Efgartigimod for the Treatment of Acute Optic Neuritis

The goal of this pilot clinical trial is to test efgartigimod alfa against placebo in adults with first-time optic neuritis (optic nerve inflammation). The main questions it aims to answer are: * Is it feasible to use efgartigimod alfa for optic neuritis? * Is it feasible to run a larger trial testing efgartigimod alfa in optic neuritis? * Does efgartigimod alfa work better than placebo in improving how quickly and how much vision returns? Participants will: * have their vision and blood tested * be asked questions about their vision * will receive standard of care treatment with steroids regardless of whether they are receiving efgartigimod alfa or not * will have periodic visits over 6 months