Immune Mediated Necrotizing Myopathy

Immune mediated necrotising myopathy (IMNM) is an autoimmune myopathy with prominent myofibre necrosis and minimal inflammation, associated with anti-SRP or anti-HMGCR antibodies.

• Subtypes: anti-SRP (severe, rapidly progressive, cardiac involvement), anti-HMGCR (statin exposure in two thirds, also occurs without), seronegative (malignancy or connective tissue disease).

• Incidence 1 in 100,000 to 300,000; adults of all ages (statin-related typically over 50), children with anti-HMGCR.

• Statin use does not need to precede anti-HMGCR disease, but exposure is a key trigger.

• Complications: severe weakness, dysphagia, cardiac involvement, interstitial lung disease (anti-SRP less), relapse.

• Subacute severe proximal weakness (weeks to months), myalgia, marked muscle atrophy over time, dysphagia, weight loss.

• No rash.

• Anti-SRP: heart failure, cardiomyopathy; anti-HMGCR: persistent weakness after statin cessation.

• CK markedly elevated (often over 5000 U/L), aldolase, AST/ALT.

• Anti-SRP and anti-HMGCR antibodies (diagnostic in context); ANA often negative.

• MRI muscle: diffuse oedema. EMG: irritable myopathy.

• Biopsy: necrotic and regenerating fibres, minimal lymphocytic infiltrate, complement deposition on capillaries.

• Cardiac evaluation (ECG, troponin, echocardiography, cardiac MRI), malignancy screen in seronegative disease and older adults.

• Stop statins permanently.

• Initial therapy: prednisolone 1 mg/kg/day or methylprednisolone 1 g IV daily for 3 days with early addition of methotrexate 15 to 25 mg weekly or mycophenolate mofetil 1 to 1.5 g twice daily.

• IVIG 2 g/kg monthly (strong evidence in anti-HMGCR), often combined with steroids and another agent.

• Refractory: rituximab 1 g on days 1 and 15 (best for anti-SRP), cyclophosphamide, tacrolimus, plasma exchange.

• Treat cardiac and pulmonary involvement per standard care.

• Taper slowly under CK and strength monitoring; relapse during taper is common.

• Physiotherapy, dysphagia care, bone protection, Pneumocystis prophylaxis, vaccination.