Necrotizing Autoimmune Myopathy

Abstract

Necrotizing autoimmune myopathy (NAM) is categorized as one of the idiopathic immune-mediated myopathies. It is characterized by subacute predominantly proximal weakness, elevated serum creatine kinase, and a muscle biopsy showing predominantly necrotic and regenerating fibers with absent or minimal inflammation. In over 50% of cases, the weakness is severe, and about 40% of patients have distal muscle weakness. Dyspnea and respiratory muscle weakness are common and can lead to some patients developing respiratory failure requiring intubation. Risk factors associated with NAM include the use of statin medications, neoplasm, and connective tissue disease. A subset of NAM patients have SRP-IgG and/or HMGCR-IgG antibodies although neither of these antibodies are 100% sensitive or specific. NAM patients are felt to be pharmacologically more treatment refractory than other immune-mediated myopathies and often require treatment with more than two immunotherapeutic agents. Commonly, patients are treated with a combination of corticosteroids, IVIG, and an oral steroid-sparing immunosuppressant agent (such as methotrexate or mycophenolate mofetil). Despite the severe weakness, the outcome can be favorable in over 50% of cases although many will have some residual weakness.