Does IMNM Cause Severe Muscle Pain? Evaluation and Care

While immune-mediated necrotizing myopathy (IMNM) is classically defined by progressive muscle weakness, it can also cause significant muscle pain (myalgia). Although the medical term “necrotizing” refers to muscle-fiber injury or death and is sometimes assumed to be a painless process, research confirms that muscle pain is a recognized—though variable—symptom of IMNM, which can sometimes be severe ^[1].

It is important to understand that pain by itself does not prove the disease is active or flaring. However, if you are experiencing severe or new muscle pain, you should report it to your rheumatologist so they can properly evaluate your symptoms in combination with other tests ^[2].

When to Seek Urgent Care

Severe muscle pain can sometimes be a sign of a medical emergency. Seek urgent medical assessment if you experience:

• Dark or “cola-colored” urine, or a marked decrease in urination (signs of a serious condition called rhabdomyolysis)
• Rapidly progressing weakness, such as suddenly being unable to stand or use a limb
• New difficulty swallowing or breathing
• High fever or severe systemic illness

Pain Frequency by IMNM Subtype

The likelihood of experiencing muscle pain with IMNM can vary depending on your specific disease subtype. The following statistics come from specific observational studies and represent estimates from those groups of patients, rather than a strict rule for everyone:

• Seronegative IMNM: This refers to patients whose blood tests do not detect the currently recognized IMNM antibodies (anti-SRP or anti-HMGCR). In one study of 117 IMNM patients, 62.5% of those in the seronegative group reported muscle pain when their symptoms first appeared, compared to 23.3% of the antibody-positive group ^[1].
• Anti-SRP IMNM: Muscle pain is a frequent feature in this subtype. One study tracking patients with anti-SRP antibodies found that 54% experienced myalgia ^[3].
• Anti-HMGCR IMNM: Symptoms in this group can be highly variable. Some patients experience significant pain alongside weakness and elevated muscle enzymes ^[2]. Conversely, other patients may only experience painless, progressive weakness despite having active muscle injury and high enzyme levels ^[4].

Evaluating Muscle Pain

Severe muscle pain should not be ignored, as it can occur alongside objective evidence of active muscle injury and degeneration ^[2]. During an active disease phase, your pain may be accompanied by increasing weakness, abnormalities on a muscle MRI (magnetic resonance imaging), and elevated levels of creatine kinase (CK), an enzyme that leaks into the blood when muscle fibers are damaged ^[2].

However, because pain intensity alone cannot confirm disease activity or rule out other causes, your care team will look at the full picture. Muscle pain can also result from unrelated issues, such as:

• Viral infections ^[5]
• Radiculopathy (pain caused by an irritated or pinched nerve) ^[6]
• Side effects from other medications, particularly statins or certain other drugs

To determine the cause of your pain, your doctor will likely evaluate your symptoms in combination with your CK trends, physical strength, and potentially an MRI or EMG (electromyography, a test of electrical nerve and muscle function). Always review all of your current medications with your clinician, and do not stop taking any prescribed treatments—including immunosuppressants or statins—without their guidance.