Optic neuritis (ON) often occurs at the presentation of multiple sclerosis (MS), neuromyelitis optica spectrum disorders (NMOSD), and myelin oligodendrocyte glycoprotein (MOG) antibody-associated disease (MOGAD). The recommended treatment of high-dose corticosteroids for ON is based on a North American study population, which did not address treatment timing or antibody serostatus. The Acute Optic Neuritis Network (ACON) presents a global, prospective, observational study protocol primarily designed to investigate the effect of time to high-dose corticosteroid treatment on 6-month visual outcomes in ON. Patients presenting within 30 days of the inaugural ON will be enrolled. For the primary analysis, patients will subsequently be assigned into the MS-ON group, the aquapotin-4-IgG positive ON (AQP4-IgG+ON) group or the MOG-IgG positive ON (MOG-IgG+ON) group and then further sub-stratified according to the number of days from the onset of visual loss to high-dose corticosteroids ( days-to-Rx ). The primary outcome measure will be high-contrast best-corrected visual acuity (HC-BCVA) at 6 months. In addition, multimodal data will be collected in subjects with any ON (CIS-ON, MS-ON, AQP4-IgG+ON or MOG-IgG+ON, and seronegative non-MS-ON), excluding infectious and granulomatous ON. Secondary outcomes include low-contrast best-corrected visual acuity (LC-BCVA), optical coherence tomography (OCT), magnetic resonance imaging (MRI) measurements, serum and cerebrospinal fluid (CSF) biomarkers (AQP4-IgG and MOG-IgG levels, neurofilament, and glial fibrillary protein), and patient reported outcome measures (headache, visual function in daily routine, depression, and quality of life questionnaires) at presentation at 6-month and 12-month follow-up visits. Data will be collected from 28 academic hospitals from Africa, Asia, the Middle East, Europe, North America, South America, and Australia. Planned recruitment consists of 100 MS-ON, 50 AQP4-IgG+ON, and 50 MOG-IgG+ON. This prospective, multimodal data collection will assess the potential value of early high-dose corticosteroid treatment, investigate the interrelations between functional impairments and structural changes, and evaluate the diagnostic yield of laboratory biomarkers. This analysis has the ability to substantially improve treatment strategies and the accuracy of diagnostic stratification in acute demyelinating ON., Trial Registration: ClinicalTrials.gov, identifier: NCT05605951., Competing Interests: SA has received speaker honoraria from Alexion, Bayer, and Roche. JB reports payment for consultation from Horizon Therapeutics, Alexion, Antigenomycs, BeiGene, Chugai, Clene Nanomedicine, Genentech, Reistone Bio, Roche, Imcyse, and TG; grants from Alexion, Novartis, and the National Institutes of Health. In addition, JB has a patent on Compositions and methods for the treatment of neuromyelitis optica. YB has received speaker honoraria from Novartis, Roche, Genzyme-Sanofi, Merck, and Biogen. EC has received reimbursement for developing educational presentations, educational and research grants, consultation fees, and/or travel stipends from Biogen Argentina, Genzyme Argentina, Merck Argentina and LATAM, Roche Argentina and LATAM, Raffo, Novartis Argentina, LACTRIMS, and The Guthy-Jackson Charitable Foundation. JC has served on advisory boards for Horizon, Roche, and UCB. CC has received honoraria for speaking from Bayer and research funding from Novartis. JD has received royalties from Wolters Kluwer, Neurology—UpToDate and from Medlink Neurology as contributing author, from Athena Diagnostics for the use of Ma2 as an autoantibody test, and from Euroimmun for the use of NMDA-receptor, GABA(B)-receptor, GABA(A)-receptor, DPPX, and IgLON5 as autoantibody tests and has received research support from Advance Medical (allosteric modulation of NMDAR) SAGE Therapeutics, Instituto Carlos III/FEDER (FIS PI20/00197, CIBERER CB15/00010, Proyectos Integrados de Excelencia, PIE 16/00014 and AC18/00009), Agencia de Gestio d'Ajuts Universitaris i de Recerca (AGAUR), CERCA Programme Generalitat de Catalunya, ERA-NET NEURON, La Caixa Foundation Health Research Award, Pablove Foundation Childhood Cancer Grant, Safra Foundation, Sage therapeutics, Cellex