Adjusted mean change in optic nerve conduction velocity (NCV) at Week 24 for the affected eye from the baseline of unaffected fellow eye as determined by FF-VEP. Adjusted for the baseline latency of fellow eye.

ITT population: all randomized subjects who received at least 1 dose of study treatment (BIIB033 or placebo). Last observation carried forward (LOCF) imputation was used if Week 24 data were missing.

Adjusted mean percentage change in thickness of the RNFL at Week 24 for the affected eye from the baseline of unaffected fellow eye as determined by SD-OCT. Percentage change is calculated as (affected eye - baseline of fellow eye)/baseline of fellow eye*100. Adjusted for the baseline RNFL thickness.

ITT population: all randomized subjects who received at least 1 dose of study treatment (BIIB033 or placebo) and a valid RNFL assessment at Baseline. LOCF imputation was used if Week 24 data were missing.

Adjusted mean percentage change in thickness of the RNFL at Week 24 for the affected eye from the baseline of unaffected fellow eye as determined by SD-OCT. Percentage change is calculated as (affected eye - baseline of fellow eye)/baseline of fellow eye*100. Adjusted for the baseline RNFL thickness.

Per-protocol Population: participants from the ITT population who completed the study, did not miss more than 1 dose of BIIB033 or placebo, and did not receive MS-modifying therapies during the study period, which were prohibited per protocol. LOCF imputation was used if Week 24 data were missing.

Adjusted mean change in thicknesses of the RGCL/IPL at Week 24 for the affected eye from the baseline of unaffected fellow eye as determined by segmentation of SD-OCT. Adjusted for the baseline RGCL/IPL thickness.

ITT population: all randomized subjects who received at least 1 dose of study treatment (BIIB033 or placebo) with a valid RGCL/IPL assessment at Baseline. LOCF imputation was used if Week 24 data were missing.

Adjusted mean change in thicknesses of the RGCL/IPL at Week 24 for the affected eye from the baseline of unaffected fellow eye as determined by segmentation of SD-OCT. Adjusted for the baseline RGCL/IPL thickness.

Per-protocol Population: participants from the ITT population who completed the study, did not miss more than 1 dose of BIIB033 or placebo, and did not receive MS-modifying therapies during the study period, which were prohibited per protocol. LOCF imputation was used if Week 24 data were missing.

Adjusted mean change in LCLA at Week 24 from baseline as determined by 1.25% and 2.5% low contrast Sloan letter charts, adjusted for the baseline LCLA value. The fellow eye is the reference eye for the inter-eye asymmetry. The range for LCLA assessment is 0-60.

ITT population: all randomized subjects who received at least 1 dose of study treatment (BIIB033 or placebo) with an LCLA assessment at Baseline. LOCF imputation was used if Week 24 data were missing.

Adjusted mean change in LCLA at Week 24 from baseline as determined by 1.25% and 2.5% low contrast Sloan letter charts, adjusted for the baseline LCLA value. The fellow eye is the reference eye for the inter-eye asymmetry. The range for LCLA assessment is 0-60.

Per-protocol Population: participants from the ITT population who completed the study, did not miss more than 1 dose of BIIB033 or placebo, and did not receive MS-modifying therapies during the study period, which were prohibited per protocol. LOCF imputation was used if Week 24 data were missing.

Adjusted mean change in optic nerve conduction velocity (NCV) at Week 24 for the affected eye from the baseline of unaffected fellow eye as determined by FF-VEP. Adjusted for the baseline latency of fellow eye.

Per-protocol Population: participants from the ITT population who completed the study, did not miss more than 1 dose of BIIB033 or placebo, and did not receive multiple sclerosis (MS)-modifying therapies during the study period, which were prohibited per protocol. LOCF imputation was used if Week 24 data were missing.

An AE was any untoward medical occurrence that did not necessarily have a causal relationship with this treatment. An SAE was any untoward medical occurrence that at any dose: resulted in death; in the view of the Investigators, placed the subject at immediate risk of death (a life-threatening event); however, this did not include an event that, had it occurred in a more severe form, might have caused death; required inpatient hospitalization or prolongation of existing hospitalization; resulted in persistent or significant disability/incapacity; resulted in a congenital anomaly/birth defect; any other medically important event that, in the opinion of the Investigators, could have jeopardized the subject or may have required intervention to prevent one of the other outcomes listed in the definition above.

Safety population: all participants who received at least 1 dose of study treatment.

One pre-dose pharmacokinetic (PK) sample and 1 post-dose PK sample (approximately between 1 and 3 hours after the end of IV infusion) were collected for all participants on Day 1 and at Weeks 4 through 20 (every 4 weeks). Additionally, only 1 PK sample was collected at Week 24 and Week 32. (There was no dosing on Week 24 and Week 32, so only one blood sample for BIIB033 concentration was taken.) Samples collected at early termination visits were treated as predose samples for the next scheduled visit.

PK analysis population: all participants who received at least 1 dose of BIIB033 and had at least 1 serum concentration data on record. n=number of participants with a sample at given timepoint.