Incidence and prevalence of immune-mediated necrotizing myopathy in adults in Olmsted County, Minnesota

Introduction/AimsImmune-mediated necrotizing myopathy (IMNM) is considered a rare subtype of the immune-mediated myopathies, but its incidence and prevalence are unknown. In this study we aimed to determine the incidence and prevalence of IMNM among adults in Olmsted County, Minnesota. MethodsWe identified adult patients with IMNM, as defined by the 2016 European Neuromuscular Centre diagnostic criteria, among residents of Olmsted County, Minnesota over a 20-year period. ResultsSeven patients fulfilled the inclusion criteria. Six patients were tested for IMNM antibodies: four were anti-3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR) positive, one was anti-signal recognition particle positive, and one was seronegative. The incidence of IMNM during 2010-2019 was 8.3 per million person-years. The prevalence of IMNM in 2010 was 1.85 per 100000 people at least 50years of age. Median age at symptom onset was 64 (range, 52-86) years and median time from symptom onset to diagnosis was 3 (range, <1 to 156) months. Statin use among anti-HMGCR IMNM patients, but not the entire IMNM cohort, was higher than in controls (P =.024). Two IMNM patients developed cancers. The incidence of malignancy in IMNM was not higher than that of the general population. Treatment outcome was favorable in all patients except for one patient with delayed treatment and another with insufficient therapy. Among three deceased patients, one died of cancer and another died of IMNM-related cardiorespiratory complications. DiscussionIMNM is a rare disease. Its prevalence is one tenth that of inclusion-body myositis in Olmsted County, Minnesota. IMNM patients in our cohort were not at higher risk for developing cancer.

Automated summary

This study aimed to determine the incidence and prevalence of immune-mediated necrotizing myopathy (IMNM) in adults in Olmsted County, Minnesota. The results showed an incidence rate of 8.3 per million person-years and a prevalence rate of 1.85 per 100,000 people in 2010. IMNM patients did not have a higher risk of developing cancer compared to the general population.