27 yo graduate student was referred with chief complaint of double vision of 2 weeks duration, patient noticed persistent tingling of all the fingers of the left hand. patient said he felt like there was ants crawling on the left side of face and the left half of tongue and felt both of his legs had become weaker lately. scotoma in the upper field of the left eye, weakness of the left medial rectus muscle, coarse horizontal nystagmus on left lateral gaze,mild weakness of left central facial muscles, deep tendon reflex tremor were normal on right and more lively on the left, there was a left extensor plantar response. patient admitted to hospital 4 months later, because of difficulty in walking and speech had become thickened, wide based ataxic gait , minor slurring of speech ,bilateral tremor in the finger to nose test and disorganization of rapid alternating movements. mri revealed numerous lesions, lumbar puncture showed 56 mg of protein with a relatively increased level of gamma globulin, several oligloclonal bands in CSF.what is the differential diagnosis