There are several things that this could be I am thinking. Has your husband had his thyroid values checked? FT4, T3, and TSH. Sometimes Graves disease can look similar to this... I was recently diagnosed with Graves, my mind was mush. I couldn't remember things for more than a few minutes. Your quadriceps atrophy as do your shoulder muscles... I started falling a lot. It's worth checking out. Graves patients also lose a lot of weight, fast.
Also sounds like it could be Lyme's disease. I know a man who was diagnosed with ALS (Lou Geherig's disease), when it was actually Lyme.
Neither of these conditions would stand out on an xray, mri, or standard blood tests. You have to be looking for them specifically.
Will be very interested to see what the MRI results are.
Feel free to email me if you'd like more info on my Graves ordeal. I will tell you it was a long slow decline for me (but not everyone.. sometimes it's very sudden).. but the last couple of years I saw several specialists, none of whom diagnosed me. I finally saw a show on TV, went to my GP asked for the full thyroid panel. Next business day Dr. is calling me all in a panic about my values.. called at least 3 times. It's been a rocky road.
My point is don't give up. Keep trying to figure it out yourself too. Sometimes we have to be our best advocates on getting the proper treatment and diagnosis.
(((((Ninja)))))