Anti-SRP antibody-positive polymyositis complicated by neuromyelitis optica spectrum disorder: a case report and literature review.

Neuromyelitis optica spectrum disorder (NMOSD) is an autoimmune inflammatory demyelinating disease primarily affecting the optic nerves and spinal cord. Polymyositis (PM) is an idiopathic inflammatory myopathy characterized by proximal muscle weakness. The anti-signal recognition particle (SRP) antibody is a myositis-specific autoantibody. We herein present a case of anti-SRP antibody-positive PM that developed 6 years after the onset of NMOSD. A 52-year-old woman was diagnosed with NMOSD 6 years previously based on left visual disturbance, a high signal intensity and contrast enhancement of the optic nerve on short τ inversion recovery (STIR)-magnetic resonance imaging (MRI), and anti-aquaporin 4 antibody positivity. She received methylprednisolone pulse therapy followed by oral prednisolone (PSL) at a starting dose of 40 mg daily. Three years later, due to recurrent numbness in the left lower limb and difficulty in reducing the PSL dose to ≤10 mg/day, satralizumab was initiated. At 52 years old, she developed myalgia and muscle weakness in both thighs. PM was diagnosed based on an elevated serum creatine kinase level, anti-SRP antibody positivity, high signal intensities in the right triceps and bilateral adductor muscles on STIR-MRI, and muscle biopsy findings. Treatment with high-dose PSL and tacrolimus markedly attenuated her symptoms. Satralizumab was continued for NMOSD stabilisation. Previous studies reported the coexistence of NMOSD and autoimmune diseases; however, NMOSD with PM/DM is rare. We described a case of NMOSD with anti-SRP antibody-positive PM and provided a literature review.