• Elina Tan, Jacinta Knight, Steffi Khonasti, David Nolan, Benjamin McGettigan, Chris Bundell, Merrilee Needham, and Anna Brusch.
• Sir Charles Gairdner Hospital, QEII Medical Centre, Perth, Western Australia, Australia.
• Intern Med J. 2023 Oct 1; 53 (10): 184618531846-1853.

BackgroundAnti-3-hydroxy-3-methylglutaryl CoA reductase (HMGCR) antibodies are associated with a subtype of immune-mediated necrotising myopathy (IMNM).AimsTo determine clinical associations of anti-HMGCR antibodies for anti-HMGCR-associated IMNM (HMGCR-IMNM) among a cohort of patients in Western Australia and to determine whether serial HMGCR antibody levels parallel disease activity.MethodsAdult patients with positive anti-HMGCR antibodies detected by enzyme-linked immunosorbent assay between January 2015 and November 2019 were included. Symptoms, examination findings, imaging findings and blood test results were reviewed retrospectively using patient records and laboratory database results.ResultsAmong 26 patients with positive anti-HMGCR antibodies, 23 were diagnosed with HMGCR-IMNM representing a positive predictive value (PPV) of 88%. Myopathy was frequently severe at diagnosis with limb weakness graded as Medical Research Council score 3 or below in 78% of patients, bulbar muscle weakness in 39% and an average creatine kinase (CK) at diagnosis of 7986 U/L. The majority (83%) required at least two therapies to maintain remission, 48% had at least one flare of disease and 57% did not achieve CK normalisation. Correlation between CK and anti-HMGCR antibody level at diagnosis was low (r = 0.04). Anti-HMGCR antibodies fell with treatment in 10 of 12 patients, but remained persistently positive in 83% of patients.ConclusionsThe PPV of anti-HMGCR antibodies for HMGCR-IMNM in this Western Australian cohort is 88%. Patients typically present with proximal limb weakness, dysphagia and markedly elevated CK, and, despite multiagent immunosuppression, a significant number of patients have evidence of persistent biochemical myositis. Anti-HMGCR antibodies did not correlate with CK levels at diagnosis.© 2023 Royal Australasian College of Physicians.

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