medications that are necessary. Routine blood tests to look for drug-induced muscle injury may be helpful as well.

Treatment

Treatment varies, depending on the type of myositis.

• Idiopathic inflammatory myopathies. For polymyositis and dermatomyositis, doctors usually begin treatment with a corticosteroid drug, such as prednisone (sold as a generic) or methylprednisolone (Solu-Medrol, others). 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. Other immunosuppressant medications may be recommended to treat dermatomyositis and polymyositis, including cyclosporine, rituximab, mycophenolate mofetil or cyclophosphamide. Unfortunately, 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 or prevent worsening weakness.

  When myositis accompanies another autoimmune disease (such as SLE), treatment of the underlying disease may be helpful; otherwise, treatment of the myositis is similar to that of polymyositis and dermatomyositis.

• 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  and take nonprescription drugs for pain. Your doctor may also prescribe prednisone to reduce inflammation associated with the infection. For pyomyositis, your doctor may recommend drainage of the abscess by making an incision or by inserting a needle.  In addition, he or she will prescribe antibiotics to fight the infection.
• Benign acute myositis. Your child’s doctor will prescribe medication for pain. No other treatment is necessary, because the illness usually gets much better within a few days.
• Myositis ossificans. Your doctor may wait to see whether the bony lump disappears on its own. If it doesn’t, he or she may recommend surgery to remove the lump.
• Drug-induced myositis. Your doctor will discontinue any medication thought to 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

Prognosis

The 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 or breathing 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. Medications taken to treat these muscle diseases may cause complications that affect prognosis.  For example, serious infections may complicate the use of immune suppressing medications.
• 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 recommend surgery to remove it.
• Drug-induced myositis. The prognosis is very good. In most cases, symptoms subside when the drug is stopped.

Additional Info

National Institute of Arthritis and Musculoskeletal and Skin Diseases
Information Clearinghouse
National Institutes of Health
1 AMS Circle
Bethesda, MD 20892-3675
Phone: 301-495-4484
Toll-Free: 1-877-226-4267
Fax: 301-718-6366
TTY: 301-565-2966
http://www.niams.nih.gov/

National Institute of Neurological Disorders and Stroke
P.O. Box 5801
Bethesda, MD 20824
Phone: 301-496-5751
Toll-Free: 1-800-352-9424
TTY: 301-468-5981
http://www.ninds.nih.gov/

American Autoimmune Related Diseases Association (AARDA)
22100 Gratiot Ave.
East Detroit, MI 48021
Phone: 586-776-3900
Toll-Free: 1-800-598-4668
http://www.aarda.org/

Muscular Dystrophy Association
3300 E. Sunrise Drive
Tucson, AZ 85718
Toll-Free: 1-800-572-1717
http://www.mdausa.org/

The Myositis Association
1233 20th St., NW
Suite 402
Washington, DC 20036
Phone: 202-887-0088
Toll-Free: 1-800-821-7356
Fax: 202-466-8940
http://www.myositis.org/

American College of Rheumatology
1800 Century Place
Suite 250
Atlanta, GA 30345-4300
Phone: 404-633-3777
http://www.rheumatology.org/index.asp