The goal of this observational study is to longitudinally investigating subjects with inaugural acute optic neuritis (ON). The main questions it aims to answer are: * Does the time to corticosteroid treatment affect the visual outcome at 6 months in subjects with acute multiple sclerosis (MS)-, aquaporin 4-IgG positive (AQP4-IgG+) and myelin-oligodendrocyte-glycoprotein-IgG positive (MOG-IgG+) ON? * How differ clinical, structural, and laboratory biomarkers in subjects with acute ON, including clinical isolated syndrome (CIS), MS-ON, AQP4-IgG+ON, MOG-IgG+ON and seronegative non-MS-ON? Participants will undergo * clinical examination, including clinical history, neurovisual and neurological tests * serum and cerebrospinal fluid examination * optical coherence tomography (OCT) * magnetic resonance imaging (MRI) * assessment of depression, pain, quality of life through validated questionnaires Researchers will compare subjects with MS-ON, AQP4-IgG+ON, MOG-IgG+ON and other ON (CIS, seronegative non-MS-ON) to detect diagnostic and predictive markers for the disease course.