Therapeutic plasma exchange for the treatment of refractory necrotizing autoimmune myopathy

INTRODUCTION: Necrotizing autoimmune myopathy (NAM) is strongly associated with pathognomonic autoantibodies targeting 3‐hydroxy‐3‐methylglutaryl‐coenzyme A reductase (HMGCR) or signal recognition particle (SRP), whose levels in turn are correlated with serum creatine kinase (CK) and necrosis. Thus, NAM may be amenable to therapeutic plasma exchange (TPE) to remove pathogenic antibodies and improve patient symptoms. METHODS: A retrospective case series and literature review of patients presenting with NAM and undergoing treatment with TPE was performed. Clinical data including patient demographics, symptoms, physical exam findings, muscle biopsy, lower extremity imaging, prior therapy, and duration from diagnosis to TPE initiation were collected retrospectively for adult patients with NAM treated with TPE after failing to respond to immunomodulatory therapy. Laboratory data including change in CK levels and myositis‐specific antibody titers from baseline were measured in some patients. RESULTS: Six patients (median age at diagnosis 52.5 years, interquartile range [IQR] 35.8‐64.5 years, four male/two female) underwent a median of 7.5 (IQR: 5‐10) TPE procedures with 5% albumin as replacement. All patients exhibited a statistically significant reduction in CK level from pre‐TPE baseline (range: 43.0%‐58.7% reduction). Responses in this cohort were best in patients with antibodies targeting HMGCR and SRP, which are most strongly associated with NAM. These results compare favorably to a literature review of NAM patients (n = 19) treated with TPE, who also exhibited positive clinical and laboratory responses across varying treatment lengths. CONCLUSION: TPE can play a role in the management of NAM, particularly in patients with HMGCR or SRP antibodies who are refractory to pharmacologic immunosuppression.