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

INTRODUCTION/AIMS:: Immune-mediated necrotizing myopathy (IMNM) is considered a rare subtype of the immune-mediated myopathies, but its incidence and prevalence are unknown. This study aimed to determine the incidence and prevalence of IMNM in the adults in Olmsted County, Minnesota.

METHODS

: We identified adult patients with IMNM defined by the 2016 European Neuromuscular Centre diagnostic criteria among Olmsted County, Minnesota, residents over a 20-year period

RESULTS

: Seven patients fulfilled the inclusion criteria. Six patients were tested for IMNM antibodies: 4 were anti 3-Hydroxy-3-Methylglutaryl-CoA Reductase (HMGCR)-positive, 1 was anti-signal recognition particle (SRP)-positive and 1 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 100,000 people ≥50 years. Median age at symptom onset was 64 years (range: 52-86) and median time from symptom onset to diagnosis was 3 months (range <1-156). Statin use among anti-HMGCR IMNM patients, but not the entire IMNM cohort, was higher than in controls (P=0.024). 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 1 with delayed treatment and one with insufficient therapy. Among 3 deceased patients, 1 died from cancer while 2 died from IMNM-related cardiorespiratory complications.

DISCUSSION:: IMNM 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.