Anti-SRP immune-mediated necrotizing myopathy from childhood to adulthood: A case series highlighting rituximab-responsive disease

ObjectivesAnti-signal recognition particle (SRP) myositis is a rare subset of immune-mediated necrotizing myopathy (IMNM). It is characterized by proximal muscle weakness, markedly elevated serum creatine kinase (CK) levels, and poor response to conventional therapies. Evidence for optimal management remains limited, particularly regarding the long-term outcomes of rituximab treatment.MethodWe retrospectively reviewed anti-SRP myositis patients who received rituximab at Seoul National University Hospital. Diagnosis was based on clinical features, elevated CK levels, and positivity for anti-SRP antibodies. Demographic, clinical, laboratory, radiological, histopathological, and treatment data were collected.ResultsFive patients, aged between 4 and 71 years, exhibited proximal muscle weakness, elevated CK levels, and variable disease severity. Rituximab induction followed by maintenance therapy led to a significant improvement in muscle strength and normalization of CK and aldolase levels. In patients with mild or subclinical phenotypes, aldolase elevations often preceded clinical relapses. Relapses during follow-up were managed with rituximab re-administration, which restored muscle strength and biochemical markers. MRI, EMG, and muscle biopsy findings were consistent with myopathic changes.ConclusionsThis case series highlights the heterogeneity in age, disease severity, and clinical course of anti-SRP myositis and the efficacy of rituximab in refractory and relapsing cases. Multimodal monitoring, including CK, aldolase, CD19+ B-cell counts, and clinical assessment, is essential for the early detection of relapses. Our findings support rituximab as a valuable component of individualized treatment strategies for anti-SRP myositis, though larger studies are warranted to establish standardized protocols and clarify long-term outcomes.