Immune-Mediated Necrotizing Myopathy (IMNM)

Simply Put

Immune-Mediated Necrotizing Myopathy (IMNM), also referred to as Necrotizing Autoimmune Myopathy (NAM), one of the Inflammatory Myopathies, is a very rare, autoimmune muscle disease that can present much like polymyositis and causes muscle inflammation and weakness in the skeletal muscles on both sides of the body (symmetrical), commonly those closest to the body’s core (proximal), such as the hips, thighs, upper arms, shoulders, and neck.

Muscle weakness, which is often severe, can develop over a period of days, weeks, or months. This weakness often leads to difficulty climbing stairs, rising from a chair or from the floor, turning in bed, lifting, combing hair, brushing teeth, or reaching over the head for something like an item on a shelf. There is no cure for necrotizing autoimmune myopathy.

Causes of IMNM may be associated with certain medications, especially statins used to control cholesterol, certain autoantibodies, cancer, viral infections, or other connective tissue diseases.

Clarifying terms

There is a difference between Necrotizing Myopathy and Immune-Mediated Necrotizing Myopathy (IMNM).

Necrotizing myopathy refers to a muscle disorder that involves necrosis or muscle cell death as seen on the muscle biopsy. The necrosis can be caused by a variety of reasons and may include toxic myopathies, drug-induced myopathies, muscular dystrophies, thyroid disease, etc.

Autoimmune Necrotizing Myopathy (NAM) / Immune-Mediated Necrotizing Myopathy (IMNM) – These terms may be used interchangeably. IMNM may look the same as necrotizing myopathies, but they have other features that prove they are autoimmune. Also, IMNM often responds to steroid and immune suppression therapy.

Who is affected by IMNM?

Immune-Mediated Necrotizing Myopathy is a rare disease. A rare disease in the U.S. is defined as a condition affecting fewer than 200,000 people.

Here is some additional information:

  • IMNM has been recently reclassified (ENMC) according to myositis-specific autoantibodies including anti-HMCGR, anti-SRP, and antibody negative. Some prior classicifications, and some still used today, would consider this polymyositis.
  • Age of onset ranges from 30 to 70 years of age in the reported cases.

Other organ involvement and IMNM

IMNM may affect other organ systems of the body, referred to as a systemic disease.

Extramuscular organ involvement

  • Respiratory muscles may be involved.
  • Interstitial lung disease (ILD) has been reported.
  • Cardiac involvement has been reported.

Potential associations with IMNM

IMNM may be associated with:

  • Antibodies such as anti-SRP, anti-HMGCR, anti-mitochondrial antibodies, and on more rare occasions, anti-Jo-1
  • With cancer
  • With viral infections, or
  • With other connective tissue diseases like scleroderma

Autoantibody Associations

Immune-mediated necrotizing myopathy, according to recent findings, may not be one disease but rather can be divided on the basis of the presence of distinct autoantibodies; anti-signal recognition particle (anti-SRP) or anti-hydroxy-3-methylglutaryl-CoA reductase (anti-HMGCR) antibodies. These two antibodies are now considered diagnostic criteria for necrotizing autoimmune myopathy/immune-mediated necrotizing myopathy. In the absence of myositis-specific antibodies, the muscle biopsy remains necessary for a NAM/IMNM diagnosis.

Anti-HMGCR-positive patients can be further subdivided into those with and without statin exposure, the latter of which may be particularly refractory to immunosuppressive therapy. Anti-HMGCR is the enzyme targeted by statins, and it is suggested that statins may trigger IMNM, however, some patients with IMNM have never taken statin medications.

Anti-SRP autoantibodies were the first myositis autoantibodies identified as specifically associated with IMNM.

Pain and IMNM

Myalgia (muscle pain) is a reality for many patients with immune-mediated necrotizing myopathy.

Looking to order the autoantibody labs?

Physicians looking to order labs for the anti-HMGCR and anti-SRP antibodies, we have included links below.
(MSU does not endorse any particular lab or facility. This information is posted for educational convenience.)



Polymyositis: The evolution of this diagnosis in light of recent research
by Dr. Lisa Christopher-Stine

While this session is about polymyositis, Dr. Lisa Christopher-Stine goes into a lot of detail about IMNM as well. This is a great resource to understand IMNM.

Signs & Symptoms of IMNM

  • Symmetric, proximal muscle weakness, like found with polymyositis (weakness is often severe). With IMNM, the CK/CPK levels (muscle enzymes) can be highly elevated, some more than 10 times the normal limit.
  • Weakness in other muscles, including breathing and swallowing muscles, and distal muscles (furthest from the body’s core)
  • Muscle pain (myalgia)
  • Inability or difficulty climbing stairs, getting into a car and rising from a seated position
  • Difficulty washing, combing or brushing one’s hair, or raising arms to lift an item such as putting a plate in a cabinet
  • Dysphagia (trouble swallowing)
  • Fatigue, which can be debilitating
  • Weight Loss

Diagnosing Immune-Mediated Necrotizing Myopathy (IMNM)

Which doctors do I see?

