Favorable Response in Statin-Naive Paraneoplastic Anti-HMGCR Antibody-Associated Myopathy to Single Dose of Rituximab and Persistent Remission With Management of Underlying Lung Cancer.

Anti-HMGCR myopathy is a subtype of immune-mediated necrotizing myopathy, typically associated with exposure to statins, although a sizable minority in some cohorts are statin-naive. Although the clinical features of acute- or subacute-onset symmetrical proximal muscle weakness mimic those of other idiopathic inflammatory myopathies, necrotizing myopathy is distinguished by the histopathological findings of muscle fiber necrosis and regeneration with little to no accompanying inflammation. Several recent studies of patients with anti-HMGCR myopathy have identified a slightly increased risk of cancer. Most patients require aggressive immunotherapy, usually as a combination of 2 or 3 immunosuppressant drugs. We report a case of a statin-naive paraneoplastic anti-HMGCR myopathy, who unlike other reported cases, responded to a single dose of 1000 mg of intravenous rituximab and subsequent chemoradiation therapy for an underlying lung cancer, despite failing to completely respond to prior high-dose oral prednisone and methotrexate.