Treatment of Inflammatory Myelitis and Optic Neuritis With Early vs Rescue Plasma Exchange (TIMELY-PLEX)

Study Population and Setting The proposal will recruit participants presenting to participating sites with severe ON or severe TM to two separate sub-trials. The detailed inclusion and exclusion criteria for each sub-trial are listed below: Optic Neuritis Sub-Trial: Inclusion criteria: * ≥18 years of age * MRI orbits demonstrating evidence of new T2 hyperintensity and/or post-gadolinium contrast enhancement of the optic nerve(s) and meeting the clinical criteria for Optic Neuritis * Visual acuity 20/200 or worse * Within 8 days of onset of visual symptoms * Able to initiate PLEX within 72h of the first dose of HDCS (if randomized to the "Early PLEX" treatment arm) * Able to sign and date informed consent form * Willingness to comply with all study procedures and availability for the duration of the study Exclusion criteria: * Evidence of prior episode of optic neuritis in the affected eye (by history or ophthalmological evaluation) * Ophthalmological comorbidity that would significantly affect best corrected visual acuity or visual fields * Pregnancy * Presence of any contraindication to receiving HDCS or PLEX, including, but not limited to, hemodynamic instability, significant bleeding/coagulopathy, or sepsis. * Any medical condition that, in the opinion of the investigator, may interfere with the patient's participation in the trial, pose any added risk for the patient, or confound the assessment of the patient (including but not limited to concurrent neurological dis…