Foundation, and La Caixa Health Foundation. EF has served on advisory boards for Alexion, Genentech, Horizon Therapeutics, and UCB. He has received speaker honoraria from Pharmacy Times. He received royalties from up-to-date. EF was a site primary investigator in a randomized clinical trial on Inebilizumab in neuromyelitis optica spectrum disorder run by Medimmune/Viela-Bio/Horizon Therapeutics. EF has received funding from the NIH (R01NS113828). EF is a member of the medical advisory board of the MOG project. EF is an editorial board member of the Journal of the Neurological Sciences and Neuroimmunology Reports. A patent has been submitted on DACH1-IgG as a biomarker of paraneoplastic autoimmunity. CF participates in a regional medical board advisory of Alexion. CG-A has received grants from Biogen Colombia. JH reports grants for OCT research from the Friedrich-Baur-Stiftung and Merck, personal fees, and non-financial support from Celgene, Janssen, Bayer, Merck, Alexion, Horizon, Novartis, Roche, Biogen, and non-financial support of the Guthy-Jackson Charitable Foundation, all outside the submitted work. JH was partially funded by the German Federal Ministry of Education and Research [(DIFUTURE), Grant Numbers 01ZZ1603[A-D] and 01ZZ1804[A-H]]. HK has received a grant from the National Research Foundation of Korea; consultancy/speaker fees or research support from Alexion, Aprilbio, Altos Biologics, Biogen, Celltrion, Daewoong, Eisai, GC Pharma, HanAll BioPharma, Handok, Horizon Therapeutics (formerly Viela Bio), Kolon Life Science, MDimune, Mitsubishi Tanabe Pharma, Merck Serono, Novartis, Roche, Sanofi Genzyme, Teva-Handok, and UCB; and is a coeditor for the Multiple Sclerosis Journal and an associated editor for the Journal of Clinical Neurology. CLM reports consultancy fees for Chiesi Farmaceutici, Regulatory PharmaNet, and Thenewway Srl and received speaker honoraria and/or travel support for meetings from Santhera Pharmaceuticals, Chiesi Farmaceutici, Regulatory Pharma Net, Thenewway Srl, First Class Srl, and Biologix. ML-P has received funding for travel and speaker honoraria from Novartis, Biogen, and Roche. MLei was funded by NHS (Myasthenia and Related Disorders Service and National Specialized Commissioning Group for Neuromyelitis Optica, UK) and by the University of Oxford, UK. She has been awarded research grants from the UK association for patients with myasthenia—Myaware and the University of Oxford. She has received speaker honoraria or travel grants from Biogen Idec, Novartis, Argenx, UCB, and the Guthy–Jackson Charitable Foundation. MLei serves on scientific or educational advisory boards for UCB Pharma, Argenx, and Viela/Horizon. SL has received consulting fees and speaker honoraria from Biogen, Novartis, TEVA, Genzyme, Sanofy, and Merck. PL has received reimbursement for developing educational presentations, educational and research grants, consultation fees, and/or travel stipends from Biogen Argentina and LATAM, Genzyme Argentina, Merck Argentina, Roche Argentina, Novartis Argentina, and LACTRIMS. AL has served as a Biogen, Bristol Myers Squibb, Merck Serono, Novartis, Roche, Sanofi/Genzyme, and Teva Advisory Board Member, has received congress and travel/accommodation expense compensations, or speaker honoraria from Biogen, Merck Serono, Mylan, Novartis, Roche, Sanofi/Genzyme, Teva, and Fondazione Italiana Sclerosi Multipla (FISM), her institutions received research grants from Novartis and Sanofi/Genzyme. SMo reports consultancy fees (Invex Therapeutics); advisory board fees (Invex therapeutics, Gensight); and speaker fees (Heidelberg engineering, Chugai-Roche Ltd., Allergan, Santen, Chiesi, and Santhera), all outside the submitted work. RM serves on scientific advisory boards for Alexion, Horizon Therapeutics, Roche, and UCB has received speaker honoraria from Alexion, Biogen, Horizon Therapeutics, Novartis, Roche, and Sanofi Genzyme, has received support for attending scientific meetings by Merck and Euroimmun, has received speaker honoraria from Biogen and Novartis. SMa received speaker honoraria for presenting at scientific meetings by Novartis and Biogen. SMo reports consultancy fees (Invex Therapeutics); advisory board fees (Invex therapeutics, Gensight); and speaker