Objective To describe the clinical and epidemiological characteristics of patients presenting optic neuritis as the initial symptom of some demyelinating diseases, divided among multiple sclerosis (MS), neuromyelitis optica spectrum disorder (NMOSD), and myelin oligodendrocyte glycoprotein-immunoglobulin G (MOG-IgG)-associated disorders (MOGADs). The gene discussed is MOG; the disease is myeloid sarcoma.