Immune-Mediated Necrotizing Myopathy (IMNM)

Who gets Immune-Mediated Necrotizing Myopathy (IMNM)?

Usually adults. Three patterns are recognised by antibody: anti-HMGCR, which is closely associated with statin exposure and can begin months or years after starting one; anti-SRP, which tends to be the most severe and often affects younger patients; and antibody-negative disease, which requires a search for an underlying cancer or connective tissue disease.

What are the symptoms of Immune-Mediated Necrotizing Myopathy (IMNM)?

Not everyone has every symptom below, and having one does not confirm the diagnosis. These are the features that most often lead to assessment:

• Severe, rapidly progressive weakness of the shoulders and hips over weeks
• Difficulty rising from a chair, climbing stairs, or lifting the arms — often to the point of needing help
• Muscle wasting, sometimes visible within months
• Muscle pain, though weakness is far more prominent
• Difficulty swallowing
• Very high CK levels on blood testing
• Breathlessness where respiratory muscles or lungs are affected
• Little or no rash, distinguishing it from dermatomyositis

How is Immune-Mediated Necrotizing Myopathy (IMNM) diagnosed?

A markedly raised CK, often above 5,000 and sometimes far higher, is characteristic and prompts urgent assessment.

Anti-HMGCR and anti-SRP antibody testing establishes the subtype and, in the case of anti-HMGCR, the link to statin exposure.

Muscle biopsy shows necrosis and regeneration of fibres with sparse inflammatory infiltrate — a pattern distinct from dermatomyositis and polymyositis. MRI identifies affected muscles and guides the biopsy.

Other causes of very high CK — rhabdomyolysis, hypothyroidism, muscular dystrophy — are excluded, and in antibody-negative disease cancer screening is arranged.

How is Immune-Mediated Necrotizing Myopathy (IMNM) treated?

Any statin is stopped, but stopping it is not sufficient: unlike simple statin toxicity, this disease continues and progresses without immunosuppression.

Treatment usually needs more than one agent from the outset: corticosteroids with methotrexate, azathioprine or mycophenolate, and intravenous immunoglobulin, which is particularly effective in anti-HMGCR disease and is often used early rather than reserved.

Rituximab is used in resistant disease, especially with anti-SRP antibodies.

Recovery of strength is slower than in other forms of myositis, and treatment is usually needed for years. Physiotherapy is essential throughout.

Living with Immune-Mediated Necrotizing Myopathy (IMNM)

This form takes longer to treat and relapses more readily than dermatomyositis or polymyositis, so treatment is rarely stopped early. CK is monitored alongside strength — both matter, and improvement in the blood test alone is not sufficient reason to reduce treatment. Statins are avoided permanently in anti-HMGCR disease, and cardiovascular risk is managed by other means.

When should you see a rheumatologist?

Severe weakness developing over weeks with a very high CK needs urgent assessment. Anyone who develops progressive weakness on a statin, or whose weakness persists after stopping one, should be assessed rather than reassured.

Common questions about Immune-Mediated Necrotizing Myopathy (IMNM)

This page is general information to support a consultation. It is not a diagnosis, and it does not replace an assessment by a doctor who has examined you.