BACKGROUND
Juvenile dermatomyositis has come to encompass several subtypes based on an emerging correlation between autoantibodies and clinical presentation. We hypothesised that myopathological findings may align with clinico-serological subtypes, potentially indicating differences in pathogenesis.
METHODS
We studied a large cohort of 101 muscle biopsies from JDM patients in the UK JDM Cohort and Biomarker Study using the international JDM score tool and performing histological analysis of dominant fiber pathology. Autoantibody data were available for the majority of patients and were correlated with histological findings.
RESULTS
Major autoantibody groups in our cohort were anti-TIF1γ (18/101), -NXP2 (15/101), -MDA5 (11/101), -Mi2 (5/101), and -PmScl (6/101). JDM biopsy severity scores varied within antibody groups except for MDA5 with consistently low, and Mi2 with consistently high scores. Dominant fiber pathology was grouped under 8 descriptive labels (minimal change (24/101), diffuse endomysial macrophage infiltrates (40/101), perifascicular atrophy (22/101), macrophage rich necrosis (6/101), scattered necrosis (2/101), clustered necrosis (2/101), inflammatory fiber invasion (2/101), chronic myopathic change (1/101)). All major autoantibody groups showed a mix of fiber pathologies with the exception of MDA5, which consisted predominantly of minimal change biopsies.
CONCLUSION
JDM patients demonstrate a range of muscle biopsy findings in our cohort with perfascicular atrophy represented in only about one third of biopsies. Dominant fiber pathology or severity scores do not clearly predict autoantibody groups. Heterogeneity of muscle histology in JDM is not fully understood but may indicate differences in activation of inflammatory signaling pathways in muscle between patients.
