Clinicopathological features of anti-HMGCR and anti-SRP myopathies that do not satisfy the EULAR/ACR criteria of inflammatory myopathies.

Abstract

The clinicopathological features of immune-mediated necrotizing myopathy (IMNM) sometimes mimic muscular dystrophy, complicating accurate diagnosis. The European League Against Rheumatism (EULAR)/American College of Rheumatology (ACR) classification criteria of idiopathic inflammatory myopathies (IIMs) are superior in terms of sensitivity and specificity; however, the sensitivity is reported to be relatively low in IMNM. We examined the clinicopathological characteristics and the prognoses of anti-3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) antibody-positive and anti-signal recognition particle (SRP) antibody-positive cases that do not satisfy the EULAR/ACR classification criteria. Seventy-one anti-HMGCR antibody-positive cases and 94 anti-SRP antibody-positive cases were included. We compared the clinicopathological characteristics of the groups that were not classified as IIMs (<possible group) with those classified as IIMs (definite-possible group) on the basis of the EULAR/ACR classification criteria. For the <possible group whose follow-up data were available, their clinical features were assessed up to 24 months. Nineteen anti-HMGCR antibody-positive cases (26%) and 19 anti-SRP antibody-positive cases (20%) were classified as the <possible group. The <possible group had milder weakness, less frequent dysphagia, lower serum creatine kinase and aldolase levels and fewer necrotic fibres and granular p62-positive fibres than the definite-possible group. Of the 26 cases in the <possible group that could be followed up, only three cases were classified as the definite-possible group, and the other cases remained in the <possible group during the follow-up period. Some IMNM cases showing a clinicopathologically mild phenotype and a benign course cannot be classified as IIMs by the EULAR/ACR classification criteria, and antibody testing is crucial for accurate diagnosis.