Abstract
Background: In juvenile idiopathic inflammatory myopathies (IIMs), morphological characteristic features of distinct subgroups are not well defined. New treatment strategies require a precise diagnosis of the subgroups in IIM, and, therefore, knowledge about the pathomorphology of juvenile IIMs is warranted. Methods: Muscle biopsies from 15 patients (median age 8 (range 3–17) years, 73% female) with IIM and seven controls were analyzed by standard methods, immunohistochemistry, and transmission electron microscopy (TEM). Detailed clinical and laboratory data were accessed retrospectively. Results: Proximal muscle weakness and skin symptoms were the main clinical symptoms. Dermatomyositis (DM) was diagnosed in 9/15, antisynthetase syndrome (ASyS) in 4/15, and overlap myositis (OM) in 2/15. Analysis of skeletal muscle tissues showed inflammatory cells and diffuse upregulation of MHC class I in all subtypes. Morphological key findings were COX-deficient fibers as a striking pathology in DM and perimysial alkaline phosphatase positivity in anti-Jo-1-ASyS. Vascular staining of the type 1 IFN-surrogate marker, MxA, correlated with endothelial tubuloreticular inclusions in both groups. None of these specific morphological findings were present in anti-PL7-ASyS or OM patients. Conclusions: Morphological characteristics discriminate IIM subtypes in juvenile patients, emphasizing differences in aetiopathogenesis and supporting the notion of individual and targeted therapeutic strategies.
Highlights
Idiopathic inflammatory myopathy (IIM) is the most common form of myopathy in adult patients
Classification of IIM has been based on clinical, serological, and morphological features that lead to further definition of distinct subgroups, such as dermatomyositis (DM), antisynthetase syndrome (ASyS), immune-mediated necrotizing myopathy (IMNM), and sporadic inclusion body myositis [3,4,5,6]
Controls were obtained from seven age-matched children, who received a muscle biopsy because of suspected neuromuscular disorder or metabolic disease confirmed by biopsy and further clinical follow-up
Summary
Idiopathic inflammatory myopathy (IIM) is the most common form of myopathy in adult patients. Classification of IIM has been based on clinical, serological, and morphological features that lead to further definition of distinct subgroups, such as dermatomyositis (DM), antisynthetase syndrome (ASyS), immune-mediated necrotizing myopathy (IMNM), and sporadic inclusion body myositis (sIBM) [3,4,5,6]. Juvenile idiopathic inflammatory myopathy (jIIM) has many features in common with adult IIM. Both age groups harbor distinct clinical phenotypes, autoantibody findings, and associated outcomes [10,11,12,13]. A stringent classification of jIIM, including clinical, serological, and morphological features, similar to the one in adult patients, does not exist. In juvenile idiopathic inflammatory myopathies (IIMs), morphological characteristic features of distinct subgroups are not well defined. Dermatomyositis (DM) was diagnosed in 9/15, antisynthetase syndrome (ASyS)
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