Azathioprine in MOGAD

Inclusion Criteria: * Age ≥ 18 years * First attack of documented acute demyelinating syndrome of the central nervous system, within the past 3 months, whatever the severity or the clinical phenotype * Tested positive for MOG-Ab, confirmed in a centralized lab (Lyon referral centre) * Ability of the subject to understand the purpose and risks of the study and provide signed and dated written informed consent. * Patients should be beneficiary of health care coverage under the social security system * Female patients of childbearing potential should have effective contraception throughout the course of treatment and for at least three months after stopping treatment. Exclusion Criteria: * Hypersensitivity to azathioprine or steroids * Active infections or cancer (including tuberculosis, hepatitis, herpes and VZV) * Psychosis not controlled by treatment * Seriously impaired bone marrow functions: Lymphocyte count \< 1000/ml and or Polynuclear neutrophil count \< 1500/ml * Seriously impaired hepatic functions: ALT and/or AST \> 3N * Seriously impaired renal functions: GFR \< 29 ml/min/1.73m² * Any live vaccine in the past 3 months or planned during the RCT and RCT+6months * Thiopurine methyltransferase (TPMT) phenotype deficient or intermediate, with enzymatic activity \< 16 nmol/h/ml * Unable to complete an MRI (e.g. due to pacemaker, severe claustrophobia, hypersensitivity to contrast media, or who lack adequate peripheral venous access) * Necessary use of a xanthine oxidase inhibitor (Allopurinol, Oxipurinol, Thiopurinol, Febuxotat,…) * Necessary use of angiotensin-converting-enzyme inhibitor, cotrimoxazole, cimetidine and indometacine * Necessary use of an aminosalicylate derivates * Necessary use of any another immunosuppressive therapy, different than azathioprine, or steroids * Necessary use of cytotoxic therapy * Necessary use of any other medical illness or disability that, in the opinion of the investigator, would compromise effective trial participation * Current enrollment or a plan to enroll in any interventional clinical study in which an investigational treatment or approved therapy is use within 5 half-lives prior to baseline. Participation in a non- interventional study can be allowed as long as this participation does not interfere with this protocol or is not likely to affect the subject's ability to comply with the protocol. * For subjects coming back from strongyloidiasis endemic regions, a parasitology screening examination will be performed on faeces, and that appropriate treatment will be performed prior to administration of corticosteroids * Patients with Lesch Nyhan syndrome * Asian patients (probable mutation of the gene NUDT1) * Female subjects who have a positive a positive urinary or blood pregnancy test result, are pregnant or are currently breast feeding * Inability to comply with study requirements * Person under legal protection or deprived of liberty