Seronegative Immune-Mediated Necrotizing Myopathy: A Case Report

Idiopathic inflammatory myopathies (IIMs) are a group of chronic autoimmune disorders characterized by proximal skeletal muscle weakness. One subtype of the IIMs is immune-mediated necrotizing myopathy (IMNM). IMNM can be further classified according to its autoantibody presence, including anti-3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR), anti-signal recognition particle (SRP), and seronegative.  Here, we describe the case of a 61-year-old Caucasian female with a prior history of distant lung cancer and current statin use presenting with a subacute onset of bilateral proximal lower extremity muscle weakness and markedly elevated creatinine kinase (CK) and amino transaminases. In the acute inpatient setting, she underwent successful treatment with corticosteroids that were eventually discontinued and replaced with azathioprine three months after hospital admission. At that point, she had attained a 60% increase in muscle strength.