Myositis (Inflammatory Muscle Disorders): Causes, Symptoms, Diagnosis, Treatment

The inflammatory muscle disorders are a group of rare diseases marked primarily by muscle inflammation and weakness. In general, muscle inflammation is referred to as “myositis,” regardless of the cause – however, myositis is frequently used as shorthand to refer to common causes of inflammatory muscle disorders or idiopathic inflammatory myopathies (IIM).

Types of myositis

Different forms of myositis include:

• Polymyositis
• Dermatomyositis
• Inclusion Body Myositis
• Autoimmune Necrotizing Myopathy

Causes of myositis

The white blood cells of the immune system generally protect us from things in the environment, like bacteria and viruses. In myositis, the immune system mistakenly injures muscle tissue, causing muscle weakness and other related conditions.

It is not clear why these conditions develop, or what sets it off for most people, though there are clues in some cases. While there is no cure for myositis, there are a variety of treatments that can help patients manage their symptoms successfully.

Symptoms of myositis

The most common symptom of myositis is muscle weakness. The weakness often develops slowly, and can be subtle at first. The symptoms of weakness depend on the group of muscles affected – most often, myositis affects the muscles closest to the trunk, at the shoulders and hips. The weakness in myositis is generally not associated with pain, but some muscle pain can be seen in about a third of patients. It can be hard to distinguish fatigue from weakness, and the two often go together.

People often notice difficulty with certain tasks. When the arms are affected, people can have trouble washing, brushing, or blow drying their hair, reaching things on high shelves, or hanging up an overcoat. When their legs are affected, they can have trouble getting up from a low chair, getting out of a car, or climbing steps. Patients can trip and be unable to catch themselves. Sometimes, in severe cases, the muscles of the throat or diaphragm can weaken, leading to trouble swallowing or shortness of breath.

Patients with inclusion body myositis have a slightly different pattern of weakness, and they may have more involvement of their hands – they may report dropping things, or having difficulty carrying heavy items like bags of groceries or pots and pans.

Beyond weakness, there are many things that can also occur in myositis patients, and may even be the main feature of disease for some.

These include:

• Inflammatory arthritis: Pain and swelling of joints, generally worse with inactivity and early in the morning, associated with prolonged stiffness. The arthritis can resemble or overlap with rheumatoid arthritis
• Interstitial lung disease: Inflammatory lung disease, leading to cough or shortness of breath
• Rash: There are several characteristic rashes in dermatomyositis, often with a dark red or purple hue. These commonly are itchy, and can affect the scalp, around the eyes, the chest, and the hands.
• Mechanic’s hands: This is a thickening of skin on the sides of the fingers, often associated with uncomfortable cracking/splitting of the skin
• Raynaud’s phenomenon: An abrupt, episodic decrease in blood flow to one, several or many fingers at a time, associated with a change in skin color to blue or white. It typically occurs after exposure to cold, and can be uncomfortable
• Nail bed/cuticle inflammation: There can be redness or pain along the nailbeds

The particular mix of symptoms can be very different from person to person, and can make the diagnosis hard to pin down initially.

Diagnosing myositis

There are a number of tests that can be done to look for evidence of myositis. The most common are blood tests, specifically, ones looking for evidence of muscle damage. Normally, the cells of the body turn over slowly as part of your body’s general maintenance program, which fixes damaged cells and grows new ones. When cells are damaged, they leak out some proteins, which can be found in the blood. Since this is always happening all over the body, there is a low level of these proteins in the bloodstream all the time. Different parts of the body have different proteins - when there is an infection or some other injury to one particular type of cell in the body, there can be a spike in the proteins from that particular cell in the blood.

The most common blood tests for muscle proteins are:

• Creatine kinase or creatine phosphokinase (CK or CPK)
• Aldolase
• Lactate dehydrogenase (LDH)
• Myoglobin

Anything that causes muscle injury, even vigorous exercise, can make the blood levels of these proteins go up. In most patients with myositis, the levels of these proteins go up when the disease is out of control, and improve with treatment. However, some patients with severe disease only have slight increases in muscle proteins, or even normal levels, so the tests must be interpreted by a doctor in light of the patient’s whole clinical presentation.

