This sounds like it may be what's called optic neuritis. This can be a precursor to multiple sclerosis. An MRI is an appropriate test in this case and if they didn't find any lesions on the brain that is fortunate.
A lab workup may be warranted, however, if this does not seem like optic neuritis to your ophthalmologist. Alternatively, you may consider a second opinion from another ophthalmologist or neurologist if there doesn't seem to be any consensus.
Optic neuritis responds well to IV steroids for quicker resolution, but the majority of people regain good, useful vision regardless (it just takes longer without steroids).
Does that address your concern?
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DISCLAIMER: My opinion is solely informative and does not constitute a formal medical opinion or recommendation. For a formal medical opinion and/or recommendation you must be examined by your doctor.