fees (Heidelberg engineering, Chugai-Roche Ltd., Allergan, Santen, Chiesi, and Santhera). All outside the submitted work. FC receives ongoing research support from the National Multiple Sclerosis Society (NMSS), the American Academy of Neurology (AAN), and Deutsche Gesellschaft für Neurologie (DGN). JPa has received support for scientific meetings and honorariums for advisory work from Merck Serono, Novartis, Chugai, Alexion, Roche, Medimmune, Argenx, UCB, Mitsubishi, Amplo, Janssen, and Sanofi. Grants from Alexion, Roche, Medimmune, UCB, and Amplo biotechnology. Patent ref P37347WO and license agreement Numares multimarker MS diagnostics Shares in AstraZeneca. Acknowledges Partial funding by Highly specialized services NHS England. SP is a named inventor on filed patents that relate to functional AQP4/NMO-IgG assays and NMO-IgG as a cancer marker, was consulted for Alexion and MedImmune, has received research support from Grifols, MedImmune, and Alexion, all compensation for consulting activities is paid directly to Mayo Clinic. SR has received research funding from the National Health and Medical Research Council (Australia), the Petre Foundation, the Brain Foundation (Australia), the Royal Australasian College of Physicians, and the University of Sydney. She was supported by an NHMRC Investigator Grant (GNT2008339). She serves as a consultant on an advisory board for UCB and Limbic Neurology and has been an invited speaker for Biogen, Excemed, and Limbic Neurology. AS received personal compensation for consulting, serving on a scientific advisory board, speaking, or other activities with Merck-Serono, Sanofi, Biogen, Roche, Novartis, Alexion, Janssen, and Horizon Therapeutics. DS received a grant from the National Multiple Sclerosis Society and serves on several advisory committees for the Multiple Sclerosis International Federation in unpaid roles. MS received speaker honoraria from Teva Pharmaceuticals and has received funding from the German Research Foundation, Federal Ministry of Education and Research and Federal Ministry for Economic Affairs and Energy, Volkswagen Stiftung, and Berlin Institute of Health. He is holding patents for the 3D printing of computed tomography models and is a shareholder of PhantomX and MSC3D. All unrelated to this work. PSu has served on advisory boards for Horizon Therapeutics, Viridian Therapeutics, Invex Therapeutics, Kriya Therapeutics, and GenSight Biologics. He receives research support from the NIH, DOD, Horizon, Invex, and Viridian. AM has received a grant for Biopas Laboratories and reports speaking fees from Chiesi. HZ received research grants from Novartis and speaking fees from Bayer Healthcare, unrelated to this project. FP served on the scientific advisory boards of Novartis and MedImmune; received travel funding and/or speaker honoraria from Bayer, Novartis, Biogen, Teva, Sanofi-Aventis/Genzyme, Merck Serono, Alexion, Chugai, MedImmune, and Shire; is an associate editor of Neurology: Neuroimmunology & Neuroinflammation; is an academic editor of PLoS ONE; consulted for Sanofi Genzyme, Biogen, MedImmune, Shire, and Alexion; received research support from Bayer, Novartis, Biogen, Teva, Sanofi-Aventis/Geynzme, Alexion, and Merck Serono; and received research support from the German Research Council, Werth Stiftung of the City of Cologne, German Ministry of Education and Research, Arthur Arnstein Stiftung Berlin, EU FP7 Framework Program, Arthur Arnstein Foundation Berlin, Guthy-Jackson Charitable Foundation, and NMSS. HS-K received speaker honoraria from Roche. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 Asseyer, Asgari, Bennett, Bialer, Blanco, Bosello, Camos-Carreras, Carnero Contentti, Carta, Chen, Chien, Chomba, Dale, Dalmau, Feldmann, Flanagan, Froment Tilikete, Garcia-Alfonso, Havla, Hellmann, Kim, Klyscz, Konietschke, La Morgia, Lana-Peixoto, Leite, Levin, Levy, Llufriu, Lopez, Lotan, Lugaresi, Marignier, Mariotto, Mollan, Ocampo, Cosima Oertel, Olszewska, Palace, Pandit, Peralta Uribe, Pittock, Ramanathan, Rattanathamsakul, Saiz, Samadzadeh, Sanchez-Dalmau, Saylor, Scheel, Schmitz-Hübsch, Shifa, Siritho, Sperber, Subramanian, Tiosano, Vaknin-Dembinsky, Mejia Vergara, Wilf-Yarkoni, Zarco, Zimmermann, Paul and Stiebel-Kalish.)