AQP4 and myeloid sarcoma: In this case, conventional pathogen examination and two mNGS results are negative, gave us quite important diagnostic ideas, basically excluded intracranial infection, need to consider other possible non-infectious diseases such as neuromyelitis optica spectrum disorder (NMOSD), multiple sclerosis (MS), and autoimmune encephalitis, etc.NMOSD is characterized by positive AQP4 antibody, long-segment myelitis (≥ 3 vertebrae) and optic neuritis, and is diagnosed by AQP4 detection and spinal cord MRI.