Myositis with nemaline rods: Clinical features and treatment outcomes

Background: Several idiopathic inflammatory myopathies (IIM), originally classified under polymyositis, evolved into distinct entities. Herein, we describe clinical features and treatment outcomes of a potentially novel clinico-histopathological entity: myositis with nemaline rods (MNR). Methods: We performed retrospective chart review of the Mayo Clinic electronic medical records to identify patients with nemaline rods and muscle inflammation on biopsy, not fitting into any IIM subgroup, and extracted clinical and laboratory data. Additional immunohistochemical studies were performed on muscle biopsies. Results: Eleven patients were identified, with mean age at onset 61.8 years (SD = 9.8), seven were female. MNR had a distinctive phenotype with rapidly progressive weakness, predominantly affecting swallowing, axial, and proximal limb muscles, often with prominent myalgia. No patients had monoclonal gammopathy. Creatine kinase levels were elevated in 73% of patients. Muscle histopathology revealed nemaline rods in non-atrophic and atrophic fibers. Inflammation was often endomysial, where both CD4+ and CD8+ T cells invaded nonnecrotic muscle fibers. A significant proportion of CD8+ T cells were KLRG1+. Of patients with follow-up, 6/8 responded to immunosuppressive treatment with some going into remission. Although MNR shares features with sporadic late-onset nemaline myopathy (SLONM) and inclusion body myositis (IBM), it has important distinctions. Unlike SLONM, MNR shows endomysial inflammation and no monoclonal protein; unlike IBM, MNR exhibits distinct clinical features, rapid progression, and responsiveness to immunotherapy. Conclusions: MNR is associated with a unique combination of clinical and histopathological features, responding well to treatment in most cases. Future studies are needed to explore underlying mechanisms and identify serological biomarkers.