Pediatric Immune-Mediated Necrotizing Myopathy: A Single-Center Retrospective Cohort Study.

BACKGROUND

Immune-mediated necrotizing myopathy (IMNM) is a type of idiopathic inflammatory myopathies (IIMs), and the data concerning the phenotypes of pediatric IMNM are very limited. The present study aimed to elucidate the characteristics of pediatric IMNM.

METHODS

We examined 116 pediatric patients with IIMs through a muscle biopsy-oriented registration study. Anti-signal recognition particle (anti-SRP) and anti-3-hydroxy-3-methylglutaryl-CoA reductase (anti-HMGCR) antibodies were detected via an immunoblot assay. A retrospective clinical, imaging, and myopathological analysis of 55 pediatric patients with IMNM was conducted.

RESULTS

The cohort included 38 girls and 17 boys with a median age of 7 years. Acute/subacute onset occurred in 40.0% and chronic onset in 60.0% of the patients. Proximal and neck weakness were common symptoms. The frequencies of anti-HMGCR, anti-SRP, and seronegative myopathies were 61.8%, 20.0%, and 18.2%, respectively. Chronic onset was more common in anti-HMGCR myopathy than in anti-SRP myopathy (P = 0.003). Thigh magnetic resonance imaging revealed generalized muscle edema, and the severity of fatty infiltration correlated with disease duration. Necrotizing myopathy was most common among patients with seronegative IMNM, followed by patients with anti-HMGCR and anti-SRP myopathies. Dystrophic pathology was most common among patients with anti-SRP myopathy, followed by patients with anti-HMGCR myopathy and seronegative IMNM. After steroids combined with multiple immunosuppressant therapies, 18 of 39 (46.2%) patients achieved complete or partial remission. The percent of complete remission was significantly lower in patients with anti-HMGCR myopathy compared with those with seronegative or anti-HMGCR myopathies (P = 0.030).

CONCLUSIONS

IMNM is common in Chinese pediatric patients with IIMs. Most patients have anti-HMGCR antibodies, are more commonly female, have chronic onset and proximal weakness, lack other organ manifestations, have disease course-related muscle fatty infiltration, and have a poor response to immunosuppression.