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

Edited by: Justin Willer, MD

Citation: Wang Y, Yang M, Zhao Y, et al. Pediatric Immune-Mediated Necrotizing Myopathy: A Single-Center Retrospective Cohort Study. Pediatr Neurol. 2025;167:33-41. doi:10.1016/j.pediatrneurol.2025.03.002

Summary: Given the rarity, there remains a dearth of systematic data on pediatric immune myopathies. This is particularly true of subtypes other than dermatomyositis. This retrospective observational study of pediatric autoimmune necrotizing myopathies (IMNM) provides clinical, serologic, radiologic, pathologic, and treatment related data on a large Chinese cohort.

The group identified 116 patients through a muscle biopsy registration program. Of these, 55 (56%) were determined to have IMNM and then retrospective analysis of data was performed. The IMNM group showed a median age of onset of 7 with a female preponderance (69%) and chronic onset (60%). Initial symptoms consisted of proximal weakness (76.3%) with relatively low incidence of non-muscle manifestations (rash, arthralgia, fever, ILD).

Serologic testing showed a majority of IMNM patients with anti-HMGCR antibody (69%) and smaller proportions with anti-SRP antibody (20%) or seronegative (18%). Muscle MRI most commonly showed generalized muscle edema with lower incidence of myofascial or subcutaneous edema. Fatty replacement was also common and was correlated with SRP antibody and duration of disease. Muscle pathology most commonly showed an overall dystrophic appearance and was more common in anti-SRP antibody related disease. MHC I upregulation and sarcolemmal MAC deposition was also common.

Follow up data was available in 39 of 55 patients. All received high dose steroids and most (87%) received multiple immunotherapies (IVIg, methotrexate, rituximab, tacrolimus, cyclophosphamide, and azathioprine). Follow up data was available from between 6-90 months. While most improved in strength and muscle enzyme levels, most (24/39) were considered refractory or not responsive to treatment. 

Comments: Limitations of the study include that this is a single center, retrospective, observational study. The high proportion of IMNM in this cohort may be related to selection bias versus a unique proportion of this Chinese cohort. 

This is one of the largest cohort studies of pediatric IMNM and provides useful data on clinical characteristics of this relatively rare group.