Editors' Note: Clinical Subgroups and Factors Associated With Progression in Patients With Inclusion Body Myositis

Dr. Michelle and investigators from the Johns Hopkins Myositis Center retrospectively analyzed clinical and pathologic data from 335 patients with inclusion body myositis (IBM) to identify unique phenotypes of the condition. The distinct clinical and pathologic characteristics of each demographic subgroup may be useful in clinical trial design and prognostication. Among the key findings, patients meeting criteria for IBM were diagnosed after a mean delay of 5 years, with fewer than half of patients (43%) with muscle biopsies demonstrating all 3 pathologic hallmarks (endomysial inflammation, rimmed vacuoles, and mononuclear invasion). Based on the limited sensitivity of muscle biopsy, Dr. Stenzel and colleagues agree with the authors that one should emphasize the clinical examination in making a diagnosis of IBM and supplement these observations with more advanced diagnostic testing and sequencing of mitochondrial DNA in muscle biopsy samples. Dr. Michelle et al. note that immunohistochemical testing and mitochondrial testing are not available in most US laboratories and their specificity in differentiating IBM from polymyositis may be limited. They stress the value of the 2011 European Neuromuscular Centre consensus diagnostic criteria, which combine clinical and pathologic features to establish the diagnosis. An update to these 2011 criteria is anticipated shortly.