Abstract
AimJuvenile idiopathic inflammatory myopathies have been recently reclassified into clinico‐serological subgroups. Myopathological correlates of the subgroups are incompletely understood.MethodsWe studied muscle biopsies from 101 children with clinically and serologically defined juvenile idiopathic inflammatory myopathies from the UK JDM Cohort and Biomarker Study by applying the international JDM score tool, myopathological review and C5b‐9 complement analysis.ResultsAutoantibody data were available for 90/101 cases with 18/90 cases positive for anti‐TIF1γ, 15/90 anti‐NXP2, 11/90 anti‐MDA5, 5/90 anti‐Mi2 and 6/90 anti‐PmScl. JDM biopsy severity scores were consistently low in the anti‐MDA5 group, high in the anti‐Mi2 group, and widely distributed in the other groups. Biopsies were classified histologically as perifascicular atrophy (22/101), macrophage‐rich necrosis (6/101), scattered necrosis (2/101), clustered necrosis (2/101), inflammatory fibre invasion (2/101), chronic myopathic change (1/101), diffuse endomysial macrophage infiltrates (40/101) and minimal change (24/101). MDA5 cases segregated with the minimal change group and showed no capillary C5b‐9‐deposition. The Mi2 group displayed high severity scores and a tendency towards sarcolemmal complement deposition. NXP2 and TIF1γ groups showed a variety of pathologies with a high proportion of diffuse endomysial macrophage infiltrates and a high proportion of capillary C5b‐9 deposition.ConclusionWe have shown that juvenile idiopathic inflammatory myopathies have a spectrum of histopathological phenotypes and show distinct complement attack complex deposition patterns. Both correlate in some cases with the serological subtypes. Most cases do not show typical histological features associated with dermatomyositis (e.g. perifascicular atrophy). In contrast, more than half show relatively mild histopathological changes.
Highlights
Juvenile idiopathic inflammatory myopathies (IIM) are a group of diseases comprising immunologically mediated inflammatory muscle diseases in children
White Black South Asian Other Clinical features, median [interquartile range] Age at onset Time from disease onset to diagnosis, Time from diagnosis to biopsy, On steroids at biopsy, n (%) CMAS at biopsy Reported clinical diagnosis, n (%) Definite or probable juvenile dermatomyositis Juvenile dermatomyositis overlap with scleroderma Juvenile dermatomyositis overlap with polyarthritis Definite or probable Juvenile polymyositis Focal myositis Mixed connective tissue disease Other idiopathic inflammatory myopathy Antibody Status, n Myositis-specific autoantibodies, n (%) Anti-TIF1c Anti-NXP-2 Anti-MDA5 Anti-Mi2 Anti-SRP Anti-PL7 Anti-SAE Myositis-associated autoantibodies, n (%) Anti-PM-Scl Anti-U1RNP Anti-Scl70 No identifiable autoantibodies, n (%)
None of the minimal changes (MC) or diffuse endomysial macrophage infiltration’ pattern (DEMI) cases had Mi2 antibodies. These results suggest that the distinction between MC and DEMI may be biologically significant
Summary
Juvenile idiopathic inflammatory myopathies (IIM) are a group of diseases comprising immunologically mediated inflammatory muscle diseases in children. This group of diseases is being re-appraised and reclassified based on an improved understanding of the autoantibody landscape and clinical phenotypes. In the traditional clinicopathological classification, juvenile IIMs include juvenile dermatomyositis, polymyositis and cases that ‘overlap’ with other connective tissue diseases. Initial correlation between the clinical findings and autoantibody groups revealed subphenotypes allowing a refined clinical description of these diseases [1].
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