Inclusion Criteria:
1. Provision of signed and dated informed consent form
2. Stated willingness to comply with all study procedures and availability for the duration of the study
3. Adults aged 18 years or older
4. Diagnosed with a first episode of optic neuritis, based on clinical presentation (i.e. typical features such as pain with eye movements, color vision changes, subacute presentation, and visual acuity loss) and confirmed by contrast enhancement or T2 hyperintensity of the optic nerve on MRI brain or orbits using a 1.5T MRI scanner or greater
5. Onset of optic neuritis-related vision changes (does not include headache, eye pain, or pain with eye movements), as defined by decreased visual acuity, subjectively reported blurred vision, or optic nerve enhancement on MRI brain or orbits, within 10 days (inclusive) of enrollment. If optic neuritis is bilateral, then enrollment must occur within 10 days of vision changes in the first affected eye.
6. Best-corrected high contrast visual acuity (HCVA) in the worse affected eye on the Early Treatment Diabetic Retinopathy Study (ETDRS) eye chart of logMAR 0.48 (20/60) or worse.
7. For females of reproductive potential: negative urine or serum pregnancy test at screening or use of highly effective contraception for at least 1 month prior to screening and agreement to use such a method during study participation and for an additional 8 weeks after the end of efgartigimod administration
8. For males of reproductive potential: use of condoms or other methods to ensure effective contraception with partner
Exclusion Criteria:
1. Current pregnancy or lactation
2. Known allergic reactions or intolerance to efgartigimod, methylprednisolone, prednisone, or gadolinium or any of their components
3. Known diagnosis of optic neuropathy preceding the current episode of optic neuritis
4. Evidence of a systemic disease other than MS, NMOSD, or MOGAD that might be associated with the optic neuritis
5. Receiving systemic immunomodulatory or immunosuppressive therapy at the time of enrollment or planned receipt within 3 weeks of treatment. Initiation of immunotherapy more than 3 weeks after the second dose of efgartigimod is not an exclusion criterion and is permitted.
6. Known diagnosis of CNS demyelinating disease (MS, NMOSD, MOGAD) prior to present attack.
7. Any visually-significant ocular pathology (i.e. retinal problems, cataracts, glaucoma etc.) in the affected eye that led to known best-corrected visual acuity deficits in participants prior to onset of optic neuritis. Congenital color-blindness is not disqualifying.
8. Alternative explanation for visual changes detected on fundoscopic exam and slit lamp examination.
9. Enrollment in another clinical study involving an investigational treatment given within 2 months of enrollment in the present study.
10. Contraindication to MRI or plasma exchange
11. Has received \>3 days of high-dose steroids (IV or PO) for the treatment of the current episode of acute optic neuritis by the time of randomization. Randomization may occur at the latest on the next day after completion of 3rd dose of steroids.
12. Known HIV disease or common variable immunodeficiency
13. History of malignancy unless considered cured by adequate treatment with no evidence of recurrence for ≥1 year before the first administration of IMP. Adequately treated participants with the following cancers may be included at any time:
1. Basal cell or squamous cell skin cancer
2. Carcinoma in situ of the cervix
3. Carcinoma in situ of the breast
4. Incidental histological finding of prostate cancer (TNM stage T1a or T1b)
14. Clinically significant uncontrolled active or chronic bacterial, viral, or fungal infection
15. Clinically significant recent major surgery (within 1 month of screening), or intends to have surgery during the study
16. Any conditions or circumstances that in the opinion of the investigator may put the participant at undue risk, confound the results of the study, or otherwise make the participant unsuitable for the study.
