The Myositis Overlap Conundrum: Differentiating Polymyositis from Inclusion Body Myositis

We describe the case of a 77-year-old woman who presented with progressive weakness in both lower extremities, dysphagia to both solids and liquids, and severe weight loss. Her recent experience with statin-induced rhabdomyolysis was initially considered to be associated with her high creatine kinase (CK) level. Nevertheless, she became weaker and experienced dysphagia even after the cancellation of statin therapy. Lab tests indicated that there were constantly high CK (3347 U/L) and positive anti-PM-Scl-100 antibody, which indicated an autoimmune inflammatory myopathy. Lower limb MRI revealed diffuse myositis, and muscle biopsy revealed active myopathy with inflammation and rimmed vacuoles, characteristic of polymyositis/inclusion body myositis (IBM). She received high-dose corticosteroids and then intravenous immunoglobulin (IVIg) and methotrexate as treatment for polymyositis. Although she experienced an improvement in her limb weakness, she was still dysphagic. The case demonstrates that the diagnosis of inflammatory myopathies in older people is complex, and there is a need to identify inclusion body myositis early in the course to inform proper management.