Experimental: Immediate Azathioprine (1st attack)

Treatment will be started at 2mg/kg or at 1mg/kg if the patient has a partial activity which would be increased slowly according the 6-TGN activity and clinical and biological tolerance at Week 2 for patient with partial activity or M1 for patient without TPMT activity deficit.~Only for patient with partial deficit and whose 6-TGN activity is low, azathioprine would be increased at 3mg/kg/d at Week 6 and without exceeding a total daily dose of 150 mg.

Experimental: Immediate Rituximab (1st attack)

Once the inclusion criteria are validated, the first injection will be performed according to the injection protocol (Annex 4). Fifteen day later, the second injection will be performed.~The next visit during a consultation with PI or his collaborators will be scheduled 1 week ± 2 days later, and patients will be advised to contact the PI if any neurologic symptoms or symptoms of adverse event occurs in the meantime.

Active_comparator: Standard Care: delayed treatment (2nd attack)

Patients will be treated according to standard of care after their 1st attack. In case of relapse:~* before 3 months, IV methylprednisolone (30 mg/kg/j not exceeding 1g/day) will be administered for 3 days. There won't be any change of the initial treatment which will be pursued.~* after 3 months, IV methylprednisolone (30 mg/kg/j not exceeding 1g/day) will be administered for 3 days with an oral relay of prednisolone (1mg/kg/day not exceeding 60 mg/day) during 3 months with then a slowly tapered dose (reduction of 25% every week for 4 weeks). Azathioprine or Rituximab might be proposed as per local clinician experience