Introduction Neuromyelitis optica (NMO) is characterized by inflammatory demyelinating lesions of the spinal cord and optic nerves from an autoimmune response against water channel aquaporin-4 (AQP4). We report 2 patients with transient hyperCKemia associated with NMO suggesting possible skeletal muscle damage. Methods: Patient 1 was a 72-year-old man who presented with muscle soreness and elevated serum creatine kinase (CK) preceding an initial attack of NMO. Patient 2 was a 25-year-old woman with an established diagnosis of NMO who presented with diffuse myalgias, proximal upper extremity weakness, and hyperCKemia. Muscle biopsies were obtained for histopathologic evaluation, protein gel electrophoresis, immunofluorescence, and complement staining. Results: In both patients the muscle showed only mild variation in fiber diameter. There were no inflammatory changes or muscle fiber necrosis, though there was reduced AQP4 expression and deposition of activated complement. Conclusions: Complement-mediated sarcolemmal injury may lead to hyperCKemia in NMO. Muscle Nerve 50: 859–862, 2014
