Persistent HyperCKemia Beyond Post-Viral Myositis: A Case of Anti-HMGCR Immune-Mediated Necrotizing Myopathy

Anti-3-hydroxy-3-methylglutaryl-coenzyme A reductase (anti-HMGCR) immune-mediated necrotizing myopathy (IMNM) is a rare autoimmune myopathy that should be suspected in patients with progressive proximal muscle weakness and marked creatine kinase (CK) elevation persisting despite statin discontinuation, although distinguishing it from more common conditions such as post-viral myositis and toxic statin-associated myopathy remains a major diagnostic challenge. We report the case of a 64-year-old man receiving long-term atorvastatin therapy who presented with progressive proximal muscle weakness, dysphagia, and severe hyperCKemia following a self-limited flu-like illness, initially raising post-viral myositis as the leading hypothesis. Despite immediate statin discontinuation and aggressive intravenous hydration, severe hyperCKemia persisted during the first week of supportive management alone, and the neurological deficits did not improve, prompting further investigation and empirical immunosuppression. Muscle magnetic resonance imaging demonstrated multifocal inflammatory muscle edema, while extensive investigation for infectious, metabolic, endocrine, neoplastic, and autoimmune causes was unrevealing. Muscle biopsy showed a necrotizing myopathy with pauci-inflammatory features, and anti-HMGCR antibodies, which had not been included in the initial extended myositis panel and required a dedicated request, were subsequently identified, confirming statin-associated IMNM. Treatment with high-dose corticosteroids followed by methotrexate produced progressive biochemical improvement and complete recovery of muscle strength and swallowing function, with CK falling to 376 U/L (N 30-200 U/L) at the last follow-up (four months after admission). The principal educational value of this case lies in the diagnostic reasoning rather than in the diagnosis itself: it was the failure of the clinical and biochemical course to follow the anticipated trajectory, rather than the initial presentation, that prompted reconsideration of the working hypothesis. Dysphagia, less commonly reported in post-viral myositis and toxic statin-associated myopathy, together with persistent severe hyperCKemia despite statin withdrawal, should prompt evaluation for anti-HMGCR IMNM.