Background and Objectives: First identified in 2010, anti-HMGCR necrotizing myopathy produces progressive and debilitating muscle weakness. Diagnosis and treatment can be delayed, as features are suggestive of a hereditary myopathy. Although it is more often described in adult populations, its epidemiology, including its incidence, remains unknown. It has not been reported in very young children.
Methods: We describe the evaluation and management of young boy found to have anti-HMGCR myopathy.
Results: A 5-year-old boy with previously normal motor strength presented with a 3-month history of progressive muscle weakness predominantly affecting his legs. Over this time period, he had lost the ability to run and was struggling to arise from the floor and climb stairs. He occasionally reported some pain in his proximal legs. His serum CPK was 12,306 U/L. There was no family history of neuromuscular disease or autoimmune conditions. While a hereditary myopathy, such as Duchenne muscular dystrophy, was considered, the relative rapid decline in his strength prompted dual consideration for an acquired inflammatory myopathy. He was started on oral prednisolone while awaiting the results of both a hereditary myopathy gene panel and muscle autoantibodies, to include anti-HMGCR IgG. His anti-HMGCR level was elevated at > 550 (Negative < 20). The hereditary myopathy panel of 211 genes was negative for disease-associated variants. His prednisolone was continued, and he was also treated with rituximab and started on monthly IVIG. His strength recovered to normal after 8 months of ongoing treatment. His CPK normalized over the same time. Anti-HMGCR IgG level declined to a nadir of 130 after 5 months of treatment. Discussion: Anti-HMGCR myopathy should be considered alongside of hereditary myopathies in children presenting with subacute progressive weakness and hyperCKemia. Early treatment with immunomodulatory agents, including rituximab, improves strength and disease biomarkers.