Anti-3-Hydroxy-3-Methylglutaryl-Coenzyme A Reductase (HMGCR) Immune-Mediated Necrotizing Myopathy With Subsequent Diagnosis of Mantle Cell Lymphoma.

Anti-3-hydroxy-3-methylglutaryl-coenzyme A reductase (anti-HMGCR) immune-mediated necrotizing myopathy (IMNM) is a subtype of autoimmune necrotizing myopathy that presents with progressive proximal weakness and persistent creatine kinase (CK) elevation, which may continue despite statin discontinuation. Its relationship with hematologic malignancy is not well established. A 67-year-old woman developed markedly elevated CK levels after starting high-intensity statin. Despite statin withdrawal, CK levels remain persistently elevated, accompanied by progressive symmetrical proximal weakness. Anti-HMGCR IgG was strongly positive, while other autoimmune serologies were unrevealing. A diagnosis of statin-associated anti-HMGCR IMNM was established, and treatment with mycophenolate mofetil and intravenous immunoglobulin resulted in partial biochemical improvement. Five weeks after initiation of immunosuppressive therapy, routine laboratory testing showed rapidly rising leukocytosis. Further evaluation revealed mantle cell lymphoma with bone marrow involvement, IGH::CCND1 rearrangement, and TP53 deletion, consistent with high-risk disease. In this context, continued clinical and laboratory monitoring is appropriate in inflammatory myopathy, and new hematologic abnormalities warrant further evaluation.