P.042 Anti-HMG Coenzyme A reductase antibody (anti-HMGCR) myopathy: case review of two pediatric patients from a single center

Background: Necrotizing anti-HMGCR myopathy is rare in children. Pediatric cases are not typically associated with statin use or malignancy. Methods: Retrospective chart review (January 2009 to December 2023) identified cases of anti-HMGCR myopathy at our hospital. Results: Two patients were identified. Patient A, presented at 8 yo with a 2 year history of proximal muscle weakness. His CK was 4,840 U/L (normal <205 U/L) with a high anti-HMGCR antibody titre. His Childhood Myositis Assessment Scale (CMAS) score was 33/52. Monthly IVIG was started and his muscle strength and CK improved. Two years later, weekly methotrexate was started for persistent mild CK elevation (602 to 869 U/L). At 11 years old, 3 years after diagnosis, his CMAS score was 47 and he could participate in soccer with mild fatigue. Patient B, presented at 8 yo with acute proximal weakness, rash and CMAS 13/52. His CK was 20,185 U/L with elevated anti-HMGCR antibody titre. He received oral corticosteroids, weekly methotrexate and monthly IVIG. At 10 yo, 2 years after diagnosis, he is asymptomatic with CMAS 51. He is maintained on methotrexate monotherapy. Conclusions: Anti-HMGCR antibody myopathy requires prompt diagnosis to obviate muscle necrosis and long-term complications. Our patients showed clinical and CMAS improvement with treatment.