Method: The investigators will conduct a multicentric prospective study including all patients of 18 years or older, with no prior history of demyelinating disorders and presenting with a diagnosis of optic neuritis in Martinique, Guadeloupe, French Guiana, Saint-Martin and Saint-Barthélemy. Patients will first undergo a full neuro-ophthalmic examination which includes visual acuity, contrast vision, color vision, slit-lamp anterior segment and fundus examination as well as automatized visual field and optical coherence tomography of the optic nerves and retina. Patients will then be admitted to the Neurology and Ophthalmologic Department of the University Hospital of Martinique for optic neuritis emergency treatment, 3-Tesla brain and medullar MRIs, and ancillary testing. Specific NMO antibodies (AQP-4 and MOG) will be tested in all patients. Neuro-ophthalmic examination will be repeated after 3 days of IV steroids in order to decide on further treatment. Patients will be further monitored at 1, 6 and 12 months so as to determine the most likely etiology of optic neuritis with the aid of MS and NMO diagnosis criteria.