Immediate Versus Delayed Treatment With Azathioprine or Rituximab in Anti-MOG Antibodies Associated Acute Demyelinating Syndromes in Children: a Randomized Controlled Clinical Trial

Inclusion Criteria: * Children \< 18 years old and ≥ 6 years old at baseline * Children weight ≥ 20 kg * All ADS with confirmed anti-MOG-Abs at onset including any acute neurologic symptom with a duration of more than 24H of inflammatory causes (including optic neuritis, transverse myelitis, rhombencephalitis, ADEM, NMOSD) Without any previous treatment other than steroids * Informed consent signed by both parents and the child * Expanded Disability Status Scale (EDSS) \< 5.5 * Affiliated to French social security regime Exclusion Criteria: * Current infection with SARS-COV2 (positive PCR) * Any prior allergy to azathioprine or rituximab with hypersensitivity to active substances, murine proteins or to any of the excipients. * Any prior history of uncontrolled cancer during the last 2 years * Uncontrolled infections (Hepatitis B, C and HIV) * Any prior history of cardiac dysfunction and/or hypertension * Any progressive or non-relapsing form demyelinating diseases * Any previous treatment with natalizumab, daclizumab, fingolimod, methotrexate, cyclosporine, mycophenolate mofetil, rituximab in the last 6 months, or determined by the treating physician to have residual immune suppression from these or other immunosuppressive treatments * CD4+, CD8+, or CD19+ absolute cell count, wbc, neutrophiles in blood at screening below lower limits of normal (LLN) * Creatinine\>30µmol/L * Platelets \<70 000mm3 * Haemoglobin \< 8g/dL * Acute renal insufficiency (clearance \< 30 ml/min) * Prior documented history of hemostase perturbation (TP and/or TCA more than twice of the witnesse's TP and/or TCA) * Prior documented history of increased liver enzyme level (ASAT and/or ALAT) \> 2N. * TP \<70% * Total bilirubin \> 2N * Any patient with allopurinol treatment and immunosupressive treatment with concomitant use of xanthine oxidase inhibitors (e.g. allopurinol, oxipurinol/thiopurinol, febuxostat) * Patients with two inactive TPMT or NUDT15 alleles (homozygous deficient or double heterozygote) * Pregnancy or lactating woman or wish for future pregnancy * Refusal to have a highly effective contraception during traitment and for one year (12 months) after the end of the experimental treatment * participation to another interventional study within 5 half-lives prior to baseline. * Active, severe infections (including tuberculosis, HBV and HCV, HIV, herpes, VZV, EBV and CMV) * Psychosis not controlled by treatment * Patients with Lesch Nyhan syndrome * Pheochromocytoma * Scleroderma * Untreated peptic ulcer * Myasthenia gravis * Any other medical illness or disability that, in the opinion of the investigator, would compromise effective trial participation