[Analysis of clinical and muscle pathological features of juvenile idiopathic inflammatory myopathy]

Objective: To investigate the clinical manifestations, muscle imaging features, and muscle pathological characteristics of juvenile idiopathic inflammatory myopathies (IIM). Methods: A retrospective cohort study was conducted including 76 children with IIM admitted to Children's Medical Center, Peking University First Hospital from January 2012 to January 2024. Clinical manifestations, laboratory findings, muscle imaging features, and muscle pathological characteristics were compared between the major subtypes of dermatomyositis and immune-mediated necrotizing myopathy (IMNM). Independent sample t-test or Mann-Whitney U test, and χ² test or Fisher's exact test were used to compare differences in characteristics between groups. Results: Among 76 children with IIM, 30 were males and 46 were females, with the age of onset at 5.0 (3.0, 8.0) years. Of these 76 children, 57 children were diagnosed with dermatomyositis, 15 children with immune-mediated necrotizing myopathy (IMNM) and 4 children with overlap myositis. The proportion of heliotrope rash, Gottron's sign, and antinuclear antibody positivity were all higher in the dermatomyositis group than those in the IMNM group (63% (36/57) vs.1/15, 47% (27/57) vs. 0/15, and 68% (32/47) vs. 5/14, respectively, all P<0.05), whereas serum creatine kinase (CK), alanine aminotransferase, aspartate aminotransferase, lactate dehydrogenase levels were all lower (all P<0.05). The proportion of myositis specific autoantibody positivity was also lower in the dermatomyositis group than in the IMNM group (52% (24/46) vs. 13/14, P=0.006). Muscle MRI showed that subcutaneous soft tissue edema was more frequently observed in the dermatomyositis group than in the IMNM group (28% (14/50) vs. 0/14, P=0.005), whereas muscle fatty infiltration was less common in the dermatomyositis group (16% (8/50) vs. 9/14, P<0.001). On muscle pathology, perifascicular muscle fiber atrophy and perivascular inflammatory cell infiltration, as well as sarcoplasmic myxovirus resistance protein A expression on perifascicular myofibers, were more common in the dermatomyositis group (78% (28/36) vs. 1/7, 89% (32/36) vs.1/7 and 10/14 vs. 0/4, respectively, all P<0.05), while muscle fiber necrosis, perimysial hyperplasia, and endomysial hyperplasia were more common in the IMNM group (7/7 vs. 33% (12/36), 2/7 vs. 0/36 and 4/7 vs. 3% (1/36), respectively, all P<0.05). Conclusions: There are differences between the 2 major juvenile IIM subtypes in terms of clinical manifestations, muscle MRI findings, and muscle pathological features. Dermatomyositis is more likely to present with characteristic skin rashes and perifascicular muscle fiber pathological changes, whereas IMNM is characterized by markedly elevated serum CK levels and prominent muscle fiber necrosis, and is more frequently associated with muscle fatty infiltration.