There are nonspecific indicators for inflammation, called the erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP). These studies are often elevated in myositis patients, but a significant number of patients never have any elevation in these markers, despite having active disease.

Blood markers

A very important advance for myositis has been the identification of blood markers, called antibodies, that appear only in myositis. There are many of these markers that have been identified, and the list continues to grow. Some markers have clear clinical associations, such as an increased risk of lung disease or arthritis. They can reveal subgroups of patients with similar symptoms, and potentially help doctors and researchers understand the differences between those patients better.

They can also be useful in patients whose symptoms are in the early stages, not neatly defined, or when the main initial symptom is something other than muscle weakness, like lung disease. When present, these tests can shine a light on the underlying condition, and lead to a faster diagnosis. While it is not fully clear if, or how, these can guide treatment at this point, there are some signs that they may be helpful soon.

Additional testing

Additional tests to confirm the diagnosis include:

• Electromyography and nerve conduction studies (EMG/NCS)
• Muscle biopsy
• Skin biopsy
• Magnetic resonance imaging (MRI) - a relatively newer technique

The EMG/NCS are specialized tests done by neurologists, examining the function of nerves, muscles, and the ability of nerves to communicate with muscles using small electrical signals. There is a pattern of changes that develop when patients have myositis, so seeing this pattern can help confirm the diagnosis in a patient who has weakness.

A muscle biopsy can show cellular and molecular evidence of muscle damage by the immune system, or rule out other causes of muscle injury, and is an important part of confirming the diagnosis of myositis. In patients with dermatomyositis, a skin biopsy can help as well, for similar reasons.

MRI can be helpful to look at muscle in a less invasive fashion. When muscle is injured, the muscle swells up slightly, and holds on to water- this pattern of muscle swelling can be seen with an MRI. When that pattern is evident, that is another sign that myositis may be present.

Therapy and treatment

Treatment of myositis should always include regular exercise and physical therapy. It is somewhat counterintuitive to recommend exercise to myositis patients, whose muscle is already injured by their immune system. However, recent research has shown that physical therapy can help maintain and improve muscle function in myositis patients, without significant risk of harm. Physical therapy is especially important in inclusion body myositis, since medicines used in other forms of myositis generally do not work well. Of course, every patient’s situation is unique, and their personal course of therapy should be discussed with a physician before starting.

There is no specific diet or food that is proven to help with myositis, but a healthy diet is important for general well-being, and in particular in patients with a chronic illness.

Myositis medications

The medicines used to treat myositis most often work by blocking the white blood cells of the immune system in some way, since these are causing the underlying damage to muscle tissue. Steroids, like prednisone, are the most common treatment. They work very quickly and can be very effective, but have a number of side effects that accumulate the longer patients are on treatment, especially with high doses. Since myositis treatment is mostly long-term, there is a need to additional medications for “steroid sparing,” to allow for less use of steroids, or to use in place of steroids.

While immunosuppressive medications like methotrexate or azathioprine are not approved by the FDA for use in myositis, they have been frequently incorporated into treatment of myositis for many years. If additional agents are required, mycophenolate mofetil, intravenous immune globulin, tacrolimus, or rituximab have all been used. Importantly, immunosuppressive therapy does not work well in inclusion body myositis, and so the risks of therapy do not exceed the benefits in most patients. Choosing the right therapy is a complex decision, and must be done by a patient in conjunction with their doctor after discussing the risks and benefits of each option.

There are a number of clinical trials for myositis patients that are evaluating different, promising medicines in these diseases. Hopefully, this will lead to more options for patients who continue to struggle with the conventional medicines currently available.

Conclusion

Myositis is a disease that can present in a variety of different forms, and can affect the whole body, not only the muscle. This can make it hard to make a diagnosis, but comprehensive assessment using all the tools available can make this possible, and hopefully will make it easier in the coming years. While there is no cure for myositis, most patients can manage their symptoms using existing therapies, and the pace of research in this area continues to accelerate, hopefully leading to more therapies and better outcomes for myositis patients in the near future.

Updated: 9/6/2019

Authors

David R. Fernandez, MD, PhD

Assistant Attending Physician, Hospital for Special Surgery

Stephen A. Paget, MD, FACP, FACR
Physician-in-Chief Emeritus, Hospital for Special Surgery
Stephen A. Paget Rheumatology Leadership Chair