Anti-HMGCR myopathy mimicking facioscapulohumeral muscular dystrophy.

Introduction: Statin use can lead to various muscle-related issues, including benign creatine kinase (CK) elevations, myalgias, toxic myopathies, rhabdomyolysis, and immune-mediated necrotizing myositis (IMNM), which primarily affects older males. IMNM presents with proximal muscle weakness, elevated CK levels, and specific antibodies.

Case presentation: We describe a 72-year-old patient with muscle weakness persisting for over 3 years after statin therapy. Initially suspected to have a genetic disorder, further testing revealed elevated anti-3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR) antibodies, indicating immune-mediated myopathy. Despite the absence of inflammatory changes on biopsy, the patient responded positively to immune therapy.

Conclusion: This case highlights challenges in diagnosing immune-mediated myopathy, especially in older patients with atypical presentations. Testing for HMGCR antibodies can aid in diagnosis, particularly when inflammatory markers are absent. Awareness of red flags, such as delayed symptom onset and response to prednisone, is crucial for accurate diagnosis and management.