Due to the rarity of Immune-Mediated Necrotizing Myopathy (IMNM), diagnosis and treatment are usually managed by a specialty center with experience in managing inflammatory myopathies and may include rheumatologists, neurologists, and neuromuscular specialists. This may be in coordination with a primary care physician (PCP). Consultation with other specialists may be required depending on your symptoms, other coexisting illnesses, and other organ involvement and may include: cardiologist, pulmonologist, oncologist, endocrinologist, and pain management.

Exercise is important for myositis patients. Therefore, you may also see a physical therapist, occupational therapist, and, if you have trouble swallowing (dysphagia), you may also see a speech-language pathologist.

Clinical and Physical Exam

A complete medical and family history is important in helping to diagnosis a rare and complex disease such as IMNM. We suggest writing a detailed medical and family history that you can share with your medical team and that can be used for all future appointments.

The physical exam and a complete drug history are also very important.

Blood Testing

Your doctor will likely order various blood tests when suspecting IMNM in order to measure autoimmune and inflammatory markers and to check for myositis-specific and myositis-associated antibodies, cancer markers, and other antibodies. Some of these tests may include:

  • Creatine Kinase (CK or CPK) (muscle enzymes)
  • Aldolase
  • ALT and AST
  • LD (Lactate dehydrogenase)
  • Sed Rate (also called ESR or Erythrocyte sedimentation rate)
  • ANA (Antinuclear Antibodies panel)
  • HMGCR and SRP antibody testing (found in some, but not all).
EMG and Nerve Conduction Study (NCS)

An EMG (Electromyography) is a test that checks the health of the muscles and the nerves that control the muscles. With IMNM, the EMG may show an irritable myopathy pattern as with other inflammatory myopathies. EMG testing can help distinguish between weakness due to muscle disease from weakness due to nerve problems. Nerve conduction study is usually done during the same visit and is a test to see how fast electrical signals move through a nerve.

MRI (Magnetic Resonance Imaging)

MRI is being used more frequently and can show changes suggesting muscle inflammation. Physicians may also use MRI to choose the best muscle for biopsy.

Muscle Biopsy

The muscle biopsy is important for a diagnosis of IMNM.  A doctor will remove a small piece of muscle tissue, either via a needle biopsy or an open surgical biopsy, and send it to a lab for testing. Typically, the muscle biopsy will show necrosis with little-to-no inflammation.

Your doctor will choose the weakest muscle, careful to avoid the site of any recent EMG, for biopsy. An MRI can be helpful in locating the best muscle to biopsy.

Other Testing

The above tests are not an exhaustive list your physician may use. Other testing will depend upon your symptoms and coexisting illnesses. Some examples are below.

Cancer Screening

Since cancer is a potentially associated cause of IMNM, your doctor may perform several cancer screenings including those that are age and gender appropriate.

If you are experiencing difficulty swallowing (dysphagia)

Barium Swallow Test

A barium swallow is a test that may be used to determine the cause of painful swallowing, difficulty with swallowing, abdominal pain, bloodstained vomit, or unexplained weight loss. Barium sulfate is a metallic compound that shows up on X-rays and is used to help see abnormalities in the esophagus and stomach.

If there is lung involvement

CT Scan (computed tomography) and Chest X-ray

A chest X-ray and CT scan of the lungs can help identify lung disease associated with myositis such as Interstitial Lung Disease (ILD).

Pulmonary Function Tests (PFT’s)

When the lungs are involved, your physician may order PFTs, a group of tests that measure how well your lungs work. This includes how well you’re able to breathe and how effective your lungs are able to bring oxygen to the rest of your body.

Challenges of diagnosing IMNM

Differentiating IMNM from other muscular dystrophies and toxic and other necrotizing myopathies may be difficult, in part due to lack of, or very minimal inflammation found on muscle biopsy that is a hallmark of the inflammatory myopathies.

It is important to rule out other more common causes of symptoms and other forms of necrotizing myopathies.

Diagnosing Myositis Myositis Antibodies

Treating Immune-Mediated Necrotizing Myopathy (IMNM)

In the case of drug-mediated IMNM, such as with statins, it was once thought that discontinuing the drug would eliminate symptoms of IMNM within weeks or months, however, in recent years, it has become recognized that discontinuing the drug does not always eliminate symptoms, especially when it involves an autoimmune process. A standard course of treatment, such as those used for other inflammatory myopathies and other autoimmune diseases are often used for IMNM.

You may start to see improvement after several weeks of high-dose corticosteroids. Your physician will likely conduct a follow up with both a physical exam to determine if muscle strength has improved as well as another CK/CPK blood test to check to see if the levels have decreased.

The goals of treatment for IMNM are to eliminate or reduce inflammation, restore muscle performance and strength, reduce morbidity, and improve a patient’s quality of life.

There are no approved therapies for IMNM, aside from steroids, which is a standard treatment for inflammatory myopathies, which are not recommended for long-term use.

Finding a combination of medications is often needed for successful treatment.

Discontinue statin medication

If you are taking a statin medication your doctor will likely discontinue it.


The initial treatment is typically corticosteroids such as prednisone or prednisolone and may be dosed by the patient’s weight at 1 mg per kg. Corticosteroids address inflammation and simultaneously help suppress the overactive immune system.

