Case Presentation: Patient presented to the ED in June 2019 with 6 month history of progressive proximal bilateral upper and lower extremity weakness, swallowing difficulty and 100 lbs. weight loss over 3 months. Exam was significant for decreased strength of bilateral hip flexors, knee flexors and knee extensors with intact reflexes. Labs were significant for CK elevation to 8000, elevated transaminases, troponin elevation to 1.65 without chest pain, ESR 22 & CRP 19 (mild elevation), SPEP without monoclonal proteins, myopathy and myositis panels were negative. CT Chest performed as part of malignancy workup demonstrated a pleural based mass which was biopsied and showed benign fibroconnective tissue. EMG was performed and consistent with myositis and length dependent neuropathy. Muscle biopsy was obtained and concerning for necrotizing myopathy. Patient was seen by rheumatology and started on immunosuppression induction with IVIG and Mycophenolate. Prior to discharge, patient was transitioned to prednisone. Unfortunately in rehab, patient experienced prolonged episodes of palpitations with associated shortness of breath. EKG showed new 1st degree AV block with bifasciular block suggestive of myocardial involvement. Patient was transferred to the hospital and underwent cardiac MRI showing nearly diffuse hyperintensity on T2 mapping consistent with diffuse myocardial inflammation. Ejection fraction was 55%. Rheumatology was consulted and patient had prednisone discontinued and restarted on IVIG, Mycophenolate and IV Solumedrol. Endomyocardial biopsy was performed which showed no obvious inflammation. EP study was without inducible arrhythmia, thus no ICD was recommended. Patient discharged back to rehab with diagnosis of immune-mediated necrotizing myopathy with likely cardiac involvement despite negative biopsy results (biopsy taken after re-initiation on induction immunosuppression), with follow-up in 2 weeks with repeat cardiac MRI.

Discussion: Inflammatory myopathies are autoimmune diseases characterized by muscle inflammation resulting in elevated serum creatinine kinase and distinctive biopsies. Included in this group of myopathies include polymyositis (PM), dermatomyositis (DM), inclusion body myositis and immune-mediated necrotizing myopathy (IMNM). Annual incidence of inflammatory myopathies worldwide is about 2 cases per million. Inflammatory myopathies have a bimodal distribution of occurrence, with the first in childhood and the second in middle age. Immune-mediated necrotizing myopathy is unique in that environmental factors play a role in triggering the immune mechanism. IMNM is also commonly misdiagnosed as polymyositis or dermatomyositis, due to proximal muscle weakness. But, with a close history and physical, an astute clinician can differentiate IMNM from PM or DM due to its severe, rapidly progressive weakness with few (if any) extramuscular manifestations. PM and DM, on the other hand, are characterized by progressive, symmetrical proximal muscle weakness but commonly involve the cardiopulmonary system.

Conclusions: The case presented is a rare case of IMNM with cardiac conduction system abnormalities, but commonly immune-mediated necrotizing myopathy, an inflammatory myopathy difficult to differentiate from PM or DM, is characterized by severe, rapidly progressive muscle weakness with very high serum CK and few extramuscular manifestations.