Acute Optic Neuritis Network: an International Study That Invesitages Subjects With a First-ever Episode of Acute Inflammation of the Optic Nerve

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.