Intravenous (IV) steroids, such as methylprednisolone, may also be used.

Steroids have many side effects including weight gain and redistribution of body fat to the face, neck, and abdomen; elevated blood sugars, mood swings, thinning of the skin, cataracts, and osteoporosis.

Additional drugs to suppress the immune system may be added to the steroids, sometimes at the beginning of treatment and other times once the determination has been made that steroids have shown to be effective.

Since steroids, especially in high doses and over prolonged periods, are known to have many adverse side effects and may cause many other health issues, the introduction of steroid-sparing medications should be considered when appropriate.

Note: Corticosteroids should NOT be confused with anabolic steroids, used by some weightlifters and athletes, to enhance performance.

Immunosuppressive Drugs

Steroid-sparing agents, such as immunosuppressive drugs, have an added benefit as they often reduce or eliminate the need for steroids while also suppressing the overactive immune system. However, these medications are not without their own risks.

Some immunosuppressive drugs used with NAM include mycophenolate mofetil (Cellcept), azathioprine (Imuran), and methotrexate.

Other Treatments

Immunoglobulin Therapy (IVIg, SubQ Ig)

Immunoglobulin comes from antibodies extracted from the plasma from thousands of blood donors. IVIg is the intravenous (IV) method. SubQ Ig is also available, and can be approved for some, and is infused quicker under the skin and eliminates the need for continuous home nursing.

Observational studies show that statin-exposed anti-HMGCR-positive patients often improve dramatically with intravenous immunoglobulin, whereas statin-naive anti-HMGCR patients may be refractory to any immunosuppressive therapy.

Rituximab (Rituxan)

Rituximab belongs to a class of drugs called monoclonal antibodies. It is given intravenously (IV).

Observational studies show that patients anti-SRP patients usually respond well to treatment with rituximab.

Managing Immune-Mediated Necrotizing Myopathy (IMNM)

Day-to-day management of IMNM can take many forms. Adapting to a “new normal” can be challenging and require many lifestyle changes.

Exercise Program

It was once thought exercise caused damage to myositis patients. This has now been proven false through research.

We now know there are massive benefits in having an approved exercise program. This may start with a doctor referring you to physical therapy or rehabilitation center.  Exercise plans that are supervised by your medical and healthcare team are most effective so you do not injure yourself.

Before starting any type of exercise plan, talk to your doctor first.

Diet and Nutrition

Eating a well-balanced diet high in protein may be beneficial for those with IMNM, and is for your overall health.

Corticosteroids have many side effects including weight gain and redistribution of body fat to the face, neck, and abdomen, elevated blood sugars, and osteoporosis, so you may need to make adjustments in your diet for these conditions.

Ask your doctor about nutrition and a referral to a dietician or nutritionist.


When living with Immune-Mediated Necrotizing Myopathy (IMNM), having a good support system in place is important.

IMNM is chronic illness and some people in your life may not understand the challenging aspects of the symptoms, treatments, and stress involved in managing this complex, sometimes even invisible illness. It can feel lonely. Depression and anxiety can also be difficult aspects of this disease which may be exacerbated by the use of steroids.

We offer several online support options for patients, family members, caregivers, and friends of those living with IMNM. Having an emotional outlet is helpful, and our groups are also educational, resourceful, and we even try to add some fun group activities to help members connect.

MSU provides a free, monthly online video support session just for myositis patients. This is a chance to meet others face-to-face, discuss challenges, share tips, and just talk to others who completely understand. View our Events page for upcoming support sessions.

Have you been diagnosed with IMNM?

Get the support you deserve. Join the Myositis Support Community.

Safe, private myositis community for patients and caregivers.

Immune-Mediated Necrotizing Myopathy In Focus

What does IMNM look like?

IMNM patients may require the use of assistive devices for mobility such as a cane, walker, or wheelchair, either for short period like during flares or for long-term muscle weakness and damage.

When the lungs are involved, some may wear oxygen.

Some may look perfectly healthy on the outside, but on the inside, they are very sick and struggling on many levels.

IMNM Prognosis

The prognosis of Immune-Mediated Necrotizing Myopathy depends on many varying factors including patient response to treatment, the severity of disease and its manifestations, the presence of pulmonary or cardiac involvement, and associated antibodies or malignancy.

Some IMNM patients respond very well to treatment, while others may not. Finding the right treatment, or a combination of treatments can take months-to-years.

Long-term steroid use may be a major source of morbidity.

Research and Clinical Trials

Research helps us better understand diseases and can lead to advances in diagnosis and treatment. As there are currently no approved treatments for IMNM, research is important, as are patients participating in clinical trials.

We have partnered with Antidote to help make your clinical trial match easier.

Simply Put

“Simply Put” is a service of Myositis Support and Understanding, to provide overviews of Myositis-related medical and scientific information in understandable language.

MSU volunteers, who have no medical background, read and analyze often-complicated medical information and present it in more simplified terms so that readers have a starting point for further investigation and consultation with healthcare providers. The information provided is not meant to be medical advice of any type.

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