Necrotizing Autoimmune Myopathy

Necrotizing autoimmune myopathy (NAM) is a type of immune-mediated myopathy characterized by myofiber necrosis but with no or minimal inflammatory cell infiltrate on muscle biopsy and specific antibodies. Anti-signal recognition particle (SRP) and antihydroxy-3-methylglutaryl-CoA reductase (HMGCR) autoantibodies are closely associated with NAM and define unique subtypes of patients. Importantly, the new European Neuromuscular Centre criteria recognize antiSRP myopathy, anti-HMGCR myopathy, and autoantibody-negative NAM as three distinct subtypes of NAM. Anti-SRP myopathy patients have more severe muscle involvement, with a better response to immunosuppressive regimens that include rituximab.In contrast, antiHMGCR myopathy is often associated with statin exposure with a good response to IVIG, even as monotherapy. Both anti-SRP and anti-HMGCR myopathy tend to be most severe in younger patients.Furthermore, children with these forms of NAM may present with dystrophy-like features which are potentially reversible with immunosuppressant treatment. Some patients with NAM develop severe weakness soon associated with fatty replacement of muscle, suggesting that intense and early immunosuppressant therapy may provide the best chance to avoid long-term disability.