Hang in there!
Let me share with you that this has happened to me twice in my life. Same symptoms - loss of vision was definitely the most predominant and scary one. Happened in 1998 (I was 25) and again in 2008 (I was 35). Had MRIs done both times - with and without contrast. In '98, my doctor seemed convinced that they would find I had MS due to a strong family history of it on both sides. They also told me the MRI would check for brain tumors, aneurysms or other trauma. The MRI came back "showing nothing" (which was a big joke between my doctor and me for a long time - "Told you there'd be nothing between your ears"), but they diagnosed me with retro-bulbar optic neuritis (swelling of the optic nerve behind the eye). They had no idea what caused the swelling, which caused the vision loss, which caused the other symptoms, other than to say that it was possible that some kind of virus got into the nerve.
In '08 when it happened I was referred to a neuro-ophthalmologist. Again, they determined that my optic nerves were swollen and this time, raised. The neuro referred me for another MRI and this time I also got a carotid artery scan (did this to see if there was potentially a tear in the artery which could cause blood to not flow properly to the brain) and an echocardiogram/TEE (this was to determine if a potential hole in my heart was causing the issues), and the echo was internal. I had to swallow a camera. Not fun! They did not tell me this ahead of time so it was just dumb luck that I have a wonderful husband who demands to go with me to every appointment. Otherwise they woud have had to admit me because I wouldn't have been able to drive. Again - all these tests simply showed that the optic nerve was swollen and they could not determine why. The neuro essentially wrote it off to that and the possibility of migraines (which I've never had).
All this to say - don't let your wife get discouraged if they don't find anything. The tests are a pain and the possibilities will scare you, but just listen to the doctors and follow their lead. If you aren't happy, get a second opinion. When I had my routine eye exam with my ophthalmologist last Feb, the optic nerves were still swollen and raised. He did a bunch of other tests there in his office - retinal scans, visual fields, etc - and they cannot figure out why they appear to be permanently swollen. Don't know if we'll ever know. I anticipate hearing the same news when I go next week.
Anyway - sorry for the novel but just wanted you to know that you're not alone.
Good luck, and she'll be in my prayers!