Fatal Cardiomyopathy Secondary to Seronegative Immune-Mediated Necrotizing Myopathy: A Case Report

BACKGROUND Idiopathic inflammatory myopathies (IIM) are disorders of autoimmune inflammation of muscle tissue. IIM subtypes include polymyositis, dermatomyositis, inclusion body myositis, and immune-mediated necrotizing myopathy (IMNM); which can be further subdivided based on serology. These disorders primarily affect skeletal muscle, and cardiac involvement is uncommon. We present a case of a patient with progressive cardiomyopathy as a rare complication of seronegative IMNM. CASE REPORT A previously healthy 35-year-old man presented with proximal muscle weakness and myalgias. Initial workup revealed elevated creatinine kinase and inflammatory markers, with magnetic resonance imaging findings of proximal muscle inflammation; muscle biopsy was diagnostic of IMNM. Serologic testing was negative for anti-SRP and anti-HMGCR, consistent with seronegative IMNM. Initial treatment included high-dose glucocorticoids, methotrexate, and intravenous immunoglobulin. He subsequently developed acute heart failure complicated by ventricular tachycardia. Additional treatment modalities included rituximab, mycophenolate mofetil, and cyclophosphamide. Cardiac positron emission tomography demonstrated an inflammatory myocardial process, and endomyocardial biopsy demonstrated interstitial fibrosis with macrophages. Despite treatment and advanced cardiac therapies, the patient developed progressive cardiogenic shock and cardiac arrest, ultimately resulting in death. CONCLUSIONS This is a case of fatal, treatment refractory cardiomyopathy secondary to seronegative IMNM. In patients with seronegative IMNM, clinical cardiac involvement likely conveys a poor prognosis. It is not understood what treatments are effective in this patient population, presenting opportunities for further study.