Clinical presentation and treatment of a case of anti-signal recognition particle–positive immune-mediated necrotizing myopathy

Inflammatory myopathies present with proximal muscle weakness and elevated creatine kinase levels. Immune-mediated necrotizing myopathy (IMNM) is distinguished by muscle necrosis with minimal lymphocytic infiltrates on biopsy. Those with IMNM may have the signal recognition particle (SRP) antibody, hydroxy-3-methylglutaryl-CoA reductase (HMGCR) antibody, or neither. We present a case of anti-SRP-positive IMNM in a patient with chronic proximal muscle weakness and dysphagia, which improved with empiric intravenous immunoglobulin. Muscle biopsy showed major histocompatibility complex-I positivity, confirming immune-mediated necrotizing myopathy. Ongoing steroid treatment with physical rehabilitation and neurology follow-up is planned. Although corticosteroids remain standard first-line therapy, early intravenous immunoglobulin or rituximab should be considered in statin-naive anti-HMGCR and anti-SRP disease. Cardiac screening is recommended given the frequency of extramuscular cardiac involvement in IMNM.