Statin-Induced Immune-Mediated Necrotizing Myopathy Resulting in Proximal Muscle Weakness

Statin-induced immune-mediated necrotizing myopathy (IMNM) is a subtype of IMNM linked to exposure to statins and is characterized by positive anti-hydroxymethylglutaryl (HMG) coenzyme A reductase (HMGCR) antibodies. Although rare, this entity has become increasingly recognized as a cause of proximal muscle weakness, especially with the widespread use of statin therapy. Unlike typical statin-associated muscle symptoms, IMNM myopathy often causes severe muscle injury, and muscle weakness persists or sometimes worsens following the withdrawal of statin therapy. Medical practitioners need to keep a high index of clinical suspicion for statin-induced IMNM in patients taking statins who present with muscle weakness. The disease can be debilitating, and treatment strategies are not well established despite the advances that have been made in the diagnosis. Here we present the clinical characteristics and disease course of two cases of statin-induced IMNM. Both patients presented with progressive proximal muscle weakness and myalgias while on long-term statin therapy without significant improvement in their symptoms following the withdrawal of statin therapy. IMNM was suspected, and both patients were found to have high titers of anti-HMG coenzyme A reductase antibodies and demonstrated microscopic features consistent with a diagnosis of IMNM on muscle biopsy. The patients experienced significant disability due to muscle weakness and required a protracted course of escalated immunosuppressive therapy. Although rare, IMNM should be suspected in patients taking statins who present with muscle weakness that fails to improve or worsens when statins were stopped. Early diagnosis and institution of immunosuppressive therapy are important to prevent the progression of the disease.