Background/Purpose: Immune-mediated necrotizing myopathy (IMNM) is an aggressive subtype of idiopathic inflammatory myopathy (IIM) associated anti-3-hydroxy-3-methylglutaryl coenzyme A reductase (anti-HMGCR) antibodies. Recently there have been reports of increased statin-associated IMNM in certain Native American groups. To address this, this research compared the proportion, characteristics, and clinical outcomes of Native American and non-Native American adults diagnosed with IIM and IMNM from the same Southwest region.

Methods: This is a retrospective single institution study approved by the Institutional Review Board. Myositis diagnosis was established based on the 2017 European Alliance of Associations for Rheumatology/American College of Rheumatology Classification Criteria for Adult and Juvenile Idiopathic Inflammatory Myopathies, and statin-associated IMNM diagnosis required the presence of anti-HMGCR antibodies. We identified 147 patients with IIM, with 32 patients who self-identified as Native American and 115 patients who self-identified as non-Native American. The proportion of IIM and IMNM in each group, the relevant clinical and laboratory parameters, and the outcomes were compared.

Results: Table 1 shows the baseline characteristics for the two groups. Compared to non-Native Americans, Native Americans with IIM had higher incidence of hyperlipidemia (HLD) (75% compared to 49.6%; p=0.015), statin exposure at diagnosis (71.9% compared to 40.0%; p=0.002), pre-existing diabetes mellitus (DM) (62.5% compared to 34.8%; p=0.044), and esophageal dysmotility (93.8% compared to 77.4%; p=0.042). Additionally, Native Americans were found to have significantly higher frequency of positive anti-HMGCR antibody, with 46.9% compared to 16.5% in non-Native Americans (p=0.0007). Native Americans also had higher frequency of positive anti-RNP antibody, with 15.6% compared to 2.6% in non-Native Americans (p=0.013). There was no significant difference in the clinical outcomes and treatment received between the two groups (Table 2). When statin-associated IMNM was specifically compared between the two groups, there were no significant differences in IMNM clinical features or outcomes. This has not been reported before.

Conclusion: Statin-associated IMNM is increased in Native Americans compared to non-Native Americans in the American Southwest. There may be underlying genetic or environmental components to explain the findings above, including certain HLA types, an increased prevalence of diabetes mellitus, hyperlipidemia, and statin exposure. However, the manifestations and outcomes of statin-associated IMNM in Native Americans and non-Native Americans appear to be similar. Given that the outcome of statin-associated IMNM depends on early initiation of appropriate therapy and that Native Americans historically have experienced healthcare disparities, it is particularly important that statin-associated IMNM in this population is recognized and treated early.

Table 1. Inflammatory Myositis in Native Americans and Non-Native Americans

Table 2. Clinical Outcomes of Statin-Associated Immune-Mediated Necrotizing Myopathy in Native Americans and Non-Native Americans

Figure 1. Muscle Edema by Magnetic Resonance Imaging in a Native American Patient Diagnosed with Immune-Mediated Necrotizing Myopathy

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/statin-associated-immune-mediated-necrotizing-myopathy-in-native-americans/